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Summation on muscles


I am learning myology and encountered 2 problems in tetanus and summation:

  1. Unfused tetanus is just a continual summation of twitches if I am not mistaken. However, is it a MUST for summation / unfused tetanus to constantly increasing muscle tension from the photo (the graph) i.e. is this the definition?

Since we are in unfused tetanus when we hold something, and I can't feel my biceps are constantly increasing in contraction / tension. (to my understanding, complete tetanus is when we are lifting up very heavy things only as it utilizes too much ATP)

  1. Can I say the muscles are in tetanus during contracting (lifting), and also in tetanus during constantly contracted state (keep the thing holding up) Since tetanus is just a frequent action of sliding filaments, which is common in both actions?

Also, during tetanus, I don't need to shorten my muscles anymore (just holding something still), but myosin heads are still power-stroking to shorten the sarcomere/muscle, may I ask how to explain this?

Thank you very much.


Smooth Muscle

Smooth muscle is a type of muscle tissue which is used by various systems to apply pressure to vessels and organs. Smooth muscle is composed of sheets or strands of smooth muscle cells. These cells have fibers of actin and myosin which run through the cell and are supported by a framework of other proteins. Smooth muscle contracts under certain stimuli as ATP is freed for use by the myosin. The amount of ATP released depends on the intensity of the stimuli, allowing smooth muscle to have a graded contraction as opposed to the “on-or-off” contraction of skeletal muscle.


Motor Units

As you have learned, every skeletal muscle fiber must be innervated by the axon terminal of a motor neuron in order to contract. Each muscle fiber is innervated by only one motor neuron. The actual group of muscle fibers in a muscle innervated by a single motor neuron is called a motor unit. The size of a motor unit is variable depending on the nature of the muscle.

A small motor unit is an arrangement where a single motor neuron supplies a small number of muscle fibers in a muscle. Small motor units permit very fine motor control of the muscle. The best example in humans is the small motor units of the extraocular eye muscles that move the eyeballs. There are thousands of muscle fibers in each muscle, but every six or so fibers are supplied by a single motor neuron, as the axons branch to form synaptic connections at their individual NMJs. This allows for exquisite control of eye movements so that both eyes can quickly focus on the same object. Small motor units are also involved in the many fine movements of the fingers and thumb of the hand for grasping, texting, etc.

A large motor unit is an arrangement where a single motor neuron supplies a large number of muscle fibers in a muscle. Large motor units are concerned with simple, or “gross,” movements, such as powerfully extending the knee joint. The best example is the large motor units of the thigh muscles or back muscles, where a single motor neuron will supply thousands of muscle fibers in a muscle, as its axon splits into thousands of branches.

There is a wide range of motor units within many skeletal muscles, which gives the nervous system a wide range of control over the muscle. The small motor units in the muscle will have smaller, lower-threshold motor neurons that are more excitable, firing first to their skeletal muscle fibers, which also tend to be the smallest. Activation of these smaller motor units, results in a relatively small degree of contractile strength (tension) generated in the muscle. As more strength is needed, larger motor units, with bigger, higher-threshold motor neurons are enlisted to activate larger muscle fibers. This increasing activation of motor units produces an increase in muscle contraction known as recruitment. As more motor units are recruited, the muscle contraction grows progressively stronger. In some muscles, the largest motor units may generate a contractile force of 50 times more than the smallest motor units in the muscle. This allows a feather to be picked up using the biceps brachii arm muscle with minimal force, and a heavy weight to be lifted by the same muscle by recruiting the largest motor units.

When necessary, the maximal number of motor units in a muscle can be recruited simultaneously, producing the maximum force of contraction for that muscle, but this cannot last for very long because of the energy requirements to sustain the contraction. To prevent complete muscle fatigue, motor units are generally not all simultaneously active, but instead some motor units rest while others are active, which allows for longer muscle contractions. The nervous system uses recruitment as a mechanism to efficiently utilize a skeletal muscle.


Summation on muscles - Biology

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  • the impulse arrives at the end bulb,
  • chemical transmitter is released from vesicles (each of which contains 5,000 - 10,000 molecules of acetylcholine) and diffuses across the neuromuscular cleft,
  • the transmitter molecules fill receptor sites in the membrane of the muscle & increase membrane permeability to sodium,
  • sodium then diffuses in & the membrane potential becomes less negative,
  • and, if the threshold potential is reached, an action potential occurs, an impulse travels along the muscle cell membrane, and the muscle contracts.


Some muscles (skeletal muscles) will not contract unless stimulated by neurons other muscles (smooth & cardiac) will contract without nervous stimulation but their contraction can be influenced by the nervous system. Thus, the nervous and muscle systems are closely interconnected. Let's now focus on muscle - what is its structure & how does it work.


Highly magnified view of a neuromuscular junction (Hirsch 2007).

  • excitability - responds to stimuli (e.g., nervous impulses)
  • contractility - able to shorten in length
  • extensibility - stretches when pulled
  • elasticity - tends to return to original shape & length after contraction or extension
  • skeletal:
    • attached to bones & moves skeleton
    • also called striated muscle (because of its appearance under the microscope, as shown in the photo to the left)
    • voluntary muscle
    • smooth (photo on the right)
      • involuntary muscle
      • muscle of the viscera (e.g., in walls of blood vessels, intestine, & other 'hollow' structures and organs in the body)
      • muscle of the heart
      • involuntary


      Skeletal muscle structure

      Skeletal muscles are usually attached to bone by tendons composed of connective tissue. This connective tissue also ensheaths the entire muscle & is called epimysium. Skeletal muscles consist of numerous subunits or bundles called fasicles (or fascicles). Fascicles are also surrounded by connective tissue (called the perimysium) and each fascicle is composed of numerous muscle fibers (or muscle cells). Muscle cells, ensheathed by endomysium, consist of many fibrils (or myofibrils), and these myofibrils are made up of long protein molecules called myofilaments. There are two types of myofilaments in myofibrils: thick myofilaments and thin myofilaments.


      Source: Wikipedia

      Skeletal muscles vary considerably in size, shape, and arrangement of fibers. They range from extremely tiny strands such as the stapedium muscle of the middle ear to large masses such as the muscles of the thigh. Skeletal muscles may be made up of hundreds, or even thousands, of muscle fibers bundled together and wrapped in a connective tissue covering. Each muscle is surrounded by a connective tissue sheath called the epimysium. Fascia, connective tissue outside the epimysium, surrounds and separates the muscles. Portions of the epimysium project inward to divide the muscle into compartments. Each compartment contains a bundle of muscle fibers. Each bundle of muscle fiber is called a fasciculus and is surrounded by a layer of connective tissue called the perimysium. Within the fasciculus, each individual muscle cell, called a muscle fiber, is surrounded by connective tissue called the endomysium. Skeletal muscles have an abundant supply of blood vessels and nerves. Before a skeletal muscle fiber can contract, it has to receive an impulse from a neuron. Generally, an artery and at least one vein accompany each nerve that penetrates the epimysium of a skeletal muscle. Branches of the nerve and blood vessels follow the connective tissue components of the muscle of a nerve cell and with one or more minute blood vessels called capillaries (Source: training.seer.cancer.gov).


      The cell membrane of a muscle cell is called the sarcolemma, and this membrane, like that of neurons, maintains a membrane potential. So, impulses travel along muscle cell membranes just as they do along nerve cell membranes. However, the 'function' of impulses in muscle cells is to bring about contraction. To understand how a muscle contracts, you need to know a bit about the structure of muscle cells.


      Skeletal muscle is the muscle attached to the skeleton. Hundreds or thousands of muscle fibers (cells) bundle together to make up an individual skeletal muscle. Muscle cells are long, cylindrical structures that are bound by a plasma membrane (the sarcolemma) and an overlying basal lamina and when grouped into bundles (fascicles) they make up muscle. The sarcolemma forms a physical barrier against the external environment and also mediates signals between the exterior and the muscle cell.

      The sarcoplasm is the specialized cytoplasm of a muscle cell that contains the usual subcellular elements along with the Golgi apparatus, abundant myofibrils, a modified endoplasmic reticulum known as the sarcoplasmic reticulum (SR), myoglobin and mitochondria. Transverse (T)-tubules invaginate the sarcolemma, allowing impulses to penetrate the cell and activate the SR. As shown in the figure, the SR forms a network around the myofibrils, storing and providing the Ca 2+ that is required for muscle contraction.

      Myofibrils are contractile units that consist of an ordered arrangement of longitudinal myofilaments. Myofilaments can be either thick filaments (comprised of myosin) or thin filaments (comprised primarily of actin). The characteristic 'striations' of skeletal and cardiac muscle are readily observable by light microscopy as alternating light and dark bands on longitudinal sections. The light band, (known as the I-band) is made up of thin filaments, whereas the dark band (known as the A-band) is made up of thick filaments. The Z-line (also known as the Z-disk or Z-band) defines the lateral boundary of each sarcomeric unit. Contraction of the sarcomere occurs when the Z-lines move closer together, making the myofibrils contract, and therefore the whole muscle cell and then the entire muscle contracts (Source: Davies and Nowak 2006).

      The SARCOLEMMA has a unique feature: it has holes in it. These "holes" lead into tubes called TRANSVERSE TUBULES (or T-TUBULES for short). These tubules pass down into the muscle cell and go around the MYOFIBRILS. However, these tubules DO NOT open into the interior of the muscle cell they pass completely through and open somewhere else on the sarcolemma (i.e., these tubules are not used to get things into and out of the muscle cell). The function of T-TUBULES is to conduct impulses from the surface of the cell (SARCOLEMMA) down into the cell and, specifically, to another structure in the cell called the SARCOPLASMIC RETICULUM.


      A muscle fiber is excited via a motor nerve that generates an action potential that spreads along the surface membrane (sarcolemma) and the transverse tubular system into the deeper parts of the muscle fiber. A receptor protein (DHP) senses the membrane depolarization, alters its conformation, and activates the ryanodine receptor (RyR) that releases Ca 2+ from the SR. Ca 2+ then bind to troponin and activates the contraction process (Jurkat-Rott and Lehmann-Horn 2005).


      Sarcoplasmic reticulum (SR) membranes in close proximity to a T-tubule. 'RyR' are proteins the aid in the release of calcium from the SR, 'SERCA2' are proteins that aid in the transport of calcium into the SR (Brette and Orchard 2007).

      The SARCOPLASMIC RETICULUM (SR) is a bit like the endoplasmic reticulum of other cells, e.g., it's hollow. But the primary function of the SARCOPLASMIC RETICULUM is to STORE CALCIUM IONS. Sarcoplasmic reticulum is very abundant in skeletal muscle cells and is closely associated with the MYOFIBRILS (and, therefore, the MYOFILAMENTS). The membrane of the SR is well-equipped to handle calcium: there are "pumps" (active transport) for calcium so that calcium is constantly being "pumped" into the SR from the cytoplasm of the muscle cell (called the SARCOPLASM). As a result, in a relaxed muscle, there is a very high concentration of calcium in the SR and a very low concentration in the sarcoplasm (and, therefore, among the myofibrils & myofilaments). In addition, the membrane has special openings, or "gates", for calcium. In a relaxed muscle, these gates are closed and calcium cannot pass through the membrane. So, the calcium remains in the SR. However, if an impulse travels along the membrane of the SR, the calcium "gates" open &, therefore, calcium diffuses rapidly out of the SR & into the sarcoplasm where the myofibrils & myofilaments are located. This, as you will see, is a key step in muscle contraction.

      Myofibrils are composed of 2 types of myofilaments: thick and thin. In skeletal muscle, these myofilaments are arranged in a very regular, precise pattern: thick myofilaments are typically surrounded by 6 thin myofilaments (end view). In a side view, thin myofilaments can be seen above and below each thick myofilament.


      Myofibril cross-section showing arrangement of thick and thin myofilaments.
      Bar = 100 nm. Image from Widrick et al. (2001)


      Source: Tskhovrebova and Trinick (2003).

      Each myofibril is composed of many subunits lined up end-to-end. These subunits are, of course, composed of myofilaments and are called SARCOMERES. The drawings above & below show just a very small section of the entire length of a myofibril and so you can only see one complete SARCOMERE.

      In each sarcomere, thin myofilaments extend in from each end. Thick myofilaments are found in the middle of the sarcomere and do not extend to the ends. Because of this arrangement, when skeletal muscle is viewed with a microscope, the ends of a sarcomere (where only thin myofilaments are found) appear lighter than the central section (which is dark because of the presence of the thick myofilaments). Thus, a myofibril has alternating light and dark areas because each consists of many sarcomeres lined up end-to-end. This is why skeletal muscle is called STRIATED MUSCLE (i.e., the alternating light and dark areas look like stripes or striations). The light areas are called the I-BANDS and the darker areas the A-BANDS. Near the center of each I-BAND is a thin dark line called the Z-LINE (or Z-membrane in the drawing below). The Z-LINE is where adjacent sarcomeres come together and the thin myofilaments of adjacent sarcomeres overlap slightly. Thus, a sarcomere can be defined as the area between Z-lines.

      Thick myofilaments are composed of a protein called MYOSIN. Each MYOSIN molecule has a tail which forms the core of the thick myofilament plus a head that projects out from the core of the filament. These MYOSIN heads are also commonly referred to as CROSS-BRIDGES.

      The MYOSIN HEAD has several important characteristics:

      • it has ATP-binding sites into which fit molecules of ATP. ATP represents potential energy.
      • it has ACTIN-binding sites into which fit molecules of ACTIN. Actin is part of the thin myofilament and will be discussed in more detail shortly.
      • it has a "hinge"at the point where it leaves the core of the thick myofilament. This allows the head to swivel back and forth, and the "swivelling" is, as will be described shortly, what actually causes muscle contraction.

      The actin molecules (or G-actin as above) are spherical and form long chains. Each thin myofilament contains two such chains that coil around each other. TROPOMYOSIN molecules are lone, thin molecules that wrap around the chain of ACTIN. At the end of each tropomyosin is an TROPONIN molecule. The TROPOMYOSIN and TROPONIN molecules are connected to each other. Each of these 3 proteins plays a key role in muscle contraction:

      • ACTIN - when actin combines with MYOSIN HEAD the ATP associated with the head breaks down into ADP. This reaction released energy that causes the MYOSIN HEAD to SWIVEL.
      • TROPOMYOSIN - In a relaxed muscle, the MYOSIN HEADS of the thick myofilament lie against TROPOMYOSIN molecules of the thin myofilament. As long as the MYOSIN HEADS remain in contact with TROPOMYOSIN nothing happens (i.e., a muscle remains relaxed).
      • TROPONIN - Troponin molecules have binding sites for calcium ions. When a calcium ion fills this site it causes a change in the shape and position of TROPONIN. And, when TROPONIN shifts, it pulls the TROPOMYOSIN to which it is attached. When TROPOMYOSIN is moved, the MYOSIN HEAD that was touching the tropomyosin now comes in contact with an underlying ACTIN molecule.

      1 - Because skeletal muscle is voluntary muscle, contraction requires a nervous impulse. So, step 1 in contraction is when the impulse is transferred from a neuron to the SARCOLEMMA of a muscle cell.

      2 - The impulse travels along the SARCOLEMMA and down the T-TUBULES. From the T-TUBULES, the impulse passes to the SARCOPLASMIC RETICULUM.

      3 - As the impulse travels along the Sarcoplasmic Reticulum (SR), the calcium gates in the membrane of the SR open. As a result, CALCIUM diffuses out of the SR and among the myofilaments.

      4 - Calcium fills the binding sites in the TROPONIN molecules. As noted previously, this alters the shape and position of the TROPONIN which in turn causes movement of the attached TROPOMYOSIN molecule.

      5 - Movement of TROPOMYOSIN permits the MYOSIN HEAD to contact ACTIN (Animations: Myofilament Contraction & Breakdown of ATP and cross-bridge movement).

      6 - Contact with ACTIN causes the MYOSIN HEAD to swivel.

      7 - During the swivel, the MYOSIN HEAD is firmly attached to ACTIN. So, when the HEAD swivels it pulls the ACTIN (and, therefore, the entire thin myofilament) forward. (Obviously, one MYOSIN HEAD cannot pull the entire thin myofilament. Many MYOSIN HEADS are swivelling simultaneously, or nearly so, and their collective efforts are enough to pull the entire thin myofilament).

      8 - At the end of the swivel, ATP fits into the binding site on the cross-bridge & this breaks the bond between the cross-bridge (myosin) and actin. The MYOSIN HEAD then swivels back. As it swivels back, the ATP breaks down to ADP & P and the cross-bridge again binds to an actin molecule.

      9 - As a result, the HEAD is once again bound firmly to ACTIN. However, because the HEAD was not attached to actin when it swivelled back, the HEAD will bind to a different ACTIN molecule (i.e., one further back on the thin myofilament). Once the HEAD is attached to ACTIN, the cross-bridge again swivels, SO STEP 7 IS REPEATED.

      As long as calcium is present (attached to TROPONIN), steps 7 through 9 will continue. And, as they do, the thin myofilament is being "pulled" by the MYOSIN HEADS of the thick myofilament. Thus, the THICK & THIN myofilaments are actually sliding past each other. As this occurs, the distance between the Z-lines of the sarcomere decreases. As sarcomeres get shorter, the myofibril, of course, gets shorter. And, obviously, the muscle fibers (and entire muscle) get shorter.

      Skeletal muscle relaxes when the nervous impulse stops. No impulse means that the membrane of the SARCOPLASMIC RETICULUM is no longer permeable to calcium (i.e., no impulse means that the CALCIUM GATES close). So, calcium no longer diffuses out. The CALCIUM PUMP in the membrane will now transport the calcium back into the SR. As this occurs, calcium ions leave the binding sites on the TOPONIN MOLECULES. Without calcium, TROPONIN returns to its original shape and position as does the attached TROPOMYOSIN. This means that TROPOMYOSIN is now back in position, in contact with the MYOSIN HEAD. So, the MYOSIN head is no longer in contact with ACTIN and, therefore, the muscle stops contracting (i.e., relaxes).

      So, under most circumstances, calcium is the "switch" that turns muscle "on and off" (contracting and relaxing). When a muscle is used for an extended period, ATP supplies can diminish. As ATP concentration in a muscle declines, the MYOSIN HEADS remain bound to actin and can no longer swivel. This decline in ATP levels in a muscle causes MUSCLE FATIGUE. Even though calcium is still present (and a nervous impulse is being transmitted to the muscle), contraction (or at least a strong contraction) is not possible.

      Animations illustrating muscle contraction:

      2 - Myosin head energized via myosin-ATPase activity which converts the bound ATP to ADP + Pi
      3 - Calcium binds to troponin
      4 - Tropomyosin translocates to uncover the cross-bridge binding sites
      5 - The energized myosin binding sites approach the binding sites
      6 - The first myosin head binds to actin
      7 - The bound myosin head releases ADP + Pi, flips and the muscle shortens
      8 - The second myosin head binds to actin
      9 - The first myosin head binds ATP to allow the actin and myosin to unbind
      10 - The second myosin head releases its ADP + Pi, flips & the muscle shortens further
      11 - The second myosin head binds to ATP to allow the actin and myosin to unbind
      12 - The second myosin head unbinds from the actin, flips back and is ready for the next cycle
      13 - The cross-bridge cycle is terminated by the loss of calcium from the troponin
      14 - Tropomyosin translocates to cover the cross-bridge binding sites
      15 - The calcium returns to the sarcoplasmic reticulum, the muscle relaxes & returns to the resting state

      Types of contractions:

      Twitch - the response of a skeletal muscle to a single stimulation (or action potential):

      • latent period - no change in length time during which impulse is traveling along sarcolemma & down t-tubules to sarcoplasmic reticulum, calcium is being released, and so on (in other words, muscle cannot contract instantaneously!)
      • contraction period - tension increases (cross-bridges are swivelling)
      • relaxation period - muscle relaxes (tension decreases) & tends to return to its original length

      An important characteristic of skeletal muscle is its ability to contract to varying degrees. A muscle, like the biceps, contracts with varying degrees of force depending on the circumstance (this is also referred to as a graded response). Muscles do this by a process called summation, specifically by motor unit summation and wave summation.

      Motor Unit Summation - the degree of contraction of a skeletal muscle is influenced by the number of motor units being stimulated (with a motor unit being a motor neuron plus all of the muscle fibers it innervates see diagram below). Skeletal muscles consist of numerous motor units and, therefore, stimulating more motor units creates a stronger contraction.

      Wave Summation - an increase in the frequency with which a muscle is stimulated increases the strength of contraction. This is illustrated in (b). With rapid stimulation (so rapid that a muscle does not completely relax between successive stimulations), a muscle fiber is re-stimulated while there is still some contractile activity. As a result, there is a 'summation' of the contractile force. In addition, with rapid stimulation there isn't enough time between successive stimulations to remove all the calcium from the sarcoplasm. So, with several stimulations in rapid succession, calcium levels in the sarcoplasm increase. More calcium means more active cross-bridges and, therefore, a stronger contraction. (Wiley animation)

      If a muscle fiber is stimulated so rapidly that it does not relax at all between stimuli, a smooth, sustained contraction called tetanus occurs (illustrated by the straight line in c above & in the diagram below).

      • involuntary muscle innervated by the Autonomic Nervous System (visceral efferent fibers)
      • found primarily in the walls of hollow organs & tubes
      • spindle-shaped cells typically arranged in sheets
      • cells do not have t-tubules & have very little sarcoplasmic reticulum
      • cells do not contain sarcomeres (so are not striated) but are made up of thick & thin myofilaments. Thin filaments in smooth muscle do not contain troponin.
      • calcium does not bind to troponin but, rather, to a protein called calmodulin. The calcium-calmodulin complex 'activates' myosin which then binds to actin & contraction (swivelling of cross-bridges) begins.


      Two types of smooth muscle:

        • found in the walls of hollow organs (e.g., small blood vessels, digestive tract, urinary system, & reproductive system)
        • multiple fibers contract as a unit (because impulses travel easily across gap junctions from cell to cell) &, in some cases, are self-excitable (generate spontaneous action potentials & contractions)
          2 - multiunit smooth muscle
          • consists of motor units that are activated by nervous stimulation
          • found in the walls of large blood vessels, in the eye (adusting the shape of the lens to permit accommodation & the size of the pupil to adjust the amount of light entering the eye), & at the base of hair follicle (the 'goose bump' muscles)

          Brette, F., and C. Orchard. 2007. Resurgence of cardiac T-tubule research. Physiology 22: 167-173.

          Davies, K. E., and K. J. Nowak. 2006. Molecular mechanisms of muscular dystrophies: old and new players. Nature Reviews Molecular Cell Biology 7: 762-773.

          Hirsch, N. P. 2007. Neuromuscular junction in health and disease. British Journal of Anaesthesia 99: 132-138.

          Jurkat-Rott, K., and F. Lehmann-Horn. 2005. Muscle channelopathies and critical points in functional and genomic studies. Journal of Clinical Investigation 115: 2000-2009.

          Tskhovrebova, L., and J. Trinick. 2003. Titin: properties and family relationships. Nature Reviews Molecular Cell Biology 4: 679-689.

          Widrick, J. J., J. G. Romatowski , K. M. Norenberg , S. T. Knuth , J. L. W. Bain , D. A. Riley , S. W. Trappe , T. A. Trappe , D. L. Costill , and R. H. Fitts. 2001. Functional properties of slow and fast gastrocnemius muscle fibers after a 17-day spaceflight. Journal of Applied Physiology 90: 2203-2211.


          Properties of Muscles

          The Muscle cells, also known as muscle fibers, are the fundamental units of human muscles. Humans have three types of muscle: skeletal, smooth and cardiac. The muscle cells share eight properties in general that distinguish them from other cells. Those eight properties of muscles are as follows:
          1) Excitability
          2) Contractility
          3) All or none law
          4) Refractory period
          5) Summation
          6) Tetanus
          7) Fatigue
          8) Rigor mortis
          Now we shall go for some discussion:

          Excitability

          Ability of a tissue to respond to stimulation is called Excitability. Like nerve fibers, when a muscle fiber is stimulated by an adequate stimulus, it becomes active. That means, it generates an action potential which is transmitted throughout the sarcolemma. Excitability as the properties of muscles is expressed by two factors—rheobase and chronaxie. Rheobase is the threshold of intensity of electric current capable of exciting the tissue no matter how long it is given. Whereas Chronaxie is the threshold of duration required to excite a tissue when the strength of stimulus is double the rheobase. Excitability of a tissue is inversely proportional to its chronaxie. Skeletal muscles and multi-unit smooth muscles are normally excited through nervous stimulation only. But cardiac muscle and single unit smooth muscles are auto-excitable .

          Contractility

          Contractility is a unique property of muscles by virtue of which when a muscle is excited, it contracts followed by relaxation. Skeletal muscles contract more quickly and forcefully than other muscles. So, their efficiency or capacity of taking work load is quite greater. On the other hand cardiac and smooth muscles show slow, continuous contractions.

          All or none law

          According to this law of properties of muscles, when a muscle is stimulated, either it responds maximally or it does not respond at all. If the stimulus is adequate in strength the response will be maximum and a further increase in the intensity of stimulus will not raise the degree of contraction any more if other conditions remain same. On the other hand, if the stimulus is inadequate, it will totally fail to bring out a response and the muscle will not contract at all.

          Refractory period

          Refractory period is the time gap during which a second stimulus fails to excite a tissue. In this property of muscles, in order to produce two successive responses, the second stimulus must fall after the refractory period from the first stimulus. In mammalian skeletal muscles, the refractory period is about 0.002 sec. Due to the short refractory period the skeletal muscles show abridgment of contractions on repetitive stimulation. The refractory periods of cardiac and smooth muscles are much longer than skeletal muscles. In case of cardiac muscle, the refractory period is longest and extends throughout the contraction and relaxation periods.

          Summation

          When a skeletal muscle is stimulated by two stimuli in rapid succession, the contractile responses of the two stimuli are added together to produce a greater response this properties of muscles are called summation. When two successive stimuli are given in a way that the second stimulus falls during the relaxation or contraction phase of the first response, the second response is super imposed on the first response producing greater force of contraction this is called summation of contractions. If the second stimulus falls within the latent period of the first response, the effects of two stimuli are completely fused to produce a single contractile response which is greater than that would occur due to a single stimulus this is called summation of stimuli.

          Tetanus

          During tetanus the muscle remains contracted throughout the period of stimulation and it does not get a chance at all to relax. Tetanus is the properties of muscle that sustained muscular contraction without intervening periods of relaxation. When the frequency of repetitive stimuli is lower so that each successive stimulus falls within the period of previous relaxation, the muscle shows incomplete relaxations between gradually increasing contractions this is called incomplete tetanus. Thus incomplete tetanus is due to summation of repetitive contractions whereas tetanus is due to summation of repetitive stimuli. Because of their long refractory periods cardiac and smooth muscles are not easily totalized.

          Fatigue

          Fatigue is one of the properties of muscles which is a state of temporary loss of excitability of a tissue due to overwork. When a muscle is stimulated repeatedly its contractility diminishes gradually and in due course it fails to respond. This happening is called fatigue. Muscular fatigue is developed due to two main reasons—(i) lack of supply of oxygen and food, and (ii) accumulation of waste products. Generally a fatigued muscle recovers the ability to contract after a period of rest.

          Rigor mortis

          Stiffening of the body after death due to a state of rigidity of muscles is called rigor mortis and are the terminal properties of muscles. It occurs 2-3 hours after death when all muscles of the body go into a state of contracture and become rigid without any stimulation. Rigor mortis disappears 24-48 hours after death when muscle proteins are decomposed due to autolysis caused by release of lysosomal enzymes.


          Muscles: Meaning and Groups | Biology

          In this article we will discuss about the meaning and groups of muscles.

          Meaning of Muscles:

          Muscles are essential to the body to provide stability and mobility. They are present in almost all the parts of the body and the type of muscle that is present in a particular part of the body is functionally very well suited for that region. Like neurons, even muscles are excitable tissues.

          The classification of muscle can be done based on various criteria:

          i. Functionally, they can be classified into voluntary and involuntary.

          ii. Histologically into striated and non-striated.

          Groups of Muscles:

          The three groups of muscles present in the body are:

          A. Skeletal Muscles:

          iii. Attached to the skeleton through tendon.

          iv. Composed of many muscle fibers, which are parallel to each other in arrangement.

          v. Each of the muscle fiber in turn is made of many sarcomeres, which are also arranged serially.

          vi. For the muscle to act, the impulse has to come from CNS (brain or spinal cord).

          Sarcomere (Fig. 2.16):

          i. Band A is present in the center is made up of myosin proteins. In the center of the A band, there is narrow gap that is known as H band. In the center of the H band is the M line. A band (anisotropic band) is so- called because light cannot pass through this band.

          ii. Extending from the Z line on either side of the A band is I band. This is called I band (isotropic band) because light can pass through this band.

          iii. I band is composed of actin proteins. In addition to this, in I band there will be presence of troponin and tropomyosin proteins.

          iv. Under resting conditions though there is some amount of overlap between bands A and I, there is some gap in the center and this is H band.

          v. During contraction, I band slides over the A band and bring about the shortening of the muscle fiber.

          vi. Myosin and actin proteins are the contractile proteins in a sarcomere.

          vii. In addition to these proteins, the other proteins present are troponin and tropomyosin. These are called as regulatory proteins.

          viii. The protoplasmic membrane namely the sarcolemmal membrane covers the muscle fiber. This membrane shows dipping in at specific parts that is at the junction of bands A and I. This part of the sarcolemmal membrane is known as T tubule (transverse tubule).

          ix. Sarcoplasmic reticulum is placed horizontally and between the two consecutive T tubules. These are known as L tubules (longitudinal tubules). The ends of the L tubules are dilated and are known as lateral cisterns.

          x. The function of the T tubule is to conduct the impulse through the muscle fiber and that of L tubules (terminal cisterns of this) is to store and release of calcium ions during the process of contraction.

          xi. Two T tubules and one L tubule together constitute sarcoplasmic triad.

          Microscopic examination also shows two other important structures in the muscle. The sarcolemmal sheath covering the muscle dips into the muscle forming the T tubules. The action potentials produced at the neuromuscular junction travel along the sarcolemmal membrane enter the interior of the muscle fiber along the T tubules.

          The other structure is the longitudinal tubules (L tubules) with their expanded ends as the lateral cisterns. They closely surround the myofibrils. These form the sarcoplasmic reticulum. The expanded portion (terminal cistern) stores ionic calcium, which plays important role in excitation-contraction coupling of muscle.

          Excitation-Contraction Coupling:

          Events during Excitation-Contraction Coupling:

          A muscle action potential reaches the T tubule by traveling along the sarcolemmal membrane. This triggers the release of the calcium ions from the cisterns of the longitudinal tubules. The calcium ions occupy the C part of the troponin molecule. This in turn brings about a conformational change in the tropomyosin molecule.

          This change is responsible for exposing the active site on the actin filaments. The head of the myosin filament gets attached to the active sites and stepwise it gets attached and detached to the active sites. In this process, the actin filament is drawn inwards towards the center of the sarcomere. This requires energy and it is supplied by the breakdown of ATP.

          Myosin head itself acts as ATPase (actin myosin ATPase) and ATP is broken down to ADP and high energy PO4 is released. The number of cross bridges occupied depends on the amount of ionic calcium available. Greater the amount of ionic calcium, greater will be the number of binding between actin and myosin and, therefore, greater will be the force or tension developed.

          During this process, the width of the H band decreases and hence the width of the sarcomere decreases (Figs 2.17a to c). The width of the A band remains unchanged. There may be overlapping of the actin filaments at the center of the sarcomere. This is known as the sliding filament theory of muscle contraction.

          Immediately following this, the calcium ions are actively pumped back into the L tubules by means of calcium pump. The pumping of calcium into cisterns also requires expenditure of energy.

          Thus ATP has two important roles in the muscle:

          (1) it is necessary for muscle contraction and

          (2) Also necessary for muscle relaxation (Figs 2.18 and 2.19). This action of ATP is known as the plasticizer action of ATP.

          Four important changes occur in the muscle when it is stimulated to contract:

          1. Electrical—in the form of muscle action potential

          2. Mechanical—in the form of muscle contraction.

          3. Chemical—in the form of breakdown of ATP and creatnine phosphate

          4. Thermal—in the form of heat production.

          When a muscle is made to contract, two types of contractions may be noted.

          During an isotonic type of muscle contraction, the length of the muscle fiber decreases but the tension in the muscle fiber remains the same. When a weight is lifted leads to certain amount of external work is done, and this is an example for isotonic contaction.

          In isometric type of muscle contraction, the length of the muscle fiber remains the same but the tension developed in the muscle is increased. Example for isometric contraction is pushing against a wall. Walking is a good example for both isometric and isotonic type of contraction. The muscles of the limb which is on the ground contract isometrically to support the body weight and the muscles in limb which is lifted up to move contract isotonically.

          Chemical changes that occur in the muscle during muscular contraction:

          The immediate source of energy supply for muscle contraction is by the breakdown of adenosine triphosphate (ATP). During muscular contraction, it is found out that the ATP content of the muscle is not markedly decreased. This shows that ATP is not only broken down but it is also getting synthesized.

          The PO4 (high energy phosphate), required for the resynthesis of ATP from ADP, is obtained from the breakdown of creatine phosphate. During repeated contraction of the muscle, the required energy can also come from the breakdown of glucose or glycogen. Free fatty acids can also provide energy for muscular contraction.

          A certain amount of heat is released even when the muscle is at rest. This is known as the resting heat. When the muscle is made to contract, some amount of heat is generated known as the heat of shortening. During relaxation of the muscle, the heat that is produced is known as the heat of relaxation. These can be measured by using thermocouples.

          A gastrocnemius-sciatic (GS) nerve preparation is used to study the properties of skeletal muscle contraction. When a threshold stimulus is applied to the sciatic nerve, the muscle responds by contraction, which can be recorded on a moving drum. The recording is known as a simple muscle twitch. There is a short time lag between the application of the stimulus and the onset of contraction. This duration is known as the latent period.

          Causes for the latent period are:

          1. The time taken for the nerve action potential to reach the neuromuscular junction.

          2. The time taken for the release of ACh.

          3. Time taken for the production of the muscle action potential, etc.

          Following this, the muscle starts contracting and the contraction reaches its maximum. This duration from the onset of contraction until the peak of contraction is known as the contraction period. After the contraction peak, the muscle fibers start relaxing.

          The duration from the peak of contraction until the complete relaxation is called relaxation period. The total time required for the twitch period (from the moment of application of stimulus to the relaxation of muscle is complete), will be approximately about 100 milliseconds.

          The latent period is approximately 10 milliseconds. The first half of the latent period is absolute refractory period. After the first stimulus, whatever is the intensity of the second stimulus applied during this period, it will not have any effect on the muscle.

          Following the absolute refractory period, during the contraction phase, if a second stimulus is applied, a bigger contraction is obtained. This effect is known as wave summation. The effect of two stimuli is added together resulting in a bigger contraction.

          If a second stimulus is applied during the relaxation period of the first response, a second contraction is obtained the second stimulus will not allow the muscle to relax completely before another contraction starts. The response is termed as superposition.

          If a second stimulus of the same strength is applied after the complete relaxation period for the first response, the curve obtained is bigger than the first response. This is known as the beneficial effect.

          The beneficial effect is due to:

          i. Increase in the ionic calcium available at the actin and myosin level.

          ii. Slight decrease in the viscosity of the muscle proteins.

          iii. Slight increase in the temperature due to the previous contraction.

          iv. Slight fall in pH in muscle.

          Instead of a second stimulus after the relaxation has started, if a number of stimuli are applied one following the other at very short intervals during the contractile phase, the responses for the different stimuli get added up.

          This results in a sustained contraction called as tetanus (tetanic type of a response). This type of a response can be produced in the skeletal muscle fibers. Since the cardiac muscle fiber has a long absolute refractory period, it cannot be tetanized.

          It is applicable to all the types of muscle fibers. The law states that force of contraction in the muscle is directly proportionate to the initial length of the muscle fiber within physiological limits. Greater the initial length, greater will be the force of contraction. This can be demonstrated by performing experiments on a gastrocnemius-sciatic preparation.

          Muscle contractions are recorded when the muscle is preloaded or when it is after-loaded state. It is observed that the height of the contraction obtained is much larger when the muscle is preloaded than when it is after-loaded.

          Preloading a muscle will increase its initial length unlike when it is after-loaded wherein the load starts acting on the muscle only after muscle starts contracting. This will not alter the initial length of the muscle fiber.

          When a muscle is repeatedly stimulated, the amplitude of the response gradually gets decreased. The work done by the muscle gradually decreases and a stage is reached when the muscle fails to respond. The relaxation becomes incomplete. When this happens, it shows that the muscle has undergone fatigue.

          Which is the seat of fatigue?

          In an isolated GS preparation, the seat of fatigue is the neuromuscular junction. It is due to exhaustion of acetylcholine. This can be proved by directly stimulating the muscle after the GS preparation has failed to produce a response when stimulated through the nerve. When the muscle is stimulated directly, the muscle responds again.

          The motor nerve is not the seat of fatigue. Recording action potentials from the nerve of a GS preparation which has demonstrated fatigue can prove this. The muscle might have undergone complete fatigue but action potentials can still be recorded from the nerve fiber. This shows that the nerve is not the seat of fatigue.

          In the case of human beings, the seat of fatigue is muscle itself. This can be proved by finger ergography. Blood pressure cuff is tied over the upper arm and the sling of the ergometer is hooked to the index finger. The person is asked to repeatedly lift the weight attached to the instrument till the muscles get tired.

          This performance can be recorded and the duration for which the exercise was carried out can be noted. During the exercise, the blood flow to the exercising muscle gets increased and this washes away the metabolic products that are produced. Next the whole procedure is repeated by inflating the blood pressure cuff so that the venous return is prevented.

          Inflation of cuff prevents the metabolic waste getting washed away. Hence they get accumulated at the muscle itself. The duration at the end of which fatigue sets in is noted and compared. The fatigue sets in early when the cuff is in inflated state suggests that the seat of fatigue is the muscle itself and it is due to accumulation of the metabolic waste.

          Muscular contraction is associated with production of lactic acid. As more lactic acid accumulates at the actin and myosin site, it prevents the sliding mechanism. The site of fatigue in the CNS is the synapse.

          Fatigue can be postponed in the human body. During exercise, adrenaline is secreted. This in turn increases the blood glucose and free fatty acid levels in the circulation. These supply the necessary fuel for muscular contraction. It also increases the blood flow to the muscle tissue by bringing about vasodilatation. Thus adrenaline postpones fatigue and this action of adrenaline is called Orbelli’s effect.

          B. Smooth Muscle:

          i. This is another type of muscle present in the body.

          ii. It is non-striated that is there are no definite cross- striations in the muscle fibers.

          iii. Thick and thin filaments are present with no regular arrangement of the filaments.

          iv. It is supplied by nerve fiber belonging to autonomic nervous system.

          v. Hence the function of these muscles is not under voluntary control.

          Types of Smooth Muscle (Table 2.6):

          There are two types of smooth muscle namely visceral/single unit/unitary smooth muscle and multi-unit smooth muscle.

          Visceral Smooth Muscle:

          i. In this type of muscle, the propagation of action potential is from cell to cell. That is the whole of the muscle acts as a single unit (structural syncitium).

          ii. It shows spontaneous development of action potential.

          iii. Present in the walls of GI tract, uterus, urinary bladder, etc.

          Multi-unit Smooth Muscle:

          i. Each fiber is almost similar to the skeletal muscle fiber but there are no definite cross-striations.

          ii. There is no cell to cell propagation of impulse just like what is seen in skeletal muscle fiber.

          iii. There is no spontaneous activity in the muscle fiber.

          iv. Present in iris, and other examples are ciliaris muscle, erector pilorum muscle, etc.

          The activity of the visceral smooth muscle is influenced by:

          i. Impulses coming along the autonomic nervous system.

          ii. Stretch of the smooth muscle.

          iii. Hormones adrenaline, thyroxine, etc. acting on it.

          iv. Local factors like hypoxia, hypercapnia, acidosis, and other inorganic ions, like potassium, sodium, etc.

          v. Apart from, ICF calcium, even ECF calcium has role to play in the process of contractions.


          Summation on muscles - Biology

          Motor units:
          All motor neurones leading to a muscle fibre end in branches. Each leading to a neuromuscular junction. Nerve impulses pass ing down this motor neurone will trigger contractions in all the muscles fibres found at the end of the branches. A motor unit i s: A motor neurone and all its muscle fibres.


          Each muscle fibre within a motor unit either contracts or does not contract maximally: ‘ALL OR NOTHING LAW’ (either the neurone wi l l fire a response or it won't. There is no partial response).

          It should be obvious that different activities require a different strength of contraction – dart/shot put.

          The strength of a contraction depends largely on the number of units recruited and the size of the units involved.

          This is called spatial summation. To create a greater force of contraction the brain recruits more and larger motor units.

          Even in resting muscle skeletal muscles are in a partial state of contraction called ‘tonus’ maintained by the activation of a few motor units at a time


          Muscles

          Muscles generate force. In humans they comprise 40–50% of the total body weight. Muscle is:

          • excitable: it receives and responds to stimuli
          • contractile: it shortens and thickens to do work
          • extensible: it can be stretched passively when relaxed
          • elastic: it returns to its original shape after contraction and extension.

          Note that muscles only do work when they contract: contraction is the active process.

          Most muscles associated with the skeleton are arranged in opposing, antagonistic pairs: when one contracts the other is passively stretched (e.g. the biceps and the triceps bending and straightening the human elbow). In bivalve molluscs, the closing adductor muscles work against the elastic spring ligament in the hinge of the shell.

          Functions of Muscle

          There are three major functions for muscle:

          (1) motion, including locomotion

          (2) maintenance of posture

          (3) heat generation (particularly in homoiotherms): approximately 85% of human body heat is generated by muscle contractions brief involuntary muscle contractions (shivering) generate heat when the animal is cold.

          Muscle types in Mammals

          There are three types of muscle tissue.

          (1) Striped (striated) muscle moves the skeleton often called voluntary (because its movement is usually voluntary) or skeletal muscle.

          (2) Cardiac muscle is a special type of striped muscle in the walls of the heart contraction is involuntary.

          (3) Smooth muscle surrounds the walls of internal organs such as the gut, bladder, blood vessels and the uterus contraction is involuntary.

          Striped muscle

          • The disposition of striped (skeletal or striated) muscles and their names is best studied using an illustrated guide. Striped muscles are surrounded by superficial fasciae composed of connective and adipose tissues: they provide a route for nerves and blood vessels.
          • Deep fascia is connective tissue which holds muscles together and separates them into functioning units. The entire muscle is also wrapped in fibrous connective tissue: the epimysium.
          • Bundles of muscle cells (fasciculi) are covered by perimysium. Endomysium penetrates into the fascicles and surrounds and separates the muscle cells.
          • The epimysium, perimysium and endomysium contribute collagenous fibers to tendons which attach muscle to the periosteum of bone.
          • Tendons can be flat or broadly cylindrical. Muscles are richly vascularized to deliver oxygen and glucose and to remove waste products they are also extensively innervated.
          • The muscle fibers (or cells, bounded by a plasma membrane or sarcolemma) contain contractile organelles called myofibrils each has a bundle of myofilaments divided lengthways into repeating structural units called sarcomeres.
          • The sarcomeres are divided into bands of different densities, giving the striped appearance characteristic of this muscle type.

          There are two kinds of myofilaments:

          X-ray crystallography shows that, in cross-section, the filaments line up in hexagonal array. Actin filaments are held by their attachment to the Z lines (made up of the protein α-actinin). I bands are regions containing actin filaments alone the light H band in the middle of each sarcomere is where only the heavy myosin filaments are present the myosin and actin filaments overlap in the dark A bands.

          Sliding filament Model

          • When a striped muscle contracts, the filaments (which do not change length) slide past each other. In each sarcomere, the many globular myosin heads which project laterally along each end of the heavy myosin filament attach to the actin filament and change conformation.
          • The myosin pulls at the actin filaments adjacent to it. The myosin heads have been energetically charged, adopting a conformation in which they can bind to actin.

          • This binding elicits the conformational change that provides the force for filament sliding and exposes an ATP-binding site.
          • ATP binding causes an allosteric (shape) change that promotes detachment of the head from actin. Dephosphorylation of ATP provides the energy to re-establish the actin binding thus the process is repeated many times (each using one ATP molecule) and the myosin pulls along the actin filament in a ratchet fashion.
          • Since the ends of each myosin filament pull in opposite directions, towards the sarcomere center, the myosin pulls the two actin regions closer and with them the Z lines: thus the whole muscle contracts. (Actin–myosin systems are found elsewhere: e.g. pulling chromosomes apart at mitosis.)

          Energy for Contraction

          • The energy for contraction is provided by ATP: the myosin heads have an ATPase, the active site of which is exposed upon actin binding.
          • The ATP synthesized and stored in muscle can only sustain a few seconds of vigorous exercise. Striped muscles contain high levels of phosphocreatine: this breaks down to release energy [some of which is used to make more ATP from adenosine diphosphate (ADP) and the phosphate is released]:

          Longer-term energy needs must be supplied by cellular respiration:

          • breakdown of glycogen (via glucose) to pyruvate with the synthesis of ATP, used as above: this is glycolysis and does not require oxygen
          • oxidation of pyruvate, in the presence of oxygen, to yield carbon dioxide, water and large amounts of ATP (38 molecules of ATP following the total oxidation of one glucose molecule: this is aerobic respiration).
          • In the absence of oxygen, the pyruvate is normally converted to lactate: this is anaerobic metabolism. In heavy exercise, the lactate spills over into the blood.
          • In humans, the glycogen/lactic acid system can provide energy for about 40 seconds of maximum muscle activity. The lactate constitutes an ‘oxygen debt’ which must later be paid off by oxidizing the lactate to carbon dioxide and water.
          • Aerobic respiration can continue more or less indefinitely, provided oxygen and glucose (or another fuel such as fats) are available.

          Regulation of Contraction

          • Regulation of contraction is effected primarily by the proteins tropomyosin and troponin. Tropomyosin blocks the actin sites to which myosin will attach.
          • Troponin has sites which bind calcium: this changes the troponin shape, so allowing the troponin to displace (‘push aside’) the tropomyosin in order that actin and myosin can interact.
          • Neuromuscular transmitter release (acetylcholine from the endings of motor neurons innervating striped muscle in mammals) triggers an action potential in the plasma membrane (sarcolemma) of the muscle fiber (cell).
          • The T (transverse)tubule system is continuous with the cell membrane (sarcolemma), and runs through the muscle fibers close to the Z lines. Thus action potentials can be rapidly transmitted into the fibers.
          • The sarcoplasmic reticulum is a second system which encloses the myofibrils. It contains calcium ions, and by sequestering them it keeps the concentration around the myofibrils low.
          • The reticulum expands near the Z lines of each sarcomere where the reticulum is in intimate contact with the T tubule system (although the two systems are not physically continuous).
          • The action potential is transmitted along the membranes of the T tubules (propagation is similar to that along the plasma membrane of a neuron) and stimulates calcium release from the sarcoplasmic reticulum, so that the intracellular level of calcium increases 100-fold the calcium initiates contraction by binding to troponin.
          • Immediately after the action potential has activated the system, the calcium is pumped back into the sarcoplasmic reticulum (an ATP-dependent process) acetylcholinase (or acetylcholinesterase) breaks down the neuromuscular transmitter acetylcholine.
          • Each contraction lasts for a few tenths of a second, until the calcium pumps have reduced the intracellular calcium pool to a point where the contractile apparatus is no longer operational repeated action potentials cause long-term contraction or muscle tonus or tetanus.

          Cardiac muscle

          Cardiac muscle resembles striped muscle in that it has similar assemblies of actin and myosin and has a striated appearance. The fibers are shorter and have branched ends. Contraction is spontaneous and involuntary. Gap junctions in the intercalated disks that join the branched cells longitudinally transmit action potentials so that the whole of the muscle contracts synchronously.

          Smooth muscle

          Smooth, involuntary muscle is also made up of actin and myosin, but the molecules are arranged much less regularly so there are no striations (thus ‘smooth’). Actin–myosin interactions are similar to those in striped muscle, although the actin filaments are attached to dense bodies rather than to Z lines. Calcium is not stored in a sarcoplasmic reticulum, but in the extracellular fluid. Contractions tend to be slower and more prolonged. In hollow organs such as the gut, the muscle fibers are arranged in sheets with a longitudinal orientation outside and a circular orientation inside.

          In vertebrates, smooth muscle fibers are divisible into two major types.

          (1) Multi-unit smooth muscle fibers (e.g. in the iris of the eye): here nerve impulses reach each fiber from a motor nerve end-plate.

          (2) Visceral smooth muscle fibers (e.g. gut and uterus walls): here only a few fibers have motor nerve end-plates, the action potential flowing to adjacent fibers through gap junctions. Such muscle fibers will contract spontaneously when stretched.

          Muscle contraction physiology

          • For most vertebrate striped muscles, a motor neuron branches so that there are several motor end-plates each on one of several muscle fibers (cells): the neuron plus its fibers is the motor unit.
          • A nerve impulse causes all fibers in the unit to contract simultaneously. The more fibers per unit, the less the control which can be exercised (some eyeball muscles may have only two or three fibers per neuron while locomotory muscles may have 400–500 fibers per neuron).
          • Contraction of a fiber may be initiated by a single nerve impulse, or several impulses with a summating effect may be necessary.
          • There is a latent period of a few milliseconds between the arrival of the impulse and actin–myosin interaction. Tension develops during the contraction period and then decreases during the longer relaxation period.
          • If a muscle works against a constant load so that the muscle shortens, this is isotonic contraction (e.g. lifting this book from the desk) or positive work if the muscle pulls against an immovable object (e.g. pulling at a locked door), this is isometric contraction: internal shortening and tension develop as the elastic elements in the muscle are stretched but the muscle does not shorten.

          Phasic and tonic Muscles

          Most striped muscles in vertebrates are phasic (twitch) muscles. Each fiber has one motor nerve end-plate. When the motor unit is stimulated by its neuron, a single stimulus with a threshold intensity leads to a twitch which is all-ornothing because the action potential spreads rapidly throughout the whole fiber. Gradation in muscle tension is facilitated by:

          • imposing a second twitch on the first before the latter’s effects have decayed (not all the myosin heads are activated during the time available for the first twitch) this is temporal summation or
          • delivering impulses to reach the motor unit so rapidly that no relaxation can occur between action potentials, so eliciting a plateau of tension or state of tetanus which persists until the impulses cease or the muscle fatigues or
          • progressively recruiting more motor units to contract: this is spatial summation. Some muscle fibers (but not the same ones continuously) will always be contracted in a given muscle: this is called tonus and allows the muscle to be held firm.

          Slow phasic fibers are used to maintain posture and in endurance activities: they are often rich in myoglobin and are dark red in color. Such fibers are rich in mitochondria. Their slow contraction allows complete, aerobic oxidation of fuels and they fatigue only slowly. Fast phasic (glycolytic) fibers are used for quick bursts of movement they lack myoglobin and have fewer mitochondria their metabolism is often anaerobic. The two fiber types may be mixed (as in most mammalian muscles) or separated (the red and white meat in chicken or herring).

          Tonic muscles are like phasic muscles with slow fibers, except that they have many nerve end-plates per fiber and do not follow the all-or-nothing phenomenon seen in a phasic muscle fiber twitch. Action potentials do not spread far and gradation in contraction is effected by more frequent impulses. Bivalve (e.g. oyster) adductor muscles contain phasic and tonic fibers, the former snapping the valves of the shell closed while the tonic fibers keep it closed.

          Arthropod Muscles

          Arthropod muscles (e.g. in crab claws) have very few neurons per muscle, and a given neuron may innervate several muscles. Each fiber may have multiple end-plates from each motor neuron, and up to five neurons may innervate one fiber: at least one neuron is inhibitory whereas the others facilitate varying degrees of slow or fast contraction. Combinations of motor stimulation can thus permit a gradation of action.

          Muscle work and Power

          Muscle force reflects the number of actin–myosin cross-bridges formed, in turn dependent on the number of fibers within the muscle, roughly proportional to its cross-sectional area: Force a Cross-sectional area of the muscle Force of contraction per unit cross-sectional area is usually between 4 and 6 kg cm–2. Muscles produce most force when contracting around their resting length.

          Work is the product of force and the distance through which the force works (force × distance): it is more or less constant per gram of muscle, but large muscles will, of course, generate more force. In some muscles, the fibers are arranged longitudinally (fusiform pattern) which permits contraction over a greater distance compared with muscles with the pennate or feathered pattern of fibers where the fibers are arranged to allow shorter fiber contraction, but the larger number of fibers generate much more force. Thus large muscles (e.g. the quadriceps of the human thigh), with a large cross-sectional area and therefore force, which can contract significant distances (e.g. about one-third the resting length), can do large amounts of work compared with small, short muscles.

          Power is the rate of doing work. Very small muscles (e.g. those of the eyeball in a small rodent) are very powerful in that they contract very quickly: the power output per unit weight of muscle is higher than that of the equivalent, larger muscles of a larger mammal. Muscles develop their maximum power at intermediate velocities: the actual velocity depends on whether the fibers are phasic or tonic, but efficiency is at a maximum at 20–30% of maximum velocity. Energy is lost because of inefficiencies in the energy interconversion processes: much will be lost as heat and to overcome internal friction in the muscles

          Evolutionary Aspects

          Somatic muscles in vertebrates are associated with the body wall and appendages (fins and limbs) and are usually striped visceral muscles are associated with the gut and are usually smooth. Fish somatic muscles mainly comprise segmental myomeres which facilitate the undulation of swimming. Paired fins are moved by a dorsal abductor muscle (pulling the fin up) and a ventral adductor muscle (pulling it down). On land, the segmental arrangement of muscles is largely lost. The fin abductor and adductor muscles become divided to form the limb muscles attached to the limb girdles, and additional muscles develop in the limbs themselves.


          Summation on muscles - Biology

          Sometimes a single EPSP is strong enough to induce an action potential in the postsynaptic neuron, but often multiple presynaptic inputs must create EPSPs around the same time for the postsynaptic neuron to be sufficiently depolarized to fire an action potential. This process is called summation and occurs at the axon hillock, as illustrated in Figure 1. Additionally, one neuron often has inputs from many presynaptic neurons—some excitatory and some inhibitory—so IPSPs can cancel out EPSPs and vice versa. It is the net change in postsynaptic membrane voltage that determines whether the postsynaptic cell has reached its threshold of excitation needed to fire an action potential. Together, synaptic summation and the threshold for excitation act as a filter so that random “noise” in the system is not transmitted as important information.

          Figure 1. A single neuron can receive both excitatory and inhibitory inputs from multiple neurons, resulting in local membrane depolarization (EPSP input) and hyperpolarization (IPSP input). All these inputs are added together at the axon hillock. If the EPSPs are strong enough to overcome the IPSPs and reach the threshold of excitation, the neuron will fire.

          Brain-computer interface

          Amyotrophic lateral sclerosis (ALS, also called Lou Gehrig’s Disease) is a neurological disease characterized by the degeneration of the motor neurons that control voluntary movements. The disease begins with muscle weakening and lack of coordination and eventually destroys the neurons that control speech, breathing, and swallowing in the end, the disease can lead to paralysis. At that point, patients require assistance from machines to be able to breathe and to communicate. Several special technologies have been developed to allow “locked-in” patients to communicate with the rest of the world. One technology, for example, allows patients to type out sentences by twitching their cheek. These sentences can then be read aloud by a computer.

          A relatively new line of research for helping paralyzed patients, including those with ALS, to communicate and retain a degree of self-sufficiency is called brain-computer interface (BCI) technology and is illustrated in Figure 2. This technology sounds like something out of science fiction: it allows paralyzed patients to control a computer using only their thoughts. There are several forms of BCI. Some forms use EEG recordings from electrodes taped onto the skull. These recordings contain information from large populations of neurons that can be decoded by a computer. Other forms of BCI require the implantation of an array of electrodes smaller than a postage stamp in the arm and hand area of the motor cortex. This form of BCI, while more invasive, is very powerful as each electrode can record actual action potentials from one or more neurons. These signals are then sent to a computer, which has been trained to decode the signal and feed it to a tool—such as a cursor on a computer screen. This means that a patient with ALS can use e-mail, read the Internet, and communicate with others by thinking of moving his or her hand or arm (even though the paralyzed patient cannot make that bodily movement). Recent advances have allowed a paralyzed locked-in patient who suffered a stroke 15 years ago to control a robotic arm and even to feed herself coffee using BCI technology.

          Despite the amazing advancements in BCI technology, it also has limitations. The technology can require many hours of training and long periods of intense concentration for the patient it can also require brain surgery to implant the devices.

          Figure 2. With brain-computer interface technology, neural signals from a paralyzed patient are collected, decoded, and then fed to a tool, such as a computer, a wheelchair, or a robotic arm.

          Watch this video in which a paralyzed woman use a brain-controlled robotic arm to bring a drink to her mouth, among other images of brain-computer interface technology in action.



          Answers, BIO 2310, Muscle Tissue

          1. Capable of contraction and relaxation. It functions to produce movement, maintain posture, support, guard exits/entrances (e.g. sphincter), and maintain body temperature.

          2. Skeletal muscle is attached to skeleton, is striated, voluntary and causes body movement. Cardiac muscle is heart muscle, is striated with intercalated discs, is involuntary and causes heart pumping. Smooth muscle is found in the wall of tubular viscera and is not striated, is involuntary and causes mixing & movement called peristalsis.

          3. Connective tissue around groups of muscles or filling spaces if fascia. Epimysium is connective tissue around a single muscle, perimysium is connective tissue around fascicles, fascicles are bundles of muscle cells, a tendon is connective tissue cord attaching muscle to (periosteum of) bone, aponeurosis is a broad sheet-like tendon.

          4. Skeletal muscle must have nerve supply to function and has an excellent blood supply.

          5. Sarcolemma is muscle cell membrane, myofiber is muscle cell, myofibril is the striated cylinders in the muscle cell, myofilaments are the contractile proteins. A band is the dark colored region, I band is light. Z lines separate the myofibril into sarcomeres which are comprised of thin myofilaments attached to the Z lines called actin and the thick myosin myofilaments. The sarcomere is the functional unit of muscle contraction because it squeezes together during contraction from the myosin pulling on the actin. The sarcoplasmic reticulum with its expanded regions called terminal cisternae are the muscle cell’s version of an endoplasmic reticulum. It functions to store calcium ions. Tropomyosin is a thin ribbon-like protein that wraps around actin and blocks myosin from attaching its head to the actin. It prevents contraction. Troponin is a small protein that acts like the glue holding the tropomyosin in place. Troponin has a binding site for calcium. Transverse tubules are inward extensions of the sarcolemma into the interior of the cell.

          6. Stimulation of the muscle cell’s sarcolemma travels into the cell through the T-tubules causing calcium release from the sarcoplasmic reticulum. The calcium binds to troponin causing it to release the tropomyosin which can then move out of the way. Now, the myosin head can form a cross bridge binding to actin. The myosin head is energized with the binding of ATP and swivels toward the center of the sarcomere causing the power stroke. This causes the sarcomere to squeeze together. ATP is also needed for the actin & myosin to release from each other so that relaxation can occur. ATP is also needed for putting the calcium back into the sarcoplasmic reticulum because it is active transport.

          7. All skeletal muscle cells need a motor neuron (movement nerve cell) to provide stimulation for contraction. There is a gap between the distal end of the neuron and the muscle cell and this is the neuromuscular junction. A chemical called acetylcholine is released from the neuron to bridge the gap and take the stimulation to the muscle cell. The motor neuron plus how ever many muscle cells it supplies is the motor unit. It may be one neuron and one muscle cells for the motor unit in areas where your movement is precise (e.g. eye movement) or one neuron for 500 muscle cells where your movement is not precise (e.g. lower back muscles).

          8. It is really the ‘on-off’ switch. It binds troponin causing the physical blocker, the tropomyosin, to move out of the way.

          9. A little bit of ATP is present in this state in the muscle cell. More can be quickly manufactured by converting creatine phosphate to ATP. Rapidly, but inefficiently, you can make ATP from anaerobic metabolism. As long as oxygen supply is sufficient, you can very efficiently make a lot of ATP from aerobic metabolism, a slow process.

          10. Oxygen debt is to restore the ATP aerobically and to remove lactic acid (end-product from anaerobic metabolism) from muscle cells. Glycogen debt is to restore glucose stores and the best way to restore these is to eat carbohydrates.

          11. Lack of ATP. Lactic acid also contributes to the soreness of these muscles.

          12. Latent period, Contraction period, Relaxation period

          13. All stimuli strong enough to cause a muscle twitch will cause identical muscle twitches. However, the all-or-none principle applies to the muscle CELL only, not the entire muscle.

          14. For a small contraction of your biceps muscle, some (say 10%) of the muscle cells will do their “all.” For a bigger contraction of your biceps muscle (say 60%) more muscle cells contract maximally. For a maximal contraction of the whole biceps muscle, all of the muscle cells will be contracting maximally.

          15. Multiple motor unit summation = spatial summation and occurs when many muscle cells or motor units contract at the same time making a bigger whole muscle contraction (as is described for number 14). Temporal summation = wave summation and is when muscle cells contract repeatedly and rapidly, so that the next contraction is occurring before the previous one has totally relaxed. Examples of temporal summation include incomplete tetanus (repeated contraction due to repeated stimuli with a little bit of contraction between each stimulus) and complete tetanus (sustained contraction with no relaxation). Treppe is the bigger muscle twitch that is achieved upon warming up for exercise. Asynchronous motor unit summation is when not all muscle cells are working at the same time so that some can rest while others are contracting. This allows posture muscles to be contracted all day without tiring, because the motor units take turns. Muscle tone is when some of the motor units are contracting making the muscle firm, but not enough are contracting to result in movement.

          16. Isometric contractions occur when you pick up something that is too heavy. While your muscle is working and creating tension, it is not shortening. Isotonic contractions result in shortening, as in bending your elbow.

          17. Slow fibers are fatigue resistant and are red. They have excellent blood supply and myoglobin for oxygen storage (think of dark meat of chicken). Therefore they are geared toward aerobic metabolism and while this is not fast these fibers do not run out of ATP and do not fatigue (think of the chicken walking around all day long). Fast fibers are fatiguable and are white. They do not have great blood supply and do not have myoglobin. They are geared toward anaerobic metabolism. They can make the ATP very quickly (think of the breast meat of chicken and the chicken flying quickly to a tree when being chased) but will run out of it soon and cannot endure (the chicken cannot fly long distances, but the goose has dark meat for the breast, why?). Intermediate are more fatigue-resistant fast fibers. You can get these through endurance training, but the fast and slow fibers are genetically determined.


          Assessment

          Muscle disease may be detected by assessing whether the muscle groups can withhold or overcome the efforts of the physician to pull or push or by observing the individual carrying out isolated voluntary movements against gravity or more complex and integrated activities, such as walking. The weakness of individual muscles or groups of muscles can be quantified by using a myometer, which measures force based on a hydraulic or electronic principle. Recordings of contraction force over a period of time are valuable in determining whether the weakness is improving or worsening.

          The assessment of muscle weakness (and wasting) is directed toward discovering evidence of muscle inflammation or damage. These changes are discerned by blood tests or by measuring alterations of the electrical properties of contracting muscles. Another investigative tool is the muscle biopsy, which provides muscle specimens for pathological diagnosis and biochemical analysis. Muscle biopsies can be taken with a needle or during a surgical procedure.


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