Organization Of The Brain Brain Regions

The Cerebral Cortex. A number of experimental approaches have developed to study the basis of behavior in the brain. One of these has been to study the role of brain regions in behavior. The brain is composed of distinct substructures. The most general categorization scheme separates the brain into five segments called lobes (Figure 1). From front to back these include the frontal, parietal and occipital lobes and the cerebellum. The temporal lobe is on the lateral surface of the brain. The outermost surface of the brain is called the cortex; this part of the brain has expanded in size the most in higher animals and is thought to be responsible for the high level of intelligence in nonhuman and human primates. Areas of the cortex are specialized so that specific physiological functions are mediated by cells in defined cortical regions. For example, visual processes occur in cells located on the surface of the back of the brain in the occipital lobe. In front of this region, on the border between the parietal and frontal lobes, is the area that controls movement, which is called the motor cortex. The area of the brain that controls sensation, the sensory cortex, is just in front of the motor cortex. The area in front of

Figure 1

The Lobes of the Brain.

The brain consists of several major sections or lobes. These include the frontal, parietal, occipital, temporal, and cerebellum.

Figure 1

The Lobes of the Brain.

The brain consists of several major sections or lobes. These include the frontal, parietal, occipital, temporal, and cerebellum.

the sensory cortex in the most frontal portion of the brain, the frontal cortex, is involved in cognitive functions and thinking. It is the evolution of this region that is believed to be responsible for superior cognitive functioning in humans.

The Thalamus. Information processing includes sensory information that comes in from sense organs (for example, eyes, ears, tongue) to the brain through the spinal cord or directly through cranial nerves (nerve cells connected directly to the brain). This in-coming information from sense organs goes to a central relay station called the thalamus. The thalamus is specialized, much like the cerebral cortex, in that defined areas receive input that is specific to a sensory modality. For example, input from the eyes through the optic nerve goes to a region of the thalamus called the dorsal lateral geniculate nucleus. This area of the thalamus, in turn, sends the information transmitted from sense organs to the appropriate area of the cortex. For example, the lateral geniculate sends visual information to the area of the cortex specialized for vision, which is located in the occipital lobe. Similarly, the cerebral cortex sends commands to the effector systems (usually muscles) that act on the environment through the same relay system. As one can easily see, the thalamus is a very important structure for the coordination of inputs and outputs from the brain. Thus, degenerative diseases of this structure are very debilitating, as would be drugs that specifically altered the function of this structure.

The Brain Stem. Other areas of the brain are responsible for life processes of which we are not usually aware. These processes are generally controlled by the part of the brain called the brain stem, which is located between the spinal cord and the cerebral hemispheres of the brain. The brain stem contains the cell bodies for centers that maintain heart rate, blood pressure, breathing, and other involuntary or unconscious life-sustaining processes. A number of psychoactive agents have actions on NEURONS located in the brain stem. For example, OPIATES such as morphine or heroin have a direct inhibitory effect on the brain stem respiration (breathing) centers. This is why heroin OVERDOSES are often fatal—since the breathing centers stop working. Drugs that do not affect neurons in this area, such as marijuana, are seldom life-threatening. A significant part of the reticular formation is also located in the brain stem. This system sends outputs into the brain and down the spinal cord. It regulates arousal by increasing or decreasing the brain's responses to environmental events. Thus, morphine decreases pain by altering the sensitivity of brain cells involved in pain perception. The brain stem is important in the control of pain and also contains the cell bodies for some important nerve cells involved in the euphoric and depressant actions of drugs.

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