Actions at Neuromuscular Junctions (NMJs)
How muscles work continued ...
As explained on the previous page
(The
Anatomy of Neuromuscular Junctions), muscles
are controlled by the nervous system - which consists
of nerve cells called neurones.
Neuromuscular junctions (NMJs) are the locations
at, and and the means by which the motor neurones
of the nervous system instruct the muscle cells of the muscular system to take actions.
This page summarises how motor
neurones excite skeletal muscle fibers.
There are 4 steps to remember, these are listed
below the following diagram of the structures
involved:
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Above: The actions that occur at a
neuromuscular junction (NMJ) |
Neuromuscular Junction Actions:
- Release of ACh
When a nerve pulse reaches a synaptic end bulb,
it triggers release of the neurotransmitter
acetylcholine (ACh) from synaptic
vesicles that contain acetylcholine
(ACh). ACh then diffuses across the
synaptic cleft between the motor neurone
and the motor end plate - as shown above.
- Activation of
ACh receptors
The motor end plate contains receptors
onto which the free ACh binds after diffusing
across the synaptic cleft.
This binding of ACh to ACh receptors in the
motor end plate causes ion channels
to open & so allow the sodium (Na+)
ions to flow across the membrane into the muscle
cell.
(Although the movement of sodium (Na+)
ions is mentioned an illustrated, the opening
of the ion channel does also allow other cations
to pass across the membrane. A cation is a positively-charged
ion, which has fewer electrons than protons,
is known as a "cation" because it
is attracted to cathodes. In the case of a simple
description of actions at a neuromuscular junction
it is generally sufficient to remember the movement
of sodium (Na+) ions .)
- Generation of
muscle action potential
The flow of sodium (Na+)
ions across the membrane into the muscle cell
generates a muscle action potential.
This action potential then travels along the sarcolemma
and through the T-Tubules.
(Action Potentials and how they are generated
and transmitted is a topic usually covered in
further detail as part of study of the Nervous
System.)
- Breakdown of
ACh
The ACh that is released at Step (1.)
is only available to take part in step (2.)
for a short time before it is broken down by
an enzyeme called acetylcholinesterase
(AChE). This breakdown of ACh occurs
within the synaptic cleft.
Understanding of the processes listed above enables
one to also understand the effects of some toxins
and drugs that interfere with theses processes,
either disabling the body or changing its behaviour
for a specific intentional purpose e.g. to relax
skeletal muscle during surgery. For example, some
drugs containing anticholinesterase agents reduce
the rate of action of AChE and are sometimes used
(at low doses) to strengthen wak muscle contractions
Next: Read about more detailed
physiology of muscle contraction, or physiology
of muscle relaxation.
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