字幕表 動画を再生する 英語字幕をプリント For bodies to produce movement muscle contraction must occur. Skeletal muscle contraction is considered voluntary. This means that we normally are aware of the movements that we're making. To cause a contraction our nervous system sends a signal down an alpha motor neuron to the muscle that we want to contract. Remember back to how skeletal muscle is made up. Within a whole muscle there are individual muscle fibers that are contained within facicles. Each muscle fiber has one connection with an axon terminal. These connections between the axon terminal and the muscle fiber is called the neuromuscular junction. Take a look at the following diagram. On top is a nerve fiber and below are three muscle fibers. While each muscle fiber contains only one neuromuscular junction, a single neuron can actually innervate multiple muscle fibers. Let's take a closer look at the connection between an axe on terminal and a muscle fiber which again is known as a neuromuscular junction. So let's take a closer look at a neuromuscular junction. What you have here is an alpha motor neuron that meets the motor end plate of a muscle fiber. Now when an action potential runs along the axon of a neuron like this it eventually reaches the axon terminal. What this does is it stimulates the release of extracellular calcium which then enters the axon terminal. This calcium causes the translocation of neurotransmitter filled vesicles or in this case acetylcholine filled vesicles and these vesicles move to the end of the axon filled terminal. At this point the vesicle fuses with the membrane of the axon terminal allowing the neurotransmitters or acetylcholine into the synapse. From that point, the acetylcholine actually binds to receptors on the motor end plate of the muscle fiber. Now what this does is causes an influx of sodium ions into the actual muscle fiber and this activates an action potential down the length of the muscle fiber. Meanwhile there enzymes that are found in the synapse between the axon terminal and the motor end plate called acetylcholinesterase and they're shown by the blue objects down here. What those enzymes do is remove any excess acetylcholine that's found in the synapse. This prevents any excess stimulation at the motor end plate. The acetylcholine is then taken back up into the axe on terminal where it can be stored for future release. As the action potential runs down the length of the cell membrane also known as the sarcolemma it comes into contact with small valleys that are found within the sarcolemma and these are called t-tubules Essentially these t-tubules invaginate into the cell allowing for the action potential to come in contact with what's called the sarcoplasmic reticulum. The sarcoplasmic reticulum is a storage area for calcium. It sequesters calcium and holds it there until that action potential can cause the release of calcium into the cell and thus causing the activation between actin and myosin and the shortening the sarcomere which results in contraction.