Motor neuron
Motor neurons | |
---|---|
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] |
A motor neuron (or motoneuron or efferent neuron
A single motor neuron may innervate many
Although the word "motor neuron" suggests that there is a single kind of neuron that controls movement, this is not the case. Indeed, upper and lower motor neurons—which differ greatly in their origins, synapse locations, routes, neurotransmitters, and lesion characteristics—are included in the same classification as "motor neurons." Essentially, motor neurons, also known as motoneurons, are made up of a variety of intricate, finely tuned circuits found throughout the body that innervate effector muscles and glands to enable both voluntary and involuntary motions. Two motor neurons come together to form a two-neuron circuit. While lower motor neurons start in the spinal cord and go to innervate muscles and glands all throughout the body, upper motor neurons originate in the cerebral cortex and travel to the brain stem or spinal cord. It is essential to comprehend the distinctions between upper and lower motor neurons as well as the routes they follow in order to effectively detect these neuronal injuries and localise the lesions. [6]
Development
Motor neurons begin to develop early in
Further specification of motor neurons occurs when
Motor column | Location in spinal cord | Target |
Median motor column | Present entire length | Axial muscles |
Hypaxial motor column | Thoracic region | Body wall muscles |
Preganglionic motor column | Thoracic region | Sympathetic ganglion |
Lateral motor column | Brachial and lumbar region (both regions are further divided into medial and lateral domains) | Muscles of the limbs |
Phrenic motor column | Cervical region | Diaphragm[11] |
Anatomy and physiology
Upper motor neurons
Nerve tracts
Nerve tracts are bundles of axons as white matter, that carry action potentials to their effectors. In the spinal cord these descending tracts carry impulses from different regions. These tracts also serve as the place of origin for lower motor neurons. There are seven major descending motor tracts to be found in the spinal cord:[16]
- Lateral corticospinal tract
- Rubrospinal tract
- Lateral reticulospinal tract
- Vestibulospinal tract
- Medial reticulospinal tract
- Tectospinal tract
- Anterior corticospinal tract
Lower motor neurons
Lower motor neurons are those that originate in the spinal cord and directly or indirectly innervate effector targets. The target of these neurons varies, but in the somatic nervous system the target will be some sort of muscle fiber. There are three primary categories of lower motor neurons, which can be further divided in sub-categories.[17]
According to their targets, motor neurons are classified into three broad categories:[18]
- Somatic motor neurons
- Special visceral motor neurons
- General visceral motor neurons
Somatic motor neurons
Somatic motor neurons originate in the
- Alpha motor neurons innervate extrafusal muscle fibers, which are the main force-generating component of a muscle. Their cell bodies are in the ventral horn of the spinal cord and they are sometimes called ventral horn cells. A single motor neuron may synapse with 150 muscle fibers on average.[20] 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:[21]
- Slow (S) motor units stimulate small muscle fibers, 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 as keeping the body upright. They gain their energy via oxidative means and hence require oxygen. They are also called red fibers.[21]
- Fast fatiguing (FF) motor units stimulate larger muscle groups, which apply large amounts of force but fatigue very quickly. They are used for tasks that require large brief bursts of energy, such as jumping or running. They gain their energy via glycolytic means and hence do not require oxygen. They are called white fibers.[21]
- Fast fatigue-resistant motor units stimulate moderate-sized muscles groups that do not react as fast as the FF motor units, but can be sustained much longer (as implied by the name) and provide more force than S motor units. These use both oxidative and glycolytic means to gain energy.[21]
In addition to voluntary skeletal muscle contraction, alpha motor neurons 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 motor neurons in the spinal cord, which cause extrafusal muscle fibers to contract and thereby resist further stretching. This process is also called the stretch reflex.
- Beta motor neurons innervate intrafusal muscle fibers of muscle spindles, with collaterals to extrafusal fibres. There are two types of beta motor neurons: Slow Contracting- These innervate extrafusal fibers. Fast Contracting- These innervate intrafusal fibers.[22]
- Gamma motor neurons innervate intrafusal muscle fibers found within the muscle spindle. They 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. There are two types of gamma motor neurons: Dynamic- These focus on Bag1 fibers and enhance dynamic sensitivity. Static- These focus on Bag2 fibers and enhance stretch sensitivity.[22]
- Regulatory factors of lower motor neurons
- Size Principle – this relates to the soma of the motor neuron. This restricts larger neurons to receive a larger excitatory signal in order to stimulate the muscle fibers it innervates. By reducing unnecessary muscle fiber recruitment, the body is able to optimize energy consumption.[22]
- Persistent Inward Current (PIC) – recent animal study research has shown that constant flow of ions such as calcium and sodium through channels in the soma and dendrites influence the synaptic input. An alternate way to think of this is that the post-synaptic neuron is being primed before receiving an impulse.[22]
- After Hyper-polarization (AHP) – A trend has been identified that shows slow motor neurons to have more intense AHPs for a longer duration. One way to remember this is that slow muscle fibers can contract for longer, so it makes sense that their corresponding motor neurons fire at a slower rate.[22]
Special visceral motor neurons
These are also known as branchial motor neurons, which are involved in facial expression, mastication, phonation, and swallowing. Associated cranial nerves are the oculomotor, abducens, trochlear, and hypoglossal nerves.[18]
Branch of NS | Position | Neurotransmitter |
---|---|---|
Somatic | n/a | Acetylcholine |
Parasympathetic | Preganglionic | Acetylcholine |
Parasympathetic | Ganglionic | Acetylcholine |
Sympathetic | Preganglionic | Acetylcholine |
Sympathetic | Ganglionic | Norepinephrine* |
*Except fibers to sweat glands and certain blood vessels Motor neuron neurotransmitters |
General visceral motor neurons
These motor neurons indirectly innervate
In consequence, the motor command of
All vertebrate motor neurons are
Neuromuscular junctions
A single motor neuron may innervate many
The interface between a motor neuron and muscle fiber is a specialized
In
Synaptic input to motor neurons
Motor neurons receive synaptic input from premotor neurons. Premotor neurons can be 1)
See also
- Betz cell
- Central chromatolysis
- Motor dysfunction
- Motor neuron disease
- Nerve
- Sensory nerve
- Motor nerve
- Afferent nerve fiber
- Efferent nerve fiber
- Sensory neuron
References
- ^ "Afferent vs. Efferent: AP® Psych Crash Course Review | Albert.io". Albert Resources. 2019-12-02. Retrieved 2021-04-25.
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- ^ Schacter D.L., Gilbert D.T., and Wegner D.M. (2011) Psychology second edition. New York, NY: Worth
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- ^ "https://www.ncbi.nlm.nih.gov/books/NBK554616/"
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- ^ PMID 24449832.
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- PMID 23103965.
- ^ Fitzpatrick, D. (2001) The Primary Motor Cortex: Upper Motor Neurons That Initiate Complex Voluntary Movements. In D. Purves, G.J. Augustine, D. Fitzpatrick, et al. (Ed.), Neuroscience. Retrieved from "The Primary Motor Cortex: Upper Motor Neurons That Initiate Complex Voluntary Movements - Neuroscience - NCBI Bookshelf". Archived from the original on 2018-06-05. Retrieved 2017-11-30.
- ^ )
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- ^ Tortora, G. J., Derrickson, B. (2011). The Spinal Cord and Spinal Nerves. In B. Roesch, L. Elfers, K. Trost, et al. (Ed.), Principles of Anatomy and Physiology (pp. 443-468). New Jersey: John Wiley & Sons, Inc.
- ^ Fitzpatrick, D. (2001) Lower Motor Neuron Circuits and Motor Control: Overview. In D. Purves, G.J. Augustine, D. Fitzpatrick, et al. (Ed.), Neuroscience. Retrieved from "Lower Motor Neuron Circuits and Motor Control - Neuroscience - NCBI Bookshelf". Archived from the original on 2018-06-05. Retrieved 2017-11-30.
- ^ a b "CHAPTER NINE". www.unc.edu. Archived from the original on 2017-11-05. Retrieved 2017-12-08.
- ISBN 978-0-321-55980-7.
- ^ a b Tortora, G. J., Derrickson, B. (2011). Muscular Tissue. In B. Roesch, L. Elfers, K. Trost, et al. (Ed.), Principles of Anatomy and Physiology (pp. 305-307, 311). New Jersey: John Wiley & Sons, Inc.
- ^ a b c d Purves D, Augustine GJ, Fitzpatrick D, et al., editors: Neuroscience. 2nd edition, 2001 "The Motor Unit - Neuroscience - NCBI Bookshelf". Archived from the original on 2018-06-05. Retrieved 2017-09-05.
- ^ S2CID 21582283.
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Sources
- 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.