The Central Nervous System
Take a moment to become aware of all the stimuli in your
environment. Notice all the colors, shapes, odors, sounds,
and so on. How many of these sensory stimuli are you usu-
ally aware of?
±e RAS is inhibited by sleep centers located in the hypothala-
mus and other neural regions, and is depressed by alcohol, sleep-
inducing drugs, and tranquilizers. Severe injury to this system, as
might follow a knockout punch that twists the brain stem, results
in permanent unconsciousness (irreversible
). Although the
RAS is central to wakefulness, some of its nuclei are also involved
in sleep, which we will discuss later in this chapter.
±e reticular formation also has a
arm. Some of its
motor nuclei project to motor neurons in the spinal cord via the
Contains visual (superior colliculi) and auditory (inferior colliculi) reﬂex centers
Contains subcortical motor centers (substantia nigra and red nuclei)
Contains nuclei for cranial nerves III and IV
Contains projection ﬁbers (e.g., ﬁbers of the pyramidal tracts)
Relays information from the cerebrum to the cerebellum
Cooperates with the medullary respiratory centers to control respiratory rate and depth
Contains nuclei of cranial nerves V–VII
Contains projection ﬁbers
Relays ascending sensory pathway impulses from skin and proprioceptors through nuclei cuneatus and gracilis
Contains visceral nuclei controlling heart rate, blood vessel diameter, respiratory rate, vomiting, coughing, etc.
Relays sensory information to the cerebellum through inferior olivary nuclei
Contains nuclei of cranial nerves VIII–XII
Contains projection ﬁbers
Site of decussation of pyramids
(pp. 450–451)—A functional system:
Maintains cerebral cortical alertness (reticular activating system)
Filters out repetitive stimuli
Helps regulate skeletal and visceral muscle activity
Processes information from cerebral motor cortex, proprioceptors, and visual and equilibrium pathways
Provides “instructions” to cerebral motor cortex and subcortical motor centers, resulting in smooth, coordinated
skeletal muscle movements
Responsible for proper balance and posture
, and help control skeletal muscles during
coarse limb movements. Other reticular motor nuclei, such as
the vasomotor, cardiac, and respiratory centers of the medulla,
are autonomic centers that regulate visceral motor functions.
Check Your Understanding
The limbic system is sometimes called the emotional-visceral
brain. Which part of the limbic system is responsible for the
When Taylor begins to feel drowsy while driving, she opens
her window, turns up the volume of the car stereo, and sips
ice-cold water. How do these actions keep her awake?
For answers, see Appendix H.