Biology I - Nervous System - Motor unit

Motor Unit

• Lower motor neurons: efferent neurons that synapse on skeletal muscle

Motor unit refers to the lower motor neuron + skeletal muscle cells controlled by neuron. muscle cells are activated together as a unit.

neuromuscular junction: synapses between lower motor neuron and skeletal muscle cell

Lower motor neruons in cranial nerves control skeletal muscles of head and neck. LMN in spinal nerves control skeletal muscles in trunk and limbs.

Smaller muscles have smaller motor units. Larger muscles have larger motor units. There can be hundreds of skeletal muscle cells in a single motor unit.

Motor Unit abnormalities (LMN signs)

Weakness - loss of strength of contractions of the skeletal muscles

Atrophy - loss of bulk of skeletal muscle

Fasciculations - involuntary twitches of skeletal muscle

Hypotonia - loss of tone of skeletal muscle, which is how much the muscle is contracted when trying to relax

Hyporeflexia - decreased muscle stretch reflexes