Motor development

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 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. Motor neurons' axons are efferent nerve fibers that carry signals from the spinal cord to the effectors to produce effects. 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 nerves 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.