each part of the body, one comes up with a caricature of the human being. This
caricature (distorted image) is referred to as the motor homunculus.
b. Pyramidal/Extrapyramidal Motor Systems. Various collections of neurons
and their processes carry commands for actions to the individual skeletal muscles.
Originating in the brain, these neurons and processes pass through the brain stem into
the spinal cord. In general, they are grouped into the pyramidal motor system and extra
pyramidal motor system.
(1) Since the pyramidal motor system is subject to volitional control, it can be
used for testing during medical examinations.
(2) The extra pyramidal motor system is more automatic. For the most part,
control in this system is non-volitional.
c. Modulation of Commands. Several areas of the brain act as coordinators
and modulators of the muscle activity of the body. These areas include the cerebellum
and the basal ganglia. The sequential patterns of action to produce an overall motion
appear to be programmed in the brain, particularly the cerebellum.
d. Motor Neurons. The individual motor neuron has its cell body in the
brainstem or spinal cord. The axon of the motor neuron passes out of the CNS to
become a part of the nerves going to the individual skeletal muscles.
e. Motor Units. In the skeletal muscle, the individual motor neuron (axon) has a
terminal branching so that it contacts several striated muscle fibers. The actual number
of striated muscle fibers contacted (innervated) by a single motor neuron are together
known as a motor unit.
(1) When its motor units are small (involving fewer muscle fibers), a muscle
can produce very fine actions. The extra ocular muscles are an example of this.
(2) With larger motor units, the muscle action is coarse.
(3) A variable number of motor units may be called into action at a given
moment. The number recruited is the number needed for the required action.
f. Neuromuscular Junctions. At the end of each branch of the terminal
branching of the motor neuron, is an enlargement known as the bouton.
(1) The bouton has a specific relationship with the sarcolemma of the striated
muscle fiber. There is no actual physical contact. Instead, there is a little space known
as the synaptic cleft.