Go
to the main content page
The
neuromuscular junction
The
signal for a muscle to contract originates in the nervous
system and is transmitted to the muscle at the neuromuscular
junction, a point of contact between the motor nerve and
the muscle. In higher organisms each muscle fibre is innervated
by a single motor nerve fibre; in other species (e.g., crustaceans)
inhibitory fibres are also present. As the nerve approaches
the muscle, it loses its myelin coat but remains partially
covered by processes of the Schwann cells, which elsewhere
surround the nerve and produce myelin. The nerve then branches
severaltimes, indenting the surface of the muscle to form
the end plate that occupies only a small region of the total
surface area of the muscle. The narrow (50 nanometres) synapse
separates the nerve from the muscle and contains the basement
membrane (basal lamina). In the subneural region the muscle
membrane is deeply folded, forming secondary synaptic clefts
into which the basement membrane penetrates.
The
neural signal is an electrical impulse that is conducted
from the motor nerve cell body in the spinal cord along
the nerve axon to its destination, the neuromuscular junction.
No electrical continuity exists between the nerve and the
muscle; the signal is transmitted by chemical means that
require specialized presynaptic and postsynaptic structures.
