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| Motor neuron | |
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Micrograph of the hypoglossal nucleus showing motor neurons with their characteristic coarse Nissl substance ("tigroid" cytoplasm). H&E-LFB stain.
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| Details | |
| Location | Ventral horn of the spinal cord, some cranial nerve nuclei |
| Morphology | Projection neuron |
| Function | Excitatory projection (to NMJ) |
| Neurotransmitter | UMN to LMN: glutamate; LMN to NMJ: ACh |
| Presynaptic connections | Primary motor cortex via the Corticospinal tract |
| Postsynaptic connections | Muscle fibers and other neurons |
| Identifiers | |
| NeuroLex ID | Motor Neuron |
| TA | A14.2.00.021 |
| FMA | 5867 |
| Anatomical terminology
[edit on Wikidata]
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A motor neuron (or motoneuron) is a nerve cell (neuron) whose cell body is located in the spinal cord and whose fiber (axon) projects outside the spinal cord to directly or indirectly control effector organs, mainly muscles and glands.[1] Motor neurons' axons are efferent nerve fibers that carry signals from the spinal cord to the effectors to produce effects.[2] Types of motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.
There are upper motor neurons and lower motor neurons, with the cell type described above being a lower motor neuron. Upper motor neurons are cortico-spinal interneurons that arise from the motor cortex and descend to the spinal cord where they activate the lower motor neurons through synapses. The term 'motor neuron' is usually restricted to the efferent neurons that actually innervate muscles (the lower motor neurons).
A single motor neuron may innervate many muscle fibres and a muscle fibre can undergo many action potentials in the time taken for a single muscle twitch. As a result, if an action potential arrives before a twitch has completed, the twitches can superimpose on one another, either through 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.[3]
| Branch of NS | Position | Neurotransmitter |
|---|---|---|
| Somatic | n/a | Acetylcholine |
| Parasympathetic | Preganglionic | Acetylcholine |
| Parasympathetic | Ganglionic | Acetylcholine |
| Sympathetic | Preganglionic | Acetylcholine |
| Sympathetic | Ganglionic | Norepinephrine* |
| *Except fibers to sweat glands and certain blood vessels Motor neuron neurotransmitters |
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According to their targets, motor neurons are classified into three broad categories[citation needed]:
Somatic motor neurons, which originate in the central nervous system, project their axons to skeletal muscles [4] (such as the muscles of the limbs, abdominal, and intercostal muscles), which are involved in locomotion .
Special visceral motor neurons, also called branchial motor neurons, which directly innervate branchial muscles (that motorize the gills in fish and the face and neck in land vertebrates).
General visceral motor neurons (visceral motor neurons for short) which 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 and parasympathetic), located in the peripheral nervous system (PNS), which themselves directly innervate visceral muscles (and also some gland cells).
In consequence:
It could be argued that, in the command of visceral muscles, the ganglionic neuron, parasympathetic or sympathetic, is the real motor neuron, being the one that directly innervates the muscle (whereas the general visceral motor neuron is, strictly speaking, a preganglionic neuron).[citation needed] But, for historical reasons, the term motor neuron is reserved for the CNS neuron.
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 noradrenergic, that is, they release the neurotransmitter noradrenaline. (see Table)
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 neurotransmitters that bind to postsynaptic receptors and triggers a response in the muscle fiber which leads to muscle movement.
Somatic motor neurons are the alpha efferent neurons, beta efferent neurons, and gamma efferent neurons. They are called efferent to indicate the flow of information from the central nervous system (CNS) to the periphery.
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 neurons 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.
A single motor neuron may synapse with one or more muscle fibers.[5] 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:[5]
Human lower motor neurons can be generated in vitro from embryonic stem cells and induced pluripotent stem cells. [6] They are currently being evaluated as experimental therapies in animal models of motor neuron disease or spinal cord injury. [7]
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| 関連記事 | 「development」「motor」 |