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Motor neuron

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Amotor neuron(ormotoneuronorefferent neuron[1]) is aneuronwhosecell bodyis located in themotor cortex,brainstemor thespinal cord,and whoseaxon(fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainlymusclesandglands.[2]There are two types of motor neuron –upper motor neuronsandlower motor neurons.Axons from upper motor neurons synapse ontointerneuronsin the spinal cord and occasionally directly onto lower motor neurons.[3]The axons from the lower motor neurons areefferent nerve fibersthat carry signals from thespinal cordto the effectors.[4]Types of lower motor neurons arealpha motor neurons,beta motor neurons,andgamma motor neurons.

Motor neurons
Micrographof thehypoglossal nucleusshowing motor neurons with their characteristic coarseNissl substance( "tigroid" cytoplasm).H&E-LFB stain.
Details
LocationVentral hornof thespinal cord,somecranial nerve nuclei
ShapeProjection neuron
FunctionExcitatory projection (to [
NeurotransmitterUMNtoLMN:glutamate;LMNtoNMJ:ACh
Presynaptic connectionsPrimary motor cortexvia theCorticospinal tract
Postsynaptic connectionsMuscle fibersand otherneurons
Identifiers
MeSHD009046
NeuroLexIDnifext_103
TA98A14.2.00.021
TA26131
FMA83617
Anatomical terms of neuroanatomy

A single motor neuron may innervate manymuscle fibresand a muscle fibre can undergo manyaction potentialsin the time taken for a singlemuscle twitch.Innervation takes place at aneuromuscular junctionand twitches can become superimposed as a result ofsummationor atetanic contraction.Individual twitches can become indistinguishable, and tension rises smoothly eventually reaching a plateau.[5]

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

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Motor neurons begin to develop early inembryonic development,and motor function continues to develop well into childhood.[7]In theneural tubecells are specified to either the rostral-caudal axis or ventral-dorsal axis. Theaxonsof motor neurons begin to appear in the fourth week of development from the ventral region of the ventral-dorsal axis (thebasal plate).[8]This homeodomain is known as the motor neural progenitor domain (pMN).Transcription factorshere includePax6,OLIG2,Nkx-6.1,andNkx-6.2,which are regulated bysonic hedgehog(Shh). The OLIG2 gene being the most important due to its role in promotingNgn2 expression,a gene that causes cell cycle exiting as well as promoting further transcription factors associated with motor neuron development.[9]

Further specification of motor neurons occurs whenretinoic acid,fibroblast growth factor,Wnts,andTGFb,are integrated into the variousHoxtranscription factors. There are 13 Hox transcription factors and along with the signals, determine whether a motor neuron will be more rostral or caudal in character. In the spinal column, Hox 4-11 sort motor neurons to one of the five motor columns.[9]

Motor columns of spinal cord[10]
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

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Spinal cord tracts
Location of lower motor neurons in spinal cord

Upper motor neurons

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Upper motor neuronsoriginate in themotor cortexlocated in theprecentral gyrus.The cells that make up theprimary motor cortexareBetz cells,which are giantpyramidal cells.The axons of these cells descend from the cortex to form thecorticospinal tract.[12]Corticomotorneuronsproject from the primary cortex directly onto motor neurons in the ventral horn of the spinal cord.[13][14]Their axons synapse on the spinal motor neurons of multiple muscles as well as on spinalinterneurons.[13][14]They are unique to primates and it has been suggested that their function is the adaptive control of thehandsincluding the relatively independent control of individual fingers.[14][15]Corticomotorneurons have so far only been found in the primary motor cortex and not in secondary motor areas.[14]

Nerve tracts

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Nerve tractsare bundles of axons aswhite matter,that carryaction potentialsto 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]

Lower motor neurons

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Lower motor neuronsare 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

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Somatic motor neurons originate in thecentral nervous system,project theiraxonstoskeletal muscles[19](such as the muscles of the limbs, abdominal, andintercostal muscles), which are involved inlocomotion.The three types of these neurons are thealpha efferent neurons,beta efferent neurons,andgamma efferent neurons.They are calledefferentto indicate the flow of information from thecentral nervous system(CNS) to theperiphery.

  • Alpha motor neuronsinnervateextrafusal muscle fibers,which are the main force-generating component of a muscle. Their cell bodies are in theventral hornof the spinal cord and they are sometimes calledventral 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 amotor 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 tomuscle tone,the continuous force generated by noncontracting muscle to oppose stretching. When a muscle is stretched,sensory neuronswithin themuscle spindledetect 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 thestretch reflex.

  • Beta motor neuronsinnervateintrafusal muscle fibersofmuscle 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 neuronsinnervate 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]
    • AfterHyper-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

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These are also known asbranchial motor neurons,which are involved in facial expression, mastication, phonation, and swallowing. Associated cranial nerves are theoculomotor,abducens,trochlear,andhypoglossalnerves.[18]

Branch of NS Position Neurotransmitter
Somatic n/a Acetylcholine
Parasympathetic Preganglionic Acetylcholine
Parasympathetic Ganglionic Acetylcholine
Sympathetic Preganglionic Acetylcholine
Sympathetic Ganglionic Norepinephrine*
*Except fibers tosweat glandsand certainblood vessels
Motor neuron neurotransmitters

General visceral motor neurons

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These motor neurons indirectly innervatecardiac muscleandsmooth musclesof theviscera( the muscles of thearteries): theysynapseonto neurons located ingangliaof theautonomic nervous system(sympatheticandparasympathetic), located in theperipheral nervous system(PNS), which themselves directly innervate visceral muscles (and also some gland cells).

In consequence, the motor command ofskeletaland branchial muscles ismonosynapticinvolving only one motor neuron, eithersomaticorbranchial,which synapses onto the muscle. Comparatively, the command ofvisceral musclesisdisynapticinvolving two neurons: thegeneral visceral motor neuron,located in the CNS, synapses onto a ganglionic neuron, located in the PNS, which synapses onto the muscle.

All vertebrate motor neurons arecholinergic,that is, they release the neurotransmitteracetylcholine.Parasympathetic ganglionic neurons are also cholinergic, whereas most sympathetic ganglionic neurons arenoradrenergic,that is, they release the neurotransmitternoradrenaline.(see Table)

Neuromuscular junctions

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A single motor neuron may innervate manymuscle fibresand a muscle fibre can undergo manyaction potentialsin the time taken for a singlemuscle twitch.As a result, if an action potential arrives before a twitch has completed, the twitches can superimpose on one another, either throughsummationor atetanic contraction.In summation, the muscle is stimulated repetitively such that additional action potentials coming from thesomatic nervous systemarrive before the end of the twitch. The twitches thus superimpose on one another, leading to a force greater than that of a single twitch. A tetanic contraction is caused by constant, very high frequency stimulation - the action potentials come at such a rapid rate that individual twitches are indistinguishable, and tension rises smoothly eventually reaching a plateau.[5]

The interface between a motor neuron and muscle fiber is a specializedsynapsecalled theneuromuscular junction.Upon adequate stimulation, the motor neuron releases a flood of acetylcholine (Ach)neurotransmittersfrom synaptic vesicles bound to the plasma membrane of the axon terminals. The acetylcholine molecules bind topostsynapticreceptorsfound within the motor end plate. Once two acetylcholine receptors have been bound, an ion channel is opened and sodium ions are allowed to flow into the cell. The influx of sodium into the cell causes depolarization and triggers a muscle action potential. T tubules of the sarcolemma are then stimulated to elicit calcium ion release from the sarcoplasmic reticulum. It is this chemical release that causes the target muscle fiber to contract.[20]

Ininvertebrates,depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could be either excitatory or inhibitory. Forvertebrates,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. This is howmuscle relaxantswork by acting on the motor neurons that innervate muscles (by decreasing theirelectrophysiologicalactivity) or oncholinergicneuromuscular junctions, rather than on the muscles themselves.

Synaptic input to motor neurons

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Motor neurons receive synaptic input from premotor neurons. Premotor neurons can be 1)spinal interneuronsthat have cell bodies in the spinal cord, 2)sensory neuronsthat convey information from the periphery andsynapse directly onto motoneurons,3)descending neuronsthat convey information from thebrainandbrainstem.The synapses can beexcitatory,inhibitory,electrical,orneuromodulatory.For any given motor neuron, determining the relative contribution of different input sources is difficult, but advances inconnectomicshave made it possible forfruit flymotor neurons. In the fly, motor neurons controlling the legs and wings are found in theventral nerve cord,homologous to thespinal cord.Fly motor neurons vary by over 100X in the total number of input synapses. However, each motor neuron gets similar fractions of its synapses from each premotor source: ~70% from neurons within the VNC, ~10% from descending neurons, ~3% from sensory neurons, and ~6% from VNC neurons that also send a process up to the brain. The remaining 10% of synapses come from neuronal fragments that are unidentified by current image segmentation algorithms and require additional manual segmentation to measure.[23]

See also

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References

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  1. ^"Afferent vs. Efferent: AP® Psych Crash Course Review | Albert.io".Albert Resources.2019-12-02.Retrieved2021-04-25.
  2. ^Tortora, Gerard; Derrickson, Bryan (2014).Principles of Anatomy & Physiology(14th ed.). New Jersey: John Wiley & Sons, Inc. pp.406, 502, 541.ISBN978-1-118-34500-9.
  3. ^Pocock, Gillian; Richards, Christopher D. (2006).Human physiology: the basis of medicine(3rd ed.). Oxford: Oxford University Press. pp. 151–153.ISBN978-0-19-856878-0.
  4. ^Schacter D.L., Gilbert D.T., and Wegner D.M. (2011) Psychology second edition. New York, NY: Worth
  5. ^abRussell, Peter (2013).Biology - Exploring the Diversity of Life.Toronto: Nelson Education. p. 946.ISBN978-0-17-665133-6.
  6. ^"https://www.ncbi.nlm.nih.gov/books/NBK554616/"
  7. ^Tortora, Gerard; Derrickson, Bryan (2011).Principles of Anatomy Physiology(14th ed.). New Jersey: John Wiley & Sons, Inc. pp.1090–1099.ISBN978-1-118-34500-9.
  8. ^Sadler, T. (2010).Langman's medical embryology(11th ed.). Philadelphia: Lippincott William & Wilkins. pp. 299–301.ISBN978-0-7817-9069-7.
  9. ^abDavis-Dusenbery, BN; Williams, LA; Klim, JR; Eggan, K (February 2014)."How to make spinal motor neurons".Development.141(3): 491–501.doi:10.1242/dev.097410.PMID24449832.
  10. ^Edgar R, Mazor Y, Rinon A, Blumenthal J, Golan Y, Buzhor E, Livnat I, Ben-Ari S, Lieder I, Shitrit A, Gilboa Y, Ben-Yehudah A, Edri O, Shraga N, Bogoch Y, Leshansky L, Aharoni S, West MD, Warshawsky D, Shtrichman R (2013)."LifeMap Discovery™: The Embryonic Development, Stem Cells, and Regenerative Medicine Research Portal".PLOS ONE.8(7): e66629.Bibcode:2013PLoSO...866629E.doi:10.1371/journal.pone.0066629.ISSN1932-6203.PMC3714290.PMID23874394.
  11. ^Philippidou, Polyxeni; Walsh, Carolyn; Aubin, Josée; Jeannotte, Lucie; Dasen, Jeremy S. (2012)."Sustained Hox5 Gene Activity is Required for Respiratory Motor Neuron Development".Nature Neuroscience.15(12): 1636–1644.doi:10.1038/nn.3242.ISSN1097-6256.PMC3676175.PMID23103965.
  12. ^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".Archivedfrom the original on 2018-06-05.Retrieved2017-11-30.
  13. ^abMack, Sarah; Kandel, Eric R.; Jessell, Thomas M.; Schwartz, James H.; Siegelbaum, Steven A.; Hudspeth, A. J. (2013).Principles of neural science.Kandel, Eric R. (5th ed.). New York.ISBN9780071390118.OCLC795553723.{{cite book}}:CS1 maint: location missing publisher (link)
  14. ^abcdLemon, Roger N. (April 4, 2008). "Descending Pathways in Motor Control".Annual Review of Neuroscience.31(1): 195–218.doi:10.1146/annurev.neuro.31.060407.125547.ISSN0147-006X.PMID18558853.S2CID16139768.
  15. ^Isa, T (April 2007). "Direct and indirect cortico-motoneuronal pathways and control of hand/arm movements".Physiology.22(2): 145–152.doi:10.1152/physiol.00045.2006.PMID17420305.
  16. ^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.
  17. ^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".Archivedfrom the original on 2018-06-05.Retrieved2017-11-30.
  18. ^ab"CHAPTER NINE".www.unc.edu.Archivedfrom the original on 2017-11-05.Retrieved2017-12-08.
  19. ^Silverthorn, Dee Unglaub (2010).Human Physiology: An Integrated Approach.Pearson. p. 398.ISBN978-0-321-55980-7.
  20. ^abTortora, 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.
  21. ^abcdPurves D, Augustine GJ, Fitzpatrick D, et al., editors: Neuroscience. 2nd edition, 2001"The Motor Unit - Neuroscience - NCBI Bookshelf".Archivedfrom the original on 2018-06-05.Retrieved2017-09-05.
  22. ^abcdeManuel, Marin; Zytnicki, Daniel (2011). "Alpha, Beta, and Gamma Motoneurons: Functional Diversity in the Motor System's Final Pathway".Journal of Integrative Neuroscience.10(3): 243–276.doi:10.1142/S0219635211002786.ISSN0219-6352.PMID21960303.S2CID21582283.
  23. ^Azevedo, Anthony; Lesser, Ellen; Mark, Brandon; Phelps, Jasper; Elabbady, Leila; Kuroda, Sumiya; Sustar, Anne; Moussa, Anthony; Kandelwal, Avinash; Dallmann, Chris J.; Agrawal, Sweta; Lee, Su-Yee J.; Pratt, Brandon; Cook, Andrew; Skutt-Kakaria, Kyobi (2022-12-15)."Tools for comprehensive reconstruction and analysis of Drosophila motor circuits":2022.12.15.520299.doi:10.1101/2022.12.15.520299.S2CID254736092.{{cite journal}}:Cite journal requires|journal=(help)

Sources

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