When the
brain decides to move part of the body and gives the
command to the motor
neurons to execute this movement, it is the muscles
at the end of the chain of command that ultimately contract
to move the body part concerned.
To transmit this command, the axons of these motor neurons,
emerging from the spinal cord, form a nerve that extends to
the muscles. Where the tip of each axon comes into proximity
with a muscle fibre, it forms a synapse with
that fibre. This special form of synapse between a motor neuron
axon and a muscle fibre is called a neuromuscular junction.
The arrival of a nerve
impulse at the neuromuscular junction causes thousands
of tiny vesicles (pouches) filled with a neurotransmitter called
acetylcholine to be released from the axon tip into the
synapse.
On the opposite side of the synapse, this acetylcholine
then binds to the surface of the muscle fibre at special
sites where there are large numbers of acetylcholine
receptors. |