11.12  Controls in the Human Nervous System

The human nervous system can be thought of as a series of steps or levels.
Each level is more complex than the level just below. No level is completely overpowered by upper levels, but each level is controlled or guided by the next upper level as it functions.

LEVELS OF CONTROL

a. Reflex Arc. The simplest and lowest level of control is the reflex arc (see para 11-15c). The reflex arc operates essentially on the level of the sensory input.

b. Segmental Reflexes. Segmental reflexes produce a wider reaction to a stimulus than the reflex arc. For this purpose, the nervous system is organized more complexly. Thus, information spreads to a wider area of the CNS. We can observe a greater reaction to the stimulus.

c. Medullary Hindbrain. In the hindbrainstem are to be found a number of nuclei (collections of neuron cell bodies) which monitor and control the activities of the visceral functions of the body, such as respiration, heartbeat, etc.

d. Reticular Formation. Within the substance of the brainstem is a diffuse system called the reticular formation.

This reticular formation has a facilitatory (excitatory) area and an inhibitory area. These areas monitor and control general body functions, including sleep.

e. Thalamus. In the forebrainstem is a major collection of nuclei, all together called the thalamus. The thalamus is a primary relay for information going to and from the cerebrum and cerebellum. In the lowest animals, the thalamus represents the highest level of nervous control.

f. Cerebellum. The cerebellum has been greatly developed with many functional subdivisions. All together, it is one of the most important integrators of motor activity of the body.

g. Cerebrum. In humans, the highest level of nervous control is localized in the cerebrum. It is at this level that conscious sensation and volitional motor activity are localized. Even so, we can clearly designate three levels of control within the cerebrum:

(1) Visceral (vegetative) level. This level is concerned primarily with visceral activities of the body as related to fight-or-flight, fear, and other emotions.

(2) Patterned (stereotyped) motor actions. Here, activities of the body are standardized and repetitive in nature. An example of a stereotyped pattern of muscle activity would be the sequence of muscle actions involved in walking.

(3) Volitional level. The volitional level is the highest and newest level of control. Here, unique, brand-new solutions can be created.