The Muscular System
(rev 3-10)
Muscular tissue enables the body and its parts
to move
• Movement is caused by ability of muscle
cells (called fibers) to shorten or contract
–
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Function of muscle tissue is to contract (or
shorten)
Muscle cells use energy to produce
movement
We have 3 types of muscle tissue in our
bodies
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Skeletal muscle —also called striated or
voluntary muscle
– This type muscle has stripes or striations
– Contractions can be voluntarily controlled
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• Cardiac muscle —composes bulk of
heart
– Striated muscle; involuntary (contractions
not under voluntary control)
– Characterized by unique dark bands
called intercalated disks
– Cells branch frequently
– Interconnected nature of cardiac muscle
cells allows heart to contract efficiently as a
unit
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• Smooth or visceral muscle
– Nonstriated, involuntary 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 contractions is
involuntary
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Structure of Skeletal Muscle
• Each skeletal muscle is an organ composed
of skeletal muscle cells and connective tissue
– 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 or Belly—main part of the muscle
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– Muscles attach to the bone by tendons
which are made of strong fibrous
connective tissue
– Some tendons are enclosed in synoviallined tubes, called tendon sheaths, and
are lubricated by synovial fluid
--Bursae—small synovial-lined sacs
containing a small amount of synovial fluid;
located between some tendons and
underlying bones
– Both tendon sheaths and bursae make it
easier for a tendon to slide over bone
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Microscopic structure
• Specialized contractile cells called fibers are
grouped into bundles
– Each skeletal muscle fiber contains thick
myofilaments (formed from the protein myosin)
and thin myofilaments (formed from the protein
actin)
– These myofilaments make the striations or stripes
we can see in skeletal muscle fibers
– The basic functional (contractile) unit of the
muscle cell is called a sarcomere. Sarcomeres
are separated from each other by dark bands
called Z lines
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• Sliding filament model explains mechanism of
contraction
– Thick and thin myofilaments slide toward
each other as a muscle contracts and
shorten the muscle. This is called the
sliding filament model.
• Contraction requires calcium and ATP
Functions of Skeletal Muscle:
1.Movement
2.Posture or muscle tone
3.Heat production
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• 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
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– Groups of muscles usually contract to
produce a single movement
• Prime mover —muscle whose
contraction is mainly responsible for
producing a movement
• Synergist —muscle whose contractions
help the prime mover produce a
movement
• Antagonist —muscle whose actions
oppose the action of a prime mover in a
movement
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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
• 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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• Supination--hand position with the palm turned
to the anterior position
• Pronation—hand position with the palm turned to
the posterior position
• Dorsiflexion--elevation of the dorsum or top of
the foot and
• Plantar flexion--the bottom of the foot is directed
downward
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Posture
– A specialized type of muscle contraction,
called tonic contraction, enables us to
maintain body position
• In a tonic contraction, only a few of a muscle’s
fibers shorten at one time
• Tonic contractions produce no movement of
body parts but DO hold muscles in position
– Tonic contractions maintain muscle tone
called posture
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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—a reduced body temperature
– Contraction of muscle fibers produces most of the
heat required to maintain normal body
temperature
• Energy for muscle contraction is obtained from
ATP. Some of the energy is lost as heat and
this is what maintains our body temperature
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Fatigue
• Causes reduced strength of muscle
contraction due to lack of sufficient rest
• Caused by repeated muscle stimulation
• Repeated muscular contraction uses up
the cellular ATP store and exceeds the
ability of the blood supply to replenish
oxygen and nutrients
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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 to remove the 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
• Heavy breathing after exercises helps to
return level of oxygen in blood to normal
levels
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Motor Unit
Stimulation of a muscle by a nerve impulse is
required before a muscle can produce
movement
A motor neuron is the 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 cell or cells it
innervates
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Muscle Stimulus
• A muscle will contract only if the
stimulus reaches a certain level of
intensity
– 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; this is called an all or none
response
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• 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
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Types of Skeletal Muscle Contraction
• Isotonic contractions
 Contraction of a muscle that produces
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
 Most types of body movements such as
walking and running are caused by isotonic
contractions
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Types of Skeletal Muscle Contraction
• Isometric contractions
 Isometric contractions are muscle contractions that
do not produce movement
 Although no movement occurs, tension within the
muscle increases
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Selected Skeletal Muscle Groups
Muscles of the head and neck
Muscles of the head and neck
 Facial muscles
 Orbicularis oculi—closes your eye
 Orbicularis oris—puckers your lips (kissing muscle)
 Zygomaticus—raises the corners of the mouth
(smiling muscle)
 Temporalis—chewing muscle; closes jaw
 Masseter—chewing muscle; closes jaw
 Neck and Shoulder muscles
 Sternocleidomastoid—flexes head
 Trapezius—elevates shoulders and extends head
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• Muscles that move the upper extremities
 Pectoralis major—flexes upper arm; hugging
muscle
 Latissimus dorsi—extends upper arm
 Deltoid—abducts upper arm
 Biceps brachii—flexes forearm
 Triceps brachii—extends forearm
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Muscles of the trunk
 Abdominal muscles
 Rectus abdominis—flexes trunk
• Respiratory muscles
 Intercostal muscles
 Diaphragm
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Muscles that move the lower extremities
 Iliopsoas—flexes thigh
 Gluteus maximus—extends thigh
 Hamstring muscles—flex lower leg
 Semimembranosus
 Semitendinosus
 Biceps femoris
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• Muscles that move the lower extremities
 Quadriceps femoris group—extend lower
leg
 Rectus femoris
 3 Vastus muscles
 Tibialis anterior—dorsiflexes foot
 Gastrocnemius—plantar flexes foot
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