Neurons

About two weeks after conception, a fluid-filled cavity called the neural tube begins to form on the back of the human embryo. This neural tube will sink under the surface of the skin, and the two major structures of the central nervous system (CNS) will begin to differentiate. The top part of the tube will enlarge and become the brain; the bottom part will become the spinal cord. The cavity will persist through development and become the fluid-filled central canal of the spinal cord and the four ventricles of the brain. The ventricles and the central canal contain cerebrospinal fluid, a clear plasmalike fluid that supports and cushions the brain and also provides nutritive and eliminative functions for the CNS. At birth the average human brain weighs approximately 12 ounces (350 grams), a quarter of the size of the average adult brain, which is about 3 pounds (1,200 to 1,400 grams). Development of the brain in the first year is rapid, with the brain doubling in weight in the first six months.

The development of different brain areas depends on intrinsic and extrinsic factors. Internally, chemicals called neurotrophins promote the survival of neurons (the basic cells of the nervous system that are specialized to communicate electrochemically with one another) and help determine where and when neurons will form connections and become diverse neurological structures. Externally, diverse experiences enhance the survival of neurons and play a major role in the degree of development of different neurological areas. Research has demonstrated that the greater the exposure a child receives to a particular experience, the greater the development of the neurological area involved in processing that type of stimulation. While this phenomenon occurs throughout the life span, the greatest im pact of environmental stimulation in restructuring and reorganizing the brain occurs in the earliest years of life.

Experience can alter the shape of the brain, but its basic architecture is determined before birth. The brain consists of three major subdivisions: the hindbrain (rhombencephalon, or "parallelogram-brain"), the midbrain (mesencephalon, or "midbrain"), and the forebrain (prosencephalon, or "forward brain"). The hindbrain is further subdivided into the myelen-cephalon ("marrow-brain") and the metencephalon ("after-brain"), while the forebrain is divided into the diencephalon ("between-brain") and the telencephalon ("end-brain"). To visualize roughly the locations of these brain areas in a person, one can hold an arm out, bend the elbow 90 degrees, and make a fist. If the forearm is the spinal cord, where the wrist enlarges into the base of the hand corresponds to the hindbrain, with the metencephalon farther up than the myelencephalon. The palm of the hand, enclosed by the fingers, would be the midbrain. The fingers would be analogous to the forebrain, with the topmost surface parts of the fingers being the telencephalon.

One can take the analogy a step further. If a fist is made with the fingers of the other hand and placed next to the fist previously made, each fist would represent the two cerebral hemispheres of the forebrain, with the skin of the fingers representing the forebrain's cerebral cortex, the six layers of cells that cover the two hemispheres. Finally, like close-fitting gloves, the meninges cover the cortex. The three layers of the meninges play a protective and nutritive role for the brain.

The more advanced the species, the greater the development of the forebrain in general and the cortex in particular. The emphasis here is placed on a neuroanatomical examination of the human brain, beginning with a look at the hindbrain and progressing to an investigation of the cerebral cortex. The terms "anterior" ("toward the front") and "posterior" ("toward the back") will be used frequently in describing the location of different brain structures. Additionally, the words "superior" ("above") and "inferior" ("below") will be used to describe vertical locations.

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