ANATOMY
OF THE BASAL SURFACE OF THE BRAIN, WITH THE BRAIN
STEM AND CEREBELLUM REMOVED
Removal of the brain
stem and cerebellum by a cut through the midbrain exposes the underlying
cerebral cortex, the base of the diencephalon, and the basal forebrain. Basal
hypothalamic landmarks, from caudal to rostral, include the mammillary bodies,
tuber cinereum, pituitary gland, and optic chiasm. The proximity of the
pituitary to the optic chiasm is important because bitemporal hemianopsia can
result from optic chiasm fiber damage, often an early sign of a pituitary
tumor. The genu and splenium of the corpus callosum are revealed in this view.
In the cross-section of the midbrain, the superior colliculus, cerebral
aqueduct, periaqueductal gray, red nucleus,
substantia nigra, and
cerebral peduncles are shown.
CLINICAL POINT
The olfactory bulb and
tract send connections directly into limbic forebrain structures, such as the
uncus (the primary olfactory cortex), amygdala, and other limbic regions. This
is the only sensory system with direct access to forebrain structures without
prior screening through the diencephalon. This reflects the evolutionary
importance of olfaction to functions vital for survival, such as detection of
food, defense, and reproduction. Olfactory damage can alter emotional behavior.
In addition, complex partial seizures involving the temporal lobe frequently
are accompanied by an olfactory aura. Changes in olfactory function and gene
expression may be among the earliest signs of Alzheimer’s disease.
The optic nerve,
chiasm, and tract can be seen extending toward the lateral geniculate body
(nucleus), the pulvinar, and the superior colliculus. Optic nerve damage can
result in ipsilateral blindness; optic chiasm damage can result in bitemporal
visual field deficits; and optic tract damage can result in contralateral
hemianopsia. Additional visual input from the optic tract enters the
hypothalamus and ends in the suprachiasmatic nucleus. This visual input conveys
information of total light flux and exposure, permitting visual influence over
diurnal rhythms such as the cortisol rhythm. Disruption of this diurnal input
can produce altered production of hormones such as melatonin, and metabolic
consequences such as the propensity for abdominal obesity resulting from
disruption of the diurnal cortisol rhythm.
