Part IV of the Nervous System begins with a description of the Central Nervous System, or CNS. As you will remember from the introduction to the nervous system (April, 1998), the CNS consists of the brain and spinal cord. This month's issue deals with the brain.
The brain controls intelligence, consciousness, instincts - all of the more complex neural functions. A lesser, but not unimportant, brain function accomplished through cranial nerves is the sensory and motor innervation of the head. It is about the consistency of cold oatmeal (maybe "mush," in some cases), and the average size of the brain is approximately two big handfuls of pinkish gray tissue. The average adult male's brain weighs about 1600 grams, or about three and a half pounds; and the average adult female's brain weighs 1450 grams. Roughly equal relative to average body weight differences. But brain size does not correlate to mental abilities. It is the neural complexity that makes a difference. Actually, the brains of the prehistoric Neanderthals were larger than those of modern humans, but they were technologically inferior. [We think.]
Before we really get going on this subject, let's define a couple of terms that pop up every now-and-then during brain-talk: 1) rostrally, and 2) caudally. Rostrally literally means "toward the snout" which are actually the higher areas of the brain. Lower parts of the brain are located caudally, or "toward the tail." See Figure 7-1.
Embryonic brain development begins with a simple tube - the neural tube. As the embryo grows, the neural tube begins expanding and constricting, and the caudal end develops into the spinal cord. The expansion develops vesicles (membrane-lined sacs), and constricting results in the brain "folding" so it will fit inside the skull; hence its unusual appearance.
The adult brain is described as having four regions: 1) the cerebral hemisphere, 2) the diencephalon, 3) the brain stem, and 4) the cerebellum. See Figure 7-2.
Before starting on the four regions, here is another brain anatomy diversion: the ventricles. There are four - see Figure 7-3.
They are contiguous cavities filled with a clear fluid (cerebrospinal fluid (see Note 1)). They include the lateral ventricles, formerly called the first and second ventricles, and the third and fourth ventricles which start on the top of the head and end up funneling into the spinal column, as depicted.
Ok, back to the four regions:
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Note 1 - Cerebrospinal fluid fills the ventricles and surrounds the CNS.
It cushions the brain and spinal cord.
CEREBRAL HEMISPHERE
This area (also called the cerebrum) is the upper part of the brain and makes up 83% of its total mass. A further division of this area is into five lobes: 1) frontal, 2) temporal , 3) parietal, 4) occipital, and 5) insula (see Figure 7-4). The frontal, temporal, parietal, and occipital lobes underlie corresponding skull plates. Each of these plates are ossified into a single helmet-like bone in adults, but are separate in young children to accommodate skull expansion until the brain stops growing. There is no corresponding bony plate covering the insula, as it lies deep in the brain under the temporal, parietal, and frontal lobes.
Note that the cerebral hemisphere is divided into right and left sides, separated by the longitudinal fissure. The two sides are physically alike, although not functionally equal.
For the purposes of this newsletter the lobes are relatively unimportant - used mainly as geographical locators. Of more importance (again, for purposes of this newsletter) is an organization in terms of the three largest areas of the cerebrum: 1) cerebral cortex, 2) cerebral white matter, and 3) basal nuclei.
Cerebral Cortex
This is the habitat of the conscious mind. It allows us to be aware of our environment, to initiate and control movement, to communicate, remember, and understand. It consists of gray matter, and covers the outside of the cortex, including all of its folds. It forms a relatively thin layer, but amounts to about 40% of the brain's total mass because of the abundant folds. Through structural mapping (A process which is performed by stimulating a body area and verifying where in the brain is activated, or by stimulating a spot in the brain and verifying the body response.) it has been discovered that specific motor and sensory functions are localized to specific regions, called domains. But, several higher brain functions, like memory and language occupy overlapping domains.
The cerebral cortex contains three types of functional areas: 1) motor areas for voluntary motor activities, 2) sensory areas, and 3) association areas that act mainly to integrate information from various sources for specific responses.
Motor Areas
The cortical area that controls motor functions is located in the frontal lobe. The axons running to the spinal cord to cause movements of the arms and legs, for example, are contralateral; that is, the left side of the motor cortex controls muscles on the right side, and vice versa.
Specific motor areas include:
1- Primary motor cortex (somatic motor area) - Located in the frontal lobe, this is the area for primary control of the limbs, trunk, etc. alluded to above.
2- Premotor cortex - The controller of learned motor skills of a patterned or repetitive nature; like typing, for example. Think of this area as a memory bank for skilled motor activities. In reading about weight training, I have often seen the term "muscle memory." I guess it comes from here, and is one of the reasons offered for suggesting changing, every now and then, order, intensity, and exercises - don't let those muscles get lackadaisical.
3- Frontal eye field - This area controls voluntary eye movements.
4- Broca's area - The most current information shows that this area becomes active immediately prior to all voluntary motor activities, not just for speech, as was once thought. The exact function of Broca's area is not precisely known, but it is thought that it "preplans" all voluntary movements by sending instructions to the primary motor and premotor cortex for execution.
Sensory Areas
Cortical areas involved with sensory awareness are located in the parietal, temporal, and occipital lobes. A distinct cortical area is reserved for each distinct major sense.
1) Primary somatosensory cortex - This area in the parietal lobe is involved with conscious awareness of general somatic senses - skin senses and proprioception. Sensory receptors relay information through the spinal cord to the brain and brain stem to the somatosensory cortex. At this point, the body area being stimulated is identified. [Hmm. I once, for only about a month, was unable to tell if the water was hot or not when I turned on the shower. After setting the hot/cold mix, I put out my left hand to test the water temperature; but, although I could see the water bounce off my skin, I couldn't tell whether it was hot or cold, or inbetween, except I could tell it was hot because of the rising steam. Just one of my ephemeral neurological aberrations. Could this have been a temporary disruption in my primary somatosensory cortex?]
2- Somatosensory association area - This area integrates different sensory inputs (e.g., touch and pressure) into a comprehensive evaluation of what is being felt. An example is reaching into your pocket and feeling some objects, and knowing what they are based on past memories - keys, coins, whatever. Damage to this area would make those objects identifiable visually, but not by touch.
3- Visual areas - These areas in the occipital lobe include the primary visual cortex, which receives visual information from the retina of the eye; and the visual association area, which interprets and evaluates visual information based on past experiences. Damage to this area would still allow an object to be seen, but with no understanding of what it is.
4- Auditory areas - The primary auditory cortex, located in the temporal lobe, is the part of the brain that provides conscious awareness of sounds. Sound waves trigger receptors in the inner ear from where impulses are sent to the primary auditory cortex. At this point the information is related to loudness, rhythm, and pitch (high/low notes). There is a "pitch map" in the auditory cortex.
The auditory association area, next to the primary auditory cortex, is responsible for evaluation of sound - a screech, thunder, music, etc. Memories of past sounds are apparently stored here to aid analysis of current sounds.
5- Gustatory (taste) cortex - This area is involved in conscious awareness of taste.
6- Olfactory (smell) cortex - Olfactory nerves from the nasal cavity carry impulses to this site in the brain. The olfactory cortex is located in an area of the brain known as the rhinencephalon. It connects to the limbic system, the area of brain involved in emotions; hence a smell/emotion connection. [The origins of aroma-therapy, perhaps.]
Association Areas
These are areas that integrate divergent information, as previously mentioned. We have already identified some specific association areas, but there are others:
1-Prefrontal cortex - This area in the frontal lobe is involved with reasoning and complex learning abilities (cognition) and personality. This cortex is important for abstract ideas, judgment, persistance, planning, social behavior, concern for others, and conscience. And it seems to be related to the limbic (emotional) part of the forebrain.
2- General interpretation (or gnostic) area - This is a poorly defined region that encompasses parts of the temporal, parietal, and occipital lobes. It occurs in only one hemisphere - usually the left. It receives input from the sensory association areas, and seems to be a storage area for complex patterns related to sensation. This gnostic area integrates sensory information into a comprehensive understanding, then sends the assessment to the prefrontal cortex, which adds an emotional element before deciding on a response.
An interesting idea based on recent evidence is that the gnostic and prefrontal cortex work together to construct "stories" based on our past experiences, and are used to interpret new, similar experiences. A conclusion that has been drawn is that our minds do not really interpret the world objectively, but use these stories.
3- Language comprehension areas - Recent findings suggest that language comprehension, not just speech recognition as was once thought, occurs in the prefrontal cortex (usually the left side).
4- Visceral association area - The cortex of the insula seems to be involved with conscious awareness of visceral sensations like a full bladder, full stomach, etc.).
Lateralization of Cortical Functioning
It has been noted that there is a distinct division of labor between the two hemispheres - right side controls the left side of the body, and visa versa. It has also been mentioned that the hemispheres are mostly specialized by side. In ninety percent of people the left side controls language and math abilities, and logic; while the right side is concerned with visual-spacial skills, intuition, emotion, and artistic/musical skills. In the other 10%, these functions are either reversed or shared. At any rate, the two hemispheres are more-or-less identical, structurally; and share the bulk of functions and memories. This sharing is possible because of the sharing between hemispheres, which is facilitated through "commissures." (Coming soon to your local . . . . .)
Cerebral White Matter
This layer is just beneath the gray matter of the cerebral cortex. The cerebral white matter is comprised of many axons which facilitate communication within the cerebral cortex and between it and the brain stem and spinal cord. Most of these areas are myelinated and bundled into groups called "tracts," of which there are three:
Basal Nuclei
These nuclei are located deep within the cerebral white matter, and function as neural calculators that help the cerebral cortex in controlling movements. Their importance lies in helping to start and stop voluntary movements initiated through the cortex. Additionally, they regulate the intensity of these movements. Parkinson's and Huntington's diseases are examples of the consequences of malfunctions of the basal nuclei.
THE DIENCEPHALON
This second of the four areas of the brain (return to Figure 7-2 if you have forgotten them) is at the central core of the fore-brain, and is surrounded by the cerebral hemispheres. It consists mainly of these structures:
1) The thalamus accounts for about 80% of the diencephalon, and contains roughly a dozen major nuclei, and each sends axons to the cerebral cortex. Some thalmic nuclei relay sensory information to the main sensory areas of the cortex. Every part of the brain that has any communication with the cerebral cortex relays its signals through one of the nuclei of the thalamus. The thalamus also actively processes the data passed through it.
Each nucleus receives input from its own specific source in the CNS and relays it to its own specific area in the cerebral cortex.
2) The hypothalamus is the home of the pituitary gland, which secretes a number of hormones. Functionally, the hypothalamus is the primary visceral control center. Its functions include:
1- Control the autonomic nervous system (ANS), which regulates contraction of smooth and cardiac muscles, and glandular secretions - regulates heart rate and blood pressure, movement of the digestive tube, secretion of sweat and salivary glands, and other ANS activities.
2- Center for emotional response - the hypothalamus is in the center of the limbic system (the emotional brain).
3- Body temperature regulation - the hypothalamus is the body's thermostat. It measures blood temperature, and initiates sweating or shivering, if necessary.
4- Hunger and thirst centers - sense nutrients and salts in the blood, and triggers hunger and/or thirst, as appropriate.
5- Sleep-wake cycles - the hypothalamus is the home of circadian rhythms (light-dark, wake-sleep).
6- Control of the endocrine system - The hypothalamus controls secretion of hormones by the pituitary gland.
3) The epithalamus - in the most dorsal part of the diencephalon, houses the pineal gland (or pineal body), a hormone-secreting organ. This gland secretes melatonin, which seems to signal the body to prepare for the nighttime part of the sleep-wake cycle.
THE BRAIN STEM
The brain stem consists of the : 1) midbrain, 2) pons, and 3) medulla oblongata. This brain area is sometimes called the "primitive brain" because it doesn't calculate or analyze, it just reacts - automatically. The brain stem also connects the cerebrum to the spinal cord, and is a major source of innervation of nerves to the face and head.
The Midbrain
The midbrain lies just below the diencephalon and is involved in pain relief, controls eye movements, is active in reflexive responses to sound, and helps to facilitate limb movements.
The Pons
The pons is a more-or-less round area between the midbrain and the medulla oblongata. It contains a respiratory center that controls breathing movements - inhalation/expiration, and nerve fibers that provide a path of communication between the cerebral motor cortex and the cerebellum to coordinate voluntary movements.
The Medulla Oblongata
Sometimes simply called the "the medulla," this piece of the puzzle is the end (most caudal) part of the brain stem, and merges into the spinal cord. A primary function of the medulla is relaying sensory information to the cerebellum - especially proprioceptive information from the spinal cord. Five special cranial nerves attach to the medulla for the functions of: 1)hearing and equilibrium, 2) tongue and pharynx innervation, 3) innervation of many organs in the thorax and abdomen, 4) innervation of muscles of the neck, and 5) innervation of tongue muscles.
Of primary importance in the medulla are : 1- cardiac centers that affect force and rate of heart-beat; 2- centers for regulation of blood pressure ; 3- center for controlling breathing - shared with the pons; and 4- centers regulating vomiting, hiccuping, swallowing, coughing, and sneezing.
THE CEREBELLUM
It accounts for 11% of the brain, and its functions are to smooth and coordinate body movements, and help to maintain posture and equilibrium.
The cerebellum receives a preview of intended movements being directed by the motor cerebral cortex, and compares them to movements actually taking place, and sends instructions to the cerebral cortex on how to make any necessary corrections between intended and actual movements. This process continuously makes minor adjustments as needed.
FUNCTIONAL BRAIN SYSTEMS
These are networks of neurons that function together, but that are not located together in the brain. Two of special importance are the limbic system and the reticular formation.
Limbic System
The structures of this system are scattered in the cerebral hemisphere and diencephalon. Damage (lesions) to one leads to personality changes, such as docile behavior, restlessness, emotional instability, and increased interest in fighting or eating. Damage to the second one destroys the will.
The limbic system communicates with various brain regions; that is, it can relay its output to areas that control visceral responses, like high blood pressure. The limbic system also communicates with the cerebral cortex, thus linking the "emotional brain" to the "thinking brain," which explains our emotional reactions to physical events.
The Reticular Formation
This is a formation of neurons running through the entire brain stem, which maintains consciousness and alertness.
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This brief trip through the brain is missing many details due to the complexity of the subject and the many remaining mysteries; but mostly because of my vast ignorance of the subject.
One area gave me fits: the "insula." It was sometimes identified as a fifth lobe in the cerebral cortex, albeit buried deep under it. Other references identified only four lobes, treating the insula separately, unlike the other four; but all references commonly ignored its function(s), until a neuroanatomy book and a friend who read or heard a piece on the insular brain, suggested that it is involved in Autonomic Nervous System "fight or flight" activities, including cardiovascular ramifications.
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QUESTIONS AND ANSWERS
(Q) I am experiencing a snap, crackle and pop
from several joints. Is that a problem?
J.F., New York, NY
(A) The condition you are describing is called "crepitus," and is generally not a problem. It is a common occurrence in shoulders, knees, ankles, and feet; especially as we age. It can be caused by irregular joint surfaces rubbing together, pressure changes around a joint (gas bubbles popping in synovial joints), or two ligaments coming into contact. In the absence of pain, it is generally a benign condition - note that this is an important distinction.
(Q) Is "Ensure" really the fountain-of-youth
that its manufacturers claim?
D.G., Salem, OR
(A) Of course not. According to the Nutrition Action Health Letter (May 1998): it is a largely useless baby formula for seniors. Its a mixture of sugar, oil, water, and protein with added vitamins and minerals. Now a candy-bar version is being promoted. Both products lack fiber and phytochemicals, unlike fruits and vegetables; and are thus not "complete" as advertised. "Complete Junk" in the words of the NAHL.
(Q) Have you heard about any new exercises? My
routine is getting a little stale.
J.J., Canberra, Australia
(A) Mainly variations on an old theme, and some combination exercises for people in a hurry. First is an upper chest shoulder oldie, but with a change - a seated flye, but done on in incline, and starting with arms extended and elbows slightly bent (soft), with hands holding dumbbells in a supinated position; move your hands together in an arching motion, then return to shoulder height level. Repeat until your set is completed.
Next comes a couple of modest twists to kickbacks. As you extend your elbows, either supinate (rotate wrist to palms-up position), or pronate your wrists (palms down).
For "skull crushers" (lying triceps extensions), use a reversed grip (i.e., palms are on top of the bar). That grip makes you keep your elbows in tight.
For those in a hurry, combine biceps curls with shoulder presses. Grasp a barbell with hands about shoulder width and with a reverse grip. Now do the first half of a reverse curl, and press the bar overhead. Now lower the bar to upper chest height, and then complete the curl.
Similarly, you can combine a reverse curl with an upright row. Many exercises can be put together to get through a workout quickly. You can get through a pretty complete workout by combining exercises - super-sets, tri-sets, quad-sets, giant-sets - there are limitless possibilities
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Fenner
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