Motor neuron

A motor neuron (or motoneuron or efferent neuron[1]) is a neuron whose 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.[2] There are two types of motor neuron – upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons.[3] The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors.[4] Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.

Motor neurons
Micrograph of the hypoglossal nucleus showing motor neurons with their characteristic coarse Nissl substance ("tigroid" cytoplasm). H&E-LFB stain.
LocationVentral horn of the spinal cord, some cranial nerve nuclei
ShapeProjection neuron
FunctionExcitatory projection (to NMJ)
NeurotransmitterUMN to LMN: glutamate; LMN to NMJ: ACh
Presynaptic connectionsPrimary motor cortex via the Corticospinal tract
Postsynaptic connectionsMuscle fibers and other neurons
NeuroLex IDnifext_103
Anatomical terms of neuroanatomy

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. Innervation takes place at a neuromuscular junction and twitches can become superimposed as a result of summation or a tetanic contraction. Individual twitches can become indistinguishable, and tension rises smoothly eventually reaching a plateau.[5]

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