Muscle and cell motility, muscle contract by sliding myosin and actin filaments

Muscle and cell motility

Skeletal and cardiac (striated) muscle contract by sliding myosin and actin filaments past each other in a process facilitated by ATP. Myosin contains a motor that interacts with the actin filament and allows myosin to ratchet along the actin. The filaments are arranged in a banded pattern in individual sarcomeres, which act in series. Specialized invaginations of the plasma membrane (T tubules) spread the surface depolarization to the interior of the cell to release calcium from the sarcoplasmic reticulum, initiating contraction.

Troponin and tropomyosin are specialized proteins that permit contraction of skeletal and cardiac muscle to be regulated by calcium. Skeletal muscle is a syncytium, while cardiac muscle consists of individual cells connected by intercalated disks. The organization of striated muscle is shown below:

Extrafusal muscle fibers  Extrafusal muscle fibers generate the force of contraction while intrafusal fibers (both IA and II-type fibers (receptors) serve as sensory detectors signaling muscle location (proprioception) and the rate of contraction.

Extrafusal muscle fibers 

Extrafusal muscle fibers generate the force of contraction while intrafusal fibers (both IA and II-type fibers (receptors) serve as sensory detectors signaling muscle location (proprioception) and the rate of contraction.

Smooth muscle 

Smooth muscle contraction closely resembles the cell motility exhibited in other cell types. It also occurs through the action of actin and myosin, which are arranged in a lattice-like pattern. Troponin is not present in smooth muscle.

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