MD110 Anatomy & Physiology
Chapter 7 The Muscular System
I. Introduction
A. Muscular tissue enables the body and its parts to move
1. Movement caused by ability of muscle cells (called fibers) to shorten or contract
2. Muscle cells shorten by converting chemical energy (obtained from food) into mechanical energy, which causes movement
3. Three types of muscle tissue exist in the body
a Skeletal (voluntary)
b Smooth (involuntary)
c Cardiac
II. Muscle Tissue
A. Types of muscle tissue
1. Skeletal muscle also called striated of voluntary muscle
a Is 40% to 50% of body weight (red meat attached to bones)
b Microscope reveals crosswise stripes or striations
c Contractions can be voluntarily controlled
2. Cardiac muscle composes bulk of heart
a Cardiac muscle cells branch frequently
b Characterized by unique dark bands called intercalated disks
c Interconnected nature of cardiac muscle cells allows heart to contract efficiently as a unit
3. Nonstriated muscle or involuntary muscle also called smooth or visceral muscle
a Lacks cross stripes or striations when seen under a microscope; appears smooth
b Found in walls of hollow visceral structures such as digestive tract, blood vessels, and ureters
c Contractions not under voluntary control; movement caused by contractions is involuntary
B. Function all muscle cells specialize in contraction (shortening)
III. Structure of Skeletal Muscle
A. Structure
1. Each skeletal muscle is an organ composed mainly of skeletal muscle cells and connective tissue
2. Most skeletal muscles extend from one bone across a joint to another bone
3. Parts of a skeletal muscle
a Origin attachment to the bone that remains relatively stationary or fixed when movement at the joint occurs
b Insertion point of attachment to the bone that moves when a muscle contracts
c Body main part of the muscle
4. Muscles attach to bone by tendons strong cords of fibrous connective tissue; some tendons enclosed in synovial-lined tubes and are lubricated by synovial fluid; tubes are called tendon sheaths
5. Bursae small synovial-lined sacs containing a small amount of synovial fluid; located between some tendons and underlying bones
B. Microscopic structure
1. Contractile cells called fibers grouped into bundles
2. Fibers contain thick myofilaments (containing the protein myosin) and thin myofilaments (composed of actin)
3. Basic functional unit called sarcomere; sarcomeres separated from each other by dark bands called Z lines
a Sliding filament model explains mechanism of contraction
(1) Thick and thin myofilaments slide past each other as a muscle contracts
(2) Contraction requires calcium and energy-rich ATP molecules.
IV. Movement of skeletal muscle
A. Movement
1. Muscles produce movement; as a muscle contracts, it pulls the insertion bone closer to the origin bone; movement occurs at the joint between the origin and insertion.
a Groups of muscles usually contract to produce a single movement
(1) Prime mover muscle whose contraction is mainly responsible for producing a given movement
(2) Synergist muscle whose contractions help the prime mover produce a given movement
(3) Antagonist muscle whose actions oppose the action of a prime mover in any given movement
B. Posture
1. A specialized type of muscle contraction, called tonic contraction, enables us to maintain body position
a In tonic contraction, only a few of a muscles fibers shorten at one time
b Tonic contractions produce no movement of body parts
c Tonic contractions maintain muscle tone called posture
(1) Good posture reduces strain on muscles, tendons, ligaments, and bones
(2) Poor posture causes fatigue and may lead to deformity
C. Heat production
1. Survival depends on the bodys ability to maintain a constant body temperature
a Fever an elevated body temperature often a sign of illness
b Hypothermia a reduced body temperature
2. Contraction of muscle fibers produces most of the body heat required to maintain body temperature.
V. Fatigue
A. Reduced strength of muscle contraction
B. Caused by repeated muscle stimulation without adequate periods of rest
C. Repeated muscular contraction depletes cellular ATP stores and outstrips the ability of the blood supply to replenish oxygen and nutrients
D. Contraction in the absence of adequate oxygen produces lactic acid, which contributes to muscle soreness
E. Oxygen debt term used to describe the metabolic effort required to burn excess lactic acid that may accumulate during prolonged periods of exercise; the body is attempting to return the cells energy and oxygen reserves to pre-exercise levels.
VI. Role of other body systems in movement
A. Muscle functioning depends on the functioning of many other parts of the body
1. Most muscles cause movements by pulling on bones across moveable joints
2. Respiratory, circulatory, nervous, muscular, and skeleltal systems play essential roles in producing normal movements
3. Multiple sclerosis, brain hemorrhage, and spinal cord injury are examples of how pathological conditions in other body organ systems can dramatically affect movement
VII. Motor Unit
A. Stimulation of a muscle by a nerve impulse is required before a muscle can shorten and produce movement
B. A motor neuron is the specialized nerve cell that transmits an impulse to a muscle, causing contraction
C. A neuromuscular junction is the specialized point of contact between a nerve ending and the muscle fiber it innervates
D. A motor unit is the combination of a motor neuron with the muscle cell or cells it innervates
VIII. Muscle Stimulus
A. A muscle will contract only if an applied stimulus reaches a certain level of intensity
1. A threshold stimulus is the minimum level of stimulation required to cause a muscle fiber to contract.
B. Once stimulated by a threshold stimulus, a muscle fiber will contract completely, a response called all or none
C. Different muscle fibers in a muscle are controlled by different motor units having different threshold-stimulus levels
1. Although individual muscle fibers always respond all or none to a threshold stimulus, the muscle as a whole does not
2. Different motor units responding to different threshold stimuli permit a muscle as a whole to execute contractions of graded force
IX. Types of Skeletal Muscle Contractions
A. Twitch and tetanic contractions
1. Twitch contractions are laboratory phenomenon and do not play a significant role in normal muscular activity; they are a single contraction of muscle fibers caused by a single threshold stimulus
2. Tetanic contractions are sustained and steady muscular contractions caused by a series of stimuli bombarding a muscle in rapid succession.
B. Isotonic contractions
1. Contraction of a muscle that produces movement at a joint
2. During isotonic contractions, the muscle shortens, causing the insertion end of the muscle to move toward the point of origin
3. Most types of body movements such as walking and running are caused by isotonic contractions
C. Isometric Contractions
1. Isometric contractions are muscle contractions that do not produce movement; the muscle as a whole does not shorten
2. Although no movement occurs during isometric contractions; tension within the muscle increases
X. Effects of Exercise on Skeletal Muscles
A. Exercise, if regular and properly practiced, improves muscle tone and posture, results in more efficient heart and lung functioning, and reduces fatigue.
B. Effects of exercise on skeletal muscles
1. Muscles undergo changes related to the amount of work they do.
a Prolonged disuse causes disuse atrophy
b Regular exercise increases muscle size, called hypertrophy
2. Strength training is exercise involving contraction of muscles against heavy resistance.
a Strength training increase the numbers of myofilaments in each muscle fiber, and as a result, the total mass of the muscle increases.
b Strength training does not increase the number of muscle fibers
3. Endurance training is exercise that increases a muscles ability to sustain moderate exercise over a long period; it is sometimes called aerobic training
a Endurance training allows more efficient delivery of oxygen and nutrients to a muscle via increased blood flow
b Endurance training does not usually result in muscle hypertrophy
XI. Skeletal Muscle Groups
A. Muscles of the head and neck
1. Facial muscles
a Orbicularis oculi
b Orbicularis oris
c Zygomaticus
2. Muscles of mastication (chewing)
a Messeter
b Temporal
3. Sternocleidomastoid flexes head
4. Trapezius elevates shoulders and extends head
B. Muscles that move the upper extremities
1. Pectoralis major flexes upper arm
2. Latissimus dorsi extends upper arm
3. Deltoid abducts upper arm
4. Biceps brachii flexes forearm
5. Triceps brachii extends forearm
C. Muscles of the trunk
1. Abdominal muscles
a Rectus abdominus
b External oblique
c Internal oblique
d Transverse abdominus
2. Respiratory muscles
a Intercostal muscles
b Diaphragm
D. Muscles that move the lower extremities
1. Iliopsoas flexes thigh
2. Gluteus maximus extends thigh
3. Adductor muscles adducts thigh
4. Hamstring muscles flex lower leg
a Semimembranous
b Semitendinosus
c Biceps femoris
5. Quadriceps femoris group extend lower leg
a Rectus femoris
b Vastus muscles
6. Tibialis anterior dorsiflexes foot
7. Gastrocnemius plantar flexes foot
8. Peroneous group flex foot
XII. Types of movement produced by skeletal muscle contractions
A. Flexion movement that decreases the angle between two bones at their joint: bending
B. Extension movement that increases the angle between two bones at their joint: straightening
C. Abduction movement of a part away from the midline of the body
D. Adduction movement of a part toward the midline of the body
E. Rotation movement around a longitudinal axis
F. 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
G. Dorsiflexion and plantar flexation foot movements; dorsiflexion results in elevation of the dorsum or top of the foot; during plantar flexation, the bottom of the foot is directed downward
XIII. Major Muscular Disorders
A. Myopathies muscle disorders; can range from mild to life threatening
B. Muscle injury
1. Strain injury from overexertion of trauma; involves stretching or tearing of muscle fibers
a Often accompanied by muscle pain (myalgia)
b May result in inflammation of muscle (myositis) or of muscle and tendon (fibromyositis)
c If injury is near a joint and involves ligament damage, it may be called a sprain
2. Cramps are painful muscle spasms (involuntary twitches)
3. Crush injuries result from severe muscle trauma and may release cell contents that ultimately cause kidney failure
4. Stress-induced muscle tension can cause headaches and back pain.
C. Infections
1. Several bacteria, viruses, and parasites can infect muscles
2. Poliomyelitis is a viral infection of motor nerves that ranges from mild to life threatening
D. Muscular dystrophy
1. A group of genetic disorders characterized by muscle atrophy
2. Duchenne (pseudohypertrophic) muscular dystrophy is the most common type
a Characterized by rapid progression of weakness and trophy, resulting in death by age 21
b X-linked inherited disease , affecting mostly boys
E. Myasthenia gravis autoimmune muscle disease characterized by weakness and chronic fatigue.