Motor neuron: Difference between revisions
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In [[neurology]], the term '''motor neuron''' (or '''motoneuron''') classically applies to [[neurons]] located in the [[central nervous system]] (or CNS) that project their [[axons]] outside the CNS and directly or indirectly control [[muscle]]s. The motor neuron is often associated with [[efferent neuron]], primary neuron, or [[alpha motor neuron]]s. Motor neurones are neurones that carry signals from the spinal cord to the muscles to produce movement. <ref>Schacter D.L., Gilbert D.T., and Wegner D.M. (2011) Psychology second edition. New York, NY: Worth</ref> |
In [[neurology]], the term '''motor neuron''' (or '''motoneuron''') classically applies to [[neurons]] located in the [[central nervous system]] (or CNS) that project their [[axons]] outside the CNS and directly or indirectly control [[muscle]]s. The motor neuron is often associated with [[efferent neuron]], primary neuron, or [[alpha motor neuron]]s. Motor neurones are neurones that carry signals from the spinal cord to the muscles to produce movement. <ref>Schacter D.L., Gilbert D.T., and Wegner D.M. (2011) Psychology second edition. New York, NY: Worth</ref> |
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== Anatomy and physiology == |
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{| style="float:right;" class="wikitable" border="1" |
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|- bgcolor="#cccccc"| |
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! Branch of NS |
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! Position |
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! Neurotransmitter |
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|- |
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| Somatic |
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| n/a |
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| Acetylcholine |
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|- |
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| Parasympathetic |
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| Preganglionic |
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| Acetylcholine |
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|- |
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| Parasympathetic |
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| Ganglionic |
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| Acetylcholine |
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|- |
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| Sympathetic |
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| Preganglionic |
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| Acetylcholine |
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|- |
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| Sympathetic |
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| Ganglionic |
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| Norepinephrine* |
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|- |
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| colspan="3" align="center" | <small>*Except fibers to [[sweat gland]]s and certain [[blood vessel]]s</small><br />''Motoneuron neurotransmitters'' |
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|} |
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According to their targets, motor neurons are classified into three broad categories: |
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'''Somatic motor neurons''', which originate in the central nervous system, project their [[axons]] to the target tissues, which are always skeletal muscles.<ref>{{cite book|last=Silverthorn|first=Dee Unglaub|title=Human Physiology: An Integrated Approach|year=2010|publisher=Pearson|isbn=978-0-321-55980-7|pages=398}}</ref> [[Skeletal muscles]] are involved in locomotion (such as the muscles of the limbs, abdominal, and [[intercostal muscles]]). |
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'''Special visceral motor neurons''', also called ''branchial motor neurons'', which directly innervate [[Branchiomeric musculature|branchial muscles]] (that motorize the [[gills]] in fish and the face and neck in land [[vertebrates]]). |
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'''General visceral motor neurons''' (''visceral motor neurons'' for short) which indirectly innervate [[cardiac muscle]] and [[smooth muscles]] of the [[viscera]] ( the muscles of the [[arteries]]): they [[synapse]] onto neurons located in [[ganglia]] of the [[autonomic nervous system]] ([[Sympathetic nervous system|sympathetic]] and [[parasympathetic]]), located in the [[peripheral nervous system]] (PNS), which themselves directly innervate visceral muscles (and also some gland cells). |
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In other words: |
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* the motor command of [[skeletal muscle|skeletal]] and branchial muscles is ''monosynaptic'' (involving only one motor neuron, respectively, ''somatic'' and ''branchial'', which synapses onto the muscle). |
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* the command of [[visceral muscles]] is ''disynaptic'' (involving two neurons: the ''general visceral motor neuron'' located in the CNS, which synapses onto a ganglionic neuron, located in the PNS, which synapses onto the muscle). |
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It could be argued that, in the command of visceral muscles, the ganglionic neuron, [[parasympathetic]] or [[sympathetic nervous system|sympathetic]], is the real ''motor neuron'', being the one that directly innervates the muscle (whereas the ''general visceral motor neuron'' is, strictly speaking, a ''[[preganglionic]]'' neuron). But, for historical reasons, the term motor neuron is reserved for the CNS neuron. |
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All vertebrate motor neurons are [[cholinergic]], that is, they release the neurotransmitter [[acetylcholine]]. Parasympathetic ganglionic neurons are also cholinergic, whereas most sympathetic ganglionic neurons are [[Norepinephrine|noradrenergic]], that is, they release the neurotransmitter [[noradrenaline]]. (see Table) |
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== Function == |
== Function == |
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Revision as of 18:50, 8 January 2013
 | This article needs additional citations for verification. (November 2012) |
| Motor neuron | |
|---|---|
| Details | |
| Location | Ventral horn of the spinal cord, some cranial nerve nuclei |
| Shape | Projection neuron |
| Function | Excitatory projection (to NMJ) |
| Neurotransmitter | UMN to LMN: glutamate; LMN to NMJ: ACh |
| Presynaptic connections | M1 via the Corticospinal tract |
| Postsynaptic connections | Muscle fibers and other neurons |
| Identifiers | |
| MeSH | D009046 |
| NeuroLex ID | nifext_103 |
| TA98 | A14.2.00.021 |
| TA2 | 6131 |
| FMA | 83617 |
| Anatomical terms of neuroanatomy | |
In neurology, the term motor neuron (or motoneuron) classically applies to neurons located in the central nervous system (or CNS) that project their axons outside the CNS and directly or indirectly control muscles. The motor neuron is often associated with efferent neuron, primary neuron, or alpha motor neurons. Motor neurones are neurones that carry signals from the spinal cord to the muscles to produce movement. [1]
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Function
The interface between a motor neuron and muscle fiber is a specialized synapse called the neuromuscular junction. Upon adequate stimulation, the motor neuron releases a flood of neurotransmitters that bind to postsynaptic receptors and triggers a response in the muscle fiber.
- In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could be either excitatory or inhibitory.
- For vertebrates, however, the response of a muscle fiber to a neurotransmitter can only be excitatory, in other words, contractile. Muscle relaxation and inhibition of muscle contraction in vertebrates is obtained only by inhibition of the motor neuron itself. Although muscle innervation may eventually play a role in the maturation of motor activity. This is why muscle relaxants work by acting on the motoneurons that innervate muscles (by decreasing their electrophysiological activity) or on cholinergic neuromuscular junctions, rather than on the muscles themselves.
Somatic motor neurons
Somatic motoneurons are further subdivided into two types: alpha efferent neurons and gamma efferent neurons. (Both types are called efferent to indicate the flow of information from the central nervous system (CNS) to the periphery.)
- Alpha motoneurons innervate extrafusal muscle fibers (typically referred to simply as muscle fibers) located throughout the muscle. Their cell bodies are in the ventral horn of the spinal cord and they are sometimes called ventral horn cells.
- Gamma motoneurons innervate intrafusal muscle fibers found within the muscle spindle.
In addition to voluntary skeletal muscle contraction, alpha motoneurons also contribute to muscle tone, the continuous force generated by noncontracting muscle to oppose stretching. When a muscle is stretched, sensory neurons within the muscle spindle detect the degree of stretch and send a signal to the CNS. The CNS activates alpha motoneurons in the spinal cord, which cause extrafusal muscle fibers to contract and thereby resist further stretching. This process is also called the stretch reflex.
Gamma motoneurons regulate the sensitivity of the spindle to muscle stretching. With activation of gamma neurons, intrafusal muscle fibers contract so that only a small stretch is required to activate spindle sensory neurons and the stretch reflex.
Motor units
Acording to Purves D, et al [2].A single motor neuron may synapse with one or more muscle fibers. The motor neuron and all of the muscle fibers to which it connects is a motor unit. Motor units are split up into 3 categories: slow motor units, fast fatiguing motor units, and fast fatigue-resistant motor units.
- Slow motor units are used to stimulate small muscle fibres which contract very slowly and provide small amounts of energy but are very resistant to fatigue, so they are used to sustain muscular contraction such keep the body upright.
- The Fast Fatiguing motor units are used to stimulate larger muscle groups which apply large amounts of force but fatigue very quickly. They are used for tasks that requires large brief bursts on energy such as jumping or running.
- The Fast Fatigue-Resistant motor unit stimulate moderate sized muscles groups that don't react as fast as the FF motor units, but can be sustained much longer, as implied by the name, and provide more force that the S motor unit.
See also
- Alpha motor neuron
- Central chromatolysis
- Gamma motor neuron
- Beta motor neuron
- Muscle spindle
- Nerve fiber
- Neuromuscular junction
- Motor dysfunction
- Motor unit
- Motor neuron disease
- Efferent nerve
- Nerves
References
- Sherwood, L. (2001). Human Physiology: From Cells to Systems (4th ed.). Pacific Grove, CA: Brooks-Cole. ISBN 0-534-37254-6.
- Marieb, E. N.; Mallatt, J. (1997). Human Anatomy (2nd ed.). Menlo Park, CA: Benjamin/Cummings. ISBN 0-8053-4068-8.