muscles

 muscles

There are more than 600 muscles in the human body. Most of these muscles are attached to the bones of the skeleton bytendons, 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 movementare 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.These interactions of body systems are covered in

this chapter, which focuses on the skeletal muscles.You may recall from Chapter 4 that there are two other types of muscle tissue: smooth muscle and cardiac muscle. These types of muscle tissue will be discussed in other chapters in relation to the organs of

which they are part. Before you continue, you may find it helpful to go back to Chapter 4 and review the structure and characteristics of skeletal muscle tissue.In this chapter we will begin with the gross (large) anatomy and physiology of muscles, then discuss the

microscopic structure of muscle cells and the biochemistry of muscle contraction.

MUSCLE STRUCTURE

All muscle cells are specialized for contraction. Whenthese cells contract, they shorten and pull a bone tproduce movement. Each skeletal muscle is made of thousands of individual muscle cells, which also maybe called muscle fibers Depending on the work a muscle is required to do, variable numbers of muscle fibers contract.When picking up a pencil, for example, only a small

portion of the muscle fibers in each finger muscle will contract. If the muscle has more work to do, such as picking up a book, more muscle fibers will contract to accomplish the task.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, eachattached to a different bone. The more immobile or

stationary attachment of the muscle is its origin; themore movable attachment is called the insertion. Themuscle itself crosses the joint of the two bones towhich it is attached, and when the muscle contracts itpulls on its insertion and moves the bone in a specific

direction.

MUSCLE ARRANGEMENTS

Muscles are arranged around the skeleton so as to bring about a variety of movements. The two general types of arrangements are the opposing antagonists and the cooperative synergists.

Antagonistic Muscles

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,

Synergistic muscles 

are those with the same function,or those that work together to perform a particular function. Recall that the biceps brachii flexes the forearm. The brachioradialis, with its origin on the

humerus and insertion on the radius, also flexes the forearm. There is even a third flexor of the forearm,the brachialis. You may wonder why we need three muscles 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 thefunction of the deltoid. Adducting the arm is broughtabout 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. All of these muscles are described and depicted

in the tables and figures later in the chapter. Without

antagonistic muscles, this variety of movements would

not be possible

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.

QUESTION: When the biceps contracts, what happens to its length, and what kind of force

does it exert?

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.

MUSCLE TONE

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.

EXERCISE Good muscle tone improves coordination. When

muscles are slightly contracted, they can react more rapidly if and when greater exertion is necessary.Muscles with poor tone are usually soft and flabby, but exercise will improve muscle tone.

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 shorteningof 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.

ENERGY SOURCES FORMUSCLE CONTRACTION

Before discussing the contraction process itself, let us look first at how muscle fibers obtain the energy they need to contract. The direct source of energy for muscle contraction is ATP. ATP, however, is not stored in large amounts in muscle fibers and is depleted in a few seconds.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 fiber

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.

MUSCLE FIBER—MICROSCOPIC STRUCTURE

We will now look more closely at a muscle fiber, keeping in mind that there are thousands of these cylindrical cells in one muscle.Each muscle fiber has its own motor nerve ending;the neuromuscular junction is where the motor neuron terminates on the muscle fiber. The axon terminal is the enlarged tip of the motor neuron; it contains sacs of the neurotransmitter acetylcholine (ACh). The membrane of the muscle fiber is the sarcolemma, which contains receptor sites for acetylcholine, and an inactivator called cholinesterase. The synapse (or synaptic cleft) is the small space between the axon terminal and the sarcolemma.Within the muscle fiber are thousands of individualcontracting units called sarcomeres, which are arranged end to end in cylinders called myofibrils. The structure of a sarcomere is shown in 

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[1]2), which are essential for 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.

CONTRACTION—THE SLIDINGFILAMENT MECHANISM

All of the parts of a muscle fiber and the electrical changes described earlier are involved in the contraction process, which is a precise sequence of events called the sliding filament mechanism.

In summary, a nerve impulse causes depolarization of a muscle fiber, and this electrical change enables the myosin filaments to pull the actin filaments toward the center of the sarcomere, making the sarcomere shorter.

 All of the sarcomeres shorten and the muscle fiber contracts. A more detailed description of this process is the following:1. A nerve impulse arrives at the axon terminal;

acetylcholine is released and diffuses across the

synapse.2. Acetylcholine makes the sarcolemma more permeable

to Na[1] ions, which rush into the cell.3. The sarcolemma depolarizes, becoming negative outside and positive inside. The T tubules bring the reversal of charges to the interior of the muscle

cell.4. Depolarization stimulates the release of Ca[1]2 ions

from the sarcoplasmic reticulum. Ca[1]2 ions bond to the troponin–tropomyosin complex, which shifts it away from the actin filaments.5. Myosin splits ATP to release its energy; bridges

on the myosin attach to the actin filaments and pull them toward the center of the sarcomere, thus making the sarcomere shorter 

6. All of the sarcomeres in a muscle fiber shorten—the entire muscle fiber contracts.7. The sarcolemma repolarizes: K[1] ions leave the cell, restoring a positive charge outside and a negative

charge inside. The pumps then return Na[1]ions outside and K[1] ions inside.8. Cholinesterase in the sarcolemma inactivates

acetylcholine.9. Subsequent nerve impulses will prolong contraction (more acetylcholine is released).10. When there are no further impulses, the muscle fiber will relax and return to its original length.Steps 1 through 8 of this sequence describe a single

muscle fiber contraction (called a twitch) in response to

a single nerve impulse. Because all of this takes place

in less than a second, useful movements would not be possible if muscle fibers relaxed immediately after contracting. Normally, however, nerve impulses arrive in a continuous stream and produce a sustained contraction called tetanus, which is a normal state not to

be confused with the disease tetanus. When in tetanus, muscle fibers remain contracted and are capable of effective

movements. In a muscle such as the biceps brachii that flexes the forearm, an effective movement means that many of its thousands of muscle fibers are in tetanus,a sustained contraction.

As you might expect with such a complex process, muscle contraction may be impaired in many different ways. Perhaps the most obvious is the loss of nerve impulses to muscle fibers, which can occur whennerves or the spinal cord are severed, or when a stroke

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