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BIO 156
Chapter 12
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The Skeleton and Muscles
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Structure and Function of the Human Skeleton
• Bones serve many functions and play an important role inhomeostasis. Bones provide the following:
– Provide internal support.
– Allow for movement.
– Help protect internal body parts. – Produce blood cells and platelets.
– Store fat.
– Help regulate blood calcium levels.
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• The human skeleton
consists of two parts:
– The axial skeleton
– The appendicular skeleton
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• All bones have a hard,
dense outer layer that
surrounds a less
compact central region. – Inside the bone is the
marrow cavity, filled witheither fat cells (yellow
marrow) or blood cells
and blood-producing
cells (red marrow) or
with combinations of the
two.
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• Bones are constantly remodeled in
adults to meet changing stresses
placed on them.
– Two cells are responsible for bone
remodeling:
• Osteoclasts—destroy bone• Osteoblasts—form bone
• Bone is a homeostatic organ that helpsmaintain proper levels of calcium in the
body.
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The joints permit varying degrees of mobility.
1. Immovable
2. Slightly movable3. Freely movable
Immovable Joint
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Slightly Movable Joints
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Freely Movable Joints
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Most of the bones of the human skeleton start out as
hyaline cartilage.
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• Bone fractures are repaired by fibroblasts and
osteoblasts.
• Osteoporosis involves a loss of calcium, which results
in brittle, easy-to-break bones.
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The Skeletal Muscles
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1. Bodymovements2. Maintenance of posture3. Production of body heat
Skeletalmuscles areinvolved in:
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• Skeletal muscle cells are known as muscle fibers
and are both excitable and contractile.
• Muscle fibers contain many small bundles of
contractile filaments known as myofibrils.
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• During muscle contraction, the actin filaments slide inward,
causing the sarcomeres to shorten.
– The energy for muscle contraction comes from ATP.
– ATP is replenished by creatine phosphate, glycolysis, andcellular respiration.
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• Individual skeletal muscle fibers contract after being stimulated
by an action potential.
– The strength of muscle contraction can be increased by
stimulating (recruiting) additional muscle fibers to contract. – Additional tension may be created in a muscle fiber if a
nerve impulse arrives while the muscle fiber is still
contracted.
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• Muscle tone results from the contraction of a
small number of muscle fibers that keepmuscles slightly tense.
• Two types of muscle fibers are found inskeletal muscle, slow- and fast-twitch.
– Fast-twitch fibers are found in greatest number
in muscles that perform rapid movement.
– Slow-twitch fibers are found in muscles that
perform slower motions or are involved in
maintaining posture.
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End of Chapter 12