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Essentials of Anatomy & Physiology 3/e Seeley/Stephens/Tate | |
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Chapter 7: The Muscular System
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Chapter Summary
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Chapter 7: The Muscular System
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Functions of the Muscular System
- The skeletal system functions to constrict organs and
vessels, pump blood, cause respiration, produce body movement, maintain posture,
produce movements involved in communication, and produce body heat.
Characteristics of Skeletal Muscle
- Skeletal muscle has contractility, excitability, extensibility,
and elasticity.
Structure
- Each skeletal muscle fiber is a single cell containing
numerous myofibrils.
- Myofibrils are composed of actin and myosin myofilaments.
- Sarcomeres are joined end to end to form myofibrils.
- Muscle fibers are organized into fasciculi, and fasciculi
are organized into muscles by associated connective tissue.
Membrane Potentials
- Cell membranes have a relative positive charge on the
outside and a relative negative charge inside.
- Action potentials are a brief reversal of the membrane
charge.
Nerve Supply
- Motor neurons carry action potentials to skeletal muscles,
where the neuron and muscle fibers form neuromuscular junctions.
- Neurons release acetylcholine, which binds to receptors
on muscle cell membranes, stimulates an action potential in the muscle cell,
and causes the muscle to contract.
Muscle Contraction
- Action potentials travel along T tubules to the sarcoplasmic
reticulum, where they cause the release of calcium ions.
- Calcium ions, released from the sarcoplasmic reticulum,
bind to the actin myofilaments, exposing attachment sites.
- Myosin forms cross-bridges with the exposed actin attachment
sites.
- The myosin molecules bend, causing the actin molecules
to slide past; this is the sliding filament mechanism. This process requires
ATP breakdown.
- A muscle twitch is an all-or-none response to an action
potential that exceeds the threshold; it consists of a lag phase, contraction
phase, and relaxation phase.
- Tetanus occurs when stimuli occur so rapidly that a muscle
does not relax between twitches.
- Small contraction forces are generated when small numbers
of motor units are recruited, and greater contraction forces are generated
when large numbers of motor units are recruited.
- Energy is produced by anaerobic (without oxygen) and
aerobic (with oxygen) respiration.
- After intense exercise, the rate of aerobic metabolism
remains elevated to repay the oxygen debt.
- Muscles contract either isometrically (tension increases,
but muscle length stays the same) or isotonically (tension remains the same,
but muscle length decreases).
- Muscle tone consists of a small percentage of muscle
fibers contracting isometrically and is responsible for posture.
- Muscles contain a combination of slow-twitch and fast-twitch
fibers.
- Slow-twitch fibers are better suited for aerobic metabolism,
and fast-twitch fibers are adapted for anaerobic metabolism.
- Sprinters have more fast-twitch fibers, whereas distance
runners have more slow-twitch fibers.
Smooth Muscle and Cardiac Muscle
- Smooth muscle is not striated, contracts more slowly
than skeletal muscle, is autorhythmic, and is under involuntary control.
- Cardiac muscle is striated, has intercalated disks, is
autorhythmic, and is under involuntary control.
Skeletal Muscle Anatomy