The area where a motor NERVE ends close to the MUSCLE membrane so can initiate muscle contraction. The motor-nerve ending is separated from the motor end plate by the synaptic cleft which is only 50–70 nm (see NANOMETRE) wide. When a nerve impulse arrives at the motor-nerve ending, molecules of ACETYLCHOLINE are released which cross the synaptic cleft and attach to receptors on the motor end plate. This initiates depolarisation of the muscle, which in turn initiates the process of contraction. Acetylcholinesterase (an ENZYME) rapidly breaks down the molecules of acetylcholine, thus ending their action and freeing the receptor in preparation for the next impulse.