Anatomy of Neuromuscular Junctions (NMJs)
How muscles work continued ...
As explained on the previous page
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
The next question is: What causes such movements
to occur ?
Muscles (and other tissues) are controlled by
the nervous system - which consists of nerve cells
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.
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.
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
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:
Each neuromuscular junction consists of the axon
terminal of a motor
neuron and the motor end plate of a muscle
- 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
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
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 illustrated below:
Above: The anatomy of a neuromuscular
In addition to the synaptic vesicles shown
above, there are also several mitochondria
(the structures within cells whose purpose is
This page describes the components of neuromuscular
The next page describes the chemical processes,
or "actions" that occur at neuromuscular
NEXT: Read about what happens at neuromuscular junctions...