Question

Which of the following statements is true concerning motor units?

Options

A
Motor unit neurons only transmit one action potential during a contraction
B
Large motor units are innervated first in a muscle, followed by smaller units in order to have a smooth contraction
C
The neuron releases epinephrine across the synaptic cleft in order to stimulate an action potential in the sarcolemma
D
Finger muscles are typically composed of smaller motor units

Explanation

Option A: A motor unit is made up of a motor neuron and all of the skeletal muscle fibers innervated by the neuron's axon terminals, including the neuromuscular junctions between the neuron and the fibers. Groups of motor units often work together as a motor pool to coordinate the contractions of a single muscle. Option B: Slow motor units, with higher post-synaptic motor neuron excitability, are activated first. Option C: In vertebrates, motor neurons release acetylcholine (ACh), a small molecule neurotransmitter, which diffuses across the synaptic cleft and binds to nicotinic acetylcholine receptors (nAChRs) on the cell membrane of the muscle fiber, also known as the sarcolemma. Option D: Smaller motor units are involved in many swift movements of our fingers.

A motor unit is made up of a motor neuron and all of the skeletal muscle fibers innervated by the neuron's axon terminals, including the neuromuscular junctions between the neuron and the fibers. Groups of motor units often work together as a motor pool to coordinate the contractions of a single muscle.

Slow motor units, with higher post-synaptic motorneuron excitability, are activated first.

In vertebrates, motor neurons release acetylcholine (ACh), a small-molecule neurotransmitter, which diffuses across the synaptic cleft and binds to nicotinic acetylcholine receptors (nAChRs) on the cell membrane of the muscle fiber, also known as the sarcolemma.

Smaller motor units are involved in many swift movements of our fingers. Hence, this option is correct.