450
UNIT 3
Regulation and Integration of the Body
12
to the hypothalamus, thalamus, cerebral cortex, cerebellum,
and spinal cord, making reticular neurons ideal for governing
the arousal of the brain as a whole.
For example, unless inhibited by other brain areas, the neu-
rons of the part of the reticular formation known as the
re-
ticular activating system (RAS)
send a continuous stream of
impulses to the cerebral cortex, keeping the cortex alert and
conscious and enhancing its excitability. Impulses from all the
great ascending sensory tracts synapse with RAS neurons, keep-
ing them active and enhancing their arousing effect on the cer-
ebrum. (Tis may explain why many students, stimulated by a
bustling environment, like to study in a crowded cafeteria.)
Te RAS also filters this flood of sensory inputs. Repetitive,
familiar, or weak signals are filtered out, but unusual, signifi-
cant, or strong impulses do reach consciousness. For example,
you are probably unaware of your watch encircling your wrist,
but would immediately notice it if the clasp broke. Between
them, the RAS and the cerebral cortex disregard perhaps 99% of
all sensory stimuli as unimportant. If this filtering did not occur,
the sensory overload would drive us crazy. Te drug LSD in-
terferes with these sensory dampers, promoting an o±en over-
whelming sensory overload.
by the emotional brain) and our thoughts (mediated by the
cognitive brain). As a result, we (1) react emotionally to things
we consciously understand to be happening, and (2) are con-
sciously aware of the emotional richness of our lives. Commu-
nication between the cerebral cortex and limbic system explains
why emotions sometimes override logic and, conversely, why
reason can stop us from expressing our emotions in inappro-
priate situations. Particular limbic system structures—the
hip-
pocampus
and amygdaloid body—also play a role in memory.
The Reticular Formation
Te
reticular formation
extends through the central core of
the medulla oblongata, pons, and midbrain
(Figure 12.17)
. It
is composed of loosely clustered neurons in what is otherwise
white matter. Tese neurons form three broad columns along
the length of the brain stem (Figure 12.14c): (1) the midline
raphe nuclei
(ra
9
fe;
raphe
5
seam or crease), which are flanked
laterally by (2) the
medial (large cell) group of nuclei
and (3)
the
lateral (small cell) group of nuclei
.
Te outstanding feature of the reticular neurons is their far-
flung axonal connections. Individual reticular neurons project
Table 12.1
Functions of Major Brain Regions
REGION
FUNCTION
Cerebral Hemispheres
(pp. 430–441)
 
Cortical gray matter:
Localizes and interprets sensory inputs
Controls voluntary and skilled skeletal muscle activity
Functions in intellectual and emotional processing
Basal nuclei (ganglia):
Subcortical motor centers
Help control skeletal muscle movements
Diencephalon
(pp. 441–443)
 
Thalamus:
Relays sensory impulses to cerebral cortex for interpretation
Relays impulses between cerebral motor cortex and lower (subcortical) motor centers, including cerebellum
Involved in memory processing
Hypothalamus:
Chief integration center of autonomic (involuntary) nervous system
Regulates body temperature, food intake, water balance, thirst, and biological rhythms and drives
Regulates hormonal output of anterior pituitary gland
Acts as an endocrine organ, producing posterior pituitary hormones ADH and oxytocin
Limbic System
(pp. 449–450)—A functional system:
Includes cerebral and diencephalon structures (e.g., hypothalamus and anterior thalamic nuclei)
Mediates emotional response
Involved in memory processing
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