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{{Infobox_neuron
 |neuron_name = Motor neuron
 |image_neuron = Medulla spinalis - Section - English.svg
 |caption_neuron = Section through the spinal cord.  Motor neuron projection through [[ventral root]] is shown in red.
 |location = [[Ventral horn]] of the [[spinal cord]]
 |function = Excitatory projection (to [[NMJ]])
 |neurotransmitter = [[ACh]]
 |morphology = Projection neuron
 |afferents = [[Primary motor cortex|M1]] via the [[Corticospinal tract]]
 |efferents = [[Muscle fiber]]s and other [[neuron]]s
 |GraySubject = 
 |GrayPage = 
}}
{{CMG}}



In [[vertebrate]]s, the term '''motor neuron''' (or '''motoneuron''') classically applies to [[neurons]] located in the [[central nervous system]] (CNS) which project their [[axons]] outside the CNS and directly or indirectly control [[muscle]]s. Motor neuron is often synonymous with [[efferent neuron]].

== Anatomy and physiology ==
{| cellpadding="3" border="1" style="border-collapse:collapse; float:right;"
|- 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</small><br>''Motoneuron neurotransmitters''
|}

According to their targets, motoneurons are classified into three broad categories: 

'''"Somatic motoneurons"''', which ''directly'' innervate [[skeletal muscles]], involved in locomotion (such as muscles of the limbs, abdominal and [[intercostal muscles]]).

'''“[[Special visceral motoneurons]]”''' — also called “branchial motoneurons”— which ''directly'' innervate [[Branchiomeric_musculature|branchial muscles]] (that motorize the [[gills]] in fish and the face and neck in land [[vertebrates]]).

'''“[[General visceral motoneurons]]”''' — "visceral motoneurons" for short— which ''indirectly'' innervate [[smooth muscles]] of the [[viscera]] (like the [[heart]], or 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 other words:
* the motor command of [[skeletal muscle|skeletal]] and branchial muscles is ''monosynaptic'' (involving only one motoneuron —respectively “somatic “ and “branchial”—  which synapses onto the muscle).
* the command of [[visceral muscles]] is ''disynaptic'' (involving two neurons: the “general visceral motoneuron” located in the CNS, which synapses onto a ganglionic neuron, located in the PNS, which synapses onto the muscle). 

It could be argued that, in the command of visceral muscles, the ganglionic neuron —[[parasympathetic]] or [[sympathetic]]— is the real “motoneuron”, being the one that directly innervates the muscle (while the “general visceral motoneuron” is, strictly speaking, a “[[preganglionic]]” neuron). But, for historical reasons, the term motoneuron is reserved for the CNS neuron.

All motoneurons are [[cholinergic]] (that is, release the neurotransmitter [[acetylcholine]]). Parasympathetic ganglionic neurons are also cholinergic, while most sympathetic ganglionic neurons are [[noradrenergic]] (that is, release the neurotransmitter [[noradrenaline]]). (see Table)

== Function ==

The interface between a motoneuron and muscle fiber is a specialized [[synapse]] called the [[neuromuscular junction]]. Upon adequate stimulation, the motoneuron releases a flood of [[neurotransmitter]]s that bind to postsynaptic [[receptor (biochemistry)|receptor]]s 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 either be 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 verterbrates is obtained only by inhibition of the motoneuron 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 [[electrophysiology|electrophysiological]] activity) or on [[acetylcholine|cholinergic]] [[neuromuscular junction]]s, rather than on the muscles themselves.

== Somatic motoneurons ==
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 [[peripheral nervous system|periphery]].) 
* ''[[Alpha motoneurons]]'' innervate [[extrafusal muscle fiber]]s (typically referred to simply as ''muscle fibers'') located throughout the muscle. 
* ''[[Gamma motoneurons]]'' innervate [[intrafusal muscle fiber]]s 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 neuron]]s 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 ==
A single motoneuron may synapse with one or more muscle fibers. The motoneuron and all of the muscle fibers to which it connects is a [[motor unit]].

==See also==
* [[neuromuscular junction]]
* [[muscle spindle]]
* [[motor unit]]
* [[Motor neurone disease]]
* [[Motor nerve]]
* [[Efferent nerve]]
* [[Efferent nerve]]

== References ==
* Sherwood, L. (2001). Human Physiology: From Cells to Systems (4 ed.). California: Brooks/Cole.
* Marieb, E. N., Mallatt, J. (1997). Human Anatomy (2 ed.). California: Benjamin/Cummings.

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[[Category:Efferent neurons]]

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