View source for Motor neuron

{{Short description|Nerve cell sending impulse to muscle}}
{{Infobox neuron
|name = Motor neuron
|image = Medulla_oblongata_-_posterior_-_cn_xii_-_very_high_mag.jpg
|caption = [[Micrograph]] of the [[hypoglossal nucleus]] showing motor neurons with their characteristic coarse [[Nissl substance]] ("tigroid" cytoplasm). [[LFB stain|H&E-LFB stain]].
|location = [[Anterior horn of spinal cord|Ventral horn]] of the [[spinal cord]], some [[cranial nerve nuclei]]
|function = Excitatory projection (to [
|neurotransmitter = [[Upper motor neuron|UMN]] to [[Lower motor neuron|LMN]]: [[glutamate]]; [[Lower motor neuron|LMN]] to [[Neuromuscular junction|NMJ]]: [[ACh]]
|morphology = Projection neuron
|afferents = [[Primary motor cortex]] via the [[Corticospinal tract]]
|efferents = [[Muscle fiber]]s and other [[neuron]]s
}}
A '''motor neuron''' (or '''motoneuron'''), also known as '''efferent neuron'''<ref>{{Cite web|date=2019-12-02|title=Afferent vs. Efferent: AP® Psych Crash Course Review {{!}} Albert.io|url=https://www.albert.io/blog/afferent-vs-efferent-whats-the-difference-ap-psychology-crash-course-review/|access-date=2021-04-25|website=Albert Resources|language=en-US}}</ref> is a [[neuron]] that allows for both voluntary and involuntary movements of the body through [[Muscle|muscle]]s and [[Gland|glands]]. Its [[cell body]] is located in the [[motor cortex]], [[brainstem]] or the [[spinal cord]], and whose [[axon]] (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly muscles and glands.<ref>{{Cite book|title=Principles of Anatomy & Physiology|last1=Tortora|first1=Gerard|last2=Derrickson|first2=Bryan|publisher=John Wiley & Sons, Inc.|year=2014|isbn=978-1-118-34500-9|edition=14th|location=New Jersey|pages=[https://archive.org/details/principlesofanat0000tort/page/406 406, 502, 541]|url=https://archive.org/details/principlesofanat0000tort/page/406}}</ref>  There are two types of motor neuron – [[upper motor neuron]]s and [[lower motor neuron]]s. Axons from upper motor neurons synapse onto [[interneuron]]s in the spinal cord and occasionally directly onto lower motor neurons.<ref name="OUP">{{cite book|last1=Pocock|first1=Gillian|last2=Richards|first2=Christopher D.|title=Human physiology: the basis of medicine|date=2006|publisher=Oxford University Press|location=Oxford|isbn=978-0-19-856878-0|pages=151–153|edition=3rd}}</ref> The axons from the lower motor neurons are [[efferent nerve fiber]]s that carry signals from the [[spinal cord]] to the effectors.<ref>Schacter D.L., Gilbert D.T., and Wegner D.M. (2011) Psychology second edition. New York, NY: Worth</ref> Types of lower motor neurons are [[alpha motor neuron]]s, [[beta motor neuron]]s, and [[gamma motor neuron]]s.

A single motor neuron may innervate many [[myocyte|muscle fibres]] and a muscle fibre can undergo many [[action potentials]] in the time taken for a single [[Muscle_contraction|muscle twitch]]. Innervation takes place at a [[neuromuscular junction]] and twitches can become superimposed as a result of [[Muscle_contraction#Gradation_of_skeletal_muscle_contractions|summation]] or a [[tetanic contraction]]. Individual twitches can become indistinguishable, and tension rises smoothly eventually reaching a plateau.<ref name="Russell 2013 946">{{cite book|last=Russell|first=Peter|title=Biology - Exploring the Diversity of Life|year=2013|publisher=Nelson Education|location=Toronto|isbn=978-0-17-665133-6|page=946}}</ref>

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.<ref name="P"> "https://www.ncbi.nlm.nih.gov/books/NBK554616/" </ref>

Disorders causing selective injury to motor neurons are classified as [[Motor neuron diseases|motor neuron diseases]].

== Development ==
Motor neurons begin to develop early in [[embryogenesis|embryonic development]], and motor function continues to develop well into childhood.<ref>{{Cite book|title=Principles of Anatomy Physiology|last1=Tortora|first1=Gerard|last2=Derrickson|first2=Bryan|publisher=John Wiley & Sons, Inc.|year=2011|isbn=978-1-118-34500-9|edition=14th|location=New Jersey|pages=[https://archive.org/details/principlesofanat0000tort/page/1090 1090–1099]|url=https://archive.org/details/principlesofanat0000tort/page/1090}}</ref> In the [[Neural tube#Ventral-dorsal patterning|neural tube]] cells are specified to either the rostral-caudal axis or ventral-dorsal axis. The [[axon]]s of motor neurons begin to appear in the fourth week of development from the ventral region of the ventral-dorsal axis (the [[Basal plate (neural tube)|basal plate]]).<ref name="Sadler">{{cite book|last1=Sadler|first1=T.|title=Langman's medical embryology.|date=2010|publisher=Lippincott William & Wilkins|location=Philadelphia|isbn=978-0-7817-9069-7|pages=299–301|edition=11th}}</ref> This homeodomain is known as the motor neural progenitor domain (pMN). [[Transcription factor]]s here include [[PAX6|Pax6]], [[OLIG2]], [[NKX6-1|Nkx-6.1]], and [[NKX6-2|Nkx-6.2]], which are regulated by [[sonic hedgehog]] (Shh). The OLIG2 gene being the most important due to its role in promoting [[Neurogenin#Motor neuron fate|Ngn2 expression]], a gene that causes cell cycle exiting as well as promoting further transcription factors associated with motor neuron development.<ref name=":1">{{cite journal|last1=Davis-Dusenbery|first1=BN|last2=Williams|first2=LA|last3=Klim|first3=JR|last4=Eggan|first4=K|title=How to make spinal motor neurons.|journal=Development|date=February 2014|volume=141|issue=3|pages=491–501|pmid=24449832|doi=10.1242/dev.097410|doi-access=free}}</ref>

Further specification of motor neurons occurs when [[retinoic acid]], [[fibroblast growth factor]], [[Wnt signalling pathway|Wnts]], and [[Transforming growth factor|TGFb]], are integrated into the various [[Hox gene|Hox]] transcription 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.<ref name=":1" /> 
{| class="wikitable"
|+Motor columns of spinal cord <ref name="KerkisEdgar2013">{{cite journal|vauthors = 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|title=LifeMap Discovery™: The Embryonic Development, Stem Cells, and Regenerative Medicine Research Portal|journal=PLOS ONE|volume=8|issue=7|year=2013|article-number=e66629|issn=1932-6203|doi=10.1371/journal.pone.0066629|pmid=23874394|pmc=3714290|bibcode=2013PLoSO...866629E|doi-access=free}}</ref>
|'''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<ref>{{Cite journal|last1=Philippidou|first1=Polyxeni|last2=Walsh|first2=Carolyn|last3=Aubin|first3=Josée|last4=Jeannotte|first4=Lucie|last5=Dasen|first5=Jeremy S.|title=Sustained Hox5 Gene Activity is Required for Respiratory Motor Neuron Development|journal=Nature Neuroscience|volume=15|issue=12|pages=1636–1644|doi=10.1038/nn.3242|issn=1097-6256|pmc=3676175|pmid=23103965|year=2012}}</ref>
|}

== Anatomy and physiology ==
[[File:Spinal cord tracts - English.svg |thumb|upright=2.4|Spinal cord tracts]]
[[File:Polio spinal diagram-en.svg |thumb|Location of lower motor neurons in spinal cord]]

=== Upper motor neurons ===

[[Upper motor neuron]]s originate in the [[motor cortex]]  located in the [[precentral gyrus]]. The cells that make up the [[primary motor cortex]] are [[Betz cell]]s, which are giant [[pyramidal cell]]s. The axons of these cells descend from the cortex to form the [[corticospinal tract]].<ref>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 {{cite web |url=https://www.ncbi.nlm.nih.gov/books/NBK10962/ |title=The Primary Motor Cortex: Upper Motor Neurons That Initiate Complex Voluntary Movements - Neuroscience - NCBI Bookshelf |access-date=2017-11-30 |url-status=live |archive-url=https://web.archive.org/web/20180605025217/https://www.ncbi.nlm.nih.gov/books/NBK10962/ |archive-date=2018-06-05 }}</ref> [[Primary motor cortex#Corticomotorneurons|Corticomotorneurons]] project from the primary cortex directly onto motor neurons in the ventral horn of the spinal cord.<ref name=":02">{{Cite book|title=Principles of neural science|others=Kandel, Eric R.|isbn=978-0-07-139011-8|edition=5th|location=New York|oclc=795553723|last1 = Mack|first1 = Sarah|last2 = Kandel|first2 = Eric R.|last3 = Jessell|first3 = Thomas M.|last4 = Schwartz|first4 = James H.|last5 = Siegelbaum|first5 = Steven A.|last6 = Hudspeth|first6 = A. J.|year = 2013}}</ref><ref name=":12">{{Cite journal|last=Lemon|first=Roger N.|date=April 4, 2008|title=Descending Pathways in Motor Control|journal=Annual Review of Neuroscience|language=en|volume=31|issue=1|pages=195–218|doi=10.1146/annurev.neuro.31.060407.125547|pmid=18558853|s2cid=16139768|issn=0147-006X|doi-access=free}}</ref> Their axons synapse on the spinal motor neurons of multiple muscles as well as on spinal [[interneuron]]s.<ref name=":02" /><ref name=":12" /> They are unique to primates and it has been suggested that their function is the adaptive control of the [[hands]] including the relatively independent control of individual fingers.<ref name=":12" /><ref>{{Cite journal|last=Isa|first=T|date=April 2007|title=Direct and indirect cortico-motoneuronal pathways and control of hand/arm movements.|journal=Physiology|volume=22|issue=2|pages=145–152|pmid=17420305|doi=10.1152/physiol.00045.2006}}</ref> Corticomotorneurons have so far only been found in the primary motor cortex and not in secondary motor areas.<ref name=":12"/>

=== Nerve tracts ===

[[Nerve tract]]s are bundles of axons as [[white matter]], that carry [[action potential]]s 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:<ref name=":2">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.</ref>
* [[Lateral corticospinal tract]]
* [[Rubrospinal tract]]
* [[Reticular formation#Medial and lateral tracts|Lateral reticulospinal tract]]
* [[Vestibulospinal tract]]
* [[Reticular formation#Medial and lateral tracts|Medial reticulospinal tract]]
* [[Tectospinal tract]]
* [[Anterior corticospinal tract]]

=== Lower motor neurons ===

[[Lower motor neuron]]s 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.<ref>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 {{cite web |url=https://www.ncbi.nlm.nih.gov/books/NBK10979/ |title=Lower Motor Neuron Circuits and Motor Control - Neuroscience - NCBI Bookshelf |access-date=2017-11-30 |url-status=live |archive-url=https://web.archive.org/web/20180605025217/https://www.ncbi.nlm.nih.gov/books/NBK10979/ |archive-date=2018-06-05 }}</ref>

According to their targets, motor neurons are classified into three broad categories:<ref name=":5">{{Cite web|url=http://www.unc.edu/~ears/classes/neuro/studyguides/sgcrainalnerves.html|title=CHAPTER NINE|website=www.unc.edu|access-date=2017-12-08|url-status=live|archive-url=https://web.archive.org/web/20171105182759/http://www.unc.edu/~ears/classes/neuro/studyguides/sgcrainalnerves.html|archive-date=2017-11-05}}</ref>
* Somatic motor neurons
* Special visceral motor neurons
* General visceral motor neurons

==== Somatic motor neurons ====
Somatic motor neurons originate in the [[central nervous system]], project their [[axons]] to [[skeletal muscles]]<ref>{{cite book|title=Human Physiology: An Integrated Approach|last=Silverthorn|first=Dee Unglaub|publisher=Pearson|year=2010|isbn=978-0-321-55980-7|page=398}}</ref> (such as the muscles of the limbs, abdominal, and [[intercostal muscles]]), which are involved in [[animal locomotion|locomotion]]. The three types of these neurons are the ''alpha efferent neurons'', ''beta efferent neurons'', and ''gamma efferent neurons''. They are called [[efferent nerve fiber|efferent]] to indicate the flow of information from the [[central nervous system]] (CNS) to the [[peripheral nervous system|periphery]].
* [[Alpha motor neuron]]s innervate [[extrafusal muscle fiber]]s, which are the main force-generating component of a muscle. Their cell bodies are in the [[Anterior grey column|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.<ref name=":3">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.</ref> 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:<ref name=":0">Purves D, Augustine GJ, Fitzpatrick D, et al., editors: Neuroscience. 2nd edition, 2001 {{cite web |url=https://www.ncbi.nlm.nih.gov/books/NBK10874/ |title=The Motor Unit - Neuroscience - NCBI Bookshelf |access-date=2017-09-05 |url-status=live |archive-url=https://web.archive.org/web/20180605025217/https://www.ncbi.nlm.nih.gov/books/NBK10874/ |archive-date=2018-06-05 }}</ref>
**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.<ref name=":0" />
**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.<ref name=":0" />
**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.<ref name=":0" />
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 neuron]]s 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 neuron]]s innervate [[intrafusal muscle fiber]]s of [[muscle spindle]]s, 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.<ref name=":4">{{cite journal|last1=Manuel|first1=Marin|last2=Zytnicki|first2=Daniel|title=Alpha, Beta, and Gamma Motoneurons: Functional Diversity in the Motor System's Final Pathway|journal=Journal of Integrative Neuroscience|volume=10|issue=3|year=2011|pages=243–276|issn=0219-6352|doi=10.1142/S0219635211002786|pmid=21960303|s2cid=21582283}}</ref>
*[[Gamma motor neuron]]s 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.<ref name=":4" />
* 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.<ref name=":4" />
** ''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.<ref name=":4" />  
** ''After [[Hyperpolarization (biology)|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.<ref name=":4" />

==== 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 nerve|oculomotor]], [[Abducens nerve|abducens]], [[Trochlear nerve|trochlear]], and [[Hypoglossal nerve|hypoglossal]] nerves.<ref name=":5" />
{| class="wikitable" style="float:right; margin-left:1em" border="1"
|- bgcolor="#cccccc" |
! Branch of NS
! Position
! Neurotransmitter
|-
| Somatic
| n/a
| [[Acetylcholine]]
|-
| Parasympathetic
| Preganglionic
| Acetylcholine
|-
| Parasympathetic
| Ganglionic
| Acetylcholine
|-
| Sympathetic
| Preganglionic
| Acetylcholine
|-
| Sympathetic
| Ganglionic
| [[Norepinephrine]]*
|-
| colspan="3" align="center" | {{small|*Except fibers to [[sweat gland]]s and certain [[blood vessel]]s}}<br />''Motor neuron neurotransmitters''
|}

==== General visceral motor neurons ====
These motor neurons 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).

In consequence, the motor command of [[skeletal muscle|skeletal]] and branchial muscles is ''monosynaptic'' involving only one motor neuron, either ''somatic'' or ''branchial'', which synapses onto the muscle.  Comparatively, the command of [[visceral muscles]] is ''disynaptic'' involving two neurons: the ''general 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 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)

=== Neuromuscular junctions ===
A single motor neuron may innervate many [[myocyte|muscle fibres]] and a muscle fibre can undergo many [[action potentials]] in the time taken for a single [[Muscle_contraction|muscle twitch]]. As a result, if an action potential arrives before a twitch has completed, the twitches can superimpose on one another, either through [[Muscle_contraction#Gradation_of_skeletal_muscle_contractions|summation]] or a [[tetanic contraction]]. In summation, the muscle is stimulated repetitively such that additional action potentials coming from the [[somatic nervous system]] arrive 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.<ref name="Russell 2013 946"/>

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 acetylcholine (Ach) [[neurotransmitter]]s from synaptic vesicles bound to the plasma membrane of the axon terminals. The acetylcholine molecules bind to [[postsynaptic]] [[receptor (biochemistry)|receptor]]s found 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.<ref name=":3" />

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.  This is how [[muscle relaxants]] work by acting on the motor neurons that innervate muscles (by decreasing their [[electrophysiology|electrophysiological]] activity) or on [[acetylcholine|cholinergic]] neuromuscular junctions, rather than on the muscles themselves.

=== Synaptic input to motor neurons ===
Motor neurons receive synaptic input from premotor neurons. Premotor neurons can be 1) [[Spinal interneuron|spinal interneurons]] that have cell bodies in the spinal cord, 2) [[Sensory neuron|sensory neurons]] that convey information from the periphery and [[Reflex|synapse directly onto motoneurons]], 3) [[Descending neuron|descending neurons]] that convey information from the [[Corticomotor neuron|brain]] and [[brainstem]]. The synapses can be [[Excitatory synapse|excitatory]], [[Inhibitory synapse|inhibitory]], [[Gap junction|electrical]], or [[Stomatogastric nervous system|neuromodulatory]]. For any given motor neuron, determining the relative contribution of different input sources is difficult, but advances in [[connectomics]] have made it possible for [[Drosophila melanogaster|fruit fly]] motor neurons. In the fly, motor neurons controlling the legs and wings are found in the [[ventral nerve cord]], homologous to the [[spinal 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.<ref>{{Cite journal |last1=Azevedo |first1=Anthony |last2=Lesser |first2=Ellen |last3=Mark |first3=Brandon |last4=Phelps |first4=Jasper |last5=Elabbady |first5=Leila |last6=Kuroda |first6=Sumiya |last7=Sustar |first7=Anne |last8=Moussa |first8=Anthony |last9=Kandelwal |first9=Avinash |last10=Dallmann |first10=Chris J. |last11=Agrawal |first11=Sweta |last12=Lee |first12=Su-Yee J. |last13=Pratt |first13=Brandon |last14=Cook |first14=Andrew |last15=Skutt-Kakaria |first15=Kyobi |date=2022-12-15 |title=Tools for comprehensive reconstruction and analysis of Drosophila motor circuits |url=https://www.biorxiv.org/content/10.1101/2022.12.15.520299v1 |language=en |article-number=2022.12.15.520299 |doi=10.1101/2022.12.15.520299|s2cid=254736092 |doi-access=free }}</ref>

==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 ==
{{Reflist}}

==Sources==
*{{cite book |last=Sherwood |first=L. |year=2001 |title=Human Physiology: From Cells to Systems |edition=4th |location=Pacific Grove, CA |publisher=Brooks-Cole |isbn=0-534-37254-6 }}
*{{cite book |last1=Marieb |first1=E. N. |last2=Mallatt |first2=J. |year=1997 |title=Human Anatomy |edition=2nd |location=Menlo Park, CA |publisher=Benjamin/Cummings |isbn=0-8053-4068-8 |url-access=registration |url=https://archive.org/details/humananatomy00mari }}

{{Nervous tissue}}
{{Authority control}}

[[Category:Efferent neurons]]
[[Category:Motor system]]