Brain Structure

Type of psychology: Biological bases of behavior

Biological bases of behavior

Fields of study: Biological influences on learning; nervous system;

thought

Biological influences on learning; nervous system;

thought

Different areas of the brain have specialized functions that control activities ranging

from basic biological processes to complex psychological operations. Understanding the

distinctive features of different neurological areas provides insight into why people

and other animals act, feel, and think as they do.
.

Key concepts

• cerebral cortex

• cerebral hemispheres

• forebrain

• hindbrain

• lobes

• midbrain

• neural tube

• neurons

About two weeks after conception, a fluid-filled cavity called the neural tube

begins to formon 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 fluidfilled

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 myelencephalon

(“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.