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.