THE BRAIN

THE BRAIN The brain has the size and appearance of a small cauliflower. But thanks to its 100 billion nerve cells (as many as there are stars in our galaxy!), we can think, plan, talk, imagine, and so much more.   Top View of The Brain The brain has two cerebral hemispheres. Each takes care of one side of the body, but the controls are crossed: the right hemisphere takes care of the left side, and vice versa. If each hemisphere were unfolded, it would be the size of an extra-large pizza! For this reason, the brain must fold over on itself many times to fit into the skull.

			THE NERVOUS SYSTEM The brain is something like the body’s control tower. It must be kept quickly informed of the body’s needs and of the resources available in the environment to satisfy them. For this purpose, the brain relies on a vast network of “wiring” distributed throughout the body: the nerves. Together with the brain and the spinal cord, the nerves constitute the nervous system. To distinguish the control centres from the information pathways, we divide the nervous system into two sub-systems (to see them, run your cursor over their names in the following diagram): CENTRAL NERVOUS SYSTEM The central nervous system consists of the brain and its natural extension, the spinal cord, which runs through the centre of the spinal column. The spinal column, together with the skull, provides a veritable suit of armour for the precious central nervous system. PERIPHERAL NERVOUS SYSTEM The peripheral nervous system consists of the nerves, some of which gather information while others transmit orders. The facial nerves enter and leave the brain directly through the skull. Other nerves reach the brain via the spinal cord. The nerves in the peripheral nervous system are in turn divided into two categories.

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THE BRAIN Unlike the brain’s four external lobes, the limbic lobes, also called the cingulate gyri, are visible only when a sagittal section of the brain is made. Cross-section of meninges VENTRICLES AND MENINGES The brain is the best protected organ in the body. The first layer of protection is the skull, which acts as armour shielding the brain from blows. Next come the meninges, three membranes that surround the brain to keep it from being damaged by contact with the inside of the skull. It is these membranes that become infected when someone gets meningitis, and it is because the meninges are in direct contact with the brain that meningitis is so dangerous. For even more protection, the brain (and the spinal cord) are bathed in cerebro-spinal fluid. This fluid circulates through a series of communicating cavities called ventricles. Cerebro-spinal fluid also circulates between the pia mater and the arachnoid mater of the meninges. In addition to cushioning blows, this fluid reduces the pressure at the base of the brain by causing the nerve tissue to “float”. Cerebro-spinal fluid is secreted by the choroid plexus in the upper ventricles and absorbed by the venous system at the base of the brain. As this fluid flows downward, it carries away toxic wastes and moves hormones between widely separated regions of the brain. The Ventricular System

	The parts of the central nervous system that contain grey matter (composed of neuron cell bodies) are often called nuclei or ganglia. Certain groups of axons found in the brain’s white matter are called pathways or bundles.		THE NERVOUS SYSTEM In addition to making the major distinction between the central and peripheral nervous systems, scientists also frequently subdivide the body’s nerves into two other main categories: THE SOMATIC NERVOUS SYSTEM These nerves participate in the organism’s relationship with its external environment. They send information to the brain from the body’s various sensory detectors. These nerves also enable us to respond to these stimuli by moving through our environment. THE AUTONOMIC NERVOUS SYSTEM These nerves are more involved in regulating vital internal functions. They help to maintain internal equilibrium by coordinating such activities as digestion, respiration, blood circulation, excretion, and the secretion of hormones. The autonomic nervous system in turn is divided into two categories. In the central nervous system, the “grey matter” is composed of the neurons’ cell bodies and their dense network of dendrites. The grey matter includes the centre of the spinal cord and the thin outer layer of the cerebral hemispheres, commonly known as the cortex. The white matter consists of the myelin sheathing that covers the axons of these same neurons to enable them to conduct nerve impulses more rapidly. These myelinated axons are grouped into bundles (the equivalent of nerves) that make connections with other groups of neurons.

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