(2) Motor pathways. A motor pathway is a series of nervous structures
used to transmit information from the CNS to the body. The commands for motor action
originate in the brain and descend (go down) the neuraxis to the appropriate spinal
levels. From this point, the commands pass through the nerves to the effector organs.
c. Controls. The human nervous system has several levels for control. The
lowest level is the simple reflex arc (see para 11-15c). The highest level of control is
the conscious level. From the lowest to the highest levels are several progressively
higher levels, such as the righting reflex. Thus, the processing of information and the
transmission of commands are not haphazard but very carefully monitored and
controlled. All information input and all information output are monitored and evaluated.
11-20. THE MOTOR PATHWAYS
Motor pathways begin in the brain. They descend the neuraxis in bundles of a
number of specific neuron processes called motor fiber tracts. Commands originating in
the right half of the brain leave the CNS through peripheral nerves on the left side.
Commands from the left half of the brain leave the CNS on the right side. Therefore,
the right half of the brain controls the left side of the body and the left half of the brain
controls the right side of the body. For example, the actions of the right hand are
controlled by the left half of the brain. (In those people who are right-handed, we refer
to the left half of the brain as being dominant.)
a. Pyramidal Motor Pathways. A pyramidal motor pathway is primarily con-
cerned with volitional (voluntary) control of the body parts, in particular the fine
movements of the hands. Because control is volitional, the pathways can be used for
neurological screening and testing. These pathways are called pyramidal because their
neuron processes contribute to the makeup of a pair of structures in the base of the
brain known as the pyramids.
b. Extrapyramidal Motor Pathways. An extrapyramidal pathway is primarily
concerned with automatic (nonvolitional) control of body parts for purposes of
coordination. Extrapyramidal pathways use many intermediate relays before reaching
the effector organs. The cerebellum of the brain plays a major role in extrapyramidal
pathways; the cerebellum helps to integrate patterned movements of the body.
11-21. THE SENSORY PATHWAYS
a. The body is continuously bombarded by types of information called stimuli
(stimulus, singular). Those few stimuli which are consciously perceived (in the cerebral
hemispheres) are called sensations.
b. Those stimuli received throughout the body are called the general senses.
Stimuli received by only single pairs of organs in the head (for example, the eyes) are
called special senses (for example, smell and taste).