Chapter 8 The Muscular System

Objectives List, locate in the body, and compare the structure and function of the three major types of muscle tissue Discuss the microscopic structure of a skeletal muscle sarcomere and motor unit Discuss how a muscle is stimulated and compare the major types of skeletal muscle contractions

Objectives Name, identify on a model or diagram, and give the function of the major muscles of the body discussed in this chapter List and explain the most common types of movement produced by skeletal muscles Name and describe the major disorders of skeletal muscles

Introduction Muscular tissue enables the body and its parts to move Movement caused by ability of muscle cells (called fibers) to shorten or contract Muscle cells shorten by converting chemical energy (obtained from food) into mechanical energy, which causes movement Three types of muscle tissue in body What causes movement of the body? Skeletal muscle contraction.

Muscle Tissue Skeletal muscle—also called striated or voluntary muscle Is 40% to 50% of body weight (“red meat” attached to bones) Microscope reveals crosswise stripes or striations Contractions can be voluntarily controlled What are the three types of muscle tissue? Skeletal, cardiac, and smooth. If someone weighs 120 pounds, about 50 pounds of the body weight comes from the skeletal muscles.

Muscle Tissue Cardiac muscle—comprises bulk of heart Cardiac muscle cells branch frequently Characterized by unique dark bands called intercalated disks Interconnected nature of cardiac muscle cells allows heart to contract efficiently as a unit What is the value of the interconnected nature of the cardiac muscle? It increases the efficiency of the heart muscle in pumping blood.

Muscle Tissue Smooth muscle—also called nonstriated, involuntary, or visceral muscle Lacks cross stripes or striations when seen under a microscope; appears smooth Found in walls of hollow visceral structures such as digestive tract, blood vessels, and ureters Contractions not under voluntary control; movement caused by contraction is involuntary

Muscle tissue. A, Skeletal muscle. B, Cardiac muscle. C, Smooth muscle.

Muscle Tissue Function—all muscle cells specialize in contraction (shortening)

Structure of Skeletal Muscle Major structures Each skeletal muscle is an organ composed mainly of skeletal muscle cells and connective tissue Most skeletal muscles extend from one bone across a joint to another bone What are the origin and the insertion point, and what are their roles in movement of the body? The origin of a muscle is its attachment to the more stationary bone. The insertion is the attachment to the more movable bone. When muscles contract, they pull on the more movable bone.

Structure of Skeletal Muscle Parts of a skeletal muscle Origin—attachment to the bone that remains relatively stationary or fixed when movement at the joint occurs Insertion—point of attachment to the bone that moves when a muscle contracts Body—main part of the muscle

Attachments of a skeletal muscle Attachments of a skeletal muscle. A muscle originates at a relatively stable part of the skeleton (origin) and inserts at the skeletal part that is moved when the muscle contracts (insertion).

Structure of Skeletal Muscle Muscles attach to bone by tendons—strong cords of fibrous connective tissue; some tendons enclosed in synovial-lined tubes called tendon sheaths; tendons lubricated by synovial fluid Bursae—small synovial-lined sacs containing a small amount of synovial fluid; located between some tendons and underlying bones Tendons do not tear or pull away from bone easily, yet emergency department physicians and nurses frequently see torn or severed tendon injuries. Small, fluid-filled sacs called bursae lie between some tendons and the bones beneath them. What is the role of bursae? Bursae make it easier for a tendon to slide over a bone when the muscle shortens.

Structure of Skeletal Muscle Microscopic structure and function Contractile cells, or muscle fibers—grouped into bundles and intricately arranged Fibers contain thick myofilaments (containing the protein myosin) and thin myofilaments (composed of actin) What are thick and thin myofilaments composed of? Thick myofilaments are formed from protein myosin; thin myofilaments are composed of actin.

Structure of skeletal muscle Structure of skeletal muscle. A, Each muscle organ has many muscle fibers, each containing many bundles of thick and thin myofilaments. The diagrams show the overlapping thick and thin filaments arranged to form adjacent segments called sarcomeres. During contraction, the thin filaments are pulled toward the center of each sarcomere, thereby shortening the whole muscle. B, This electron micrograph shows that the overlapping thick and thin filaments within each sarcomere create a pattern of dark striations in the muscle. The extreme magnification allowed by electron microscopy has revolutionized our concept of the structure and function of skeletal muscle and other tissues. (B, Courtesy Dr. H.E. Huxley.)

Structure of Skeletal Muscle Microscopic structure Basic functional (contractile) unit called a sarcomere Sarcomeres are separated from each other by dark bands called Z lines Sliding filament model explains mechanism of contraction Thick and thin myofilaments slide past each other as a muscle contracts Contraction requires calcium and energy-rich ATP molecules As the osteon (Haversian system) serves as the basic building block in compact bone, the sarcomere serves that function in muscle. What is required for contraction? Calcium and energy-rich ATP molecules.

Mechanisms of muscle contraction.

Functions of Skeletal Muscle Movement Muscles produce movement by pulling on bones as a muscle contracts The insertion bone is pulled closer to the origin bone Movement occurs at the joint between the origin and the insertion Describe how muscles produce movement. Muscles produce movement by pulling on bones. Contraction, or shortening, of the muscle pulls the insertion bone toward the origin bone.

Functions of Skeletal Muscle Movement Groups of muscles usually contract to produce a single movement Prime mover—muscle whose contraction is mainly responsible for producing a given movement Synergist—muscle whose contractions help the prime mover produce a given movement Antagonist—muscle whose actions oppose the action of a prime mover in any given movement When a muscle group produces movement, what are the components of the group called? Prime mover, synergist, antagonist. When antagonist muscles contract, roles are reversed and they produce opposite movement. Voluntary movement is normally smooth and free of jerks because skeletal muscles work in coordinated teams, not separately.

Functions of Skeletal Muscle Posture A type of muscle contraction called tonic contraction enables us to maintain body position In tonic contraction, only a few of a muscle’s fibers shorten at one time Tonic contractions produce no movement of body parts Good posture means that body parts are held in positions that favor best function. Bad posture causes fatigue and may lead to deformity.

Functions of Skeletal Muscle Posture Good posture (optimum body positioning) favors best body functioning Skeletal muscle tone maintains posture by counteracting the pull of gravity

Functions of Skeletal Muscle Heat production Survival depends on the body’s ability to maintain a constant body temperature Fever—an elevated body temperature—often a sign of illness Hypothermia—body temperature below normal Contraction of muscle fibers produces most of the heat required to maintain normal body temperature Heat is produced by the breakdown of adenosine triphosphate (ATP) during contractions.

Fatigue Reduced strength of muscle contraction Caused by repeated muscle stimulation without adequate periods of rest Repeated muscular contraction depletes cellular ATP stores and outstrips the ability of the blood supply to replenish oxygen and nutrients What can cause a decrease in the strength of muscle contraction? Muscle cells being repeatedly stimulated without adequate rest.

Fatigue Contraction in the absence of adequate oxygen produces lactic acid, which contributes to muscle burning Oxygen debt—the metabolic effort required to burn excess lactic acid that may accumulate during prolonged periods of exercise Labored breathing after strenuous exercise is required to “pay the debt” This increased metabolism helps restore energy and oxygen reserves to pre-exercise levels Labored breathing is required to help the body pay the oxygen debt—an example of homeostasis at work. What is the body attempting to do on the cellular level during labored breathing after exercise? To return the cells’ energy and oxygen reserves to pre-exercise levels.

Role of Other Body Systems in Movement Muscle functioning depends on the functioning of many other parts of the body Most muscles cause movements by pulling on bones across movable joints Respiratory, circulatory, nervous, muscular, and skeletal systems play essential roles in producing normal movements Normal function of one body part depends on the normal function of all other parts.

Role of Other Body Systems in Movement Muscle functioning depends on the functioning of many other parts of the body Multiple sclerosis, brain hemorrhage, and spinal cord injury are examples of how pathological conditions in other body organ systems can dramatically affect movement What are some examples of pathological conditions that might affect movement of the body? Skeletal system disorders, multiple sclerosis, brain hemorrhage, and spinal cord injury.

Motor Unit Stimulation of a muscle by a nerve impulse is required before a muscle can shorten and produce movement A motor neuron is the nerve cell that transmits an impulse to a muscle, causing contraction Specialized chemicals are released by the motor neuron in response to a nerve impulse.

Motor Unit A neuromuscular junction is the point of contact between a nerve ending and the muscle fiber it innervates A motor unit is the combination of a motor neuron and the muscle cell or cells it innervates

Motor neuron. A motor unit consists of one motor neuron and the muscle fibers supplied by its branches. (Courtesy Dr. Paul C. Letourneau, Department of Anatomy, Medical School, University of Minnesota, Minneapolis.)

Muscle Stimulus A muscle will contract only if an applied stimulus reaches a certain minimal level of intensity—called a threshold stimulus Once stimulated by a threshold stimulus, a muscle fiber will contract completely, a response called all or none What is a threshold stimulus? The minimal level of stimulation required to cause a muscle fiber to contract.

Muscle Stimulus Different muscle fibers in a muscle are controlled by different motor units having different threshold-stimulus levels Although individual muscle fibers always respond all or none to a threshold stimulus, the muscle as a whole does not Different motor units responding to different threshold stimuli permit a muscle as a whole to execute contractions of graded force How is a graded response of a muscle accomplished? Different motor units responding to different threshold stimuli permit a muscle as a whole to execute contractions of graded force.

Types of Skeletal Muscle Contraction Twitch and tetanic contractions Twitch contractions—quick, jerky responses to a stimulus—are laboratory phenomena and do not play a significant role in normal muscular activity Tetanic contractions are sustained and steady muscular contractions caused by a series of stimuli bombarding a muscle in rapid succession About 30 stimuli per second evoke tetanic contractions in certain types of skeletal muscles. Are twitch contractions significant for normal activity? No

Types of Skeletal Muscle Contraction Isotonic contractions Produce movement at a joint During isotonic contractions, the muscle changes length, causing the insertion end of the muscle to move relative to the point of origin Concentric contractions shorten muscles Eccentric contractions allow muscles to increase in length Most types of body movements such as walking and running are caused by isotonic contractions What is an isotonic contraction? A contraction that produces movement at a joint. The muscle changes length and the insertion end moves closer to the point of origin.

Types of Skeletal Muscle Contraction Isometric contractions Isometric contractions are muscle contractions that do not produce movement; the muscle as a whole does not shorten Although no movement occurs during isometric contractions, tension within the muscle increases What is an isometric contraction? A muscle contraction that does not produce movement. It increases tension within a muscle but does not change the length of the muscle. What are the differences between isotonic and isometric contractions? Isotonic contractions produce movement; isometric contractions increase tension within a muscle but do not change the length of the muscle. Pushing against a wall is an example of an isometric contraction.

Types of muscle contraction Types of muscle contraction. A, In isotonic contraction the muscle changes length, producing movement either by eccentric contraction (muscle lengthens) or concentric contraction (muscle shortens). B, In isometric contraction the muscle pulls forcefully against a load but does not shorten.

Effects of Exercise on Skeletal Muscles Exercise, if done regularly and practiced properly, improves muscle tone and posture, results in more efficient heart and lung functioning, and reduces fatigue Specific effects of exercise on skeletal muscles Muscles undergo changes related to the amount of work they normally do Prolonged inactivity causes disuse atrophy Regular exercise increases muscle size, called hypertrophy What are some of the health benefits of regular exercise? Greatly improved muscle tone, better posture, more efficient heart and lung function, less fatigue, looking and feeling better! What type of changes do muscles undergo relative to the amount of work they normally do? Disuse atrophy, hypertrophy

Effects of Exercise on Skeletal Muscles Strength training involves contraction of muscles against heavy resistance Strength training increases the number of myofilaments in each muscle fiber, and as a result, the total mass of the muscle increases Strength training does not increase the number of muscle fibers What are examples of strength training? Isometric exercises and weight lifting

Effects of Exercise on Skeletal Muscles Endurance training increases a muscle’s ability to sustain moderate exercise over a long period; sometimes called aerobic training Endurance training allows more efficient delivery of oxygen and nutrients to a muscle via increased blood flow Endurance training does not usually result in muscle hypertrophy What are the differences between strength training and endurance training? Strength training can produce muscle hypertrophy by contracting muscles against resistance. Endurance training, also called aerobic training, increases a muscle’s ability to sustain moderate exercise over a long period without muscle hypertrophy.

Skeletal Muscle Groups Muscles of the head and neck Facial muscles Orbicularis oculi Orbicularis oris Zygomaticus What are the names of the “kissing muscle” and the “smiling muscle”? Orbicularis oris is the kissing muscle, and zygomaticus is the smiling muscle.

Skeletal Muscle Groups Muscles of the head and neck Muscles of mastication Masseter Temporal Sternocleidomastoid—flexes head Trapezius—elevates shoulders and extends head Mastication muscles are among the strongest muscles in the body.

General overview of the body musculature. A, Anterior view General overview of the body musculature. A, Anterior view. B, Posterior view. (John V. Hagen.)

Muscles of the head and neck Muscles of the head and neck. Muscles that produce most facial expressions surround the eyes, nose, and mouth. Large muscles of mastication stretch from the upper skull to the lower jaw. These powerful muscles produce chewing movements. The neck muscles connect the skull to the trunk of the body, rotating the head or bending the neck. (John V. Hagen.)

Skeletal Muscle Groups Muscles that move the upper extremities Pectoralis major—flexes upper arm Latissimus dorsi—extends upper arm Deltoid—abducts upper arm Biceps brachii—flexes forearm Triceps brachii—extends forearm The upper extremity is attached to the thorax by the pectoralis major. What is the name of the “boxer’s muscle”? Triceps brachii

Skeletal Muscle Groups Muscles of the trunk Abdominal muscles Rectus abdominis External oblique Internal oblique Transversus abdominis Respiratory muscles Intercostal muscles Diaphragm Rectus abdominis flexes the spinal column. Which muscles change the size of the chest during respiration? Respiratory muscles, intercostal muscles and diaphragm

Muscles of the trunk. A, Anterior view showing superficial muscles Muscles of the trunk. A, Anterior view showing superficial muscles. B, Anterior view showing deeper muscles. (John V. Hagen.)

Skeletal Muscle Groups Muscles that move the lower extremities Iliopsoas—flexes thigh Gluteus maximus—extends thigh Adductor muscles—adduct thighs Hamstring muscles—flex lower leg Semimembranosus Semitendinosus Biceps femoris Iliopsoas normally flexes the thigh, but it can flex the trunk if the thigh is fixed/immovable, as in sit-ups.

Skeletal Muscle Groups Muscles that move the lower extremities Quadriceps femoris group—extend lower leg Rectus femoris Vastus muscles Tibialis anterior—dorsiflex foot Gastrocnemius—plantar flex foot Peroneus group—flexes foot Gastrocnemius is sometimes called the toe dancer’s muscle.

Types of Movements Produced by Skeletal Muscle Contractions Flexion—movement that decreases the angle between two bones at their joint: bending Extension—movement that increases the angle between two bones at their joint: straightening

Flexion and extension of the lower arm Flexion and extension of the lower arm. A and B, When the lower arm is flexed at the elbow, the biceps brachii contracts while its antagonist, the triceps brachii, relaxes. B and C, When the lower arm is extended, the biceps brachii relaxes while the triceps brachii contracts. (A, C, Rolin Graphics.)

Flexion and extension of the lower leg Flexion and extension of the lower leg. A and B, When the lower leg _ exes at the knee, muscles of the hamstring group contract while their antagonists in the quadriceps femoris group relax. B and C, When the lower leg extends, the hamstring muscles relax while the quadriceps femoris muscle contracts. (A, C, Rolin Graphics.)

Types of Movements Produced by Skeletal Muscle Contractions Abduction—movement of a part away from the midline of the body Adduction—movement of a part toward the midline of the body Rotation and circumduction—movement around a longitudinal axis What are examples of the types of movements listed here? Flexion: bending elbow or knee; extension: straightening elbow or knee; abduction: lowering arm; rotation: shaking head “no.”

Types of Movements Produced by Skeletal Muscle Contractions Supination and pronation—hand positions that result from rotation of the forearm; supination results in a hand position with the palm turned to the anterior position; pronation occurs when the palm faces posteriorly What are the origins of the words “supination” and “pronation”? Supine, which means lying face up and prone, which means lying face down.

Types of Movements Produced by Skeletal Muscle Contractions Dorsiflexion and plantar flexion—foot movements; dorsiflexion results in elevation of the dorsum or top of the foot; during plantar flexion, the bottom of the foot is directed downward

Examples of body movements. A, Adduction and abduction. B, Rotation Examples of body movements. A, Adduction and abduction. B, Rotation. C, Pronation and supination. D, Dorsiflexion and plantar flexion.

Major Muscular Disorders Myopathies—muscle disorders; can range from mild to life threatening Muscle injury Strain—injury from overexertion or trauma; involves stretching or tearing of muscle fibers Often accompanied by myalgia (muscle pain) May result in inflammation of muscle (myositis) or of muscle and tendon (fibromyositis) If injury is near a joint and involves ligament damage, it may be called a sprain

Major Muscular Disorders Muscle injury Cramps are painful muscle spasms (involuntary twitches) Crush injuries result from severe muscle trauma and may release cell contents that ultimately cause kidney failure Stress-induced muscle tension can cause headaches and back pain

Major Muscular Disorders Infections Several bacteria, viruses, and parasites can infect muscles Poliomyelitis is a viral infection of motor nerves that ranges from mild to life threatening Why is polio no longer a threat to the U.S. population? A successful vaccination program has halted transmission of the disease. Is polio a threat anywhere in the world? It is still contracted in Third World regions where the population has not been vaccinated, but it is becoming less and less of a threat. What are the effects of polio on the body? It is a viral infection of the nerves that control skeletal muscle movement. It can cause paralysis that may progress to death because of the muscles affected.

Major Muscular Disorders Muscular dystrophy A group of genetic disorders characterized by muscle atrophy Duchenne (pseudohypertrophic) muscular dystrophy is the most common type Characterized by rapid progression of weakness and atrophy, resulting in death by age 21 X-linked inherited disease, affecting mostly boys

Major Muscular Disorders Myasthenia gravis—autoimmune muscle disease characterized by weakness and chronic fatigue