Skeletal muscle cells contract as a result of impulses from motor neurons. The place where a motor neuron stimulates a muscle cell is called a neuromuscular junction. Your goals for learning are to examine the structure of a neuromuscular junction. To understand the sequence of events occurring at the neuromuscular junction following a stimulus. Here's what you need to know. Anatomy of a skeletal muscle cell. Definitions of resting membrane potential, action potential, and depolarization. To review the anatomy of a skeletal muscle cell, click the link button. If you use a link button, you can return to the page you started from by clicking the return button. To see definitions of terms, click the bold red words. Skeletal muscle cells are electrically insulated from each other by endomsium. In order for skeletal muscle cells to contract, each cell must be stimulated by a process of a motor neuron. Each motor neuron is a single nerve cell extending from the brain or spinal cord to the muscle where it can stimulate several muscle cells. The axon is the elongated process of the nerve cell that carries impulses to the muscle. The neuromuscular junction is the place where the terminal portion of a motor neuron axon meets the muscle cell membrane. Click the neuromuscular junction to see it in an expanded view. Now let's take a closer 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 that at the neuromuscular junction, the motor neuron and the sarco lima of the muscle cell do not actually touch. The synaptic cleft is the space between the axon terminal and a folded region of the sarmma called the motor end plate highlighted here in red. Recall that the sarmma is the general name for the muscle cell membrane. The T- tubules are invaginations of the sarcolemma penetrating deep into the interior of the cell. The terminal cesterna are specialized regions of the cycloplasmic reticulum that serve as reservoirs of calcium ions shown here as red dots. Portions of the cycloplasmic reticulum have been removed here in order to see the underlying structures. The sarcomir is the contractile unit of the muscle cell that extends from one Zline to the next. The plus and minus signs on the axon terminal sarmma 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 the neurotransmitter acetylcholine resulting in the depolarization of the motor end plate. This depolarization triggers an action potential that propagates along the sarmma and down the tubules causing contraction of the sarcomirs. On the next several pages we will look at this process in more detail. When the action potential arrives at the axon terminal, the voltage change of the membrane opens voltage regulated calcium channels, allowing calcium ions to enter the axon terminal. To see the arrival of the action potential, click the motor neuron. Click the calcium channel to see calcium ions enter the axon terminal. The calcium ions cause several synaptic vesicles to fuse with the membrane of the axon terminal. Click a synaptic vesicle to see this activity. The neurotransmitter acetylcholine contained within the vesicles is liberated by exocytosis into the synaptic clft. Additionally, calcium ions are pumped out of the axon terminal. Click a fused synaptic vesicle to start exocytosis. Acetil choline binds to receptor sites of chemically regulated ion channels on the motor end plate. This causes the channels to open permitting an influx of sodium ions and a small elux of potassium ions. This ion exchange causes a local depolarization of the motor end plate which will be indicated here as a glow. Click the acetil choline molecule to start this process. 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. Acetilchine is then broken down by the enzyme acetylcolonsterase. Click the acetylcolonsterase molecule to start this process. The deolarization of the motor end plate initiates an action potential which propagates along the sarmma in all directions and down the tubules. Click the deolarized motor end plate to initiate the action potential. The action potential causes the release of calcium ions from the terminal cesterna into the cytool. Click the terminal cesterna to see this activity. Calcium ions trigger a contraction of the muscle cell. This process will be examined in more detail in sliding filament theory. Click the muscle cell to see it contract. To see the activity at the neuromuscular junction as a continuous process, click the motor neuron. Here's a summary of what we've covered. Each skeletal muscle cell is individually stimulated by a motor neuron. The neuromuscular junction is the place where the terminal portion of a motor neuron axon meets a muscle cell membrane separated by a synaptic clft. An action potential arriving at the axon terminal brings about the release of acetylcholine which leads to depolarization of the motor end plate. Depolarization of the motor end plate triggers an action potential that propagates along the sarimma and down the T- tubules. This action potential causes the release of calcium ions from the terminal cyestern into the cytool triggering contraction of the muscle cell. 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