Neuromuscular junction (NMJ) anatomy

Axon

The long extension of the motor neuron that carries the action potential to the NMJ.

Synaptic knob

Bulb at the end of the axon that sits over the muscle fiber.

Ca²⁺ channels

Voltage-gated channels in the synaptic knob open when the action potential arrives, letting calcium into the neuron.

Synaptic vesicles

Contain acetylcholine (ACh), the neurotransmitter.

Neurotransmitter (ACh)

Released into the synaptic cleft, it binds receptors on the motor end plate (part of the sarcolemma).

Motor end plate

Specialized region of the sarcolemma with ACh receptors.

Synaptic cleft

Gap between the neuron and muscle fiber where ACh diffuses.

Sarcolemma

cell membrane of the muscle fiber; conducts action potential

T-tubules

space on the sarcolemma that carry the action potential deep into the fiber.

Sarcoplasmic Reticulum

Stores Ca²⁺,releases it when stimulated to trigger contraction.