Ninth lecture

The H zone is a relatively lighter zone in the center of the A band. In the center of the H zone, there is a narrow dark line, known as M line, produced by proteins that bind all the thick filaments in a sarcomere together.

The space between adjacent thick and thin filaments is bridged by projections known as cross-bridges. The head of the cross-bridges act as ATPase and contain a binding site for actine and another binding site for ATP.

Opposite group: Muscles are arranged in opposite groups performing antagonistic actions to: Flexor and extensor muscle. Adductor end abductor muscle. Depressor and levator muscle. Contractor and dilator muscle. Rotator muscle.

Muscle spindle: There are sensory elements within the muscle that monitor movement and send several impulses to the CNS. The highly modified muscle cells (muscle spindle) that perform this sensory function are located within each bundle of fibres in a muscle. Muscle spindle consists of two parts, contractile region resemble the main fibrils of the muscle but somewhat thinner and a central region lacks the contractile elements and warpped by processes of sensory neuron. When the central region is stretched, the sensory nerve endings are activated, sending impulses to CNS initiate the contraction.

Excitation contraction coupling: (mechanism of muscle contraction) It is the process by which an action potential initiates the muscle contraction.

1- Propagation of the action potential and release of Ca++ ions: Propagation of the action potential in the motor nerve leads to production of an end plate potential generae of an action potential at the motor end plate. Spreads on both sides of the motor end plate and excites the whole muscle fiber. The action potential spreads along the T tubules causing release of Ca++ ions from the terminal cisternae of the sarcoplasmic reticulum. The released Ca++ ions diffuse rapidly to the region of the thick and thin filaments.

2- Binding of the cross-bridges between the thick and thin. The action potential leads to release of Ca++ ions (first step) which combine with traponin molecules. Combination of Ca++ ions with traponin on the thin filament causes the trapomyosin to move away from its blocking position and thus exposing the binding sites present on actin molecules. Cross bridges from the thick (myosin) filaments combine with the binding sites on the actin.

3- Cross-bridge cycling which results in sliding of the thin filaments across the thick (myosin) filaments: Binding: Cross-bridges on the thick filaments bind to actin. Bending: Binding of the cross-bridges leads to release of energy stored in myosin (ATP by ATPase). Detachment: Detachment of the cross-bridges from the thin filaments. Return to original position: The cross-bridge returns to its original position and another cycle can occur by binding to another actin molecule and so on. Ca++ ions combine with troponin. The width of the I bands decreases and the H zones become narrower but the width of the A bands do not change.

4- Relaxation occurs when Ca++ ions are transported into the sarcoplasmic reticulum by an active process using ATP and Ca-ATPase. Removal of Ca++ ions makes troponin to return to its original state causes trapomyosin to move back and cover the binding sites on actin. Cross bridge cycles stop the thin filaments slide back to their original position and the sarcomere to return to their original length.

I. Electrical changes: Direct Indirect II. Metabolic (chemical) changes A) During contraction: 1-The head of myosin contain ATPase enzyme which catalyses the hydrolysis of ATP. ATP ADP + P (phosphorc acid) + Energy. ATPase ATP is rapidly reformed from ADP by the addition of phosphate group from creatine phosphate (Cr-P) by creatine phosphokinase enzyme. 3- To maintain muscle contraction, a steady supply of energy is derived from the following sources: Anaerobic oxidation of glucose: Glucose (ms glycogen) 2 Py. Acids + 2 ATP.

During severe exercise the muscle depends on local glycogen stores and anaerobic glycolysis to meet its energy requirements. The pyruvic acid produced is converted into lactic acid and accumulate in the blood. The advantage of anaerobic glycolysis is that it is able to supply ATP at a high rate and within a short time. The disadvantage of anaerobic glycolysis is that it provides only small amounts of high energy phosphate (2 ATP molecules Aerobic oxidation of glucose and free fatty acids: Bl. Glucose CO2 + 6H2O + 38 ATP. Free fatty acids + O2 CO2 + H2O + 44 ATP.

At low or moderate levels of activity. ATP is produced by the aerobic oxidation of blood glucose or free fatty acids (carried by the blood from the adipose tissues). Aerobic oxidation of glucose or fatty acids produces great amount of high energy phosphate (38 ATP for each molecule of glucose). In exhausted muscles, there is an emergency for the supply of ATP. This is done by combining two ADP molecules to reform one ATP molecule and one AMP (Adenosine Monophosphate) molecule. ADP + ADP ATP + AMP.

B) During recovery: At he end of muscular activity, the energy stores in the muscle (ATP, Cr-P & muscle glycogen ) is depleted and lactic acid is increased in blood. Recovery occurs by removal of lactic acid and regeneration of the energy stores. Part of the lactic acid is oxidised into CO2 and H2O. The energy produced from this oxidation is used for reformation of ATP and by turn Cr-P. Lactic acid CO2 + H2O + ATP ATP + creatine Cr-P + ADP The other part of lactic acid diffuses to the blood stream and then to the liver where it is converted into blood glucose (through the Cori cycle). Muscles take glucose from the blood stream and changes it into muscle glycogen.

III. N.B. Ph: changes in the muscle during contraction: The pH changes towards the acidic side, due to the release of phosphoric acid. ATP ADP + Phosphoric acid + Energy The pH changes towards the alkaline side due to the release of creatine from the Cr-P. ADP + Cr-P ATP + creatine The pH changes towards the acid side due to the release of lactic acid from the anaerobic oxidation of glucose. Glucose (mgglycogen) 2 lactic acid + 2 ATP.

Types of contraction in skeletal muscles: The muscles contain two elements, contractile part in addition to elastic viscous part. It is possible for contraction to isometric contraction occur without appearciable decrease in length. Contraction against a constant load, with appearciable approximation of the muscle ends is called: isotonic contraction It is possible to do negative work while contracting.

Recordering of muscle contraction: a- Simple muscle twich: In stationery drum it is represented by a line while in movable drum it is represented by a curve contain latent (refractory) period contraction period, and relaxation period.

b- Summation: Two successive stimuli: If the second stimulus occur during the relative refractory period a larger number of fibres will excited. If the second stimulus is done during the contraction period, summation of the contracted fibres exceeds the response, increase the peak level. If the second stimulus occur during the relaxation period, the muscle fibres recontract, giving a second peak which is higher than the first. If the second stimulus occur at the end of relaxation of the first curve, a second twich will obtained characterized by the absence of latent period.