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As a motor neuron approaches the surface of a muscle fiber, the myelin sheath ceases and the axon splits into a small cluster of fine terminal branches. Each terminal branch runs along a shallow groove in the muscle fiber surface. The sarcolemma (= plasma membrane) in the junctional grooves is highly folded and is often called the motor end plate (= sole plate). Many authorities, however, prefer to use the term motor end plate to refer to the terminal branches or entire neuromuscular junction.
The tips of the terminal branches, which often expand into synaptic knobs (= synaptic bulbs or terminal boutons), contain many mitochondria and secretory vesicles filled with acetylcholine (ACh). A non-myelinating Schwann cell (teloglia) covers the terminal branches and nearby sarcolemma. Between the synaptic knob and sarcolemma is a small space known as the synaptic cleft. Acetylcholine molecules, after being released from the synaptic knob, diffuse across this space and bind with receptors in the sarcolemma. The many folds in the junctional sarcolemma provide surface area for additional ACh receptors.
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