As explained on the previous page (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 to occur ?
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.
The
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 other structures such tissues, bones, limbs etc.,
that
are
controlled by the muscles.
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
fibre.
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.
* We include comment on this possibly
confusing terminology because the overlapping use of terms
used here and elsewhere may be misleading.
The Oxford Medical
Dictionary defines a neuromuscular junction as
"the meeting point of a nerve fibre and the muscle
fibre that it supplies".
These structures are illustrated below: |
