The axons of motor neurones, conveying impulses to skeletal muscle to produce contraction, divide into fine filaments terminating in minute pads called motor end-plates. At the point where the nerve reaches the muscle, the myelin sheath is absent and the fine filament passes to a sensitive area on the surface of the muscle fibre. Each muscle fibre is stimulated through a single motor end-plate, and one motor nerve has many motor end-plates. The motor end-plate and the sensitive area of muscle fibre through which it is stimulated is analogous to the synapse between neurones and is known as the neuromuscular junction. The nerve impulse is passed across the gap between the motor end-plate and the muscle fibre by the neurotransmitter, acetylcholine. The group of muscle fibres and the motor end-plates of the nerve fibre that supplies them constitute a motor unit. Nerve impulses cause serial contraction of motor units in a muscle and each unit contracts to its full capacity. The strength of the contraction depends on the number of motor units in action at a particular time.
The endings of autonomic nerves supplying smooth muscle and glands branch near their effector structure and release a neurotransmitter which stimulates or depresses the activity of the structure.
- Area where a neuron meets a muscle fiber.
- Separated by a gap called synaptic cleft.
- When an electrical signal (action potential) travels to the end of a neuron, the neuron releases a chemical message called a neurotransmitter (specifically, acetylcholine at the neuromuscular junction, also known as ach).
- The ach binds to the muscle cell, and initiates an electrical signal (action potential) there.
- This ultimately results in the muscle fiber contracting.
Figure 21 – Neuromuscular Junction (Motor Unit)