Nervous System

In humans, centralization has reached the greatest degree of specialization. The brain and spinal cord are formed early in embryonic development. At the beginning of the third week of gestation, the embryo has already formed a neural plate on the dorsal surface which eventually folds together to form a hollow tube from which the brain and spinal cord develop. During this time the 100 billion neurons found in the brain are produced-the sum total of all the neurons that the brain will ever contain in an individual's lifetime. The brain is one of the largest organs in the body and consists of three main regions: the forebrain, midbrain, and hindbrain. The cerebrum, which is the most important area for neural processing, together with the thalamus and hypothalamus, forms the forebrain. In the midbrain are centers for the receipt and integration of several types of sensory information, such as seeing and hearing. The information is then sent on to specific areas in the cerebrum to be processed. The hindbrain consists of three parts: the medulla oblongata, the pons, and cerebellum, and it functions in maintaining homeostasis and coordinating movement. The pons and medulla of the hindbrain, together with the midbrain, form the brainstem, which is the location of reflex centers such as those that control heart beat rate and breathing rate. The other part of the central nervous system, the spinal cord, serves as a pathway for nerve tracts carrying impulses to and from the brain. It acts as the site for simple reflexes such as the familiar knee jerk. If a slice were made into the spinal cord, it would show a cord with a small central canal surrounded by an area of gray matter shaped like a butterfly surrounded by white matter. The gray matter is composed of large masses of cell bodies, dendrites and unmyelinated axons; the white matter is composed of bundles of axons that are called tracts, which send information to the brain or send information away from the brain.