Lower motor neuron
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.
Glutamate released from the upper motor neurons triggers depolarization in the lower motor neurons in the anterior 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.