The Nervous System

Activity of any kind means a huge amount of information is sensed, interpreted, and directed to countless muscle fibers. The neural circuits of the body are ablaze with activity.

Nerves = large bundles of neuron fibers (axons and dendrites), can be seen with the unaided eye. They are not the same as neurons, which are tiny individual cells. Many nerves have a whitish color because they are made up mainly of axons coated with myelin (fatty layer over axons).

Neurilemma - Thin layer of cells also wrapped around most nerve cell fibers outside of the brain and spinal cord. It provides a "tunnel" through which damaged nerve cell fibers can grow when repairing themselves. When a severed finger is sewn back on its nerves are likely to regenerate--can expect feeling to creep back at rate of about l millimeter per day. (The neurons in the brain and spinal cord cannot be replaced, however. They must last a lifetime. This is probably because new connections, or "circuit" are created in the brain during learning, and if brain cells were frequently replaced, everything you learned would be wiped out day after day (Thompson, 1985).

A. The Central Nervous System = (CNS)
[Two parts - The Brain and the Spinal Cord]

1. THE BRAIN = 3 lbs. Ever increasing proportion of brain devoted to cerebral cortex in the movement from lower to higher animals.

Cerebral cortex = In humans, it is the largest brain structure. The cerebral cortex accounts for no less than 70 percent of the neurons in the central nervous system. It looks like a wrinkled walnut. It covers most of the visible portions of the brain with a mantle of gray matter (spongy tissue made up mostly of cell bodies).

Corticalization = increase in size and wrinkling of the cortex. Fact that humans are more intelligent than other animals is related to this.

Cerebral hemispheres = The cortex is composed of two (or right and left sides).

Corpus callosum = a thick band of fibers which connects the two cerebral hemispheres.

A. THE TWO SIDES OF THE BRAIN:
1. Left cerebral hemisphere = Specializes in language. ca. 95% of adults use this hemisphere for speaking, writing, and understanding language. It is superior in math, at judging time and rhythm, ordering and coordinating complex movements, especially those needed for speech. Mainly involved with analysis (breaking information into parts). It also processes information sequentially (in order, one item after the next).
2. Right cerebral hemisphere = Responds to only the simplest language and numbers. Working with the right hemisphere is a little like talking to a child who only understands a dozen words or so. To answer quest- ions, the right hemisphere must point to objects or make other nonverbal responses. Once regarded as a "minor" hemisphere. We now know it has its own talents. It is superior at perceptual skills such as recognizing patterns, faces, and melodies. It is also involved in detecting and expressing emotion. It is better at visualization and at "manipulo-spatial" skills, such as arranging blocks to match a pattern, putting together a puzzle, or drawing a picture. Superior at spatial tasks. Appears to process information simultaneously and holistically (all at once).
NOTE: While activity may focus on one hemisphere, the skills of both sides of the brain are combined in most activities. The entire brain is used at all times. Lawyers, artists, and psychologists showed no differences in a test of right and left hemisphere activity.

B. THE LOBES OF THE BRAIN:
l. The Occipital Lobes = located at the back of the brain, they are the primary visual area of the cortex. Visual images are mapped onto the cortex--but the mapped is greatly stretched and distorted. It is not like a screen in the brain. Visual information creates complex patterns of activity in nerve cells.
2. The Parietal Lobes = Located on top, just above the occipital lobes. It deals with body sensations. Touch, temperature, pressure, and other somatic, or bodily, sensations are channeled to: the somatosensory area on the parietal lobes. As a map of sensations, the cortex reflects the sensitivity of body areas, not their size.
3. The Temporal Lobes = Located on each side of the brain (temples). Auditory information projects directly to the temporal lobes, making them the site where hearing registers. If we were to stimulate the primary auditory area of a temporal lobe, our subject would "hear" a series of sounds. These sounds would increase in pitch as we moved from the top to the bottom. stimulating in another direction, we would find an orderly change in loudness. Sound qualities are clearly mapped on the surface of the cortex. And, the left temporal area also contains a language "center." Damage to this area can severely limit ability to use language.
4. The Frontal Lobes = Located in fore area. It deals with smell, speech, motor control and abstract thought. Olfactory (smell) information registers on the underside of the frontal lobes. Motor cortex = an arch of tissue running over the top of the brain directs the body's muscles. If stimulated with brief electri- cal current, muscular twitches will be observed in various parts of the body. The correspondence of motor cortex area and bodily area exists solely in terms of importance--not in size--the hands represented with more area than the feet. (This area is shown as running parallel to somatosensory area, both arching over top of the brain.) More complex behaviors are also related to the frontal lobes. When frontal brain areas are removed, animals lose the ability to judge the passage of time, to hold the solution to a problem in mind, or to respond to emotionally unpleasant situations. Damage to this area in humans alters personality and decreases emotionality. Doing intellectual tasks based on reasoning or planning also seems to rely on the frontal lobes. Patients with frontal lobe damage often get "stuck" on such tasks and repeat the same wrong answers over and over.

C. ASSOCIATION CORTEX = All other areas of the human brain, including part of all the lobes (primary sensory and motor areas make up only a small part of the cerebral cortex). The size and relative amount of association cortex increase strikingly as one ascends the evolutionary scale. It seems to process and combine information from the various senses. It is probably also related to higher mental abilities. The link of the frontal lobe with thinking skills is a good example of such abilities. Additional clues to the workings of the association cortex come from studies of humans with brain injuries.

2. THE SPINAL CORD= Acts like a cable, connects the brain to other parts of the body. Places the entire body in sensory and motor communication with the brain. Cord is made up of columns of white matter--nerve tissue made up of axons that leave the spinal cord to form peripheral nerves--30 pairs of these spinal nerves that leave the spinal cord, plus one pair leaving the bottom tip = 31 pairs of spinal nerves. And, 12 cranial nerves, that leave the brain directly.
Reflex arc = Adaptive Spinal cord capable of responding on its own--the body can react to protect itself without calling on the brain. Simplest behavior pattern = Muscle fibers (made up of effector cells) contract, causing a foot (hand, finger) to withdraw (from thorn, heat, etc.) Procedure:
1. Sensory neuron in foot detects thorn
2. and fires a message (or action potential) to the spinal cord.
3. The sensory neuron synapses (connects) with a connector neuron (or interneuron) inside the spinal cord.
4. The connector neuron in turn activates another connector cell (a motor neuron in this case) that leads back to muscle fibers.
5. The effector cells in the muscle fibers contract and cause the foot to withdraw.

NOTE: In reality more complex activity accompanies simple reflex--a counter action in balance or the shifting of weight, for example, involving many cells and several layers of spinal nerves, etc.

B. Peripheral Nervous System
Composed of nerves that carry information to and from the CNS--to and from brain and spinal cord. It has two subparts, which both coordinate the inner and outer worlds of the body:

l. Somatic nervous system = Carries messages to and from the sense organs and skeletal muscles--activates motion and response (ability to avoid falling twig) - It is the
2. Autonomic nervous system = (ANS) serves the internal organs and glands of the body--activates blood pressure, quickens the heart, etc .(while avoiding falling twig). The Autonomic nervous system can be subdivided into two branches, which are both related to emotional responses, such as crying, sweating, heart rate, and other involuntary behavior--both are active at all times, combining to determine if the body is aroused or quieted:
a. Sympathetic or "Emergency" system which arouses the body for action--prepares the body for "fight or flight" during times of danger or emotion.
b. Parasympathetic = "Sustaining" system is most active after a stressful or emotional event. Its role is to quiet the body and return it to a lower level of arousal. It also helps maintain vital functions such as heart rate, breathing, and digestion at moderate levels.

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SPLIT BRAIN - An operation in which the corpus callosum is severed
in humans or animals, resulting in a doubling of consciousness.

Each hemisphere has its own separate and private sensations; its own perceptions; its own concepts; and its own impulses to act. Both halves of the brain have the same experience at the same time. One hemisphere may not know what is happening in the other--the right hand not knowing what the left hand is doing. If a conflict arises, one hemisphere usually overrides the other. So, even though there can be serious conflicts in action, they can act completely normal.
Aphasia = an impaired ability to use language. This is the result of injury to either of two areas (which, not surprisingly, are interconnected in the brain) of the cortex particularly related to language:
Broca's area - lies on the left frontal lobe. With damage, one can read, can understand the speech of others, but they have great difficulty speaking or writing themselves. Typically, their grammar and pronunciation are poor and their speech is slow and labored. Knows what s/he wants to say, but can't utter the words (often found in stroke patients).
Wernicke's area - found on the left temporal lobe. With damage, one has difficulty with the meaning of words, not grammar or pronunciation. Often speak in incredibly roundabout ways to avoid using certain nouns.

Mindblindness (Agnosia) = This condition involves an inability to identify seen objects. If shown a candle, for instance, someone with an agnosia might describe it as a long narrow object tapering at the top. Might even draw it, but still fail to name it. If allowed to feel the object, the subject will name it immediately.
Facial agnoisa = inability to identify familiar persons. However, may identify them immediately by their voices. A brain area devoted to recognizing others is located on the underside of the occipital lobes. These areas appear to have no other function.
Humunculus = Mythical little man inside the brain that makes decisions or observes incoming information

[Notes from: Coon, Dennis. Introduction to Psychology, Exploration and Application. St. Paul: West Publishing Company, 1989.]

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