Spinal motor nerves

Motor neuron
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
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 []

A motor neuron (or motoneuron) 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. 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. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.

The term 'motor neuron' is usually restricted to the lower motor neurons, the efferent nerves that directly innervate muscles.

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.