The telencephalon is the largest part of the human brain, constituting about 85% of total brain weight, and is that portion in which all modalities are represented. Various sensory inputs (such as vision and hearing) are localized in some areas, whereas motor functions are represented in other regions and are modulated by subcortical nuclei. The telencephalon contains circuits that interrelate regions that have specific functions, such as motor or visual, with other regions called association areas. Seeing a familiar image may precipitate a cascade of neural events having olfactory, emotional, sensory, and motor components. Damage to association areas results in complex neurologic deficits. The patient may not be blind or paralyzed but may be unable to recognize sensory input (agnosia), express ideas or thoughts (aphasia), or perform complex goal-directed movements (apraxia).

Overview

The telencephalon consists of large hemispheres separated from each other by a deep longitudinal cerebral fissure. Each hemisphere has an outer surface, the cerebral cortex, which is composed of layers of cells. The cortex is thrown into elevations called gyri (singular, gyrus) that are separated by grooves called sulci (singular, sulcus). Internal to the cortex are large amounts of subcortical white matter along with aggregates of gray matter that form the basal nuclei and the amygdala. Although not parts of either the telencephalon or the basal nuclei, the subthalamic nucleus (of the diencephalon) and the substantia nigra (of the mesencephalon) have important connections that functionally link them with the basal nuclei.

Information passing into or out of the cerebral cortex must traverse the subcortical white matter. The myelinated fibers forming the white matter are organized into (1) association bundles that connect adjacent or distant gyri in one hemisphere; (2) commissural bundles that connect the hemispheres, the largest of these being the corpus callosum; and (3) the internal capsule. The internal capsule contains axons projecting to numerous downstream nuclei (corticofugal fibers) and axons conveying information to the cerebral cortex (corticopetal fibers). The terms corticofugal and corticopetal are umbrella terms that include all efferent and all afferent fibers, respectively, of the cerebral cortex.

The hippocampal complex and the amygdala are located in the walls of the temporal horn of the lateral ventricle. The axons of cells in these structures coalesce to form the fornix, stria terminalis, and amygdalofugal pathway.