The Muscular System - Design: parts of the muscular system






The muscles of the body are divided into three main types: skeletal, smooth, and cardiac. As their name implies, skeletal muscles are attached to the skeleton and move various parts of the body. They are composed of tissue fibers that are striated or striped. The alternating bands of light and dark result from the pattern of the filaments (threadlike proteins) within each muscle cell. Skeletal muscles are called voluntary muscles because a person controls their use, such as in the flexing of an arm or the raising of a foot.

There are just over 650 skeletal muscles in the whole human body. Some authorities state there are as many as 850 muscles in the body. No exact figure is available because scientists disagree about which ones are separate muscles and which ones are part of larger muscles. There is also some variability in muscular structure between individuals.

Smooth muscles are found in the stomach and intestinal walls, in artery and vein walls, and in various hollow organs. They are called involuntary muscles because a person generally cannot consciously control them. They are regulated by the autonomic nervous system (a division of the nervous system that affects internal organs such as the heart, lungs, stomach, and liver). Unlike skeletal muscles, smooth muscles have no striations or stripes.

In a vessel or organ, smooth muscles are arranged in sheets or layers. Often, there are two layers, one running circularly (around) and the other longitudinally (up and down). As the two layers alternately contract and relax, the shape of the vessel or organ changes and fluid or food is propelled along. Smooth muscles contract slowly and can remain contracted for a long period of time without tiring.

Acetylcholine (ah-see-til-KOE-leen):
Neurotransmitter chemical released at the neuromuscular junction by motor neurons that translates messages from the brain to muscle fibers.
Adenosine triphosphate (ah-DEN-o-seen try-FOS-fate):
High-energy molecule found in every cell in the body.
Aerobic metabolism (air-ROH-bic muh-TAB-uhlizm):
Chemical reactions that require oxygen in order to create adenosine triphosphate.
Antagonist (an-TAG-o-nist):
Muscle that acts in opposition to a prime mover.
Cramp:
Prolonged muscle spasm.
Fascicle (FA-si-kul):
Bundle of myofibrils wrapped together by connective tissue.
Lactic acid (LAK-tik ASS-id):
Chemical waste product created when muscle fibers break down glucose without the proper amount of oxygen.
Muscle tone:
Sustained partial contraction of certain muscle fibers in all muscles.
Myofibrils (my-o-FIE-brilz):
cylindrical structures lying within skeletal muscle fibers that are composed of repeating structural units called sarcomeres.
Myofilament (my-o-FILL-ah-ment):
Protein filament composing the myofibrils; can be either thick (composed of myosin) or thin (composed of actin).
Neuromuscular junction (nu-row-MUSS-ku-lar-JUNK-shun):
Region where a motor neuron comes into close contact with a muscle fiber.
Prime mover (or agonist):
Muscle whose contractions are chiefly responsible for producing a particular movement.
Rigor mortis (RIG-er MOR-tis):
Rigid state of the body after death due to irreversible muscle contractions.
Sarcomere (SAR-koh-meer):
Unit of contraction in a skeletal muscle fiber containing a precise arrangement of thick and thin myofilaments.
Spasm:
Sudden, involuntary muscle contraction.
Strain:
Slight tear in a muscle; also called a pulled muscle.
Synergist (SIN-er-jist):
Muscle that cooperates with another to produce a particular movement.
Tendon (TEN-den):
Tough, white, cordlike tissue that attaches muscle to bone.

Cardiac muscle, called the myocardium, is found in only one place in the body: the heart. It is a unique type of muscle. Like skeletal muscle, it is

Some of the estimated over 800 muscles in the human body. No exact figure is available because scientists disagree about which ones are separate muscles and which ones are part of larger muscles. (Illustration by Kopp Illustration, Inc.)
Some of the estimated over 800 muscles in the human body. No exact figure is available because scientists disagree about which ones are separate muscles and which ones are part of larger muscles. (Illustration by
Kopp Illustration, Inc.
)

striated. But like smooth muscle, it is involuntary, controlled by the autonomic nervous system. The myocardium is composed of thick bundles of muscle that are twisted and whorled into ringlike arrangements. Forming the walls of the chambers of the heart, the myocardium contracts to pump blood throughout the body (for a further discussion of its actions, see chapter 1).

Of the three types of muscle, skeletal are probably the most familiar. They stabilize joints, help maintain posture, and give the body its general shape. In men, they make up about 40 percent of the body's mass or weight; in women, about 23 percent. Since the term muscular system refers specifically to skeletal muscles, the remainder of this chapter will focus on them.

Structure of muscle cells

Each muscle is made of hundreds to thousands of individual muscle cells. Unlike most other cells in the body, these cells are unusually shaped: they are elongated like a cylinder or a long rod. Because of their shape, muscle cells are normally referred to as muscle fibers. Whereas most cells have a single nucleus (the part of the cell that controls its activities), muscle fibers have as many as 100 or more nuclei. The nuclei are located on the surface of the fiber, just under its thin membrane. Another difference between muscle fibers and other body cells is their size. They can extend the entire length of a muscle. For example, a muscle fiber in a thigh muscle could measure 0.0004 inch (0.001 centimeter) in diameter and 12 to 16 inches (30 to 40 centimeters) in length.

When a person dies, blood stops circulating through the body. The skeletal muscles (along with all other parts of the body) are deprived of oxygen and nutrients, including ATP. Calcium ions leak out of their storage area in the membranes of muscle fibers, causing thick myofilaments to attach to and pull thin myofilaments. While the muscle fibers still have a stored supply of ATP, the heads of thick myofilaments are able to detach from the thin myofilaments. When the supply of ATP runs out, however, the heads cannot detach and the muscle fibers stay in a contracted position.

The rigid state of muscle contraction that results is called rigor mortis. Depending on the person's physical condition at death, the onset of rigor mortis may vary from ten minutes to several hours after death. Facial muscles are usually affected first, followed by other parts of the body. Rigor mortis lasts until the muscle fibers begin to decompose fifteen to twenty-five hours after death.

Each muscle fiber is composed of hundreds of smaller filaments or threads called myofibrils (the prefix myocomes from the Latin word myos , meaning "muscle"). Each myofibril contains bundles of threadlike proteins or filaments called myofilaments, which can be either thick or thin. The larger thick myofilaments are made mostly of bundled molecules of the protein myosin. The thin myofilaments are composed of the protein actin.

In each myofibril, the thick and thin myofilaments are combined into thousands of units or segments that repeat over and over. These units are called sarcomeres. Thick myofilaments lie in the center of a sarcomere. Thin myofilaments are attached at either end of a sarcomere and extend toward the center, passing among the thick myofilaments. This regular arrangement of the varying myofilaments within each sarcomere produces the striated or striped appearance of each myofibril and, by extension, of muscle fibers.

As are most living cells, muscle fibers are soft and fragile. Even so, they can exert tremendous power without being ripped apart. The reason is that muscles are composed of different types of tissue (like all other organs in the body). In addition, those tissues are bundled together, providing strength and support. Each myofibril is enclosed in a delicate sheath or covering made of connective tissue (tissue found everywhere in the body that connects body parts, providing support, storage, and protection). Numerous sheathed myofibrils are then bundled together and wrapped with thicker connective tissue to form what is called a fascicle (from the Latin word fasciculus , meaning "a bundle"). Many fascicles are then bundled together by an even tougher coat of connective tissue to form the muscle.

Tendons

The layers of connective tissue that bundle the various parts of a muscle usually converge or come together at the end of the muscle to form a tough, white, cord-like tissue called a tendon. Tendons attach muscles to bone. Because they contain fibers of the tough protein collagen, tendons are much stronger than muscle tissue. The collagen fibers are arranged in a tendon in a wavy way so that it can stretch and provide additional length at the muscle-bone junction. As muscles are used, the tendons are able to withstand the constant pulling and tugging.

Muscles are always attached at both of their ends. The end that is attached to a bone that moves when the muscle contracts is called the insertion. The other end, attached to a bone that does not move when the muscle contracts, is called the origin. It is important to note that not all muscles are attached to bones at both ends. The ends of some muscles are attached to other muscles; some are attached to the skin.

Major muscles of the body

Skeletal muscles that support the skull, backbone, and rib cage are called axial skeletal muscles. These include the muscles of the head and neck and those of the trunk. Roughly 60 percent of all skeletal muscles in the body are axial muscles. The skeletal muscles of the limbs (arms and legs) are called distal or appendicular skeletal muscles. These include the muscles of the shoulders and arms and those of the hip and legs.

Muscle names are descriptive. Some muscles are named according to their location in the body. For example, the frontalis muscle overlies the frontal bone of the skull. Other muscles are named for their relative size. Terms such as maximus (largest), minimus (smallest), and longus (long) are often used as part of a muscle's name. Still other muscles are named for their shape. The deltoid muscle is so named because it has the shape of the Greek letter delta , which is triangular-shaped. And some muscles are named for their actions. Terms such as flexor (to flex or bend in), extensor (to extend or straighten out), adductor (to draw toward a line that runs down the middle of the body), and abductor (to draw away from a line that runs down the middle of the body) are often added as part of a muscle's name.

Please note: in the naming of the major muscles of the body on the following pages, pronunciations are provided in parenthesis when necessary.

MUSCLES OF THE HEAD AND NECK. The muscles of the face are unique: they are attached to the skull on one end and to the skin or other muscles on the other end. Muscles that are attached to the skin of the face allow people to express emotions through actions such as smiling, frowning, pouting, and kissing.

As mentioned, the frontalis (frun-TA-lis) covers the frontal bone or forehead. The temporalis (tem-po-RAL-is) is a fan-shaped muscle overlying the temporal bone on each side of the head above the ear. The orbicularis oculi (or-bik-u-LAR-is OK-u-lie) encircles each eye and helps close the eyelid. The orbicularis oris (or-bik-u-LAR-is OR-is) is the circular muscle around the lips. It closes and extends the lips.

The masseter (mas-SE-ter), located over the rear of the lower jaw on each side of the face, opens and closes the jaw, allowing chewing. The buccinator (BUK-si-na-tor), running horizontally across each cheek, flattens the cheek and pulls back the corners of the mouth. The sternocleidomastoid (ster-nokli-do-MAS-toyd), located on either side of the neck and extending from the

A section of striated muscle tissue. Skeletal muscles are composed of tissue fibers that are striated or striped. (Photograph by M. Abbey. Reproduced by permission of Photo Researchers, Inc.)
A section of striated muscle tissue. Skeletal muscles are composed of tissue fibers that are striated or striped. (Photograph by
M. Abbey
. Reproduced by permission of
Photo Researchers, Inc.
)

clavicle or collarbone to the temporal bone on the side of the head, allows the head to rotate and the neck to flex.

MUSCLES OF THE TRUNK. On the front part of the trunk or torso, the pectoralis major (pek-to-RA-lis MA-jor) are the large, fan-shaped muscles that cover the upper part of the chest. They flex the shoulders and pull the arms into the body. The rectus abdominis (REK-tus ab-DOM-i-nis) are the strap-like muscles of the abdomen, extending from the ribs to the pelvis. Better known as the stomach muscles, they flex the vertebral column or backbone and provide support for the abdomen and its many organs. The muscles making up the side walls of the abdomen are the external oblique (ex-TER-nal o-BLEEK). In addition to helping compress the abdomen, they rotate the trunk and allow it to bend sideways.

On the rear part of the trunk, the trapezius (trah-PEE-zee-us) are the kite-shaped muscles that run from the back of the neck and upper back down to the middle of the back. They raise, lower, and adduct the shoulders. The large, flat muscles that cover the lower back are the latissimus dorsi (lah-TIS-i-mus DOR-see). They adduct and rotate the arms and help extend the shoulders.

Robots and machines that move and pick up objects like humans may no longer be found only in science fiction novels and movies. Scientists have created various artificial muscles that contract and expand just like human muscles. Unlike human muscles, however, artificial muscles have no limit to their strength.

One such artificial muscle is made out of artificial silk, which is cooked and then boiled to make a rubbery, semiliquid substance. The substance is similar in structure to human muscle, composed of smaller and smaller fibers. These fibers are naturally negatively charged with electricity.

When an acid (which has a positive electrical charge) is applied to this substance, the negative and positive ions attract each other and the substance contracts. When a base material (which has a negative charge) is applied, the ions repel each other and the material expands.

The National Aeronautics and Space Administration (NASA) has plans for artificial muscles. A small NASA rover destined to explore an asteroid in 2002 will be equipped with artificial muscles. Scientists hope tests like this one will eventually lead to the creation of space robots with humanlike flexibility and movement. Beyond that, they hope artificial muscles may someday be used to replace defective muscles in humans.

MUSCLES OF THE SHOULDERS AND ARMS. The fleshy, triangular-shaped muscles that form the rounded shape of the shoulders are the deltoid (DEL-toyd). They help abduct the arm, or move it away from the middle of the body. The most familiar muscle of the upper arm is the biceps brachii (BI-seps BRAY-key-eye.) Located on the front of the upper arm, the bicep makes a prominent bulge as it flexes the elbow. On the rear portion of the upper arms is the triceps brachii (TRY-seps BRAY-key-eye). Its action is just the opposite of the biceps: it extends or straightens the forearm.

The muscles of the forearm, which move the bones of the hands, are thin and long. Of these many muscles, the flexor carpi (FLEX-or CAR-pee) bend the wrist and the flexor digitorum (FLEX-or di-ji-TOR-um) bend the fingers. The muscles that have the opposite effect, extending the wrist and fingers, are the extensor carpi and the extensor digitorum.

MUSCLES OF THE HIPS AND LEGS. Muscles of the lower limbs cause movement at the hip, knee, and foot joints. These muscles are among the largest and strongest muscles in the body. Muscles on the thigh (upper portion of the leg) are especially massive and powerful since they hold the body upright against the force of gravity.

The gluteus maximus (GLOO-tee-us MAX-i-mus) are the large muscles that form most of the flesh of the buttocks. These powerful muscles help extend the hip in activities such as climbing stairs and jumping. The adductor (ah-DUC-ter) muscles are a group of muscles that form a mass on the inside of the thighs. As their name indicates, they adduct or press the thighs together.

On the front of the thigh is a group of four muscles known collectively as the quadriceps (KWOD-ri-seps). Together, the quadriceps help powerfully extend or straighten the knee, such as when an individual kicks a soccer ball. On the back of the thigh, a group of three muscles performs the

A muscle-tendon junction. As muscles are used, the tendons are able to withstand the constant pulling and tugging. (Reproduced by permission of Photo Researchers, Inc.)
A muscle-tendon junction. As muscles are used, the tendons are able to withstand the constant pulling and tugging. (Reproduced by permission of
Photo Researchers, Inc.
)

opposite effect. Known as hamstrings (HAM-strings), these muscles flex or bend the knee.

The sartorius (sar-TOR-ee-us) is long, straplike muscle that crosses the front of the thigh diagonally from the outside of the hip to the inside of the knee. Although it is not that powerful, it does lie on upper surface of the thigh and is easily seen. The sartorius helps rotate the leg so an individual can sit in a cross-legged position with the knees wide apart.

On the back part of the lower leg is the calf muscle, properly known as the gastrocnemius (gas-trok-NEE-me-us). This diamond-shaped muscle, formed in two sections, helps extend or lower the foot, such as when an individual walks on his or her toes. The strong tendon that attaches the gastrocnemius to the heel of the foot is the well-known Achilles tendon (ah-KI-leez; in Greek mythology, a hero of the Trojan War who is killed by an arrow shot into his heel). The main muscle on the front part of the lower leg, the tibialis anterior (tib-ee-A-lis), opposes the action of the gastrocnemius. It flexes and inverts or elevates the foot. When runners and other athletes experience tenderness and pain in the front part of the lower leg, a condition commonly known as shin splints, the tibialis anterior has been strained or pulled.



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