Now let’s take a closure look at a neuromuscular junction. The swollen distal end of the motor neuron axon is called the axon terminal. Within the axon terminal are synaptic vesicles containing the neurotransmitter acetylcholine. Notice at the neuromuscular junction, the motor neuron and the sarcolemma of the muscle cell do not actually touch. The synaptic cleft is the space between the axon terminal and a folded region of the sacrolemma called the motor end plate, highlightedt here in red. Recall that the sarcolemma is the general name for the muscle cell membrane. The T tubules are invaginations of the sacrolemma penetrating deep into the interior of the cell. The terminal cisternae are specialized regions of the sarcoplasmic reticulum that serve as reservoirs of calcium ions shows here as red dots. Portions of sarcoplasmic reticulum have been removed here in order to see the underlying structures. The sarcomere is the contractile unit of the muscle cell that extends from one Z line to the next. The plus and minus signs on the axon terminal, sacrolemma, and T tubules indicate a polarized condition called the resting membrane potential. Although, plus and minus signs are not shown on the motor end plate, it is also polarized. Here’s an overview of the activity that occurs at the neuromuscular junction. An action potential arriving at the axon terminal triggers the release of a neurotransmitter acetylcholine, resulting in the depolarization of the motor end plate. This depolarization triggers an action potential that propagates along the sarcolemma and down T tubules, causing contraction of the sarcomeres. Let’s look into this process in more detail. When the action potential arrives at the axon terminal, the voltage change of the membrane opens voltage-gated calcium channels, allowing calcium ions to enter the axon terminal. The calcium ions cause several synaptic vesicles to fuse with the membrane of the axon terminal. The neurotransmitter acetylcholine contained within the vesicles is liberated by exocytosis into the synaptic cleft. Additionally, calcium ions are pumped out of the axon terminal. Acetylcholine binds to receptor sites of chemically gated ion channels on the motor end plate. This causes the channels to open, permitting an influx of sodium ions and a small efflux of potassium ions. This ion exchange causes a local depolarization of the motor end plate, which will be indicated here as a glow. Lets start this process. [Sparking Sound] Now the motor end plate is depolarized. After a brief period of time, acetylcholine diffuses away from its receptor site and the ion channel closes. Acetylcholine is then broken down by the enzyme acetylcholinesterase. The depolarization of the motor end plate initiates an action potential, which propagates along the sarcolemma in all directions and down the T tubules. The action potential causes the release of calcium ions from the terminal cisternae into the cytosol.