Figure 12-9. A neuromuscular junction.
a. In general terms, the neuromuscular junction and the synapse are
physiologically identical. Synaptic vesicles in the enlarged bouton of the motor neuron
contain the neurotransmitter acetylcholine (ACH). As an impulse reaches the bouton,
ACH is released and passes through thepresynaptic membrane into the synaptic cleft.
However, the surface of the postsynaptic membrane is in a series of longitudinal folds.
This greatly increases the surface area receptive to the ACH.
b. The motor unit is the group of striated muscle fibers innervated by the
terminal arborization (tree-like branching) of one motor neuron. The fewer the muscle
fibers found per motor unit, the more the muscle is capable of finer movements. As the
number in the motor unit increases, the muscle action is coarser. When a muscle is to
be used, the nervous system recruits just enough motor units to supply the strength
needed for the work to be done.
Section VI. THE GENERAL REFLEX AND THE REFLEX ARC
12-20. THE GENERAL REFLEX
The simplest reaction of the human nervous system is the reflex. A reflex is
defined as an automatic reaction to a stimulus.