Chapter 131

### ANATOMY OF THE BRAIN

#### Cerebrum

The cerebrum contains 83% of the brain tissue and consists of two hemispheres. The thick folds of the cerebrum are gyri, whereas the shallow grooves are sulci. Longitudinal fissures separate the two hemispheres. The corpus callosum, located at the bottom of the longitudinal fissure, is a bundle of nerve fibers that connects the hemispheres.

The cerebral cortex is a 2- to 3-mm thick layer of tissue covering the cerebrum, which contains about 40% of the brain mass. The cerebral cortex has six layers known as the neocortex. The layers are numbered I-VI, with VI being the innermost layer. Layer VI is thickest in sensory regions, whereas layer V is thickest in motor regions. All axons that leave the cortex, and enter the white matter, arise from layers III, V, and VI.

The cerebral cortex is divided into lobes:

• The frontal lobe is the site for voluntary and planned motor behaviors. The motor speech area (Broca area) is usually in the frontal lobe of the left hemisphere, regardless of which hemisphere is dominant. It is also the lobe responsible for sensory reception and integration of taste and some visual information. When the cortical control of movements is considered, the left frontal lobe controls the right side of the body, whereas the right frontal lobe controls the left side.

• The parietal lobe is concerned with sensory reception and integration of somesthetic (touch, pressure, heat, cold, pain, stretch, movement), taste, and some visual information. Damage to the right parietal lobe can cause visual–spatial deficits. Damage to the left parietal lobe may disrupt a patient’s ability to understand spoken or written language.

• Various parts of the temporal lobe are important for the sense of hearing, certain aspects of memory, and emotional behavior. The right temporal lobe is mainly involved in visual memory, whereas the left temporal lobe is mainly involved in verbal memory.

• The occipital lobe is crucial for the sense of sight.

• The insular lobe is located within the cerebral cortex, under the frontal, parietal, and temporal lobes. It plays a role in emotion and homeostasis.

#### Basal Ganglia

The basal ganglia are a group of nuclei lying deep in the subcortical white matter of the frontal lobes. It organizes the muscle-driven motor movements of the body behavior. Its major components are the caudate, putamen, and globus pallidus nuclei. The basal ganglia are functionally associated with the substantia nigra.

#### Cerebellum

The cerebellum consists of two cerebellar hemispheres connected by a narrow bridge-like vermis. Three pairs of cerebellar peduncles connect it to the brainstem (inferior peduncle to the medulla oblongata, middle peduncle to the pons, and superior peduncle to the midbrain). The cerebellum receives most of the information from the pons. Spinocerebellar tracts enter through the inferior peduncle. Motor outputs leave the cerebellum through ...

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