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Chapter 7 The Muscular System
Specialized tissue that enable the body and its parts to move.
Summary
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Functions of skeletal muscle
Structure of skeletal muscle
Contraction
Muscles
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Head and neck
Upper extremity
Trunk
Lower extremities
Types of movements
Functions of Skeletal Muscle
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Movement – muscle move bones by pulling not pushing.
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Synergists – any movement is generally accomplished by more
than one muscle. All of the muscles responsible for a particular
movement are synergists. The one that is most responsible for
the movement is the Prime Mover.
Antagonists - muscles and muscle groups usually work in pairs
– example the biceps flex your arm and its partner, the triceps,
extend your arm. The two muscles are antagonists, i.e. cause
opposite actions. When one contracts the other relaxes.
Levators – muscle that raise a body part.
Functions of Skeletal Muscle
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Maintenance of posture or muscle tone
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We are able to maintain our body position because of tonic
contractions in our skeletal muscles. These contractions don’t
produce movement yet hold our muscles in position.
Heat production – contraction of muscles produces most
of the heat required to maintain body temperature.
Structure of Skeletal Muscle
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Composed of striated muscle cells (=muscle fibers) and
connective tissue.
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Tendons anchor muscle firmly to bones.
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Most muscles attach to 2 bones that have a moveable joint
between them.
Structure of Skeletal Muscle
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Bursae – small fluid filled
sacs that lie between
some tendons and the
bones beneath them. They
are made of connective
tissue and are lined with
synovial membrane that
secretes synovial fluid.
Figure
Structure of Skeletal Muscle
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Contribution of the nervous system
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Electrochemical impulses travel from the frontal lobes of the
cerebrum via motor nerves to the muscle fibers and cause
them to contract.
Sensation is a function of the brain – impulses are integrated in
the parietal lobes of the cerebrum (conscious muscle sense)
and in the cerebellum (unconscious). These activities promote
coordination.
Structure of Skeletal Muscle
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Microscopic anatomy
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Muscle cells (fibers) are grouped in a highly organized way in the
muscle. The membrane that surrounds each muscle cell is called
the sarcolemma.
Muscle cells are filled with 2 types of fine threadlike proteins
called myofilaments:
The myofilaments are arranged in the cells in small units called
sarcomeres.
Structure of Skeletal Muscle
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Neuromuscular junction
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Spot where the axon of a motor nerve nears the muscle fiber.
The axon terminal does not touch the muscle but comes close.
The space between the axon and the muscle cell is called the
synapse.
Within the terminal end of the axon are small sacs filled with a
neurotransmitter called acetylcholine.
Muscle Contraction
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Sequence
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Electrical impulse travels down a motor neuron. When it reaches
the end, acetylcholine (chemical) is released into the synapse.
Acetylcholine binds to special receptors on the muscle cell and
causes an electrical impulse to spread over the cell.
The sarcomeres shorten and the muscle cell contracts.
Excess acetylcholine is destroyed by acetylcholinesterase.
*Myesthenia gravis
Muscle Contraction
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Muscles contract only when the stimulus from the motor
neuron is strong enough = Threshold stimulus is the
minimum level of stimulation necessary to cause a
contraction.
Additional stimuli arriving rapidly cause an increase in the
force of the muscle contraction.
Botox
Types of Skeletal Muscle Contraction
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Twitch –
Tetanic –
*Tetanus
Isotonic contraction –
Isometric contraction –
Energy for Contraction
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Muscles require energy in the form of ATP to contract.
ATP is produced by aerobic respiration.
Secondary sources
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Creatine phosphate stored in the muscle is broken down to
produce more ATP.
Glycogen stored in the muscle and liver is broken down into
glucose which is broken down to produce ATP. This process
requires O2.
Energy for Contraction
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O2 is transported to the muscle by the circulatory system
(hemoglobin in RBCs). Some O2 is stored in the muscle
by myoglobin.
Strenuous exercise can cause the muscles to “run out” of
O2 resulting in oxygen debt. Glucose is only partially
broken down to lactic acid which causes muscle fatigue.
Movements Produced by Muscle
Contractions
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Flexion – movement reduces the angle between two
bones at their joint.
Extension – opposite of flexion – increases the angle at a
joint.
Abduction – moving a body part away from the midline of
the body.
Adduction – moving a body part toward the midline of
the body.
Rotation – movement of a body part around an axis.
Movements Produced by Muscle
Contractions
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Supination – refers to hand position – movement turns
palm up to the anterior position (anatomical position).
Pronation – palm of hand turned posteriorly.
Dorsiflexion – top of foot is elevated with toes pointing
upward.
Plantar flexion – foot directed downward (standing on
your toes).
Muscles
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Head and Neck
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Frontal (frontalis) – over the frontal skull bone raise your
eyebrows.
Orbicularis oculi – closes the eye.
Orbicularis oris – around the mouth; kissing muscle.
Masseter – elevates mandible allowing use to close our mouth
and chew food.
Temporal (temporalis) – assists the masseter in closing the jaw.
Muscles
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Head and neck
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Sternocleidomastoid – flexes the head toward the chest.
Trapezius – helps elevate, lower, and adducts the shoulders
(scapula) and extend the head backwards.
Muscles
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Upper extremities
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Pectoralis major – upper anterior chest; flexor of the upper arm
Latissimus dorsi – extensor of the upper arm.
Deltoid – powerful abductor of the upper arm.
Biceps Brachii – 2-headed muscle that serves as the primary
flexor of the forearm.
Triceps brachii – 3 headed muscle on the posterior surface of the
upper arm; extensor of the elbow/forearm.
Muscles
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Trunk
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Anterior abdomen – 3 layers of muscles whose fibers run in
different directions – girdle effect.
 External oblique – outermost layer
 Internal oblique – middle layer
 Transversus abdominis – innermost layer
Rectus abdominus runs down the midline of the abdomen from
the thorax to the pubis. (6-pack)
Respiratory muscles
 Intercostal muscles – between the ribs
 Diaphragm – separates the thoracic and abdominal cavities.
Muscles
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Lower Extremities
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Gluteus maximus – forms the buttock; extensor of the thigh,
supports the torso.
Gluteus medius – abducts the femur; injection site.
Hamstring muscles – flexors of the lower leg
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Semimembranosus
Semitendinosus
Biceps femoris
Muscles
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Lower extremities
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Quadriceps femoris – covers the upper thigh, extends the leg.
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Rectus femoris
Vastus lateralis – injection site
Vastus medialis
(Vastus intermedius)
Tibialis anterior – dorsiflexes the foot
Gastocnemius – primary calf muscle, plantar flexion of the
foot.
Intramusclular Injection Sites
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Deltoid
Gluteus medius
Vastus lateralis