The small gap between the membrane of an axon terminal and the sarcolemma of a muscle fiber is called a cleft.

The small gap between the membrane of an axon terminal and the sarcolemma of a muscle fiber is known as the synaptic cleft. In neuromuscular junctions, this cleft plays a critical role in the transmission of signals from neurons to muscle fibers. When an action potential reaches the axon terminal, it triggers the release of neurotransmitters, particularly acetylcholine, into the synaptic cleft. These neurotransmitters then bind to receptors on the sarcolemma, leading to a chain of events that ultimately results in muscle contraction. The cleft is essential for the precise communication between nerve cells and muscle cells, ensuring that the signals are transmitted effectively.