Nutritional Profile Energy value (calories per serving): Moderate Protein: Low (cocoa powder) High (chocolate) Fat: Moderate Saturated fat: High Cholesterol: None Carbohydrates: Low (chocolate) High (cocoa powder) Fiber: Moderate (chocolate) High (cocoa powder) Sodium: Moderate Major vitamin contribution: B vitamins Major mineral contribution: Calcium, iron, copper
About the Nutrients in This Food Cocoa beans are high-carbohydrate, high-protein food, with less dietary fiber and more fat than all other beans, excepting soy beans. The cocoa bean’s dietary fiber includes pectins and gums. Its proteins are limited in the essential amino acids lysine and isoleucine. Cocoa butter, the fat in cocoa beans, is the second most highly saturated vegetable fat (coconut oil is number one), but it has two redeeming nutritional qualities. First, it rarely turns rancid. Second, it melts at 95°F, the temperature of the human tongue. Cocoa butter has no cholesterol; neither does plain cocoa powder or plain dark chocolate. Cocoa beans have B vitamins (thiamine, riboflavin, niacin) plus min- erals (iron, magnesium, potassium, phosphorus, and copper). All chocolate candy is made from chocolate liquor, a thick paste pro- duce by roasting and grinding cocoa beans. Dark (sweet) chocolate is made of chocolate liquor, cocoa butter, and sugar. Milk chocolate is made of choc- olate liquor, cocoa butter, sugar, milk or milk powder, and vanilla. White * These values apply to plain cocoa powder and plain unsweetened chocolate. Add- ing other foods, such as milk or sugar, changes these values. For example, there is no cholesterol in plain bitter chocolate, but there is cholesterol in milk chocolate. chocolate is made of cocoa butter, sugar, and milk powder. Baking chocolate is unsweetened dark chocolate. The most prominent nutrient in chocolate is its fat. Fat Content in One Ounce of Chocolate
| Saturated fat (g) | Monounsaturated fat (g) | Polyunsaturated fat (g) | Cholesterol (mg) | |
| Dark (sweet) | ||||
| chocolate | 5.6 | 3.2 | 0.3 | 0 |
| Milk chocolate | 5.9 | 4.5 | 0.4 | 6.6 |
| Baking chocolate | 9 | 5.6 | 0.3 | 0 |
| White chocolate | 5.5 | 2.6 | 0.3 | 0 |
The Most Nutritious Way to Serve This Food With low-fat milk to complete the proteins without adding saturated fat and cholesterol. NOTE : Both cocoa and chocolate contain oxalic acid, which binds with calcium to form cal- cium oxalate, an insoluble compound, but milk has so much calcium that the small amount bound to cocoa and chocolate hardly matters. Chocolate skim milk is a source of calcium.
Diets That May Restrict or Exclude This Food Antiflatulence diet Low-calcium and low-oxalate diet (to prevent the formation of calcium oxalate kidney stones) Low-calorie diet Low-carbohydrate diet Low-fat diet Low-fat, controlled-cholesterol diet (milk chocolates) Low-fiber diet Potassium-regulated (low-potassium) diet
Buying This Food Look for: Tightly sealed boxes or bars. When you open a box of chocolates or unwrap a candy bar, the chocolate should be glossy and shiny. Chocolate that looks dull may be stale, or it may be inexpensively made candy without enough cocoa butter to make it gleam and give it the rich creamy mouthfeel we associate with the best chocolate. (Fine chocolate melts evenly on the tongue.) Chocolate should also smell fresh, not dry and powdery, and when you break a bar or piece of chocolate it should break cleanly, not crumble. One exception: If you have stored a bar of chocolate in the refrigerator, it may splinter if you break it without bringing it to room temperature first.
Storing This Food Store chocolate at a constant temperature, preferably below 78°F. At higher temperatures, the fat in the chocolate will rise to the surface and, when the chocolate is cooled, the fat will solidif y into a whitish powdery bloom. Bloom is unsightly but doesn’t change the chocolate’s taste or nutritional value. To get rid of bloom, melt the chocolate. The chocolate will turn dark, rich brown again when its fat recombines with the other ingredients. Chocolate with bloom makes a perfectly satisfactory chocolate sauce. Dark chocolate (bitter chocolate, semisweet chocolate) ages for at least six months after it is made, as its flavor becomes deeper and more intense. Wrapped tightly and stored in a cool, dry cabinet, it can stay fresh for a year or more. Milk chocolate ages only for about a month after it is made and holds its peak flavor for about three to six months, depending on how carefully it is stored. Plain cocoa, with no added milk powder or sugar, will stay fresh for up to a year if you keep it tightly sealed and cool.
What Happens When You Cook This Food Chocolate burns easily. To melt it without mishap, stir the chocolate in a bowl over a pot of hot water or in the top of a double boiler or put the chocolate in a covered dish and melt it in the microwave (which does not get as hot as a pot on the store). Simple chemistry dictates that chocolate cakes be leavened with baking soda rather than baking powder. Chocolate is so acidic that it will upset the delicate balance of acid (cream of tartar) and base (alkali = sodium bicarbonate = baking soda) in baking powder. But it is not acidic enough to balance plain sodium bicarbonate. That’s why we add an acidic sour-milk product such as buttermilk or sour cream or yogurt to a chocolate cake. Without the sour milk, the batter would be so basic that the chocolate would look red, not brown, and taste very bitter.
How Other Kinds of Processing Affect This Food Freezing. Chocolate freezes and thaws well. Pack it in a moistureproof container and defrost it in the same package to let it reabsorb moisture it gave off while frozen.
Medical Uses and/or Benefits Mood elevator. Chocolate’s reputation for making people feel good is based not only on its caffeine content—19 mg caffeine per ounce of dark (sweet) chocolate, which is one-third the amount of caffeine in a five-ounce cup of brewed coffee—but also on its naturally occurring mood altering chemicals phenylethylalanine and anandamide. Phenylethylalanine is found in the blood of people in love. Anandamide stimulates areas of your brain also affected by the active ingredients in marijuana. (NOTE : As noted by the researchers at the Neurosci- ences Institute in San Diego who identified anandamide in chocolate in 1996, to get even the faintest hint of marijuana-like effects from chocolate you would have to eat more than 25 pounds of the candy all at once.) Possible heart health benefits. Chocolate is rich in catechins, the antioxidant chemicals that give tea its reputation as a heart-protective anticancer beverage (see tea). In addition, a series of studies beginning with those at the USDA Agricultural Research Center in Peoria, Illinois, suggest that consuming foods rich in stearic acid like chocolate may reduce rather than raise the risk of a blood clot leading to a heart attack. Possible slowing of the aging process. Chocolate is a relatively good source of copper, a mineral that may play a role in slowing the aging process by decreasing the incidence of “protein glycation,” a reaction in which sugar molecules ( gly = sugar) hook up with protein molecules in the bloodstream, twisting the protein molecules out of shape and rendering them unusable. This can lead to bone loss, rising cholesterol, cardiac abnormalities, and a slew of other unpleasantries. In people with diabetes, excess protein glycation may be one factor involved in complications such as loss of vision. Ordinarily, increased protein glyca- tion is age-related. But at the USDA Grand Forks Human Nutrition Research Center in North Dakota, agricultural research scientist Jack T. Saari has found that rats on copper-deficient diets experience more protein glycation at any age than other rats. A recent USDA survey of American eating patterns says that most of us get about 1.2 mg copper a day, considerably less than the Estimated Safe and Adequate Daily Dietary Intake (ESADDI) or 1.5 mg to 3 mg a day. Vegetarians are less likely to be copper deficient because, as Saari notes, the foods highest in copper are whole grains, nuts, seeds, and beans, including the cocoa bean. One ounce of dark chocolate has .25 mg copper (8 –17 percent of the ESADDI).
Adverse Effects Associated with This Food Possible loss of bone density. In 2008, a team of Australian researchers at Royal Perth Hos- pital, and Sir Charles Gairdner Hospital published a report in the American Journal of Clinical Nutrition suggesting that women who consume chocolate daily had 3.1 percent lower bone density than women who consume chocolate no more than once a week. No explanation for the reaction was proposed; the finding remains to be confirmed. Possible increase in the risk of heart disease. Cocoa beans, cocoa powder, and plain dark chocolate are high in saturated fats. Milk chocolate is high in saturated fats and cholesterol. Eating foods high in saturated fats and cholesterol increases the amount of cholesterol in your blood and raises your risk of heart disease. NOTE : Plain cocoa powder and plain dark chocolate may be exceptions to this rule. In studies at the USDA Agricultural Research Center in Peoria, Illinois, volunteers who consumed foods high in stearic acid, the saturated fat in cocoa beans, cocoa powder, and chocolate, had a lower risk of blood clots. In addition, chocolate is high in flavonoids, the antioxidant chemicals that give red wine its heart-healthy reputation. Mild jitters. There is less caffeine in chocolate than in an equal size serving of coffee: A five- ounce cup of drip-brewed coffee has 110 to 150 mg caffeine; a five-ounce cup of cocoa made with a tablespoon of plain cocoa powder ( 1/3 oz.) has about 18 mg caffeine. Nonetheless, people who are very sensitive to caffeine may find even these small amounts problematic. Allergic reaction. According to the Merck Manual, chocolate is one of the 12 foods most likely to trigger the classic food allergy symptoms: hives, swelling of the lips and eyes, and upset stomach.* The others are berries (blackberries, blueberries, raspberries, strawberries), corn, eggs, fish, legumes (green peas, lima beans, peanuts, soybeans), milk, nuts, peaches, pork, shellfish, and wheat (see wheat cer ea ls).
Food/Drug Interactions Monoamine oxidase (MAO) inhibitors. Monoamine oxidase inhibitors are drugs used to treat depression. They inactivate naturally occurring enzymes in your body that metabolize tyra- mine, a substance found in many fermented or aged foods. Tyramine constricts blood vessels and increases blood pressure. Caffeine is a substance similar to tyramine. If you consume excessive amounts of a caffeinated food, such as cocoa or chocolate, while you are taking an M AO inhibitor, the result may be a hypertensive crisis. False-positive test for pheochromocytoma. Pheochromocytoma, a tumor of the adrenal gland, secretes adrenalin, which the body converts to VM A (vanillylmandelic acid). VM A is excreted in urine, and, until recently, the test for this tumor measured the level of VM A in the urine. In the past, chocolate and cocoa, both of which contain VM A, were eliminated from the patient’s diet prior to the test lest they elevate the level of VM A in the urine and produce a false-positive result. Today, more finely drawn tests usually make this unnecessary. * The evidence link ing chocolate to allergic or migraine headaches is inconsistent. In some people, phenylet hylamine (PEA) seems to cause headaches similar to t hose induced by t yramine, anot her pressor amine. The PEA-induced headache is unusual in t hat it is a delayed react ion t hat usually occurs 12 or more hours after t he chocolate is eaten.... chocolate
Nutritional Profile Energy value (calories per serving): Low Protein: Trace Fat: Trace Saturated fat: None Cholesterol: None Carbohydrates: Trace Fiber: Trace Sodium: Low Major vitamin contribution: None Major mineral contribution: None
About the Nutrients in This Food Coffee beans are roasted seeds from the fruit of the evergreen coffee tree. Like other nuts and seeds, they are high in proteins (11 percent), sucrose and other sugars (8 percent), oils (10 to 15 percent), assorted organic acids (6 percent), B vitamins, iron, and the central nervous system stimulant caffeine (1 to 2 percent). With the exceptions of caffeine, none of these nutrients is found in coffee. Like spinach, rhubarb, and tea, coffee contains oxalic acid (which binds calcium ions into insoluble compounds your body cannot absorb), but this is of no nutritional consequence as long as your diet contains adequate amounts of calcium-rich foods. Coffee’s best known constituent is the methylxanthine central ner- vous system stimulant caffeine. How much caffeine you get in a cup of coffee depends on how the coffee was processed and brewed. Caffeine is Caffeine Content/Coffee Servings Brewed coffee 60 mg/five-ounce cup Brewed/decaffeinated 5 mg/five-ounce cup Espresso 64 mg/one-ounce serving Instant 47 mg/rounded teaspoon
The Most Nutritious Way to Serve This Food In moderation, with high-calcium foods. Like spinach, rhubarb, and tea, coffee has oxalic acid, which binds calcium into insoluble compounds. This will have no important effect as long as you keep your consumption moderate (two to four cups of coffee a day) and your calcium consumption high.
Diets That May Restrict or Exclude This Food Bland diet Gout diet Diet for people with heart disease (regular coffee)
Buying This Food Look for: Ground coffee and coffee beans in tightly sealed, air- and moisture-proof containers. Avoid: Bulk coffees or coffee beans stored in open bins. When coffee is exposed to air, the volatile molecules that give it its distinctive flavor and richness escape, leaving the coffee flavorless and/or bitter.
Storing This Food Store unopened vacuum-packed cans of ground coffee or coffee beans in a cool, dark cabinet—where they will stay fresh for six months to a year. They will lose some flavor in storage, though, because it is impossible to can coffee without trapping some flavor- destroying air inside the can. Once the can or paper sack has been opened, the coffee or beans should be sealed as tight as possible and stored in the refrigerator. Tightly wrapped, refrigerated ground coffee will hold its freshness and flavor for about a week, whole beans for about three weeks. For longer storage, freeze the coffee or beans in an air- and moistureproof container. ( You can brew coffee directly from frozen ground coffee and you can grind frozen beans without thawing them.)
Preparing This Food If you make your coffee with tap water, let the water run for a while to add oxygen. Soft water makes “cleaner”-tasting coffee than mineral-rich hard water. Coffee made with chlorinated water will taste better if you refrigerate the water overnight in a glass (not plastic) bottle so that the chlorine evaporates. Never make coffee with hot tap water or water that has been boiled. Both lack oxygen, which means that your coffee will taste flat. Always brew coffee in a scrupulously clean pot. Each time you make coffee, oils are left on the inside of the pot. If you don’t scrub them off, they will turn rancid and the next pot of coffee you brew will taste bitter. To clean a coffee pot, wash it with detergent, rinse it with water in which you have dissolved a few teaspoons of baking soda, then rinse one more time with boiling water.
What Happens When You Cook This Food In making coffee, your aim is to extract flavorful solids (including coffee oils and sucrose and other sugars) from the ground beans without pulling bitter, astringent tannins along with them. How long you brew the coffee determines how much solid material you extract and how the coffee tastes. The longer the brewing time, the greater the amount of solids extracted. If you brew the coffee long enough to extract more than 30 percent of its solids, you will get bitter compounds along with the flavorful ones. (These will also develop by let- ting coffee sit for a long time after brewing it.) Ordinarily, drip coffee tastes less bitter than percolator coffee because the water in a drip coffeemaker goes through the coffee only once, while the water in the percolator pot is circulated through the coffee several times. To make strong but not bitter coffee, increase the amount of coffee—not the brewing time.
How Other Kinds of Processing Affect This Food Drying. Soluble coffees (freeze-dried, instant) are made by dehydrating concentrated brewed coffee. These coffees are often lower in caffeine than regular ground coffees because caffeine, which dissolves in water, is lost when the coffee is dehydrated. Decaffeinating. Decaffeinated coffee is made with beans from which the caffeine has been extracted, either with an organic solvent (methylene chloride) or with water. How the coffee is decaffeinated has no effect on its taste, but many people prefer water-processed decaf- feinated coffee because it is not a chemically treated food. (Methylene chloride is an animal carcinogen, but the amounts that remain in coffees decaffeinated with methylene chloride are so small that the FDA does not consider them hazardous. The carcinogenic organic sol- vent trichloroethylene [TCE], a chemical that causes liver cancer in laboratory animals, is no longer used to decaffeinate coffee.)
Medical Uses and/or Benefits As a stimulant and mood elevator. Caffeine is a stimulant. It increases alertness and concentra- tion, intensifies muscle responses, quickens heartbeat, and elevates mood. Its effects derive from the fact that its molecular structure is similar to that of adenosine, a natural chemical by-product of normal cell activity. Adenosine is a regular chemical that keeps nerve cell activ- ity within safe limits. When caffeine molecules hook up to sites in the brain when adenosine molecules normally dock, nerve cells continue to fire indiscriminately, producing the jangly feeling sometimes associated with drinking coffee, tea, and other caffeine products. As a rule, it takes five to six hours to metabolize and excrete caffeine from the body. During that time, its effects may vary widely from person to person. Some find its stimu- lation pleasant, even relaxing; others experience restlessness, nervousness, hyperactivity, insomnia, flushing, and upset stomach after as little as one cup a day. It is possible to develop a tolerance for caffeine, so people who drink coffee every day are likely to find it less imme- diately stimulating than those who drink it only once in a while. Changes in blood vessels. Caffeine’s effects on blood vessels depend on site: It dilates coronary and gastrointestinal vessels but constricts blood vessels in your head and may relieve headache, such as migraine, which symptoms include swollen cranial blood vessels. It may also increase pain-free exercise time in patients with angina. However, because it speeds up heartbeat, doc- tors often advise patients with heart disease to avoid caffeinated beverages entirely. As a diuretic. Caffeine is a mild diuretic sometimes included in over-the-counter remedies for premenstrual tension or menstrual discomfort.
Adverse Effects Associated with This Food Stimulation of acid secretion in the stomach. Both regular and decaffeinated coffees increase the secretion of stomach acid, which suggests that the culprit is the oil in coffee, not its caffeine. Elevated blood levels of cholesterol and homocysteine. In the mid-1990s, several studies in the Netherlands and Norway suggested that drinking even moderate amounts of coffee (five cups a day or less) might raise blood levels of cholesterol and homocysteine (by-product of protein metabolism considered an independent risk factor for heart disease), thus increas- ing the risk of cardiovascular disease. Follow-up studies, however, showed the risk limited to drinking unfiltered coffees such as coffee made in a coffee press, or boiled coffees such as Greek, Turkish, or espresso coffee. The unfiltered coffees contain problematic amounts of cafestol and kahweol, two members of a chemical family called diterpenes, which are believed to affect cholesterol and homocysteine levels. Diterpenes are removed by filtering coffee, as in a drip-brew pot. Possible increased risk of miscarriage. Two studies released in 2008 arrived at different conclusions regarding a link between coffee consumption and an increased risk of miscar- riage. The first, at Kaiser Permanente (California), found a higher risk of miscarriage among women consuming even two eight-ounce cups of coffee a day. The second, at Mt. Sinai School of Medicine (New York), found no such link. However, although the authors of the Kaiser Permanente study described it as a “prospective study” (a study in which the research- ers report results that occur after the study begins), in fact nearly two-thirds of the women who suffered a miscarriage miscarried before the study began, thus confusing the results. Increased risk of heartburn /acid reflux. The natural oils in both regular and decaffeinated coffees loosen the lower esophageal sphincter (LES), a muscular valve between the esopha- gus and the stomach. When food is swallowed, the valve opens to let food into the stomach, then closes tightly to keep acidic stomach contents from refluxing (flowing backwards) into the esophagus. If the LES does not close efficiently, the stomach contents reflux and cause heartburn, a burning sensation. Repeated reflux is a risk factor for esophageal cancer. Masking of sleep disorders. Sleep deprivation is a serious problem associated not only with automobile accidents but also with health conditions such as depression and high blood pres- sure. People who rely on the caffeine in a morning cup of coffee to compensate for lack of sleep may put themselves at risk for these disorders. Withdrawal symptoms. Caffeine is a drug for which you develop a tolerance; the more often you use it, the more likely you are to require a larger dose to produce the same effects and the more likely you are to experience withdrawal symptoms (headache, irritation) if you stop using it. The symptoms of coffee-withdrawal can be relieved immediately by drinking a cup of coffee.
Food/Drug Interactions Drugs that make it harder to metabolize caffeine. Some medical drugs slow the body’s metabolism of caffeine, thus increasing its stimulating effect. The list of such drugs includes cimetidine (Tagamet), disulfiram (Antabuse), estrogens, fluoroquinolone antibiotics (e.g., ciprofloxacin, enoxacin, norfloxacin), fluconazole (Diflucan), fluvoxamine (Luvox), mexi- letine (Mexitil), riluzole (R ilutek), terbinafine (Lamisil), and verapamil (Calan). If you are taking one of these medicines, check with your doctor regarding your consumption of caf- feinated beverages. Drugs whose adverse effects increase due to consumption of large amounts of caffeine. This list includes such drugs as metaproterenol (Alupent), clozapine (Clozaril), ephedrine, epinephrine, monoamine oxidase inhibitors, phenylpropanolamine, and theophylline. In addition, suddenly decreasing your caffeine intake may increase blood levels of lithium, a drug used to control mood swings. If you are taking one of these medicines, check with your doctor regarding your consumption of caffeinated beverages. Allopurinol. Coffee and other beverages containing methylxanthine stimulants (caffeine, theophylline, and theobromine) reduce the effectiveness of the antigout drug allopurinol, which is designed to inhibit xanthines. Analgesics. Caffeine strengthens over-the-counter painkillers (acetaminophen, aspirin, and other nonsteroidal anti-inflammatories [NSAIDS] such as ibuprofen and naproxen). But it also makes it more likely that NSAIDS will irritate your stomach lining. Antibiotics. Coffee increases stomach acidity, which reduces the rate at which ampicillin, erythromycin, griseofulvin, penicillin, and tetracyclines are absorbed when they are taken by mouth. (There is no effect when the drugs are administered by injection.) Antiulcer medication. Coffee increases stomach acidity and reduces the effectiveness of nor- mal doses of cimetidine and other antiulcer medication. False-positive test for pheochromocytoma. Pheochromocytoma, a tumor of the adrenal glands, secretes adrenalin, which is converted to VM A (vanillylmandelic acid) by the body and excreted in the urine. Until recently, the test for this tumor measured the levels of VM A in the patient’s urine and coffee, which contains VM A, was eliminated from patients’ diets lest it elevate the level of VM A in the urine, producing a false-positive test result. Today, more finely drawn tests make this unnecessary. Iron supplements. Caffeine binds with iron to form insoluble compounds your body cannot absorb. Ideally, iron supplements and coffee should be taken at least two hours apart. Birth control pills. Using oral contraceptives appears to double the time it takes to eliminate caffeine from the body. Instead of five to six hours, the stimulation of one cup of coffee may last as long as 12 hours. Monoamine oxidase (MAO) inhibitors. Monoamine oxidase inhibitors are drugs used to treat depression. They inactivate naturally occurring enzymes in your body that metabolize tyra- mine, a substance found in many fermented or aged foods. Tyramine constricts blood vessels and increases blood pressure. Caffeine is a substance similar to tyramine. If you consume excessive amounts of a caffeinated beverage such as coffee while you are taking an M AO inhibitor, the result may be a hypertensive crisis. Nonprescription drugs containing caffeine. The caffeine in coffee may add to the stimulant effects of the caffeine in over-the-counter cold remedies, diuretics, pain relievers, stimulants, and weight-control products containing caffeine. Some cold pills contain 30 mg caffeine, some pain relievers 130 mg, and some weight-control products as much as 280 mg caffeine. There are 110 –150 mg caffeine in a five-ounce cup of drip-brewed coffee. Sedatives. The caffeine in coffee may counteract the drowsiness caused by sedative drugs; this may be a boon to people who get sleepy when they take antihistamines. Coffee will not, however, “sober up” people who are experiencing the inebriating effects of alcoholic beverages. Theophylline. Caffeine relaxes the smooth muscle of the bronchi and may intensif y the effects (and/or increase the risk of side effects) of this antiasthmatic drug.... coffee
Structure of muscle Skeletal or voluntary muscle forms the bulk of the body’s musculature and contains more than 600 such muscles. They are classi?ed according to their methods of action. A ?exor muscle closes a joint, an extensor opens it; an abductor moves a body part outwards, an adductor moves it in; a depressor lowers a body part and an elevator raises it; while a constrictor (sphincter) muscle surrounds an ori?ce, closing and opening it. Each muscle is enclosed in a sheath of ?brous tissue, known as fascia or epimysium, and, from this, partitions of ?brous tissue, known as perimysium, run into the substance of the muscle, dividing it up into small bundles. Each of these bundles consists in turn of a collection of ?bres, which form the units of the muscle. Each ?bre is about 50 micrometres in thickness and ranges in length from a few millimetres to 300 millimetres. If the ?bre is cut across and examined under a high-powered microscope, it is seen to be further divided into ?brils. Each ?bre is enclosed in an elastic sheath of its own, which allows it to lengthen and shorten, and is known as the sarcolemma. Within the sarcolemma lie numerous nuclei belonging to the muscle ?bre, which was originally developed from a simple cell. To the sarcolemma, at either end, is attached a minute bundle of connective-tissue ?bres which unites the muscle ?bre to its neighbours, or to one of the connective-tissue partitions in the muscle, and by means of these connections the ?bre affects muscle contraction. Between the muscle ?bres, and enveloped in a sheath of connective tissue, lie here and there special structures known as muscle-spindles. Each of these contains thin muscle ?bres, numerous nuclei, and the endings of sensory nerves. (See TOUCH.) The heart muscle comprises short ?bres which communicate with their neighbours via short branches and have no sarcolemma.
Plain or unstriped muscle is found in the following positions: the inner and middle coats of the STOMACH and INTESTINE; the ureters (see URETER) and URINARY BLADDER; the TRACHEA and bronchial tubes; the ducts of glands; the GALL-BLADDER; the UTERUS and FALLOPIAN TUBES; the middle coat of the blood and lymph vessels; the iris and ciliary muscle of the EYE; the dartos muscle of the SCROTUM; and in association with the various glands and hairs in the SKIN. The ?bres are very much smaller than those of striped muscle, although they vary greatly in size. Each has one or more oval nuclei and a delicate sheath of sarcolemma enveloping it. The ?bres are grouped in bundles, much as are the striped ?bres, but they adhere to one another by cement material, not by the tendon bundles found in voluntary muscle.
Development of muscle All the muscles of the developing individual arise from the central layer (mesoderm) of the EMBRYO, each ?bre taking origin from a single cell. Later on in life, muscles have the power both of increasing in size – as the result of use, for example, in athletes – and also of healing, after parts of them have been destroyed by injury. An example of the great extent to which unstriped muscle can develop to meet the demands made on it is the uterus, whose muscular wall develops so much during pregnancy that the organ increases from the weight of 30–40 g (1–1••• oz.) to a weight of around 1 kg (2 lb.), decreasing again to its former small size in the course of a month after childbirth.
Physiology of contraction A muscle is an elaborate chemico-physical system for producing heat and mechanical work. The total energy liberated by a contracting muscle can be exactly measured. From 25–30 per cent of the total energy expended is used in mechanical work. The heat of contracting muscle makes an important contribution to the maintenance of the heat of the body. (See also MYOGLOBIN.)
The energy of muscular contraction is derived from a complicated series of chemical reactions. Complex substances are broken down and built up again, supplying each other with energy for this purpose. The ?rst reaction is the breakdown of adenyl-pyrophosphate into phosphoric acid and adenylic acid (derived from nucleic acid); this supplies the immediate energy for contraction. Next phosphocreatine breaks down into creatine and phosphoric acid, giving energy for the resynthesis of adenyl-pyrophosphate. Creatine is a normal nitrogenous constituent of muscle. Then glycogen through the intermediary stage of sugar bound to phosphate breaks down into lactic acid to supply energy for the resynthesis of phosphocreatine. Finally part of the lactic acid is oxidised to supply energy for building up the rest of the lactic acid into glycogen again. If there is not enough oxygen, lactic acid accumulates and fatigue results.
All of the chemical changes are mediated by the action of several enzymes (see ENZYME).
Involuntary muscle has several peculiarities of contraction. In the heart, rhythmicality is an important feature – one beat appearing to be, in a sense, the cause of the next beat. Tonus is a character of all muscle, but particularly of unstriped muscle in some localities, as in the walls of arteries.
Fatigue occurs when a muscle is made to act for some time and is due to the accumulation of waste products, especially sarcolactic acid (see LACTIC ACID). These substances affect the end-plates of the nerve controlling the muscle, and so prevent destructive overaction of the muscle. As they are rapidly swept away by the blood, the muscle, after a rest (and particularly if the rest is accompanied by massage or by gentle contractions to quicken the circulation) recovers rapidly from the fatigue. Muscular activity over the whole body causes prolonged fatigue which is remedied by rest to allow for metabolic balance to be re-established.... muscle
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