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.
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.