Figure 1116

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Release and uptake of calcium by the sarcoplasmic reticulum during contraction and relaxation of a skeletal-muscle fiber.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

II. Biological Control Systems

11. Muscle

© The McGraw-Hill Companies, 2001



TABLE 11-1 Functions of ATP in Skeletal-Muscle Contraction

1. Hydrolysis of ATP by myosin energizes the cross bridges, providing the energy for force generation.

2. Binding of ATP to myosin dissociates cross bridges bound to actin, allowing the bridges to repeat their cycle of activity.

3. Hydrolysis of ATP by the Ca-ATPase in the sarcoplasmic reticulum provides the energy for the active transport of calcium ions into the lateral sacs of the reticulum, lowering cytosolic calcium to pre-release levels, ending the contraction, and allowing the muscle fiber to relax.

(a) Single motor unit

Neuromuscular junctions

(a) Single motor unit

Neuromuscular junctions

(b) Two motor units

To reiterate, just as contraction results from the release of calcium ions stored in the sarcoplasmic retic-ulum, so contraction ends and relaxation begins as calcium is pumped back into the reticulum (Figure 11-16). ATP is required to provide the energy for the calcium pump, and this is the third major role of ATP in muscle contraction (Table 11-1).

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