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3 types of muscles of vertebrates
- 1) Skeletal muscle appears striated voluntary, or conscious, control
- 2) Cardiac muscle- also striated, located only in the heart Involuntary control
- 3) Smooth muscle- not striated
- Involuntary control
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structures of skeletal muscle
myofibrils, saroplasmic reticulum, T tubukes, sarcomere, z line, actin, myosin
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sarcoplasmic reticulum
- SR stores calcium, important for contraction
- surrounds each myofibril
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Sarcomere
unit of a myofibril
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Actin
thin filaments, attach to Z line
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How Do Skeletal Muscles Contract?
- Relaxed muscle: tropomyosin prevents the myosin heads from attaching to actin
- Contracting muscle:
- tropomyosin moves, allowing myosin to bind to actin
- All sarcomeres of a muscle fiber shorten at the time
- ATP is required to extend myosin (sling shot)
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Sliding Filament mechanism of muscle contraction
- 1. Tropomyosin covers the binding sites, so the myosin head cannot attatch.
- 2. When the binding sites of actin are exposed, the byosin head attaches to a bind site
- 3. The myosin head flexes, pulling the thin filament past the thick filament and shortening the sarcomere
- 4. Using energy from ATP, the myosin head detaches from actin, extends, and then attaches to another actin binding site farther along on the thin filament
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All vertebrate neuromuscular junctions use the neurotransmitter acetylcholine
- Each action potential in a motor neuron releases enough acetylcholine to produce a huge excitatory postsynaptic potential in the muscle fiber, bringing its membrane potential above threshold and triggering an action potential
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- Axons stimulate muscle fibers at neuromuscular junctions (synapse)
Acetylcholine is the neurotransmitter that triggers the action potential
The action potential moves down the T tubules
to the SR
where it causes calcium ions (Ca2+) to be released from the SR
Ca2+ binds to troponin, causing it to pull tropomyosin, off the actin binding sites
- myosin heads can bind to actin
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What makes the fiber stop contracting?
- When the action potential in the muscle fiber is over, the SR stops releasing Ca2+
Active transport (req ATP) pump Ca2+ back into the SR
Ca2+ leaves the accessory proteins, which move back over the active binding sites
- Therefore, the myosin head can no longer attach to actin, and contraction stops within a few hundredths of a second
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Muscle Contractions
- motor unit : A motor neuron and all the muscle fibers that it innervates
- Strength: varies in both the number of muscle fibers stimulated and the frequency of action potentials in each fiber
- Slow twitch: less power, lasts longer, lots of mitochondria, good blood supply, more ATP, less fatigue. Thin fibers, fewer myofibrils (marathon runner)
- Fast twitch: more powerful, smaller blood supply, few mitochondria, uses mostly glycolysis. Thick fibers, more myofibrils. (sprinter)
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40.4 How Do Cardiac and Smooth Muscles Differ From Skeletal Muscle?
- Cardiac muscle powers the heart
- Single nucleus, striated, intercalated disks
- contract around 70 times each minute, enormous numbers of mitochondria
- Action potentials from the pacemaker spread rapidly through gap junctions in the intercalated discs, help together by desmosomes
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Smooth Muscle
- surrounds blood vessels and most hollow organs, including the uterus and bladder
- not striated because the thin and thick filaments are scattered throughout the cells
- single nucleus
- stretches easily, (bladder, stomach, uterus)
- can be initiated by stretching, by hormones, by signals from the autonomic nervous system, or by a combination of these stimuli
- produces slow, involuntary contraction
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40.5 What Are the Functions and Structures of Vertebrate Skeletons?
- The skeleton provides a rigid framework that supports the body and protects its internal organs
- brain and spinal cord are almost completely enclosed
- rib cage protects the lungs and the heart
- pelvic girdle supports and protects abdominal organs
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Vertebrae Skeleton
- allows locomotion: adapted for walking, running, jumping, etc
- Sensory function: bones of the middle ear transmit sound
- Blood production: red blood cells, white blood cells, and platelets in red bone marrow (sternum ribs, upper arms and legs, and hips)
- Stores calcium and phosphorus: maintaining a constant concentration in the blood
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2 types of skeleton
- The axial skeleton: includes the bones of head, vertebral column, and rib cage
- The appendicular skeleton: includes the pectoral and pelvic girdles, and the appendages attached to them
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3 types of ct in bone
cartilage, bone, ligaments
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Cartilage
- During the embryonic development of the skeleton, except for the skull and collarbone, is first formed from cartilage
- also covers the ends of bones at joints
- nose ears, larynx, trachea, and bronchi
- tough, shock-absorbing intervertebral discs
- The living cells of cartilage: chondrocytes
- Blood vessels do not penetrate, therefore very slow to repair if at all
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Bone
- hard outer shell of compact bone that encloses spongy bone in its interior
- Compact bone is dense and strong and provides an attachment site for muscle
- Spongy bone consists of an open network of bony fibers
- It is porous, lightweight, rich in blood vessels
- Contains bone marrow, where blood cells form
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There are three types of bone cells:
- Osteoblasts—bone-forming cells
- Osteocytes—mature bone cells
- Osteoclasts—bone-dissolving cell
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