Within the muscle fiber are thousands of individual contracting units called sarcomeres, which are arranged end to end in cylinders called myofibrils. The Z lines are the end boundaries of a sarcomere. Filaments of the protein myosin are in the center of the sarcomere, and filaments of the protein actin are at the ends, attached to the Z lines. Myosin filaments are anchored to the Z lines by the protein titin.

Myosin and actin are the contractile proteins of a muscle fiber. Their interactions produce muscle contraction. Also present are two inhibitory proteins, troponin and tropomyosin, which are part of the actin filaments and prevent the sliding of actin and myosin when the muscle fiber is relaxed.

Surrounding the sarcomeres is the sarcoplasmic reticulum, the endoplasmic reticulum of muscle cells. The sarcoplasmic reticulum is a reservoir for calcium ions (Ca+2), which are essential for the contraction process.

All of these parts of a muscle fiber are involved in the contraction process. Contraction begins when a nerve impulse arrives at the axon terminal and stimulates the release of acetylcholine. Acetylcholine generates electrical changes (the movement of ions) at the sarcolemma of the muscle fiber. These electrical changes initiate a sequence of events within the muscle fiber that is called the sliding filament mechanism of muscle contraction. We will begin our discussion with the sarcolemma.

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The secondary energy sources are creatine phosphate and glycogen. Creatine phosphate is, like ATP, an energy-transferring molecule. When it is broken down (by an enzyme) to creatine, phosphate, and energy, the energy is used to synthesize more ATP. Most of the creatine formed is used to resynthesize creatine phosphate, but some is converted to creatinine, a nitrogenous waste product that is excreted by the kidneys.

The most abundant energy source in muscle fibers is glycogen. When glycogen is needed to provide energy for sustained contractions (more than a few seconds), it is first broken down into the glucose molecules of which it is made. Glucose is then further broken down in the process of cell respiration to produce ATP, and muscle fibers may continue to contract.

Glucose + O2 = CO2 + H2O + ATP + heat

Look first at the products of this reaction. ATP will be used by the muscle fibers for contraction. The heat produced will contribute to body temperature, and if exercise is strenuous, will increase body temperature. The water becomes part of intracellular water, and the carbon dioxide is a waste product that will be exhaled.

Now look at what is needed to release energy from glucose: oxygen. Muscles have two sources of oxygen. The blood delivers a continuous supply of oxygen from the lungs, which is carried by the hemoglobin in red blood cells. Within muscle fibers themselves there is another protein called myoglobin, which stores some oxygen within the muscle cells. Both hemoglobin and myoglobin contain the mineral iron, which enables them to bond to oxygen. (Iron also makes both molecules red, and it is myoglobin that gives muscle tissue a red or dark color.)

During strenuous exercise, the oxygen stored in myoglobin is quickly used up, and normal circulation may not deliver oxygen fast enough to permit the completion of cell respiration. Even though the respiratory rate increases, the muscle fibers may literally run out of oxygen. This state is called oxygen debt, and in this case, glucose cannot be completely broken down into carbon dioxide and water. If oxygen is not present (or not present in sufficient amounts), glucose is converted to an intermediate molecule called lactic acid, which causes muscle fatigue.

In a state of fatigue, muscle fibers cannot contract efficiently, and contraction may become painful. To be in oxygen debt means that we owe the body some oxygen. Lactic acid from muscles enters the blood and circulates to the liver, where it is converted to pyruvic acid, a simple carbohydrate (three carbons, about half a glucose molecule). This conversion requires ATP, and oxygen is needed to produce the necessary ATP in the liver. This is why, after strenuous exercise, the respiratory rate and heart rate remain high for a time and only gradually return to normal. Another name proposed for this state is recovery oxygen uptake, which is a little longer but also makes sense. Oxygen uptake means a faster and deeper respiratory rate. What is this uptake for? For recovery from strenuous exercise.

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Within muscles are receptors called stretch receptors (proprioceptors or muscle spindles). The general function of all sensory receptors is to detect changes. The function of stretch receptors is to detect changes in the length of a muscle as it is stretched. The sensory impulses generated by these receptors are interpreted by the brain as a mental “picture” of where the muscle is.

We can be aware of muscle sense if we choose to be, but usually we can safely take it for granted. In fact, that is what we are meant to do. Imagine what life would be like if we had to watch every move to be sure that a hand or foot performed its intended action. Even simple activities such as walking or eating would require our constant attention.

At times, we may become aware of our muscle sense. Learning a skill such as typing or playing the guitar involves very precise movements of the fingers, and beginners will often watch their fingers to be sure they are moving properly. With practice, however, the movements simply “feel” right, which means that the brain has formed a very good mental picture of the task. Muscle sense again becomes unconscious, and the experienced typist or guitarist need not watch every movement.

All sensation is a function of brain activity, and muscle sense is no exception. The impulses for muscle sense are integrated in the parietal lobes of the cerebrum (conscious muscle sense) and in the cerebellum (unconscious muscle sense) to be used to promote coordination.

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There are two general types of exercise: isotonic and isometric. In isotonic exercise, muscles contract and bring about movement. Jogging, swimming, and weight lifting are examples. Isotonic exercise improves muscle tone, muscle strength, and, if done repetitively against great resistance (as in weight lifting), muscle size. This type of exercise also improves cardiovascular and respiratory efficiency, because movement exerts demands on the heart and respiratory muscles. If done for 30 minutes or longer, such exercise may be called aerobic, because it strengthens the heart and respiratory muscles as well as the muscles attached to the skeleton.

Isotonic contractions are of two kinds, concentric or eccentric. A concentric contraction is the shortening of a muscle as it exerts force. An eccentric contraction is the lengthening of a muscle as it still exerts force. Imagine lifting a book straight up (or try it); the triceps brachii contracts and shortens to straighten the elbow and raise the book, a concentric contraction. Now imagine slowly lowering the book. The triceps brachii is still contracting even as it is lengthening, exerting force to oppose gravity (which would make the book drop quickly). This is an eccentric contraction.

Isometric exercise involves contraction without movement. If you put your palms together and push one hand against the other, you can feel your arm muscles contracting. If both hands push equally, there will be no movement; this is isometric contraction. Such exercises will increase muscle tone and muscle strength but are not considered aerobic. When the body is moving, the brain receives sensory information about this movement from the joints involved, and responds with reflexes that increase heart rate and respiration. Without movement, the brain does not get this sensory information, and heart rate and breathing do not increase nearly as much as they would during an equally strenuous isotonic exercise.

Many of our actions involve both isotonic and isometric contractions. Pulling open a door requires isotonic contractions of arm muscles, but if the door is then held open for someone else, those contractions become isometric. Picking up a pencil is isotonic; holding it in your hand is isometric. Walking uphill involves concentric isotonic contractions, and may be quite strenuous. Walking downhill seems easier, but is no less complex. The eccentric isotonic contractions involved make each step a precisely aimed and controlled fall against gravity. Without such control (which we do not have to think about) a downhill walk would quickly become a roll. These various kinds of contractions are needed for even the simplest activities.

Anabolic steroids are synthetic drugs very similar in structure and action to the male hormone testosterone. Normal secretion of testosterone, beginning in males at puberty, increases muscle size and is the reason men usually have larger muscles than do women.

Some athletes, both male and female, both amateur and professional, take anabolic steroids to build muscle mass and to increase muscle strength. There is no doubt that the use of anabolic steroids will increase muscle size, but there are hazards, some of them very serious. Side effects of such self-medication include liver damage, kidney damage, disruption of reproductive cycles, and mental changes such as irritability and aggressiveness.

Female athletes may develop increased growth of facial and body hair and may become sterile as a result of the effects of a male hormone on their own hormonal cycles.

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Muscles may also be called synergists if they help to stabilize or steady a joint to make a more precise movement possible. If you drink a glass of water, the biceps brachii may be the prime mover to flex the forearm. At the same time, the muscles of the shoulder keep that joint stable, so that the water gets to your mouth, not over your shoulder or down your chin. The shoulder muscles are considered synergists for this movement because their contribution makes the movement effective.

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Antagonists are opponents, so we use the term antagonistic muscles for muscles that have opposing or opposite functions. The biceps brachii is the muscle on the front of the upper arm. The origin of the biceps is on the scapula (there are actually two tendons, hence the name biceps), and the insertion is on the radius. When the biceps contracts, it flexes the forearm, that is, bends the elbow. Recall that when a muscle contracts, it gets shorter and pulls. Muscles cannot push, for when they relax they exert no force. Therefore, the biceps can bend the elbow but cannot straighten it; another muscle is needed. The triceps brachii is located on the back of the upper arm. Its origins (the prefix tri tells you that there are three of them) are on the scapula and humerus, and its insertion is on the ulna. When the triceps contracts and pulls, it extends the forearm, that is, straightens the elbow.

Joints that are capable of a variety of movements have several sets of antagonists. Notice how many ways you can move your upper arm at the shoulder, for instance. Abducting (laterally raising) the arm is the function of the deltoid. Adducting the arm is brought about by the pectoralis major and latissimus dorsi. Flexion of the arm (across the chest) is also a function of the pectoralis major, and extension of the arm (behind the back) is also a function of the latissimus dorsi. Without antagonistic muscles, this variety of movements would not be possible.

You may be familiar with range-of-motion (or ROM) exercises that are often recommended for patients confined to bed. Such exercises are designed to stretch and contract the antagonistic muscles of a joint to preserve as much muscle function and joint mobility as possible.

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Muscles are anchored firmly to bones by tendons. Most tendons are rope-like, but some are flat; a flat tendon is called an aponeurosis. Tendons are made of fibrous connective tissue, which, you may remember, is very strong and merges with the fascia that covers the muscle and with the periosteum, the fibrous connective tissue membrane that covers bones. A muscle usually has at least two tendons, each attached to a different bone. The more immobile or stationary attachment of the muscle is its origin; the more movable attachment is called the insertion. The muscle itself crosses the joint of the two bones to which it is attached, and when the muscle contracts it pulls on its insertion and moves the bone in a specific direction.

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There are more than 600 muscles in the human body. Most of these muscles are attached to the bones of the skeleton by tendons, although a few muscles are attached to the undersurface of the skin. The primary function of the muscular system is to move the skeleton. The muscle contractions required for movement also produce heat, which contributes to the maintenance of a constant body temperature. The other body systems directly involved in movement are the nervous, respiratory, and circulatory systems. The nervous system transmits the electrochemical impulses that cause muscle cells to contract. The respiratory system exchanges oxygen and carbon dioxide between the air and blood. The circulatory system brings oxygen to the muscles and takes carbon dioxide away.

You may recall from Muscle Tissue article that there are two other types of muscle tissue: smooth muscle and cardiac muscle. Before you continue, you may find it helpful to go back to Muscle Tissue and review the structure and characteristics of skeletal muscle tissue.

Even our simplest movements require the interaction of many muscles, and the contraction of skeletal muscles depends on the brain. The nerve impulses for movement come from the frontal lobes of the cerebrum. The cerebrum is the largest part of the brain; the frontal lobes are beneath the frontal bone. The motor areas of the frontal lobes generate electrochemical impulses that travel along motor nerves to muscle fibers, causing the muscle fibers to contract.

For a movement to be effective, some muscles must contract while others relax. When walking, for example, antagonistic muscles on the front and back of the thigh or the lower leg will alternate their contractions and relaxations, and our steps will be smooth and efficient. This is what we call coordination, and we do not have to think about making it happen. Coordination takes place below the level of conscious thought and is regulated by the cerebellum, which is located below the occipital lobes of the cerebrum.

Except during certain stages of sleep, most of our muscles are in a state of slight contraction; this is what is known as muscle tone. When sitting upright, for example, the tone of your neck muscles keeps your head up, and the tone of your back muscles keeps your back straight. This is an important function of muscle tone for human beings, because it helps us to maintain an upright posture. For a muscle to remain slightly contracted, only a few of the muscle fibers in that muscle must contract. Alternate fibers contract so that the muscle as a whole does not become fatigued. This is similar to a pianist continuously rippling her fingers over the keys of the piano—some notes are always sounding at any given moment, but the notes that are sounding are always changing. This contraction of alternate fibers, muscle tone, is also regulated by the cerebellum of the brain.
Muscle fibers need the energy of ATP (adenosine triphosphate) in order to contract. When they produce ATP in the process of cell respiration, muscle fibers also produce heat. The heat generated by normal muscle tone is approximately 25% of the total body heat at rest. During exercise, of course, heat production increases significantly.

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Diffusion is the movement of molecules from an area of greater concentration to an area of lesser concentration (that is, with or along a concentration gradient). Diffusion occurs because molecules have free energy; that is, they are always in motion. The molecules in a solid move very slowly; those in a liquid move faster; and those in a gas move faster still, such as when ice absorbs heat energy, melts, and then evaporates. Imagine a green sugar cube at the bottom of a glass of water (green so that we can see it). As the sugar dissolves, the sugar molecules collide with one another or the water molecules, and the green color seems to rise in the glass. These collisions spread out the sugar molecules until they are evenly dispersed among the water molecules (this would take a very long time), and the water eventually becomes entirely green. The molecules are still moving, but as some go to the top, others go to the bottom, and so on. Thus, an equilibrium (or steady state) is reached.

Diffusion is a very slow process, but may be an effective transport mechanism across microscopic distances. Within the body, the gases oxygen and carbon dioxide move by diffusion. In the lungs, for example, there is a high concentration of oxygen in the alveoli (air sacs) and a low concentration of oxygen in the blood in the surrounding pulmonary capillaries. The opposite is true for carbon dioxide: a low concentration in the air in the alveoli and a high concentration in the blood in the pulmonary capillaries. These gases diffuse in opposite directions, each moving from where there is more to where there is less. Oxygen diffuses from the air to the blood to be circulated throughout the body. Carbon dioxide diffuses from the blood to the air to be exhaled.

Osmosis may be simply defined as the diffusion of water through a selectively permeable membrane. That is, water will move from an area with more water present to an area with less water. Another way to say this is that water will naturally tend to move to an area where there is more dissolved material, such as salt or sugar. If a 2% salt solution and a 6% salt solution are separated by a membrane allowing water but not salt to pass through it, water will diffuse from the 2% salt solution to the 6% salt solution. The result is that the 2% solution will become more concentrated, and the 6% solution will become more dilute.

In the body, the cells lining the small intestine absorb water from digested food by osmosis. These cells have first absorbed salts, have become more “salty,” and water follows salt into the cells. The process of osmosis also takes place in the kidneys, which reabsorb large amounts of water (many gallons each day) to prevent its loss in urine.

Human cells or other body fluids contain many dissolved substances (called solutes) such as salts, sugars, acids, and bases. The concentration of solutes in a fluid creates the osmotic pressure of the solution, which in turn determines the movement of water through membranes.

As an example here, we will use sodium chloride (NaCl). Human cells have an NaCl concentration of 0.9%. With human cells as a reference point, the relative NaCl concentrations of other solutions may be described with the following terms:

Isotonic—a solution with the same salt concentration as in cells. The blood plasma is isotonic to red blood cells.
Hypotonic—a solution with a lower salt concentration than in cells. Distilled water (0% salt) is hypotonic to human cells.
Hypertonic—a solution with a higher salt concentration than in cells. Seawater (3% salt) is hypertonic to human cells.

• If RBCs are placed in distilled water, more water will enter the cells than leave, and the cells will swell and eventually burst.
• If RBCs are placed in seawater, more water will leave the cells than enter, and the cells will shrivel and die.
•When RBCs are in plasma, water moves into and out of them at equal rates, and the cells remain normal in size and water content.

This knowledge of osmotic pressure is used when replacement fluids are needed for a patient who has become dehydrated. Isotonic solutions are usually used; normal saline and Ringer’s solution are examples. These will provide rehydration without causing osmotic damage to cells or extensive shifts of fluid between the blood and tissues.

The word facilitate means to help or assist. In facilitated diffusion, molecules move through a membrane from an area of greater concentration to an area of lesser concentration, but they need some help to do this.

In the body, our cells must take in glucose to use for ATP production. Glucose, however, will not diffuse through most cell membranes by itself, even if there is more outside the cell than inside. Diffusion of glucose into most cells requires a glucose transporter, which may also be called a carrier enzyme. These transporters are proteins that are part of the cell membrane. Glucose bonds to the transporter, and by doing so changes the shape of the protein. This physical change propels the glucose into the interior of the cell. Other transporters are specific for other organic molecules such as amino acids.

Active transport requires the energy of ATP to move molecules from an area of lesser concentration to an area of greater concentration. Notice that this is the opposite of diffusion, in which the free energy of molecules causes them to move to where there are fewer of them. Active transport is therefore said to be movement against a concentration gradient.

In the body, nerve cells and muscle cells have “sodium pumps” to move sodium ions (Na+) out of the cells. Sodium ions are more abundant outside the cells, and they constantly diffuse into the cell (through specific diffusion channels), their area of lesser concentration. Without the sodium pumps to return them outside, the incoming sodium ions would bring about an unwanted nerve impulse or muscle contraction. Nerve and muscle cells constantly produce ATP to keep their sodium pumps (and similar potassium pumps) working and prevent spontaneous impulses.

Another example of active transport is the absorption of glucose and amino acids by the cells lining the small intestine. The cells use ATP to absorb these nutrients from digested food, even when their intracellular concentration becomes greater than their extracellular concentration.

The process of filtration also requires energy, but the energy needed does not come directly from ATP. It is the energy of mechanical pressure. Filtration means that water and dissolved materials are forced through a membrane from an area of higher pressure to an area of lower pressure.

In the body, blood pressure is created by the pumping of the heart. Filtration occurs when blood flows through capillaries, whose walls are only one cell thick and very permeable. The blood pressure in capillaries is higher than the pressure of the surrounding tissue fluid. In capillaries throughout the body, blood pressure forces plasma (water) and dissolved materials through the capillary membranes into the surrounding tissue spaces. This creates more tissue fluid and is how cells receive glucose, amino acids, and other nutrients. Blood pressure in the capillaries of the kidneys also brings about filtration, which is the first step in the formation of urine.

These two processes are similar in that both involve a cell engulfing something, and both are forms of endocytosis, endo meaning “to take into” a cell. An example of phagocytosis is a white blood cell engulfing bacteria. The white blood cell flows around the bacterium, taking it in and eventually digesting it. Digestion is accomplished by the enzymes in the cell’s lysosomes.

Other cells that are stationary may take in small molecules that become adsorbed or attached to their membranes. The cells of the kidney tubules reabsorb small proteins by pinocytosis so that the protein is not lost in urine.

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The endoplasmic reticulum (ER) is an extensive network of membranous tubules that extend from the nuclear membrane to the cell membrane. Rough ER has numerous ribosomes on its surface, whereas smooth ER has no ribosomes. As a network of interconnected tunnels, the ER is a passageway for the transport of the materials necessary for cell function within the cell. These include proteins synthesized by the ribosomes on the rough ER, and lipids synthesized by the smooth ER.

Ribosomes are very small structures made of protein and ribosomal RNA. Some are found on the surface of rough ER, while others float freely within the cytoplasm. Ribosomes are the site of protein synthesis. The proteins produced may be structural proteins such as collagen in the skin, enzymes, or hormones such as insulin that regulate cellular processes. These proteins may function within the cell or be secreted from the cell to be used elsewhere in the body.

Our protein molecules are subject to damage, and some cellular proteins, especially regulatory proteins, may be needed just for a very short time. All such proteins must be destroyed, and this is the function of proteasomes. A proteasome is a barrel-shaped organelle made of enzymes that cut protein molecules apart (protease enzymes). Proteins that are to be destroyed, that is, those no longer needed or those that are damaged or misfolded, are tagged by a protein called ubiquitin (sort of a cellular mop or broom) and carried into a proteasome. The protein is snipped into peptides or amino acids, which may be used again for protein synthesis on ribosomes. Proteasomes are particularly important during cell division and during embryonic development, when great changes are taking place very rapidly as cells become specialized.

Many of our cells have secretory functions, that is, they produce specific products to be used elsewhere in tissues. Secretion is one task of the Golgi apparatus, a series of flat, membranous sacs, somewhat like a stack of saucers. Carbohydrates are synthesized within the Golgi apparatus, and are packaged, along with other materials, for secretion from the cell. Proteins from the ribosomes or lipids from the smooth endoplasmic reticulum may also be secreted in this way. To secrete a substance, small sacs of the Golgi membrane break off and fuse with the cell membrane, releasing the substance to the exterior of the cell. This is exocytosis, exo meaning “to go out” of the cell.

Mitochondria are oval or spherical organelles bounded by a double membrane. The inner membrane has folds called cristae. Within the mitochondria, the aerobic (oxygen-requiring) reactions of cell respiration take place. Therefore, mitochondria are the site of ATP (and hence energy) production. Cells that require large amounts of ATP, such as muscle cells, have many mitochondria to meet their need for energy. Mitochondria contain their own genes in a single DNA molecule and duplicate themselves when a cell divides. An individual’s mitochondrial DNA (mDNA) is of maternal origin, that is, from the mitochondria that were present in the ovum, or egg cell, which was then fertilized by a sperm cell. The mitochondria of the sperm cell usually do not enter the ovum during fertilization, because they are not found in the head of the sperm with the chromosomes.

Lysosomes are single-membrane structures that contain digestive enzymes. When certain white blood cells engulf bacteria, the bacteria are digested and destroyed by these lysosomal enzymes. Worn-out cell parts and dead cells are also digested by these enzymes. This is a beneficial process, and is necessary before tissue repair can begin. But it does have a disadvantage in that lysosomal digestion contributes to inflammation in damaged tissues. An excess of inflammation can start a vicious cycle, actually a positive feedback mechanism, that results in extensive tissue damage.

Many of our cells are capable of dividing, or reproducing, themselves. Centrioles are a pair of rod-shaped structures perpendicular to one another, located just outside the nucleus. Their function is to organize the spindle fibers during cell division. The spindle fibers are contracting proteins that pull the two sets of chromosomes apart, toward the ends of the original cell as it divides into two new cells. Each new cell then has a full set of chromosomes.

Cilia and flagella are mobile thread-like projections through the cell membrane; each is anchored by a basal body just within the membrane. Cilia are usually shorter than flagella, and an individual cell has many of them on its free surface. The cilia of a cell beat in unison and sweep materials across the cell surface. Cells lining the fallopian tubes, for example, have cilia to sweep the egg cell toward the uterus. The only human cell with a flagellum is the sperm cell. The flagellum provides motility, or movement, for the sperm cell.

Microvilli are folds of the cell membrane on the free surface of a cell. These folds greatly increase the surface area of the membrane, and are part of the cells lining organs that absorb materials. The small intestine, for example, requires a large surface area for the absorption of nutrients, and many of its lining cells have microvilli. Some cells of the kidney tubules also have microvilli that provide for the efficient reabsorption of useful materials back to the blood.

Endoplasmic reticulum (ER)
• Passageway for transport of materials within the cell
• Synthesis of lipids
Ribosomes
• Site of protein synthesis
Proteasomes
• Site of destruction of old or damaged proteins
Golgi apparatus

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• Synthesis of carbohydrates
• Packaging of materials for secretion from the cell
Mitochondria
• Site of aerobic cell respiration—ATP production
Lysosomes
• Contain enzymes to digest ingested material or damaged tissue
Centrioles
• Organize the spindle fibers during cell division
Cilia
• Sweep materials across the cell surface
Flagellum
• Enables a cell to move
Microvilli
• Increase a cell’s surface area for absorption

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