Download Chapter 7 The Muscular System

Chapter 7
The Muscular System
INTRODUCTION

Muscular tissue enables the body and its
parts to move

Three types of muscle tissue exist in body (see
Chapter 3)
 Movement caused by muscle cells (called
fibers) shortening, or contracting
 Muscle movement occurs when chemical
energy (obtained from food) is converted into
mechanical energy
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MUSCLE TISSUE

Types of muscle tissue (Figure 7-1)

Skeletal muscle—also called striated or
voluntary muscle
• Microscope reveals crosswise stripes or striations
• Contractions can be voluntarily controlled
 Cardiac muscle—composes bulk of heart
• Cardiac muscle fibers branch frequently
• Characterized by unique dark bands called
intercalated disks
• Interconnected nature of cardiac muscle fibers allows
heart to contract efficiently as a unit
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MUSCLE TISSUE

Types of muscle tissue (cont.)

Nonstriated muscle, or involuntary muscle—
also called smooth 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

Function—all muscle fibers specialize in
contraction (shortening)
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STRUCTURE OF SKELETAL MUSCLE

Composition—mainly striated muscle fibers
and connective tissue
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Most skeletal muscles extend from one bone across a
joint to another bone
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

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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 called tendon sheaths
Bursae—small synovial-lined sacs containing a small
amount of synovial fluid; located between some tendons
and underlying bones
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STRUCTURE OF SKELETAL MUSCLE

Microscopic structure (Figure 7-3)
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Contractile cells called fibers—grouped into bundles
Fibers contain thick myofilaments (containing the protein
myosin) and thin myofilaments (composed of actin)
Basic functional (contractile) unit called sarcomere
• Sarcomeres 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
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FUNCTIONS OF SKELETAL MUSCLE

Movement

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 the insertion
• Groups of muscles usually contract to produce a
single movement
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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
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FUNCTIONS OF SKELETAL MUSCLE
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Posture
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A specialized 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
• Tonic contractions maintain muscle tone called posture
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
Good posture reduces strain on muscles, tendons, ligaments,
and bones
Poor posture causes fatigue and may lead to deformity
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FUNCTIONS OF SKELETAL MUSCLE

Heat production
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Survival depends on the body’s ability to
maintain a constant body temperature
• Fever—an elevated body temperature—often a sign
of illness
• Hypothermia—a reduced body temperature

Contraction of muscle fibers produces most of
the heat required to maintain normal body
temperature
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FATIGUE

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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
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FATIGUE
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Contraction in the absence of adequate
oxygen produces lactic acid, which
contributes to muscle soreness
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
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ROLE OF OTHER BODY SYSTEMS IN
MOVEMENT
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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
Multiple sclerosis, brain hemorrhage, and spinal
cord injury are examples of how pathological
conditions in other body organ systems can
dramatically affect movement
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MOTOR UNIT (Figure 7-4)
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Stimulation of a muscle by a nerve impulse is
required before a muscle can shorten and
produce movement
A motor neuron is the specialized nerve that
transmits an impulse to a muscle, causing
contraction
A neuromuscular junction is the specialized point
of contact between a nerve ending and the
muscle fiber it innervates
A motor unit is the combination of a motor neuron
with the muscle fiber or fibers it innervates
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MUSCLE STIMULUS
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A muscle will contract only if an applied
stimulus reaches a certain level of intensity
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A threshold stimulus is the minimal level of
stimulation required to cause a muscle fiber to
contract
Once stimulated by a threshold stimulus, a
muscle fiber will contract completely, a
response called all or none
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MUSCLE STIMULUS
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Different muscle fibers in a muscle are
controlled by different motor units having
different threshold-stimulus levels
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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
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TYPES OF SKELETAL MUSCLE
CONTRACTION
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Twitch and tetanic contractions
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Twitch contractions are laboratory phenomena
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
Tetanic contractions are sustained and steady
muscular contractions caused by a series of
stimuli bombarding a muscle in rapid
succession
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TYPES OF SKELETAL MUSCLE
CONTRACTION
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Isotonic contractions
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Contraction of a muscle that produces movement at a
joint because the muscle changes length
During concentric contractions, the muscle shortens
insertion end of the muscle to move toward the point of
origin
During eccentric contractions, the muscle lengthens
under tension, thus moving the insertion away from the
origin
Most types of body movements such as walking and
running are produced by isotonic contractions
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TYPES OF SKELETAL MUSCLE
CONTRACTION
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Isometric contractions
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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
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EFFECTS OF EXERCISE ON SKELETAL
MUSCLES
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Exercise, if regular and properly practiced,
improves muscle tone and posture, results
in more efficient heart and lung
functioning, and reduces fatigue
Muscles change in relation to the amount
of work they normally do
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Prolonged inactivity causes disuse atrophy
Regular exercise increases muscle size, called
hypertrophy
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EFFECTS OF EXERCISE ON SKELETAL
MUSCLES
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Strength training is exercise involving contraction of muscles
against heavy resistance
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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

Endurance training is exercise that increases a muscle’s
ability to sustain moderate exercise over a long period; it is
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
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SKELETAL MUSCLE GROUPS
(Table 7-1)
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Muscles of the head and neck (Figure 7-7)
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Facial muscles
• Orbicularis oculi
• Orbicularis oris
• Zygomaticus
 Muscles of mastication
• Masseter
• Temporal
 Sternocleidomastoid—flexes head
 Trapezius—elevates shoulders and extends
head
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SKELETAL MUSCLE GROUPS
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Muscles that move the upper extremities
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Pectoralis major—flexes upper arm
 Latissimus dorsi—extends upper arm
 Deltoid—abducts upper arm
 Biceps brachii—flexes forearm
 Triceps brachii—extends forearm
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SKELETAL MUSCLE GROUPS
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Muscles of the trunk (Figure 7-8)
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Abdominal muscles
• Rectus abdominis
• External oblique
• Internal oblique
• Transversus abdominis
Respiratory muscles
• Intercostal muscles
• Diaphragm
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SKELETAL MUSCLE GROUPS
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Muscles that move the lower extremities
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Iliopsoas—flexes thigh
Gluteus maximus—extends thigh
Adductor muscles—adduct thighs
Hamstring muscles—flex lower leg
• Semimembranosus
• Semitendinosus
• Biceps femoris
Quadriceps femoris group—extend lower leg
• Rectus femoris
• Vastus muscles
Tibialis anterior—dorsiflexes foot
Gastrocnemius—plantar flexes foot
Peroneus group—flex foot
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MOVEMENTS PRODUCED BY
SKELETAL MUSCLE CONTRACTIONS
(Figures 7-9 through 7-11)
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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
Abduction—movement of a part away from the
midline of the body
Adduction—movement of a part toward the
midline of the body
Rotation—movement around a longitudinal axis
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MOVEMENTS PRODUCED BY
SKELETAL MUSCLE CONTRACTIONS
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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
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
Inversion and eversion—foot movements;
inversion moves ankle to side so that sole faces
inward toward midline; eversion turns ankle so
that the sole faces out to the side
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