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  • 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.

For bodies to produce movement muscle contraction must occur.


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肌肉收縮 (Muscle Contraction)

  • 108 11
    Cheng-Hong Liu 發佈於 2021 年 01 月 14 日