Motor neuron

Motor neurons
Micrograph of the hypoglossal nucleus showing motor neurons with their characteristic coarse Nissl substance ("tigroid" cytoplasm). H&E-LFB stain.
Details
Location	Ventral horn of the spinal cord, some cranial nerve nuclei
Shape	Projection neuron
Function	Excitatory projection (to [
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
MeSH	D009046
NeuroLex ID	nifext_103
TA98	A14.2.00.021
TA2	6131
FMA	83617
Anatomical terms of neuroanatomy [edit on Wikidata]

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.

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]

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]