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Structures of the Brain

Originally published on November 10, 2009
Most recently updated on April 10, 2013

This article takes a detailed look at the structures of the brain and what function various parts serve.

From the outside, the most obvious parts of the brain are

  • The two wrinkled cerebral hemispheres in the left and right halves of the upper brain (together called the cerebrum)
  • The cerebellum, a smaller section attached to the lower back portion of the cerebrum
  • The brainstem, which extends down from the center of the brain and in front of the cerebellum, to merge with the top of the spinal cord.

Looking at the brain from the side (see Figure 2) tells us something about why certain functions are located where they are in the system.


Figure 2. The brain seen from the side.

The brainstem is often thought of as the most primitive region of the brain because evolutionary studies have shown it to be the first area to develop in complex animals. The brainstem controls our most basic functions, many of which happen without our thinking about them at all.

Three structures make up the brainstem:

  • The medulla, which controls breathing, swallowing, blood pressure, and heart rate
  • The pons (Latin for “bridge”), which links the cerebellum to the cerebrum
  • The midbrain, which governs rudimentary vision and hearing

Running down the length of the brainstem is the reticular formation, which is responsible for wakefulness or arousal. If a brain tumor distorts the reticular formation, a comalike state can occur.

There are twelve pairs of cranial nerves. Each cranial nerve exists as a pair, one nerve for the left side of the body and the other nerve for the right side. Most of them originate in the brainstem. They are identified by numbers (I through XII). These nerves control important things such as swallowing, facial movement, the senses (vision, taste, and hearing), and neck and shoulder muscles.

Major nerves carrying information to and from the rest of the body pass through the brainstem. The nerve axons cross over in the medulla so that the left side of the brain controls the right side of the body, and vice versa. Tumors on one side of the brain may well affect movement and sensation on the opposite side of the body. (An exception is in the cerebellum, where a side of the brain sends signals to the same side’s arm and leg.)

Above the midbrain is the diencephalon, which includes the thalamus and hypothalamus. The hypothalamus is a regulatory center involved in many important functions, such as hormone secretion (including that of the nearby pituitary gland), the autonomic nervous system, eating, sleeping, temperature, emotion, and sexual behaviors. Sitting above the hypothalamus, the thalamus serves as an information processor for much of what goes to and from the brain.

The cerebellum is the lower back of the brain, beneath the cerebral hemispheres and separated from them by a fold of dura mater called the tentorium. It is about one eighth the size of the cerebrum. The cerebellum is involved in fine motor coordination and balance, continually and automatically making allowances that let the body maintain its balance. If a tumor grows in the cerebellum, a person may stagger (ataxic gait) or make jerky movements. The person may be unable to judge distances or make his or her hands do what he or she wants them to do.

The cerebrum is the part of the brain that developed most recently in evolutionary terms and is enormous in proportion to the rest of the brain in humans. It is the part of the brain involved in sensory input, thinking, reasoning, learning, and memory—the functions we associate with intelligence. The cerebrum is proportionately larger in animals that seem to be able to take in sensory information and analyze it in some way. But only in humans is it so massive and complex.

The cerebrum is made up of right and left cerebral hemispheres, with a large groove called the cerebral fissure separating the two sides. Deep inside the brain, in the middle, is the corpus callosum, a bundle of nerve fibers connecting the halves of the brain, allowing information to move back and forth between the two sides.

The cerebral tissue of the brain is called the cortex, a deeply folded area made up of billions of cell bodies whose darkish cast give it the name gray matter. The axons connected to the cell bodies extend below the cortex, forming white matter. The convolutions of the cortex are formed by deep folds that result in a more compact structure, much like the folding of a road map. It is estimated that if the cortex were unfolded, its area would be three times larger that the surface of the brain. This folding accomplishes an important conservation of space, which means many more nerve cells are concentrated in each unit of volume.

Like a road map, the cortex defines specific areas of function in the brain. Several large grooves, or fissures, outline four areas, or lobes, on each side of the brain: the frontal, temporal, parietal, and occipital lobes.

The frontal lobes have much to do with the intellect and the ability to fit into a social group, helping us plan and prioritize, concentrate and recall, and exert control over our behavior. Damage to the most forward section of the frontal lobes can cause offensive social behaviors.

Toward the back of the frontal lobe is the motor area, a strip of brain with distinct sections controlling motor activity such as swallowing, chewing, talking, and movement of the hand, legs, toes, and so on. Doctors often need to map this area of the brain by using cortical stimulation before surgery to make sure they know exactly where the functions are; otherwise, they might disturb or remove tissue that would affect those functions (see Figure 3).


Figure 3.
Functions of the Brain.

Toward the front of the parietal lobes is the sensory area, a strip of cortex going up one lobe, over the cerebral fissure, and down the other lobe, much as the motor area strip crosses the frontal lobes. The motor area controls movement; the sensory area is concerned with sensations coming in from the eyes, ears, nose, tongue, and other organs.

The occipital lobes are the visual center of the brain, making sense of information coming into the brain from the eyes. The left occipital lobe receives input from the right field of vision, and the right occipital lobe receives input from the left field of vision.

The temporal lobes are involved in a significant way with speech and language, hearing, and memory. The temporal lobes have additional complex features. The amygdala, which appears to play an important role in extreme states of excitement, aggression, fear, and anger, is connected to the hippocampus, which assists in the formation of memories from new experiences.
           
This network of complex interconnections surrounding the top of the brainstem is called the limbic system. It links our basic functions and emotions to areas of the cerebrum that have to do with those higher levels of thought and understanding that we associate with being human.

Although the lobes have overall executive functions, they do not act alone, nor does any one process reside solely in a particular lobe. The brain’s massive network of interconnections keeps communication going between the hemispheres and among the lobes, as well as among the structures below the cortex. The brain always functions as a whole.

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