Lower motor neuronal damage results in characteristic pattern of deficits, relating to the myotome affected by the damage.
Lower motor neuron injury is characterized by muscle weakness, hypotonia, hyporeflexia and muscle atrophy.
Lower motor neurons (LMNs) refers to motor neurons located in either the anterior grey column, anterior nerve roots (spinal lower motor neurons) or the cranial nerve nuclei of the brainstem and cranial nerves with motor function.
All voluntary movement relies on spinal lower motor neurons, which innervate skeletal muscle fibers.
Spinal lower motor act as a link between upper motor neurons and muscles.
Cranial nerve lower motor neurons control movements of the eyes, face and tongue.
Cranial nerve lower motor neurons contribute to chewing, swallowing and vocalization.
Damage to the lower motor neurons can lead to flaccid paralysis, absent deep tendon reflexes and muscle atrophy.
Classification of lower motor neurons are based on the type of muscle fiber they innervate.
Alpha motor neurons innervate extrafusal muscle fibers, the most numerous type of muscle fiber and the one involved in muscle contraction.
Beta motor neurons innervate intrafusal fibers of muscle spindles with collaterals to extrafusal fibers.
Gamma motor neurons innervate intrafusal muscle fibers, which together with sensory afferents compose muscle spindles, and are part of the system for sensing body position.
Glutamate released from the upper motor neurons triggers depolarization in the lower motor neurons in the anterior grey column.
Depolarization in the lower motor neurons 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.
Damage to lower motor neurons causes muscle wasting, decreased strength and decreased reflexes in affected areas.
Damage to the upper motor neuron causes muscle atrophy with long standing lesion.
LMN lesions are indicated by abnormal EMG potentials, fasciculations, paralysis, weakening of muscles, and neurogenic atrophy of skeletal muscle.
Bell’s Palsy, Bulbar palsy, Poliomyelitis and Amyotrophic lateral sclerosis (ALS) are all pathologies associated with lower motor neuron dysfunction.
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