Muscle tissues are specialized for contraction. Contractuion results from
the sliding of thin filaments of actin lengthwise between thick myosin
filaments.
Muscle tissues are specialized for contraction
- Smooth muscle: involuntary cells with tapering ends but no
cross-banding; smooth, rhythmic contractions; nuclei located centrally;
occurs in digestive organs, reproductive organs, etc.
- Cardiac muscle: involuntary fibers with cross-striations;
cylindrical in shape but branching and coming together repeatedly;
nuclei located centrally; cell boundaries marked by intercalated disks;
rhythmic contractions; occurs in heart only
- Skeletal muscle: voluntary cells with cylindrical shape;
cross-striations caused by alignment of actin and myosin fibers;
many nuclei per fiber; no cell boundaries (each fiber is thus called a
syncytium); rapid, forceful contractions, but fatigues easily;
occurs in muscles; always attaches to connective tissues
Fine structure of striated muscle:
- Protein fibers are perpendicular to alternating light and dark bands.
- Dark A-bands are made of thick filaments of the protein
myosin.
The protein troponin usually binds to myosin
to form tropomyosin.
- Light I-bands are myosin-free, but contain thin filaments of actin.
- Midway through each light band runs a dark Z-line to which
thin filaments attach. The interval from one Z-line to the next is the
unit of contraction, called a sarcomere.
- Extensions of the plasma membrane (sarcolemma)) run along
each Z-line to form transverse tubules (T-tubules).
Mechanism of contraction ("sliding filament theory"):
- T-tubules supply oxygen and calcium ions to the contracting fiber.
- When a nerve signal arrives, the membrane becomes more permeable to calcium.
- Calcium ions rush in and bind to troponin preferentially, releasing myosin.
- Myosin molecules, shaped somewhat like golf clubs, now bind to actin.
- The heads of the myosin molecules now rotate, moving the actin fibers
and causing them to slide along the myosin fibers.
- After its head rotates, each myosin fiber detaches from its actin fiber.
The head resumes its original position and forms a new attachment to
another actin fiber.
- Myosin fibers are staggered from one another; some rotate against their
actin fibers while others are releasing their holds and recovering to form
new attachments. In this way, muscle contraction is smooth and even
instead of jerky.
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