Muscle & Exercise Physiology

Three Sources of Energy for Muscle Contraction

1)      The first source of energy that is used to reconstitute the ATP is the substance phosphocreatine, which carries a high-energy phosphate bond similar to the bonds of ATP.

2)      The second important source of energy, which is used to reconstitute both ATP and phosphocreatine, is a process called glycolysis-the breakdown of glycogen previously stored in the muscle cells.

3)      The third and final source of energy is oxidative metabolism, which means combining oxygen with the end products of glycolysis and with various other cellular foodstuffs to liberate ATP.

More than 95% of all energy used by the muscles for sustained long-term contraction is derived from oxidative metabolism.

Explanations:
Sarcoplasmic reticulum (SR) is the internal tubular structure that is the site of Ca2+ storage and release for excitation-contraction coupling. It has terminal cisternae that make intimate contact with the T tubules in a triad arrangement.

an analysis of the lever systems of the body depends on knowledge of the following:
(1) the point of muscle insertion;
(2) its distance from the fulcrum of the lever;
(3) the length of the lever arm;
and (4) the position of the lever.

Fast Fibers (Type II, White Muscle)
 The following are characteristics of fast fibers:
1. Fast fibers are large for great strength of contraction.
2. Fast fibers have an extensive sarcoplasmic reticulum for rapid release of calcium ions to initiate con- traction.
3. Large amounts of glycolytic enzymes are present in fast fibers for rapid release of energy by the glycolytic process.
4. Fast fibers have a less extensive blood supply than slow fibers because oxidative metabolism is of sec- ondary importance.
5. Fast fibers have fewer mitochondria than slow fibers, also because oxidative metabolism is secondary.
A deficit of red myoglobin in fast muscle gives it the name white muscle.

[Ganong 24th p-115]

Events occurring in skeletal muscle  during prolonged fasting
➤ Decreased use of ketone body to spare it for brain.
➤ Use of fatty acid almost exclusively.
➤ Decreased protein catabolism with decreased urinary nitrogen loss.
Events occurring in brain
➤ Uptake and utilization of ketone bodies

During strenuous exercise, O2 consumption and CO2 formation can increase as much as 20-fold.
Yet, in the healthy athlete, alveolar ventilation ordinarily increases almost exactly in step with the increased level of oxygen metabolism.
The arterial PO2, PCO2, and pH remain almost exactly normal. In trying to analyze what causes the increased ven-

1. The resting membrane potential is about -80 to -90 millivolts in skeletal fibers, about 10 to 20 millivolts more negative than in neurons.
2. The duration of the action potential is 1 to 5 milliseconds in skeletal muscle, about five times as long as in large myelinated nerves.
3. The velocity of conduction is 3 to 5 m/sec, about 1/13 the velocity of conduction in the large myelinated nerve fibers that excite skeletal muscle.

Isotonic Contractions
Isotonic contractions maintain constant tension in the muscle as the muscle changes length. This can occur only when a muscle’s maximal force of contraction exceeds the total load on the muscle. Isotonic muscle contractions can be either concentric (muscle shortens) or eccentric (muscle lengthens).

Isometric Contractions
In contrast to isotonic contractions, isometric contractions generate force without changing the length of the muscle . This is typical of muscles found in the hands and forearm: the muscles do not change length, and joints are not moved, so force for grip is sufficient. An example is when the muscles of the hand and forearm grip an object; the joints of the hand do not move, but muscles generate sufficient force to prevent the object from being dropped.

Explanations:
There are three superfamilies of molecular motor proteins in the cell that use the energy of ATP to generate force, movement, or both. Myosin is the force generator for muscle cell contraction. Cellular myosins can also interact with the cytoskeleton (primarily thin filaments) to participate in contraction as well as movement of cell contents. Kinesins and cellular dyneins are motor proteins that primarily interact with microtubules to move cargo around the cells.

Treatment of malignant hyperthermia
a.rapid cooling
b. the administration of dantrolene, a drug that antagonizes ryanodine receptors, which inhibits calcium ion release for the sarcoplasmic reticulum and thereby attenuating muscle contraction.

Reference: Ganong 26^th; Figure-2.5

Myosin-thick filament