Human Physiology, 14th edition (2016)

The Central Nervous System 219

sending fibers to the hippocampus. Through these interconnec-

tions, the limbic system and the hypothalamus appear to cooper-

ate in the neural basis of emotional states.

Studies of the functions of these regions include electri-

cal stimulation of specific locations, destruction of tissue (pro-

ducing lesions ) in particular sites, and surgical removal, or

ablation, of specific structures. These studies suggest that the

hypothalamus and limbic system are involved in the following

feelings and behaviors:

1. Aggression. Stimulation of certain areas of the amygdala

produces rage and aggression, and stimulation of particu-

lar areas of the hypothalamus can produce similar effects.

2. Fear. Fear can be produced by electrical stimulation of

the amygdala and hypothalamus, and surgical removal of

the limbic system can result in an absence of fear. Mon-

keys are normally terrified of snakes, for example, but

they will handle snakes without fear if their limbic sys-

tem is removed. Humans with damage to their amygdala

have demonstrated an impaired ability to recognize facial

expressions of fear and anger. These and other studies sug-

gest that the amygdala is needed for fear conditioning.

3. Feeding. The hypothalamus contains both a feeding cen-

ter and a satiety center. Electrical stimulation of the for-

mer causes overeating, and stimulation of the latter will

stop feeding behavior in experimental animals.

4. Sex. The hypothalamus and limbic system are involved in

the regulation of the sexual drive and sexual behavior, as

shown by stimulation and ablation studies in experimen-

tal animals. The cerebral cortex, however, is also critically

important for the sex drive in lower animals, and the role

of the cerebrum is even more important for the sex drive in

humans.

5. Goal-directed behavior (reward and punishment sys-

tem). Electrodes placed in particular sites between the

Limbic System and Emotion

The parts of the brain that appear to be of paramount importance
in the neural basis of emotional states are the hypothalamus (in
the diencephalon) and the limbic system. The limbic system
consists of a group of forebrain nuclei and fiber tracts that form
a ring around the brain stem ( limbus 5 ring). Among the com-
ponents of the limbic system are the cingulate gyrus (part of
the cerebral cortex), the amygdaloid nucleus (or amygdala ), the
hippocampus, and the septal nuclei ( figs.  8.15 and 8.18 ). The
cingulate gyrus is a thick area of cortex that surrounds the cor-
pus callosum and is involved in emotions (particularly negative
emotions associated with pain and fear) and motivation. Studies
demonstrate that the anterior insula is activated together with
the anterior cingulate cortex during emotional experiences.
The limbic system was once called the rhinencephalon,
or “smell brain,” because it is involved in the central process-
ing of olfactory information. This may be its primary function
in lower vertebrates whose limbic system may constitute the
entire forebrain. It is now known, however, that the limbic sys-
tem in humans is a center for basic emotional drives. The limbic
system was derived early in the course of vertebrate evolution,
and its tissue is phylogenetically older than the cerebral cortex.
There are thus few synaptic connections between the cerebral
cortex and the structures of the limbic system, which perhaps
helps explain why we have so little conscious control over our
emotions.
There is a closed circuit of information flow between the lim-
bic system and the thalamus and hypothalamus ( fig. 8.15 ) called
the Papez circuit. (The thalamus and hypothalamus are part of the
diencephalon, described in a later section.) In the Papez circuit,
a fiber tract, the fornix, connects the hippocampus to the mam-
millary bodies of the hypothalamus, which, in turn, project to the
anterior nuclei of the thalamus. The thalamic nuclei, in turn, send
fibers to the cingulate gyrus, which then completes the circuit by

Figure 8.15 The limbic system. The

left temporal lobe has been removed in this

figure to show the structures of the limbic system

(green). The limbic system consists of particular

nuclei (aggregations of neuron cell bodies) and

axon tracts of the cerebrum that cooperate in

the generation of emotions. The hypothalamus,

though part of the diencephalon rather than the

cerebrum (telencephalon), participates with the

limbic system in emotions.

Corpus

callosum

Cingulate
gyrus

Septal
nucleus

Preoptic
nucleus
Olfactory bulb

Olfactory tract

Cortex of right

hemisphere

Hypothalamus

Hippocampus

Amygdala

Mammillary

body

Thalamus Fornix