Anatomy of Neuromuscular Junctions (NMJs)
How muscles work continued ...
As explained on the page about the sliding-filament theory of muscle action, muscles contract and relax as a result of two different types of filaments (called thick filaments, and thin filaments) moving backwards and forwards across each other.
The next question is: What causes such movements ?
Muscles (and other tissues) are controlled by the nervous system - which consists of nerve cells called neurones. Of the three types of neurones (motor neurones, sensory neurones and relay neurones), motor neurones instruct skeletal muscle cells to perform the series of actions that lead to sliding filaments and hence muscle contraction.
A single motor neurone together with all of the muscle fibers (muscle cells) to which it is attached, and therefore stimulates, is called a motor unit.
- Stimulation of the one neurone of a motor unit results in simultaneous contraction of all of the muscle cells in that motor unit.
number of muscle cells in a motor unit, i.e. the
ratio of motor neurones (nerve cells) to muscle
cells, varies according to the type and function
of the muscle of which it is a part.
For example, delicate muscles of facial expression are not developed for high load-bearing actions but rather to convey a huge variety of subtle movements of skin. These muscles therefore consist of motor units containing only a few (sometimes less than 10) muscle cells each. However, larger more powerful muscles such as the biceps brachii and the gastrocnemius contain motor units of up to 2000 muscle cells each.
Neuromuscular junctions (NMJs) are the locations and means by which the motor neurones of the nervous system instruct the muscle cells of the muscular system to take actions - actions that, in turn, lead to the movement of muscles and the attached structures such tissues, bones, limbs etc..
Anatomical Description of a NMJ:
- The Motor Neurone Part:
The long processes of neurones are called 'axons'. As the axon of a motor neurone enters the structure of skeletal muscle it forms many branches called 'axon terminals'. There is a bulbous swelling called a 'synaptic end bulb' at the extreme / end of each axon terminal. Each synaptic end bulb contains many synaptic vesicles, each of which contains an important chemical neurotransmitter called 'acetylcholine', which is often abbreviated to simply 'ACh'.
- The Muscle Fiber Part:
The part of the sarcolemma of the muscle cell that is in closest proximity to the synaptic end bulb is called the 'motor end plate'.
The 'Synapse' or 'Neuromuscular Junction' (NMJ):
The area between the axon terminal (of the neurone - nerve cell) and the sarcolemma (the plasma membrane sheath that forms the outer-layer of the muscle cell) is called the 'synaptic cleft'.
In terms of the anatomy of the nervous system, the tiny gap across which nerve impulses pass from one neuron (nerve cell) to the next is called a 'synapse'. A neuromuscular junction (also known, and sometimes referred to as a "myoneural junction") is the equivalent gap between a motor-neuron and the motor end plate of a muscle cell to which it is attached. However, in the case of the NMJ, some texts refer to the tiny physical gap as the 'synaptic cleft' and use the term 'neuromuscular junction' to refer to the interaction of the nerve and muscle cells more generally* and so also serve as the heading under which the processes that occur at the NMJ are described.
These structures are shown below:
Above: The anatomy of a neuromuscular junction (NMJ)
In addition to the synaptic vesicles shown above, there are also several mitochondria (the structures within cells whose purpose is energy storage and release).
This page describes the components of neuromuscular junctions.
The next page describes the chemical processes, or 'actions' that occur at neuromuscular junctions.
NEXT: Read about what happens at neuromuscular junctions ...