Information

Why do the neuron pathways decussate?


I learn about the brain right now, and there are crossing of neuron pathways everywhere; in the thalamus, the medulla oblongata, the pyramidal tract… And I don't understand why? What is the reason to connect - for example - the right foot to the left part of your brain?


Crossing or decussation is a lot more robust against wiring errors than their seemingly simpler same-sided wiring counterparts. See here for the research carried out to establish this and for more detail.


Why Decussate? Topological Constraints on 3D Wiring

Many vertebrate motor and sensory systems “decussate” or cross the midline to the opposite side of the body. The successful crossing of millions of axons during development requires a complex of tightly controlled regulatory processes. Because these processes have evolved in many distinct systems and organisms, it seems reasonable to presume that decussation confers a significant functional advantage—yet if this is so, the nature of this advantage is not understood. In this article, we examine constraints imposed by topology on the ways that a three-dimensional processor and environment can be wired together in a continuous, somatotopic, way. We show that as the number of wiring connections grows, decussated arrangements become overwhelmingly more robust against wiring errors than seemingly simpler same-sided wiring schemes. These results provide a predictive approach for understanding how 3D networks must be wired if they are to be robust, and therefore have implications both for future large-scale computational networks and for complex biomedical devices. Anat Rec, 291:1278–1292, 2008. © 2008 Wiley-Liss, Inc.


Which is the pathway for information through a neuron? dendrite → axon → cell body → axon terminals axon terminals → axon → cell body → dendrite dendrite → cell body → axon → axon terminals cell body → dendrite → axon → axon terminals

Dendrites are the extensions of the soma/cell body that serve to carry the nerve impulse towards the cell body.

Axon is the single large extension of the cell body and carries the nerve impulse away from the cell body towards the axon terminals.

The axon terminals of presynaptic neurons synapse with dendrites of the postsynaptic neuron. Hence, the nerve impulse is carried from the axon terminal to the dendrites at the synapse. The process continues and the nerve impulse is transmitted from one neuron to the next.

dendrite → cell body → axon → axon terminals

The pathway for information through neuron are dendrite → cell body → axon → axon terminals and this because Dendrite is the input zone of nerve cell where neuron can receive information from another nerve cell.

Cell body is the part of the neuron that contain nucleus and it is link to the dendrites and it send signals to the axon.

Axon is a long nerve cell that join the cell body at a terminal called axon hilock and this conduct electrical impulses.

Axon terminal contain synaptic vessicles and it is synapse to other terminal and it is the out put zone of neuron. It is the out put zone where information is expell out


How Neural Pathways are Discovered?

Neural pathways were discovered on the brain of cadaver. These pathways were discovered during the examination of the brain as these pathways are large and long. They are easily identifiable on macroscopic examination of the brain.

For example, corpus callosum can be seen while dissecting the brain of a cadaver. It communicates information between two cerebral hemispheres. Axons which forms neural pathways are of two types,

Myelinated
axons make the pathways appear bright because myelin contains fat content.
Unmyelinated axons make the pathway appear grey because they don’t contain any
fat content.


Neuron Classification

Stocktrek Images / Getty Images

There are three main categories of neurons. They are multipolar, unipolar, and bipolar neurons.

  • Multipolar neurons are found in the central nervous system and are the most common of the neuron types. These neurons have a single axon ​and many dendrites extending from the cell body.​​
  • Unipolar neurons have one very short process that extends from a single cell body and branches into two processes. Unipolar neurons are found in spinal nerve cell bodies and cranial nerves.
  • Bipolar neurons are sensory neurons consisting of one axon and one dendrite that extend from the cell body. They are found in retinal cells and olfactory epithelium.

Neurons are classified as either motor, sensory, or interneurons. Motor neurons carry information from the central nervous system to organs, glands, and muscles. Sensory neurons send information to the central nervous system from internal organs or from external stimuli. Interneurons relay signals between ​motor and sensory neurons.​


Hormones Can Be Classified Based on Their Solubility and Receptor Location

Most hormones fall into three broad categories: (1) small lipophilic molecules that diffuse across the plasma membrane and interact with intracellular receptors and (2) hydrophilic or (3) lipophilic molecules that bind to cell-surface receptors (Figure 20-2). Recently, nitric oxide, a gas, has been shown to be a key regulator controlling many cellular responses. We discuss this important regulator later in this chapter. Here we briefly describe the three main types of hormones later we discuss the mechanisms that regulate their synthesis, release, and degradation.

Figure 20-2

Some hormones bind to intracellular receptors others, to cell-surface receptors. (a) Steroid hormones, thyroxine, and retinoids, being lipophilic, are transported by carrier proteins in the blood. After dissociation from these carriers, such hormones (more. )

Lipophilic Hormones with Intracellular Receptors

Many lipid-soluble hormones diffuse across the plasma membrane and interact with receptors in the cytosol or nucleus. The resulting hormone-receptor complexes bind to transcription-control regions in DNA thereby affecting expression of specific genes (see Figure 20-2a). Hormones of this type include the steroids (e.g., cortisol, progesterone, estradiol, and testosterone), thyroxine, and retinoic acid (see Figure 10-65).

All steroids are synthesized from cholesterol and have similar chemical skeletons. After crossing the plasma membrane, steroid hormones interact with intracellular receptors, forming complexes that can increase or decrease transcription of specific genes (see Figure 10-68). These receptor-steroid complexes also may affect the stability of specific mRNAs. Steroids are effective for hours or days and often influence the growth and differentiation of specific tissues. For example, estrogen and progesterone, the female sex hormones, stimulate the production of egg-white hormones in chickens and cell proliferation in the hen oviduct. In mammals, estrogens stimulate growth of the uterine wall in preparation for embryo implantation. In insects and crustaceans, α-ecdysone (which is chemically related to steroids) triggers the differentiation and maturation of larvae like estrogens, it induces the expression of specific gene products.

These two thyroid hormones stimulate increased expression of many cytosolic enzymes (e.g., liver hexokinase) that cata-lyze the catabolism of glucose, fats, and proteins and of mitochondrial enzymes that catalyze oxidative phosphorylation.

Retinoids are polyisoprenoid lipids derived from retinol (vitamin A). They perform multiple regulatory functions in diverse cellular processes. Retinoids regulate cellular proliferation, differentiation, and death, and they have numerous clinical applications. Their diverse effects reflect, at least in part, the multiplicity of retinoid derivatives, the existence of two different classes of receptors that form heterodimers, and differences in their cis-acting regulatory sites on DNA. During development retinoids act as local mediators of cell-cell interaction. For instance, during the formation of motor neurons in the chick, one class of motor neurons generates a retinoid signal which regulates the number and type of neighboring motoneurons.

Water-Soluble Hormones with Cell-Surface Receptors

Because water-soluble signaling molecules cannot diffuse across the plasma membrane, they all bind to cell-surface receptors. This large class of compounds is composed of two groups: (1) peptide hormones, such as insulin, growth factors, and glucagon, which range in size from a few amino acids to protein-size compounds, and (2) small charged molecules, such as epinephrine and histamine (see Figure 21-28), that are derived from amino acids and function as hormones and neurotransmitters.

Many water-soluble hormones induce a modification in the activity of one or more enzymes already present in the target cell. In this case, the effects of the surface-bound hormone usually are nearly immediate, but persist for a short period only. These signals also can give rise to changes in gene expression that may persist for hours or days. In yet other cases water-soluble signals may lead to irreversible changes, such as cellular differentiation.

Lipophilic Hormones with Cell-Surface Receptors

The primary lipid-soluble hormones that bind to cell-surface receptors are the prostaglandins. There are at least 16 different prostaglandins in nine different chemical classes, designated PGA – PGI. Prostaglandins are part of an even larger family of 20 carbon𠄼ontaining hormones called eicosanoid hormones. In addition to prostaglandins, they include prostacyclins, thromboxanes, and leukotrienes. Eicosonoid hormones are synthesized from a common precursor, arachidonic acid. Arachidonic acid is generated from phospholipids and diacylglycerol.

Many prostaglandins act as local mediators during paracrine and autocrine signaling and are de-stroyed near the site of their synthesis. They mod-ulate the responses of other hormones and can have profound effects on many cellular processes. Certain prostaglandins cause blood platelets to aggregate and adhere to the walls of blood vessels. Because platelets play a key role in clotting blood and plugging leaks in blood vessels, these prostaglandins can affect the course of vascular disease and wound healing aspirin inhibits their synthesis by acetylating (and thereby irreversibly inhibiting) prostaglandin H2 synthase. Other prostaglandins initiate the contraction of smooth muscle cells they accumulate in the uterus at the time of childbirth and appear to be important in inducing uterine contraction.

Recent studies have shown that a family of plant steroids, called brassinosteroids, regulates many aspects of development. These lipophilic compounds, like prostaglandins, act through cell-surface receptors.


Footnotes

Electronic supplementary material is available online at https://doi.org/10.6084/m9.figshare.c.5427713.

Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

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Postsynaptic potential

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Postsynaptic potential (PSP), a temporary change in the electric polarization of the membrane of a nerve cell (neuron). The result of chemical transmission of a nerve impulse at the synapse (neuronal junction), the postsynaptic potential can lead to the firing of a new impulse.

When an impulse arrives at a synapse from an activated neuron (presynaptic neuron), a chemical substance called a neurotransmitter is released causing the opening of channel-shaped molecules in the membrane of the resting neuron (postsynaptic neuron). Ions flowing through the channels create a shift in the resting membrane polarization, which usually has a slightly more negative charge inside the neuron than outside. Hyperpolarization—that is, an increase in negative charge on the inside of the neuron—constitutes an inhibitory PSP, because it inhibits the neuron from firing an impulse. Depolarization—a decrease in negative charge—constitutes an excitatory PSP because, if the neuron reaches the critical threshold potential, it can excite the generation of a nerve impulse (action potential).

The PSP is a graded potential that is, its degree of hyperpolarization or depolarization varies according to the activation of ion channels. The ability to integrate multiple PSPs at multiple synapses is an important property of neurons and is called summation. Summation may be either spatial, in which signals are received from many synapses at once, or temporal, in which successive signals are received from the same synapse. Spatial and temporal summation can occur simultaneously.

The equivalent of the PSP at nerve-muscle synapses is called the end-plate potential.


Resumen

Introducción

La esclerosis lateral amiotrófica (ELA) es la enfermedad degenerativa de las motoneuronas más frecuente. Aunque un pequeño porcentaje de los casos de ELA tienen un origen familiar y son secundarios a mutaciones en genes concretos, a la gran mayoría de ellos se les presupone un origen multifactorial, sin que su patogenia haya sido completamente aclarada. No obstante, en los últimos años varios estudios han aumentado el conocimiento sobre la patogenia de la enfermedad, planteando la cuestión de si se trata de una proteinopatía, una ribonucleinopatía, una axonopatía o una enfermedad del microambiente neuronal.

Desarrollo

En el presente artículo revisamos los trabajos publicados tanto en pacientes como en modelos animales de ELA y discutimos la implicación de los principales procesos celulares que parecen contribuir a su patogenia (procesamiento génico, metabolismo de proteínas, estrés oxidativo, transporte axonal y relación con el microambiente neuronal).

Conclusiones

Aunque la patogenia de la ELA dista de estar aclarada, los estudios recientes apuntan a la idea de que hay unos desencadenantes iniciales que varían de unos sujetos a otros, y unas vías finales de degeneración de las motoneuronas que están implicadas en la mayor parte de los casos de enfermedad.


How is your brain similar to other objects? For example, how is your brain like a bowl of Jell-O? How is it different? Are they both soft? Do they have layers? Can they store information? Do they use electricity? Do they contain chemicals? Give each person a different object. Each person must make a list of similarities and differences between their object and a brain.

Although it's not too difficult to describe what the brain does, it's not too easy to act it out. Try to describe the functions of the brain and nervous system with this game of "Brain Charades."

Write down words that describe brain functions on small pieces of paper. This table of words will help you get started:

Mix the papers in a bowl, bag or a hat. A player should pick a paper out of the bowl then act out the function. Everyone else should try to guess what the player is acting out. Actors must remain silent. When someone guesses the action, write the word on the board. Another player should select a new word and act it out. Repeat the game until all of the words have been identified correctly.


Watch the video: Αντιδράσεις για το αυστηρό νομοσχέδιο για τις πορείες - Κεντρικό Δελτίο Ειδήσεων 2162020. OPEN TV (December 2021).