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The human brain is a complex aggregate of billions of cells working together to process stimuli, to monitor needs, and to direct behavior. Developmentally, the brain begins at the most rostral extension of the neural tube; it bends over and convolutes as it expands within the confines of the skull (cranium). The brain's expansion is disproportionate relative to the growth of the spinal cord, the most caudal extension of the central nervous system. Figure 1 illustrates the development of the human brain, showing its major subdivisions.

There are three major sections of the brain: the prosen cephalon or forebrain, the mesencephalon or midbrain, and the rhombencephalon or hindbrain. The forebrain is the largest and most expansive and is made up of two subdivisions: the telencephalon (endbrain) and the diencephalon (interbrain). Telencephalic structures account for about 75% of the weight of the entire human central nervous system. These structures include the two cerebral hemispheres that are connected by a mass of crossing fiber tracts (the corpus callosum). The surface of the hemispheres is a multicellular layer of brain tissue about 4.5 centimeters thick, called the cerebral cortex. The cortex is divided into subregions according to gross anatomical landmarks called sulci and gyri. The largest subregions are called lobes, of which there are four in each hemisphere: frontal, parietal, temporal, and occipital. The location of the four lobes and other major brain structures can be seen in Figures 1 and 2. The occipital lobes have visual functions. The temporal lobes are important for audition, learning and memory, and, on the left side of the brain, for understanding language. The parietal lobes control visuospatial and so-matosensory functions, and at the junction with the temporal lobe, the left parietal cortex is important for language comprehension. Frontal cortex is polysensory; it is known to be important in movement, impulse control, emotional behavior, problem solving, and, on the left side, language expression.

In the cerebral hemispheres the cortex has a laminar architecture with the different neuronal cell types organized in layers. From an evolutionary standpoint, the layered cor-

Prosencephalon

Telencephalon Diencephalon

Mesencephalon \

Rhombencephalon

Optic vesicle

Rhom Rhom esicle

Mesencephalon

Rhombencephalon

Rhombencephalon

Telencephalon

Rhombencephalon Cerebellum Pons

Mesencephalon

Telencephalon

Diencephalon

Spinal cord

Mesencephalon

Diencephalon Optic vesicle

Rhombencephalon Cerebellum Pons

Mesencephalon

Diencephalon

Telencephalon

Diencephalon

Telencephalon

Temporal lobe Diencephalon

Prosencephalon Cerebellum Medulla

Diencephalon

Mesencephalon

Prosencephalon Cerebellum Medulla

Telencephalon

Pituitary Optic stalk Telencephalon

Temporal lobe Diencephalon

Telencephalon

Mesencephalon

Rhombencephalon Cerebellum Medulla

Mesencephalon

Frontal lobe

Rhombencephalon Cerebellum Medulla

Puititary stalk Optic stalk Olfactory bulb

Olfactory bulb

Temporal lobe

Frontal lobe

Olfactory bulb

Temporal lobe

Pons Medulla

Occipital lobe Cerebellum

Newborn

Parietal lobe

Telencephalon

Occipital lobe Cerebellum

Figure 1. The development of the human brain, showing its major subdivisions.

Pons Medulla

Newborn

Cingulate gyrus

Parietal cortex

Frontal cortex

Olfactory bulb

Amygdala area

Hippocampal

Medulla oblongata

Figure 2. The location of the four lobes and other major brain structures of the adult brain.

Corpus callosum

Fornix

Cingulate gyrus

Parietal cortex

Frontal cortex

Corpus callosum

Fornix

Amygdala area

Hippocampal

Thalamus

_ Hypothalamus Colliculi

Occipital cortex

Olfactory bulb

Medulla oblongata

Figure 2. The location of the four lobes and other major brain structures of the adult brain.

Cerebellum

Thalamus

_ Hypothalamus Colliculi

Occipital cortex

Cerebellum tical areas have changed in complexity across the phyloge-netic scale. Cortical nerve cell bodies collectively appear gray, thus accounting for the fact that cerebral cortex is commonly called gray matter. Likewise, nerve fibers emanating from the cell bodies, because of their collective white appearance subcortically, have been referred to as white matter. These fibers connect with other nerve cells that are aggregated in clusters called subcortical nuclei. In the tel-encephalon, the subcortical nuclei include the septum, the amygdaloid complex, and nuclei of the basal ganglia (caudate, putamen, and globus pallidus). Septal and amygdala regions are intimately connected to each other and are important in emotional and motivational functions. The basal ganglia are concerned largely with various aspects of motor control.

The cerebral hemispheres are attached to the dien-cephalon by massive fiber bundles, the corona radiata. Major structural components present in the diencephalon include the thalamus (a way station for incoming neurons); the subthalamus (a way station between the thalamus and the cortex); the hypothalamus (literally, "under the thalamus"); and the epithalamus (containing the pineal body and the habenular complex).

The middle section of the developing brain is called the mesencephalon or midbrain. At maturity the mesen-cephalon resembles its early embryonic form more closely than do either the prosencephalon or the rhombencephalon. The mesencephalon is made up of three main parts, the tectum (containing auditory and visual relay stations called the inferior and superior colliculi), the tegmentum (containing the midbrain reticular formation that activates attention, the substantia nigra that subserves motor functions, and numerous other nuclear groups), and the crus cerebri (a descending bundle of fibers).

The third major section of the brain, part of which eventually exits into the spinal cord at the base of the skull, is the rhombencephalon or hindbrain. It is composed of two subparts, the metencephalon (consisting of the pons and cerebellum) and the myelencephalon (the medulla oblon-gata). The cerebellum is a prominent eminence; it is the center for motor skills and also subserves certain types of learned activities. The pons and medulla oblongata contain clusters of cranial nerve nuclei that connect the nerves going to and from the face and head. Because of the shape and position of the pons and medulla at the base of the brain, they often are referred to as the brain stem, although this term usually includes structures in the midbrain and lower diencephalon as well.

The various components of the brain are interconnected through a very complicated network of neuronal pathways, and neurons are in continuous communication (through specialized chemicals called neurotransmitters). Nuclei within the brain seldom act autonomously. Instead, several nuclei and their fiber tracts may act together to organize and modulate complex behaviors. The functions subserved by these many diverse structures and systems are generally similar in all normal, healthy adults. Sensory systems regulate information coming from outside and inside the body; attentional systems not only keep us alert, but also allow us to ignore stimulus information that may be irrelevant and to rest when we need to; motor systems regulate how we respond and move about; and emotional and motivational systems monitor drives and needs and homeosta-sis. Other systems help us to learn and to remember or forget. Together, the functioning brain is essential to every aspect of life and consciousness.

Marlene Oscar-Berman

Boston University School of Medicine and Department of Veterans Affairs Health Care System

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