| Lower motor neuron | |
|---|---|
| Identifiers | |
| FMA | 84632 |
| Anatomical terms of neuroanatomy | |
Lower motor neurons (LMNs) aremotor neurons located in either theanterior grey column,anterior nerve roots (spinal lower motor neurons) or thecranial nerve nuclei of thebrainstem andcranial nerves with motor function (cranial nerve lower motor neurons).[1] Many voluntary movements rely on spinal lower motor neurons, which innervateskeletal musclefibers and act as a link betweenupper motor neurons andmuscles.[2][3] Cranial nerve lower motor neurons also control some voluntary movements of the eyes, face and tongue, and contribute to chewing, swallowing and vocalization.[4] Damage to lower motor neurons often leads tohypotonia,hyporeflexia,flaccid paralysis as well asmuscle atrophy andfasciculations.
Lower motor neurons are classified based on the type of muscle fiber they innervate:[5]
Glutamate released from the upper motor neurons triggersdepolarization in the lower motor neurons in the anterior grey column, which in turn causes an action potential to propagate the length of theaxon to theneuromuscular junction whereacetylcholine is released to carry the signal across thesynaptic cleft to the postsynaptic receptors of themuscle cell membrane, signaling the muscle to contract.
Damage to lower motor neurons,lower motor neuron lesions (LMNL) causemuscle wasting (atrophy), decreased strength and decreasedreflexes in affected areas. These findings are in contrast to findings inupper motor neuron lesions. LMNL is indicated by abnormalEMG potentials,fasciculations,paralysis,weakening of muscles, and neurogenicatrophy of skeletal muscle.Bell's palsy,bulbar palsy,poliomyelitis andamyotrophic lateral sclerosis (ALS) are all pathologies associated with lower motor neuron dysfunction.[6][7]
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