Editing Motor neuron (section)

== 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=9780071390118|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}}</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|pages=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 |pages=2022.12.15.520299 |doi=10.1101/2022.12.15.520299|s2cid=254736092 }}</ref>