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,
and neuromuscular junctions (NMJs) are the locations at,
and and 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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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 neurmuscular 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
(coming soon). |
