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Lower motor neurons (LMNs) are the motor neurons connecting the brainstem and spinal cord to muscle fibers, bringing the nerve impulses from the upper motor neurons out to the muscles. A lower motor neuron's axon travels through a foramen and terminates on an effector (muscle).
Classification
The axons of lower motor neurons are a type of motor fibers. Lower motor neurons are classified based on the type of muscle fiber they innervate:
- Alpha motor neurons (α-MNs) innervate extrafusal muscle fibers, the most numerous type of muscle fiber and the one most involved in muscle contraction.
- Gamma motor neurons (γ-MNs) innervate intrafusal muscle fibers, which are involved with muscle spindles and the sense of body position.
Physiology
Glutamate released from the upper motor neurons triggers depolarization in the lower motor neurons in the ventral horn which in turn causes an action potential to propagate the length of the axon to the neuromuscular junction where acetylcholine is released to carry the signal across the synaptic cleft to the postsynaptic receptors of the muscle cell membrane, signaling the muscle to contract.
Consequences of lesions
Damage to lower motor neurons (lower motor neurone lesions) is indicated by abnormal EMG potentials, fasciculations, paralysis, weakening of muscles, and neurogenic atrophy of skeletal muscle.
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