The analogy between the nervous system and a telephone communication system includes a large central exchange. This central exchange receives incoming messages, routes them to the proper points to be interpreted, and then generates outgoing messages. In the nervous system the brain functions as a central exchange.
The brain integrates incoming stimuli and coordinates behavior. Most of its functions, however, are not essential for life. In experimental animals the removal of most of the brain (except for parts of the brain stem) does not cause the animal to die, though it does debilitate him. Even after complete severing of the upper spinal cord, an animal is still capable of reflex behavior.
As you read the text, try to answer the following questions.
How do structures in the brain influence behavior?
How does the brain function in vision, audition, bodily movement?
What parts of the brain control learning? Thinking? Memory?
How do human beings pay attention to their surroundings?
The largest part of the brain is the cerebrum. There are two cerebral hemispheres covered with intricate convoluted ridges and deep fissures. The cerebral hemispheres, mirror images of each other, hide most of the internal structure of the brain. The brain is actually a continuation of the spinal cord. The lower region of the brain -- the brainstem -- cannot be differentiated from the spinal cord. The cerebral hemispheres are folded over, hiding the upper part of this brain stem.
The central fissure runs down the side of each hemisphere. This fissure is useful as a landmark in identifying areas of the cerebrum.
LOWER MOTOR AND SENSORY FUNCTIONS OF THE BRAIN
Figure 11 shows important structures of the human brain. The brain and spinal cord are hollow structures, their centers being filled with cerebra- spinal fluid. In Figure 11 the two arrows for the spinal cord are indicating the front and back walls of the single, hollow cord. As we move up the spinal cord to the brain stem, the first important structure is the medulla. The medulla controls such functions as heart activity, breathing, and blood pressure. If this area were destroyed the person would die.
Just behind the brain stem is a baseball-sized structure called the cerebellum. The cerebellum is one of the major centers in the brain for the smooth coordination of bodily movements. If the cerebellum were removed or injured, muscular coordination would be severely hampered. A person with cerebellar damage walks with a shuffling, uncoordinated gait. The cerebellum is most important in receiving and integrating proprioceptive stimuli from the skeletal muscles and in sending impulses to the muscles to coordinate their actions.
Another very important structure is the thalamus, which serves as a relay station for incoming impulses from various receptors in the body. Fibers that carry impulses dealing with temperature sensation, sight, touch, sound, and pain, synapse in the thalamus with neurons that ascend to specific somesthetic projection areas of the cortex. (See Figure 12.)
Just below the thalamus is a tiny area called the hypothalamus (hypo = below). Although it is small, this area influences important human functions, including the expression of emotion, wakefulness, thirst, hunger, and sexuality.
Figure 11. internal structure of the human brain
Figure 12 Comparison of thalamus and RAS responses to a pain stimulus
The reticular activating system (RAS) is a network of neural fibers in the brain stem. Like the thalamus the RAS is a relay station for incoming sensory impulses. While the thalamus relays to specific cortical areas, the RAS relays to nonspecific areas (Figure 12). A loud noise, for example, would send an impulse through the thalamus to be relayed to an auditory area of the cortex. Impulses resulting from the same stimulus also reach the RAS which, in turn, sends impulses to diffuse areas throughout the brain. The RAS functions to alert or "wake up" the cortex and prepare it for more specific stimuli. While the thalamus "tells" an isolated area about a specific stimulus, the RAS tells the entire cortex to "pay attention -- something is happening out there." Damage to a person's reticular activating system results in a deep coma. The organism is still capable of receiving stimuli, but if the RAS is not functioning, the person does not come out of his coma.
THE CEREBRAL CORTEX
Most of man's awareness and conscious behavior involves the cerebral cortex. Man has a proportionately larger cortex than any other animal's. The function of the cortex is broken down into motor functions, sensory functions, and associative functions.
Figure 13 shows how the cerebral cortex is divided into four lobes -- Frontal, Parietal, Temporal and Occipital (rear). For each lobe shown in this side view, there is a duplicate lobe in the other hemisphere of the brain.
Just in front of the central fissure, at the back of the frontal lobe, lies the motor area of the cortex
Impulses that control the skeletal muscles of the body originate in the motor area. If an experimenter were to stimulate this area electrically, it would cause movements of parts of the body. Each specific part of the motor area sends impulses to specific muscles.
The top part of the brain controls the lower extremities (feet and legs); the loweer part controls the upper part of the body (head and shoulders).
Like motor functions, sensory functions are also projected on the cortex. The area for many sensory functions is located just behind the central fissure at the front of the parietal lobe
The somesthetic, or bodily, senses on the cortex receive impulses generated at receptors in the peripheral nervous system. The impulses ascend the spinal cord, enter the brain, and are relayed through the thalamus to the sensory area of the cortex. If an experimenter were to stimulate point A, a sensation in the hand would be reported. Three other sensory primary projections are shown in Figure 17. -These areas are terminal points for impulses arising in the sense organs-When stimulated by light rays, neurons of the eye send impulses to the visual cortex on the occipital lobe. Sound vibrations in the ear propagate auditory impulses that terminate in the auditory cortex on the temporal lobe. An area on the parietal lobe receives taste sensations. Refer to Figure 13 for the location of the lobes.
In addition to receiving and sending sensory and motor impulses, the brain integrates, interprets, and stores information. Of the 12 billion neurons in the human brain, the majority are involved in these associative functions.
Figure 17 Primary sensory projections of the cortex
The functions of many parts of the brain have been identified through direct stimulation or alteration
An association area in the parietal lobe deals with complex perceptual functions, especially the recognition of objects by touch. If you were blindfolded and given a pencil to hold, you could easily identify it. But a person who has damage to the parietal association area would not be able to do this. This defect is called agnosia. Ruch (1938) trained a blindfolded chimpanzee to discriminate between a pyramid and a wedge. He then removed the parietal association area. No amount of retraining could bring back the discrimination.
The lower portions of the temporal lobe affect visual associations. They receive impulses from the visual projection areas. If the visual association area is destroyed there is no impairment of a person's sight. There is, however, an inability to distinguish one form from another. If an animal is taught to discriminate between a square and a rectangle and then suffers damage in the temporal lobe, the discrimination is lost. Unlike touch discriminations, however, the lost visual discrimination can be relearned. It seems that intact parts of the brain can take over the functions of the damaged visual association areas. This phenomenon, called the equivalence-of-function principle, is found to operate for many areas of the brain.
The temporal lobes also function in memory. When Penfield stimulated certain areas of the temporal lobes, patients reported memories of past experiences (Penfield and Roberts, 1959). These memories, unlike ordinary memories, were vividly and faithfully recalled as if the patient were reliving the experience. When stimulation stopped, and then began again at the same spot, the same memory was "replayed" again from beginning to end.
The brain is limited in the number of tasks done simultaneously
When one tries to do several tasks at the same time the brain may be limited in its efficiency. For example, when one is driving a car while at the same time talking on a cell phone, several different areas of the brain are being used. This is called multitasking and can produce slower response times than if each task were done separately. Rather than double the amount of brain tissue being activated, the amount of tissue involved for each task is reduced considerably.
Marcel Just, a psychologist at Carnegie Mellon University, reported (Blakeslee, 2001) on an experiment involving 18 subjects who volunteered to have images made of their brains while engaged in multitasking. One task involved language comprehension, listening to complex sentences and judging whether they were true or false. The other task required comparing pairs of three dimensional objects and rotating them mentally to see if they were the same. The language task takes place in the temporal lobe; the spatial tasks involved the parietal lobe. When each task was done separately, 37 voxels of brain tissue were activated. When the tasks were done together, one would expect 74 voxels to be active. But only about half (42) voxels were active. Although accuracy did not suffer, it took the subjects longer. It was speculated that if the tasks were even more complicated, one of the tasks would probably be terminated in order to successfully handle the other. When driving a car, for example, if road conditions suddently became complicated, the driver would probably stop talking on the cell phone; though the person on the other end, not seeing the complications, would probably continue to blather away.
Some portions of the brain remain largely unexplored
We know least about the frontal lobes of the cortex. Though much research has been done on these areas, the results are often conflicting. It is widely agreed, though, that these areas are important for abstract thinking. Some people with frontal lobe damage have difficulty in finding more than one approach to solving problems; they cannot shift easily from one line of attack to another. An experiment performed by Jacobsen (1935) showed that monkeys with frontal lobe damage cannot solve problems involving time delay. In this experiment a monkey was shown that food was hidden in one of two wells covered by identical wooden blocks. Then the blocks were hidden by a screen for varying amounts of time. When the delay was more than about one second, the animal could not reliably uncover the well with the food. This could not be interpreted as a memory defect. When the monkey with frontal damage was allowed to watch while a single well was baited, he could always go for the food, regardless of the delay.
There is new evidence that depression may be due to a slowdown in cell growth in the brain. Yvette Sheline (1996) reported that the hippocampi of ten depressed patients were 12 to 15% smaller than a control group. The difference appears to be in the volume. Brains of deceased patients who had suffered depression show that the prefrontal cortex may be involved for bipolar disorders where neurons appeared to be less densely packed.
Now test yourself without looking back.
Give the name and function of each structure indicated by the arrows above.
1 ) Name:
2. What symptoms might be found in a patient with severe damage to number 3 above?
3. What symptoms might be found in a patient with severe damage to number 4?
4. What symptoms might be found in a patient with severe damage to number 5?
5. State the equivalence-of-function principle.
6. An area of the brain in which the problem-solving function is found is the lobe.
7. An area of the brain that seems to function in visual and tactile memory is the _ lobe.
8. An area of a person's brain has been damaged. The person is blindfolded and handed a sphere and a cube. He cannot discriminate between them. Damage has probably been done to the_________________________ lobe.
9. Damage to which two lobes might result in an impairment of visual discrimination?
8 OR MORE CORRECT shold probably do exercises anyhow
FEWER THAN 8 CORRECT -- DO EXERCISES
Here are some diagrams that you can refer to while completing the following exercises.
The upper part of the spinal cord is connected to the brainstem. The major function of the________________________ is to carry neural pulses to and from the brain.
The cortex is not essential for life. If me cortex of an animal were
removed, the animal:
a. would die;
b. would not die.
c. would not be affected by any stimulus.
d. would walk with a limp.
The external surface of me brain is divided into two cerebral
A person with cerebellar damage walks with a jerky, shuffling gait. The function of me cerebellum is to coordinate _____________________________
The vital functions of heart activity, blood pressure, and breathing are controlled in a lower portion of me brain in a structure called me
3 spinal cord
5 muscular movement
Write the numbers identifying the structures in the diagram below:
b. Spinal cord_____
c. Reticular activating system (RAS)_____
1) Relays incoming sensory
fibers to specific cortical
2) Smooths and coordinates muscular activity_____
3) Controls breathing, heartbeat, and blood pressure_____
4) Major neural pathway to me brain _____
5) One function is involved in emotion _____
a. Reticular Activating System
b. Spinal cord
1) Nonspecific arousal system _____
2) Terminal for many sensory fibers _____
3) Origin for motor fibers _____
4) Separates motor cortex from sensory cortex _____
a. Motor cortex
b. Central fissure
e. Somesthetic cortex
Most of the internal structure of the brain is hidden by the __________________________________________________ 6
When one area of the cortex is removed, another area might take
over me function of me first. This is known as:
a. the all-or-none property.
b. nondecremental principle.
c. retroactive functioning.
d. equivalence of function.
1 Refer to diagram.
a. The visual projection area is number
b. The somesthetic projection area is number
c. The motor projection area is number
6 cerebral cortex, cerebrum, or cerebral hemispheres
When Penfield stimulated the temporal lobe of a patient's brain, the
patient reported a vividly recalled experience. This shows mat the
temporal lobe has some function in
When the parietal lobe stiflers damage, a person may be unable to
discriminate between objects held in the hand but not seen. The
parietal lobe functions in_____________________________________
Certain areas of the extreme front of me brain are needed for
experimental animals to solve problems involving a time delay. If an
animal has difficulty in this type of problem solving, what lobe has
probably been damaged?
A patient with damage to the temporal lobe may not be able to tell the difference between a square and a rectangle. The temporal lobe, then, must have something to do with
NOW TAKE PROGRESS CHECK 2
1 touch, tactile
3 visual perception, visual discrimination-
Given the symptoms below, write the name of a structure in the above diagram that might be damaged and identify that structure by number.
a. Patient is in a deep coma.
b. Patient walks with a shuffling gait
c. Patient is dead. _______________________ _____
2. What is the equivalence-of-function principle? ________________________________________________________________
3. Specific sensory relay is accomplished by the______________________
4. Nonspecific sensory relay is accomplished by the ______________________
5. A loud noise would cause nerve impulses to be sent to the cortex via:
a. the thalamus.
b. the cerebellum.
c. the RAS.
d. the hypothalamus.
a. The fissure shown in number 2 is called the _____________________________ It separates the motor area from the___________________________
b. If you wanted to elicit a sensation in me hand, you would stimulate a specific part of number______
c. If you wanted to elicit foot movement, you would stimulate number_________
d. Number 4 is called the______________
e. Number 5 is called the______________ ANSWER KEY
OR MORE CORRECT GO ON TO MODULE 7
FEWER THAN 4 CORRECT INSTRUCTOR CONFERENCE
July 31, 2001