The human nervous system, Part III, begins with the definition of a nerve, followed by a functional description of a nerve.
A nerve is a cord-like organ in the PNS. Each nerve contains several axons (nerve fibers) organized essentially in bundles of bundles (see Figure 6-1). Each bundle is wrapped in connective tissue. Each axon in a nerve is surrounded by Schwann cells, and a "sack" of connective tissue - endoneurium. Groups of axons are bundled into nerve fascicles - perineurium. And finally, the whole nerve is surrounded by the epineurium.
Now that we have defined a neuron (a nerve cell), a nerve fiber (a long axon), and a nerve (a collection of nerve fibers in the PNS); we will move on to the function of nerves (through impulse conduction).
An inactive neuron is polarized, which means that its outer surface is positively charged relative to the inside of its cover (picture this as the inside and outside surfaces of the wall of a hose). In addition, a higher concentration of sodium (NA+) is found externally than internally. The opposite is true for potassium (K+) ions. When an axon is stimulated (actually dendrites attached to the neuron), the plasma membrane (the "sack") becomes more permeable, and positive sodium ions (NA+) move inside. Consequently the inside becomes less negative - called depolarization. A strong depolarizing stimulus triggers a nerve impulse (or Action Potential), and polarity is completely reversed (positive inside, and negative outside); and now the nerve impulse travels quickly down the entire length of the axon. Then the membrane repolarizes itself. Synapses are a sort of filter for the thousands of nerve impulses that are generated - some are sent on through synapses, while others are stopped.
So, how are specific behaviors manifested from impulses in neurons and neuron-chains? At their simplest, these behaviors are called reflexes, which are rapid, automatic motor responses to stimuli. The word "automatic" should be stressed. There are no thought processes involved. This is a knee-jerk behavior - like jerking back from something hot, or vomiting after ingesting something that your stomach finds particularly irritating. The reflexes are either somatic (evoking skeletal muscle contraction), or visceral (activating smooth or cardiac muscle, or glands). Reflexes are manifested through a chain of neurons called reflex arcs. Each reflex arc has five basic components which activate each other inline.
Three-neuron reflex arcs make up the basic structure of the nervous system. Sensory neurons enter the spinal cord dorsally (see Note 1); while motor axons exit ventrally (see Note 2); and the intervening interneurons are isolated to the CNS. Nerves in the PNS consist of peripheral axons of the sensory and motor neurons.
The CNS contains separate areas of gray [the little gray cells that Poirot is always talking about] and white cells (otherwise known as "matter"). Gray matter surrounds the hollow central cavity (see Note 3). Gray matter is where cell bodies are concentrated (cell bodies, dendrites, and short myelinated axons). External to the gray matter is white matter, which contains no neuronal cell bodies, but literally millions of axons. The white color is from the myelinated axons. Most of these axons run from the spinal cord to the brain, or from the brain back to the spinal cord; thus providing for communication between the two.
Next month's issue of the Newsletter will continue with the Nervous System, Part IV.
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Note 1 - Dorsal is posterior, back, or upper.
Note 2 - Ventral is anterior, or lower.
Note 3 - Hollow central cavity refers to the center of both the brain and spinal cord - it is hollow [more hollow in some than others, methinks].
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Does stress have an effect on the immune system? It seems so. Even before Hippocrates (500 B.C.), it was suspected that "the passions" had an effect on health. More recently it has been discovered that there is a molecular connection between the nervous and immune systems. In addition, it has been discovered in animal studies that impairing this link raises the risk of inflammatory diseases, arthritis, and thyroid problems.
Human studies have largely looked at heart disease and blood pressure, and a stress connection. Data is accumulating that appears to show a link between future heart disease and emotions - constant anger, depression, anxiety, loneliness. It may be that healthy people who experience these characteristics, or frequent explosions of anger, may be setting themselves up for potentially fatal results.
Some heart attacks seem to be triggered by the rapid clumping of blood platelets (the solid component of blood) as part of the "fight or flight" response to fear or anger. As these emotions soar higher, the clumping increases, with potentially disastrous consequences.
In addition to platelet aggregation, immune cells activate, blood sugar rushes to muscles to provide energy for action, heart and breathing rates increase, and blood pressure rises. These fight or flight reactions are induced by specific hormones; which, unfortunately, are not always shut off when the crisis is over. Chronic stress may be the cause. Now the immune system becomes vulnerable to infections and disease.
Those who study the stress/health connection believe that there are also diseases, other than of the heart, associated with the fight or flight reaction.
But, guess what? Many researchers consider regular EXERCISE to be the best stress reducer. Big surprise.
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More on the benefits of walking - evidence continues to accumulate that regular, daily walkers (and others who do some kind of consistent, moderate exercise live longer than those who don't (those who can be classified as homo sedentarius). The January 8, 1998 issue of the New England Journal of Medicine reported a study from the University of Virginia that revealed that older men who walked more than two miles daily, had half the risk of dying from cancer and other "stuff," than those who walked less than a mile a day.
A February 11, 1998 report from the Journal of American Medicine (JAMA) on a study at the University of Helsinki in Finland, of 16,000 twins over 19 years; found that regular exercise helped extend the lives of the participants, irrespective of genetic background. By studying twins (identical and fraternal), the impact of exercise even on those with similar genetic backgrounds was significant. For example, among twins, with one active and one sedentary; the active twin had a 56% lower risk of dying prematurely. Vigorous walking as little as six times a month, at least 30 minutes a session, seems to protect against many illnesses. Of course, there is evidence that more is better. The Centers For Disease Control and Prevention, and the American College of Sports Medicine recommend that we get 30 minutes of moderate physical activity at least five times a week to hold chronic illness at bay and to extend longevity.
There is evidence to support the idea that people who engage in more vigorous activity (e.g., jogging or swimming laps) several times a week live longest, but walking is not far behind.
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Calcium supplements are necessary for a number of people. Everyone needs calcium, but how much, and from what sources?
The best source seems to be milk and other dairy products, although there are other sources, including plant sources. But many adults don't drink milk, and just don't get enough calcium. In fact, it is thought that most adults are deficient in calcium. So, do we need to take a calcium supplement? Frequently the answer is yes.
Research has shown that calcium supplements can decrease risk of bone loss and resultant fractures, and that insufficient calcium increases risk of heart disease. Diets high in calcium seems to lower risk of heart disease, as well as that of high blood pressure and colon cancer.
It is always recommended that we get our nutritional needs (including calcium) from food, but realistically, that isn't always going to happen. For example, it is estimated that women over 50 only get half of the recommended amount of calcium; in which case supplements are recommended to make up the deficit.
So, how much calcium should we get? According to the National Institutes of Health, the recommended amount is age and gender related.
Women |
Mg |
| Up to age 24 | 1500 |
| 25-49 | 1000 |
| 50+ | 1500 |
| Pregnant/nursing | 1200-1500 |
Men |
Mg |
| Up to age 24 | 1500 |
| 25-64 | 1000 |
| 65+ | 1500 |
Selecting a calcium supplement can be a little daunting. There are over 50 commercial calcium supplements on the market.
"How do I choose," you may ask. Good question, and I will try to help. The many supplements get their calcium from a few common sources:
Some recommend calcium carbonate because it contains more calcium than other sources, so you don't have to take so many pills or tablets. Unfortunately, it isn't always easy to tell from the label what the source is. Look carefully, it may be buried in the small print.
If you take a calcium supplement with a meal (the recommended way), more calcium is absorbed, so it should contain the most calcium for the least money - calcium carbonate, as it turns out. If you take the supplement outside of a meal (not normally recommended), take the one that is absorbed better - calcium citrate.
To determine the amount of supplementary calcium that you need, there are two methods. The easy, general way is based on daily recommended amount. If your daily requirement is 1000 mg., supplement with 500 mg. If your daily requirement is 1500 mg., supplement with 1000 mg.
A more accurate method is to make a list of "heavy hitters," that is top sources of calcium, and determine how many servings of each you eat each day:
For each serving count 300 mg. of calcium.
It is recommended that you take no more than 500 mg. at a time, so space your feedings if you are supplementing with more than 500 mg. For example, Tums Extra Strength (if that should happen to be your choice) comes in tablets of 300 mg. each, so you should space them out.
Some calcium supplements contain some "extras," like vitamin D, magnesium, zinc, copper and/or manganese; but a simple multi-vitamin/multimineral supplement should suffice, as long as it contains 400 IUs vitamin D, 100 mg. of magnesium, and 15% of recommended daily intake of zinc; all for bone health.
As you have heard before in earlier Newsletters, vitamin D is critical for calcium absorption. Without it the absorption rate drops off to about 10-15%. Good sources are milk (it's fortified with D) and sunlight (the body synthesizes vitamin D from ultra-violet rays from the sun). Only five to 15 minutes of strong sunshine on your hands, arms, and face (sans sunscreen or sunblock), two or three times a week will suffice.
Food is generally a poor source of D, although fatty fish (like salmon or mackerel) contains some; but without milk or exposure to sunlight, a Multi can make a difference.
A couple of final points: look for a USP stamp on the supplement container - it gives some assurance of absorbability of a product; and when reading labels, look for "elemental" calcium, or just "calcium" to show usable calcium.
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Soil depletion - is it serious? Yes, I think you could say that. Is U.S. soil depleted of nutrients to such a degree that we are malnourished as a result? NO.
Soil depletion is an ongoing process, and has been for centuries. Farmers have had to use fertilizers, along with techniques like crop rotation to grow crops. Plants won't grow in seriously depleted soil, which of course necessitates the use of creative farming methods to continue growing crops. If soil is mineral-poor, they are added back - like phosphorus, potassium, iodine, calcium, copper, iron, selenium, flouride, molybdenum, and zinc - so that crops can thrive. At any given time, some areas are worse than others, so eat a balanced and varied diet and you shouldn't worry about being malnourished because of soil depletion.
It seems that many of those behind the theory that depleted soil is causing poor nutrition are promoting sales of minerals.
This information comes from the May 1998 issue of the University of California, Berkeley Wellness Letter.
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How to assess your risk of a heart attack, by the American Heart Association: just score points for the questions below, and total your points.
Age: Men
Age: Women
Family History:
Inactive Lifestyle:
Weight:
Smoking:
Diabetic:
Total Cholesterol Level:
HDL Level:
Blood Pressure:
Total Points:
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Why do men with pierced ears make good husbands?
Because they are used to pain and buying jewelry.
-Rita Rudner
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Hi Yuri and Natasha.
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QUESTIONS AND ANSWERS
(Q) What are the best sources of the antioxidants:
vitamins C and E, and beta-carotene?
A.J.F., Palo Alto, CA
(A) Cruciferous vegetables, like broccoli and spinach. Other sources are: sweet potatoes, carrots, and tomatoes.
(Q)Is eating chicken soup to help fight a cold
just an old wives' tale?
D.B.F. Eugene, OR
(A) Yes, but one that seems to have some merit. It is an effective decongestant. It has been reported in The New England Journal of Medicine that chicken soup contains natural bronchodilators and other "things" that reduce movement of inflammatory white blood cells to bronchial areas. Mom and Grams were right.
(Q) I have been eating a healthy amount of fiber
lately, somewhere in the neighborhood of 20-35 grams a day, but I am experiencing
frequent constipation - not exactly what I expected!
Anonymous, Phoenix, AZ
(A) What you are experiencing is not uncommon. You must ingest plenty of fluids in order for fiber to do its expected job - at least eight glasses of fluid per day; unless you have severe heart or kidney problems, in which case you should consult your doctor about this.
(Q) A friend told me that "yucca" might
help relieve my arthritis. What is it, and can it really help?
H.M.F., Roseburg, OR
(A) Conventional medicine is of little help - basically just a store of pain killers for either osteoarthritis or rheumatoid arthritis. But alternative medicine offers a different choice, which is "yucca." Does it work? It seems to for some. American indians used the juice from the yucca plant to relieve suffering from arthritis over 1000 years ago. It has been discovered that saponin (medspeak for yucca juice) protects friendly intestinal bacteria that compete with harmful micro-organisms and stop them from causing allergic reactions related to arthritis. For anyone who would like to try it, it should be found at your local health food store.
Arthritis relief may also come from an increase in folic acid, protein, zinc, vitamins C and D, and pantothenic acid in your diet. Likewise, a decrease in fat consumption may also help.
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Send questions or comments to Mike
Fenner.
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