Overview Of The Brain

Encased securely in the skull, or cranium, the brain is a roughly symmetric, soft, and delicate organ weighing about 3 lb (approximately 1400 g) in the adult human. At the lower end of the brain, the medulla oblongata of the brain stem is continuous with the spinal cord, which descends into the spinal canal below an opening in the base of the skull called the foramen magnum; thus, the CNS is a distinct and connected whole that is protected throughout by the skeletal system. The brain is also surrounded by three layers of covering tissues: the thick dura mater just below the skull, the weblike arachnoid attached to the inner surface of the dura, and the thin pia mater directly adjacent to the surface of the brain.

The brain is customarily divided into three gross anatomic segments: the cerebrum, cerebellum, and brain stem (Fig. 1). The cerebrum consists of the paired

Table I

Major Neuroanatomic Divisions

Nervous system Central Peripheral Central nervous system Brain

Spinal Cord Brain

Brain stem Midbrain Pons Medulla Cerebellum Cerebrum

Cerebral hemispheres Gray matter Cortex Basal ganglia White matter Diencephalon Thalamus Hypothalamus Cerebral cortex Frontal lobe

Temporal lobe-limbic system Parietal lobe Occipital lobe

Figure 1 The major structures of the brain, illustrated from a midline view between the two hemispheres. Reprinted with permission from Kandel, E. R., Schwartz, J. H., and Jessell, T. M., Eds. (2000). Principles of Neural Science, 4th ed., p. 9. McGraw-Hill, New York.

cerebral hemispheres, which are joined by a large white matter tract called the corpus callosum, and the diencephalon, which includes the thalamus and hypothalamus. The cerebellum is a relatively large structure situated posterior to the brain stem. The brain stem itself is made up of the midbrain, pons, and medulla (short for medulla oblongata). Also within the brain are four cavities known as ventricles: the two lateral ventricles in the cerebral hemispheres, the third ventricle between the two thalami, and the fourth ventricle between the brain stem and the cerebellum. The ventricles are filled with a clear, colorless liquid called cerebrospinal fluid (CSF). The blood supply of the brain comes from four major arteries in the neck that provide a rich source of blood to meet the high oxygen demand of the brain. There are two carotid arteries and two vertebral arteries that link together in an anastomotic structure called the circle of Willis at the base of the brain, where major arteries irrigating specific cerebral areas originate. Venous drainage is accomplished by a complex network of venous sinuses that ultimately terminate in the paired internal jugular veins in the neck.

A source of confusion to many in studying the gross anatomy of the brain is the use of the singular descriptor in discussions of what are evidently paired structures. For example, it is commonplace to refer to the "cerebrum" when in fact both cerebra are implied.

This convention, well-entrenched in the scientific literature, is maintained for simplicity because the singular serves as a convenient shorthand. In this article, use of the singular descriptor for gross anatomic regions will imply that a pair of these regions exists unless otherwise specified.

A vast number of cells are found within the brain itself. The cellular composition of the brain includes neurons, or nerve cells, which are the basic functional units of the nervous system, and glial gells, or glia, which have a variety of supporting roles. Estimates vary, but it is possible that the brain contains up to 100 billion neurons and perhaps 10 times as many glial cells. Neurons are excitable cells that function to integrate signals they receive and transmit impulses to other neurons, and each one is connected to many others at points of juncture called synapses. Thus, the brain can be considered to be a densely interconnected electrical organ. The ongoing neuronal activity that results permits an impressive range of integrative functions that the brain maintains over all bodily and mental processes.

A final anatomic distinction to be made in the brain is between the gray matter and white matter. The cell bodies of neurons are concentrated in the gray matter, which largely consists of neuronal somata and their extensive synaptic connections. In gross anatomic terms, gray matter includes both the cortex of the brain, its outer layer, and a number of structures below the cortex known as nuclei. In contrast, the white matter consists of the axons of neurons, which travel throughout the brain and allow gray matter areas to communicate with each other. The white matter is characterized by myelin, a fatty substance that imparts a glistening white appearance to the cut brain. Myelin serves to insulate axons and in so doing greatly increases the efficiency of interneuronal communication by increasing the speed of neuronal conduction. White matter tracts course within and between the hemispheres to connect gray matter areas in functionally unified networks subserving many functions.

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