How do you start a
This is an alpha motor
neuron driven event.
The alpha motor neuron soma or cell
body is located in the spinal cord.
They will project all the way out to whatever
muscle you are going to try to contract.
These are cholinergic nerves, meaning
that they will release acetylcholine.
They do this so at the
These are specialized areas on the muscle
sometimes referred to as motor end plates.
Acetylcholine is released, it binds to
a postsynaptic acetylcholine receptor.
receptors are ionotropic,
meaning that they’re going to
allow ions to pass through.
In this case, the ions pass through,
primarily sodium but not solely,
will depolarize the sarcolemma,
which is the specialized name of
the plasma membrane of muscle.
You might ask, how do you
turn off this response?
Acetylcholinesterase will break down
acetylcholine and that terminates the signal.
But the acetylcholine receptor
membrane potential change
then moves along the sarcolemma.
From the sarcolemma membrane, that
depolarization then travels down the T-tubules.
As it travels down the T-tubules,
now you’re in close proximity to
some receptors that can start the
initiation of a muscle contraction.
These particular receptors are
lined along the T-tubule.
They are known as dihydropyridine
receptors or DHP.
These dihydropyridine receptors are
mechanically tethered to another receptor
known as a calcium release channel or sometimes
referred to as a ryanodine receptor.
These ryanodine receptors are hooked
to the sarcoplasmic reticulum.
And in fact, when the
dihydropyridine receptor is engaged
by a change in voltage that
comes down the T-tubule,
it then moves off these calcium release
channels to allow calcium to spill out.
It is this calcium that spilled
out that will eventually
bind to troponin C to start
a muscle contraction.
How you stop a muscle contraction or
the spill out of calcium is to relax
or turn off the
which then turns off the calcium release
from the calcium release channels.