Difference between revisions of "Soma (biology)" - New World Encyclopedia

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Soma
Dendrite Soma Axon Nucleus Node of Ranvier Axon Terminal Schwann cell Myelin sheath
Structure of a typical neuron

Soma is the bulbous body of a neuron (nerve cell) from which one or more processes emanate (dendrites and/or axons) and which is the part of the neuron that contains the cell nucleus enclosing a conspicuous nucleolus. The soma (pl. somata or somas) is also known as a perikaryon (pl, perikary), a cyton, or simply as a neuron cell body. The word "soma" comes from the Greek σῶμα, meaning "body," while perikaryon comes from peri, meaning "around" and kary, meaning "nucleus."

Overview

Every neuron contains a soma, or cell body. A neuron or nerve cell is a highly specialized, electrically excitable cell in the nervous system that conducts nerve impulses between different parts of the body. Neurons can process and transmit information from both internal and external environments, communicating this information via chemical or electronic impulse across a synapse (the junction between cells) and utilizing the action potential—an electrical signal that is generated by means of the electrically excitable membrane of the neuron. In vertebrate animals, neurons are the core components of the brain, spinal cord, and peripheral nerves and are typically large and complex.

The three main structural regions of a typical neuron are two categories of processes (an axon, which carries an electric impulse; and one or more dendritic trees, which typically receive input) and the soma, which contains the nucleus and is the major biosynthetic center.

The axon is one of the two types of protoplasmic protrusions extending from the neuron cell bodies. The axon is a slender, armlike projection that can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length and typically conducts electrical impulses away from the neuron's cell body. The function of the axon is to transmit information to different neurons, muscles, and glands. In certain sensory neurons (pseudounipolar neurons), such as those for touch and warmth, the electrical impulse travels along an axon from the periphery to the cell body, and from the cell body to the spinal cord along another branch of the same axon. No neuron ever has more than one axon. The axon is specialized for the conduction of the particular electric impulses known as action potentials.

Dendrites are the second of the two types of protoplasmic protrusions that extrude from the cell body of a neuron. These are cellular extensions with many branches and are specialized to receive chemical signals from the axon termini of other neurons. Dendrites convert these signals into small electric impulses and transmit them to the soma. Electrical stimulation is transmitted onto dendrites by upstream neurons (usually their axons) via synapses, which are located at various points throughout the dendritic tree. Dendrites play a critical role in integrating these synaptic inputs and in determining the extent to which action potentials are produced by the neuron.

Dendrites are distinguished in general from axons by several features, including shape (dendrites often taper while axons usually maintain a constant radius), length (dendrites are restricted to a small region around the cell body while axons can be much longer), and function (dendrites usually receive signals while axons usually transmit them). All of these rules have exceptions, however. For example, while the axon is generally involved in information outflow, this region can also receive input from other neurons. Information outflow from dendrites to other neurons can also occur. And axons can be very short (and even absent) in some types of neurons. In the case of neurons without axons, the dendrites may serve the function of transmitting signals from the cell body.

The soma or perikaryon is the bulbous end of a neuron, from which the dendrites and axon branch off. The soma contains many organelles; granules called Nissl granules; and the cell nucleus as the key feature.

Both dendrites and axons tend to share the same organelles as the soma, although both lack the nucleus, and axons lack Golgi apparatus and Nissl bodies that are found in dendrites.

Structure

There are many different specialized types of neurons, and their sizes vary from as small as about 5 micrometres to over 10 millimetre for some of the smallest and largest neurons of invertebrates, respectively.

The soma (main part of the neuron in which the dendrites branch off of) contains many organelles, including granules called Nissl granules, which are composed largely of rough endoplasmic reticulum and free polyribosomes.^[1] The cell nucleus is a key feature of the soma. The nucleus is the source of most of the RNA that is produced in neurons. In general, most proteins are produced from mRNAs that do not travel far from the cell nucleus. This creates a challenge for supplying new proteins to axon endings that can be a meter or more away from the soma. Axons contain microtubule-associated motor proteins that transport protein-containing vesicles between the soma and the synapses at the axon terminals. Such transport of molecules towards and away from the soma maintains critical cell functions.

The axon hillock is a specialized domain of the neuronal cell body from which the axon originates. A high amount of protein synthesis occurs in this region, as it contains a large number of Nissl granules (which are ribosomes wrapped in RER) and polyribosomes. Within the axon hillock, materials are sorted as either items that will enter the axon (like the components of the cytoskeletal architecture of the axon, mitochondria, etc.) or will remain in the soma. In addition, the axon hillock also has a specialized plasma membrane that contains large numbers of voltage-gated ion channels, since this is most often the site of action potential initiation.^[1]

The survival of some sensory neurons depends on axon terminals making contact with sources of survival factors that prevent apoptosis. The survival factors are neurotrophic factors, including molecules such as nerve growth factor (NGF). NGF interacts with receptors at axon terminals, and this produces a signal that must be transported up the length of the axon to the nucleus. A current theory of how such survival signals are sent from axon endings to the soma includes the idea that NGF receptors are endocytosed from the surface of axon tips and that such endocytotic vesicles are transported up the axon.^[2]

References
ISBN links support NWE through referral fees

↑ ^1.0 ^1.1 (2008) Fundamental Neuroscience, 3rd, Academic Press. ISBN 978-0-12-374019-9.
↑ Delcroix JD, Valletta J, Wu C, et al. (2004). Trafficking the NGF signal: implications for normal and degenerating neurons. Prog. Brain Res. 146: 3–23.

Campbell, N. A., J. B. Reece, L. A. Urry, et al. 2008. Biology, 8th edition. San Francisco: Pearson/Benjamin Cummings. ISBN 9780805368444.

Chamberlin, S. L., and B. Narins. 2005. The Gale Encyclopedia of Neurological Disorders. Detroit: Thomson Gale. ISBN 078769150X.

Marieb, E. N. and K. Hoehn. 2010. Human Anatomy & Physiology, 8th edition. Benjamin Cummings. ISBN 9780805395693.

External links

Template:OklahomaHistology - "Slide 3 Spinal cord"

Histology: nervous tissue

Neurons (gray matter): soma, axon (axon hillock, axoplasm, axolemma, neurofibril/neurofilament), dendrite (Nissl body, dendritic spine)
types (bipolar, pseudounipolar, multipolar, pyramidal, Purkinje, granule)

Afferent nerve/Sensory nerve/Sensory neuron (GSA, GVA, Type Ia sensory fiber), Efferent nerve/Motor nerve/Motor neuron (GSE, GVE, Alpha motor neuron, Gamma motoneurons, Upper motor neuron, Lower motor neuron), Interneuron (Renshaw)

Synapses: neuropil, boutons, synaptic vesicle, neuromuscular junction, electrical synapse

Sensory receptors: Free nerve ending, Meissner's corpuscle, Merkel nerve ending, Muscle spindle, Pacinian corpuscle, Ruffini ending, Olfactory receptor neuron, Photoreceptor, Hair cell, Taste bud

Glial cells: astrocyte, ependymal cells, microglia, radial glia

Myelination (white matter): Schwann cell, oligodendrocyte, nodes of Ranvier, internode, Schmidt-Lanterman incisures, neurolemma

closely related Connective tissue: epineurium, perineurium, endoneurium, nerve fascicle, meninges

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[squire-1] 1.0 ^1.1 (2008) Fundamental Neuroscience, 3rd, Academic Press. ISBN 978-0-12-374019-9.

[2] Delcroix JD, Valletta J, Wu C, et al. (2004). Trafficking the NGF signal: implications for normal and degenerating neurons. Prog. Brain Res. 146: 3–23.

[1]

[2]

@@ Line 9: / Line 9: @@
 The three main structural regions of a typical neuron are two categories of processes (an ''[[axon]]'', which carries an electric impulse; and one or more ''dendritic trees'', which typically receive input) and the soma, which contains the nucleus and is the major biosynthetic center.
 The '''axon''' is one of the two types of protoplasmic protrusions extending from the neuron cell bodies. The axon is a slender, armlike projection that can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length and typically conducts [[action potential|electrical impulses]] away from the neuron's [[cell body]]. The function of the axon is to transmit information to different neurons, [[muscle]]s, and [[gland]]s. In certain sensory neurons ([[pseudounipolar neuron]]s), such as those for touch and warmth, the electrical impulse travels along an axon from the periphery to the cell body, and from the cell body to the spinal cord along another branch of the same axon. No neuron ever has more than one axon. The axon is specialized for the conduction of the particular electric impulses known as action potentials.
-'''Dendrites''' are the second of the two types of [[protoplasm]]ic protrusions that extrude from the cell body of a neuron. These are cellular extensions with many branches and are specialized to receive chemical signals from the axon termini of other neurons. Dendrites convert these signals into small electric impulses and transmit them to the soma. Electrical stimulation is transmitted onto dendrites by upstream neurons (usually their axons) via [[synapses]] which are located at various points throughout the dendritic tree. Dendrites play a critical role in integrating these [[synapse|synaptic inputs]] and in determining the extent to which [[action potentials]] are produced by the neuron. Some recent research has suggested that dendrites can support action potentials and release neurotransmitters, a property that was originally believed to be specific to axons, although whether these processes should be considered dendrites or axons is also not firmly established based on differing definitions used for the terms (see below).
+'''Dendrites''' are the second of the two types of [[protoplasm]]ic protrusions that extrude from the cell body of a neuron. These are cellular extensions with many branches and are specialized to receive chemical signals from the axon termini of other neurons. Dendrites convert these signals into small electric impulses and transmit them to the soma. Electrical stimulation is transmitted onto dendrites by upstream neurons (usually their axons) via [[synapses]], which are located at various points throughout the dendritic tree. Dendrites play a critical role in integrating these [[synapse|synaptic inputs]] and in determining the extent to which [[action potentials]] are produced by the neuron.
 [[Image:Complete neuron cell diagram en.svg|thumb|right|420px]]
-Dendrites are distinguished in general from axons by several features, including shape (dendrites often taper while axons usually maintain a constant radius), length (dendrites are restricted to a small region around the cell body while axons can be much longer), and function (dendrites usually receive signals while axons usually transmit them).  All of these rules have exceptions, however. For example, while the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons. Information outflow from dendrites to other neurons can also occur. And axons can be very short (and even absent) in some types of neurons. In the case of neurons without axons, the dendrites may serve the function of transmitting signals from the cell body. Both dendrites and axons tend to share the same organelles as the soma, although both lack the nucleus, and axons lack Golgi apparatus and Nissl bodies that are found in dendrites.
+Dendrites are distinguished in general from axons by several features, including shape (dendrites often taper while axons usually maintain a constant radius), length (dendrites are restricted to a small region around the cell body while axons can be much longer), and function (dendrites usually receive signals while axons usually transmit them).  All of these rules have exceptions, however. For example, while the axon is generally involved in information outflow, this region can also receive input from other neurons. Information outflow from dendrites to other neurons can also occur. And axons can be very short (and even absent) in some types of neurons. In the case of neurons without axons, the dendrites may serve the function of transmitting signals from the cell body.
-The distinction between dendrites and axons is not always clear. For example, neurons classified as unipolar (or pseudounipolar, since they originate as bipolar neurons) have one process that extends from the cell body and it forms two ends (a central process and a peripheral process, both with branches at their ends, where there are sensory endings/receptive terminals). These are chiefly sensory neurons of the [[peripheral nervous system]]. Some classify this extension as a dendrite, using the older definition of dendrites as processes that transmit impulses toward the cell body. However, functional definitions based on the generation and transmission of an impulse classify this as an axon (Marieb and Hoehn 2010).
-The diffusely branching dendrites, of which there may be hundreds near the cell body in motor neurons, provide an extensive surface area for receiving signals from other neurons and transmitting them to the cell body. These signals are not usually action potentials, as is found in axons, but rather graded potentials. Some neuron types (bipolar) have a single fused dendrite (albeit with many branches at the end) in addition to one axon. But other neuron neuron types (multipolar) have many dendrites extending form the cell body.
-Certain classes of dendrites (i.e. [[Purkinje cells]] of cerebellum, cerebral cortex) contain small projections referred to as "appendages" or "[[Dendritic spine|spines]]". Appendages increase receptive properties of dendrites to isolate signal specificity. Increased neural activity at spines increases their size and conduction which is thought to play a role in learning and memory formation. There are approximately 200,000 spines per cell, each of which serves as a postsynaptic process for individual presynaptic axons.
 The '''soma''' or perikaryon is the bulbous end of a neuron, from which the dendrites and axon branch off. The soma contains many [[organelle]]s; granules called Nissl granules; and the [[cell nucleus]] as the key feature.
+Both dendrites and axons tend to share the same organelles as the soma, although both lack the nucleus, and axons lack Golgi apparatus and Nissl bodies that are found in dendrites.
 ==Structure==