Lower motor neuron

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 terminates on an effector (muscle).

Classification

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 involved in muscle contraction.
• Gamma motor neurons (γ-MNs) innervate intrafusal muscle fibers, which together with sensory afferents compose muscle spindles. These are part of the system for sensing body position (proprioception).

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 neuron lesions (LMNL) cause decreased tone, decreased strength and decreased reflexes in affected areas. These findings are in contrast to findings in upper motor neuron lesions. LMNL is indicated by abnormal EMG potentials, fasciculations, paralysis, weakening of muscles, and neurogenic atrophy of skeletal muscle.

See also

• Upper motor neuron
• Upper motor neuron lesion
• Lower motor neuron lesion

Motor system