Actions at Neuromuscular Junctions (NMJs)
How muscles work continued ...
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.
The 4 stages are listed below the following diagram of the structures involved:
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 positivel y-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 the physiology of muscle relaxation.