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.