444
UNIT 3
Regulation and Integration of the Body
12
and eye movements when we visually follow a moving object,
even if we are not consciously looking at it. Te
inferior col-
liculi
are part of the auditory relay from the hearing receptors of
the ear to the sensory cortex. Tey also act in reflexive responses
to sound, such as the
startle reflex
which causes you to turn your
head toward an unexpected noise.
Also embedded in each side of the midbrain white matter are
two pigmented nuclei, the substantia nigra and red nucleus. Te
bandlike
substantia nigra
(sub-stan
9
she-ah ni
9
grah) is located
deep to the cerebral peduncle (Figure 12.14a). Its dark (
nigr
5
black) color reflects a high content of melanin pigment, a pre-
cursor of the neurotransmitter (dopamine) released by these
neurons. Te substantia nigra is functionally linked to the basal
nuclei (its axons project to the putamen), and many authorities
consider it part of the basal nuclear complex. Degeneration of
the dopamine-releasing neurons of the substantia nigra is the
ultimate cause of Parkinson’s disease.
cerebellar peduncles
, also fiber tracts, connect the midbrain
to the cerebellum dorsally (Figure 12.13b, c).
Running through the midbrain is the hollow
cerebral aq-
ueduct
, which connects the third and fourth ventricles (Fig-
ures 12.10 and 12.14a). It delineates the cerebral peduncles ven-
trally from the
tectum
, the midbrain’s roof. Surrounding the aque-
duct is the
periaqueductal gray matter
, which is involved in pain
suppression and links the fear-perceiving amygdaloid body and
ANS pathways that control the “fight-or-flight” response. Te
periaqueductal gray matter also includes nuclei that control two
cranial nerves, the
oculomotor
and the
trochlear nuclei
(trok
9
le-ar).
Nuclei are also scattered in the surrounding white mat-
ter of the midbrain. Te
corpora quadrigemina
(kor
9
por-ah
kwod
0
rĭ-jem
9
ĭ-nah; “quadruplets”), the largest midbrain nuclei,
raise four domelike protrusions on the dorsal midbrain surface
(Figures 12.10 and 12.13c). Te superior pair, the
superior col-
liculi
(ko
˘
-lik
9
ū-li), are visual reflex centers that coordinate head
Optic chiasma
View
(a)
View
(c)
View
(b)
Optic nerve (II)
Thalamus
Mammillary body
Oculomotor nerve (III)
Optic tract
Infundibulum
Pituitary gland
Crus cerebri of cerebral
peduncles (midbrain)
Trigeminal nerve (V)
Abducens nerve (VI)
Abducens nerve (VI)
Facial nerve (VII)
Vagus nerve (X)
Accessory nerve (XI)
Hypoglossal nerve (XII)
Ventral root of first cervical nerve
Superior colliculus
Trochlear nerve (IV)
Pons
Middle cerebellar
peduncle
Pyramid
Decussation of pyramids
(a) Ventral view
(b) Left lateral view
Spinal cord
Inferior colliculus
Trochlear nerve (IV)
Superior cerebellar peduncle
Middle cerebellar peduncle
Inferior cerebellar peduncle
Vestibulocochlear nerve (VIII)
Vestibulocochlear nerve (VIII)
Glossopharyngeal nerve (IX)
Olive
Diencephalon
Thalamus
Hypothalamus
Diencephalon
Brain stem
Thalamus
Hypothalamus
Midbrain
Pons
Medulla
oblongata
Figure 12.13
Three views of the brain stem (green) and the diencephalon (purple).
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