ABSTRACT
The nerve ending of the NMJ is packed with mitochondria and with secretory vesicles. Each of the secretory vesicles contains a few thousand mole cules of acetylcholine (ACh). A single vesicle is said to hold a quantum of ACh. On arrival of an action potential (AP) at the nerve terminal, some of these vesi cles fuse with the presynaptic membrane and discharge their contents into the synaptic cleft. The synaptic cleft measures between 30 and 50 nm across, and ACh molecules must diffuse across this gap in order to act on the muscle fiber. The density of nicotinic ACh receptors (nAChRs) is extremely high on the muscle cell membrane (sarcolemma) in the region of the motor endplate, although such protein elements cannot themselves be viewed by simple light microscopy or electron microscopy. The skeletal muscle fiber has numerous invaginations of its sarcolemma, known as junctional folds. The enzyme acetyl cholinesterase (AChE) is expressed at the surface of the muscle fiber and can also be secreted into the synaptic cleft. Using labeled antibodies that recognize AChE, one can visualize motor endplates on skeletal muscle fibers. This enzyme serves to break ACh down into acetate and choline, thereby limiting the action of the transmitter at the nAChRs. Choline is actively transported back into the αmotor neuron end-bulb by a specific choline transporter protein.
