| Motor neuron | |
|---|---|
 Micrograph of thehypoglossal nucleus showing motor neurons with their characteristic coarseNissl substance ("tigroid" cytoplasm).H&E-LFB stain. | |
| Details | |
| Location | Ventral horn of thespinal cord, somecranial nerve nuclei |
| Shape | Projection neuron |
| Function | Excitatory projection (to [ |
| Neurotransmitter | UMN toLMN:glutamate;LMN toNMJ:ACh |
| Presynaptic connections | Primary motor cortex via theCorticospinal tract |
| Postsynaptic connections | Muscle fibers and otherneurons |
| Identifiers | |
| MeSH | D009046 |
| NeuroLex ID | nifext_103 |
| TA98 | A14.2.00.021 |
| TA2 | 6131 |
| FMA | 83617 |
| Anatomical terms of neuroanatomy | |
Amotor neuron (ormotoneuron), also known asefferent neuron[1] is aneuron that allows for both voluntary and involuntary movements of the body throughmuscles andglands. Itscell body is located in themotor cortex,brainstem or thespinal cord, and whoseaxon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly muscles and glands.[2] There are two types of motor neuron –upper motor neurons andlower motor neurons. Axons from upper motor neurons synapse ontointerneurons in the spinal cord and occasionally directly onto lower motor neurons.[3] The axons from the lower motor neurons areefferent nerve fibers that carry signals from thespinal cord to the effectors.[4] Types of lower motor neurons arealpha motor neurons,beta motor neurons, andgamma motor neurons.
A single motor neuron may innervate manymuscle fibres and a muscle fibre can undergo manyaction potentials in the time taken for a singlemuscle twitch. Innervation takes place at aneuromuscular junction and twitches can become superimposed as a result ofsummation or 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]
Disorders causing selective injury to motor neurons are classified asmotor neuron diseases.
Motor neurons begin to develop early inembryonic development, and motor function continues to develop well into childhood.[7] In theneural tube cells are specified to either the rostral-caudal axis or ventral-dorsal axis. Theaxons of 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 factors here 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 variousHox 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.[9]
| 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] |
Upper motor neurons originate in themotor cortex located in theprecentral gyrus. The cells that make up theprimary motor cortex areBetz cells, which are giantpyramidal cells. The axons of these cells descend from the cortex to form thecorticospinal tract.[12]Corticomotorneurons project 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 thehands including 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 are bundles of axons aswhite matter, that carryaction 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]
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 originate in thecentral nervous system, project theiraxons toskeletal 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 calledefferent to indicate the flow of information from thecentral nervous system (CNS) to theperiphery.
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 neurons within themuscle 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 thestretch reflex.
These are also known asbranchial motor neurons, which are involved in facial expression, mastication, phonation, and swallowing. Associated cranial nerves are theoculomotor,abducens,trochlear, andhypoglossal 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 tosweat glands and certainblood vessels Motor neuron neurotransmitters | ||
These motor neurons indirectly innervatecardiac muscle andsmooth muscles of theviscera ( the muscles of thearteries): theysynapse onto neurons located inganglia of theautonomic nervous system (sympathetic andparasympathetic), located in theperipheral nervous system (PNS), which themselves directly innervate visceral muscles (and also some gland cells).
In consequence, the motor command ofskeletal and branchial muscles ismonosynaptic involving only one motor neuron, eithersomatic orbranchial, which synapses onto the muscle. Comparatively, the command ofvisceral muscles isdisynaptic involving 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)
A single motor neuron may innervate manymuscle fibres and a muscle fibre can undergo manyaction potentials in 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 throughsummation or atetanic contraction. In summation, the muscle is stimulated repetitively such that additional action potentials coming from thesomatic 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.[5]
The interface between a motor neuron and muscle fiber is a specializedsynapse called theneuromuscular junction. Upon adequate stimulation, the motor neuron releases a flood of acetylcholine (Ach)neurotransmitters from synaptic vesicles bound to the plasma membrane of the axon terminals. The acetylcholine molecules bind topostsynapticreceptors 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.[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 relaxants work by acting on the motor neurons that innervate muscles (by decreasing theirelectrophysiological activity) or oncholinergic neuromuscular junctions, rather than on the muscles themselves.
Motor neurons receive synaptic input from premotor neurons. Premotor neurons can be 1)spinal interneurons that have cell bodies in the spinal cord, 2)sensory neurons that convey information from the periphery andsynapse directly onto motoneurons, 3)descending neurons that convey information from thebrain andbrainstem. The synapses can beexcitatory,inhibitory,electrical, orneuromodulatory. For any given motor neuron, determining the relative contribution of different input sources is difficult, but advances inconnectomics have made it possible forfruit fly motor 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]
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