Physiology Lecture 11 - Damietta University PDF

Physiology Skeletal Muscle Contraction LECTURE (11) DR. El-Sawy 0  Physiology Skeletal Mu...

Physiology Skeletal Muscle Contraction LECTURE (11) DR. El-Sawy 0  Physiology Skeletal Muscle Contraction Definition:  Machines which convert the stored chemical energy into mechanical energy (work) and heat.  Ms constitutes 50% of body weight. Types : 1. Skeletal ms: 40% 2. Cardiac ms. 10 % 3. Smooth ms.  Skeletal: as they are usually attached to the skeleton.  Somatic: as they move the body (soma). Naming  Voluntary: as their contraction is under voluntary control.  Striated: as they appear striated under the microscope. 1. Movements of body as a whole or part of it e.g. one limb. Functions 2. Maintenance of body posture. 3. Control of body temperature. DR. El-Sawy 1  Physiology Skeletal Muscle Contraction  Skeletal muscle formed of thousands of muscle fibers (myofibers).  The muscle fibers: is the structural unit of the skeletal muscle.  It is elongated, multinucleated cell of about 10-100 m in diameter.  Its cytoplasm (sarcoplasm) contains the usual organelles as sarcoplasmic reticulum, Golgi apparatus, ribosomes, mitochondria, and glycogen  The cell membrane has tubular extentions called transverse tubules (T- tubules) which extend deep into muscle fibers.  They extend from one end of the muscle to the other giving muscle fiber its longitudinal striations.  Each myofibril shows an alternation of light (I) and dark (A) bands due to the special arrangement of its myofilaments.  These bands give skeletal muscle its transverse striations.  In the center of A-band, there is a lighter zone called H zone & in the center of the H-zone, there is a dark line called M-line.  In the center of I-band, there is Z-line.  The part of the myofibril located in between 2 Z-lines is called sarcomere: DR. El-Sawy 2  Physiology Skeletal Muscle Contraction  It is the functional unit of the muscles.  Formed of an A-band and half of the I-bands on each side.  It is composed of myofilaments (contractile filaments) which are:  Present in middle of sarcomere Thick filaments  Producing A band and are attached by M-line.  Present on both sides of the sarcomere Thin  Attached by Z- lines which connect the ends of thin filaments filaments from one sarcomere to the other.  The space between thick and thin filaments is bridged by projections known as cross bridges which are parts of thick filaments. DR. El-Sawy 3  Physiology Skeletal Muscle Contraction  It is formed of myosin protein molecules.  Each myosin molecule is composed of: 1) Head which contains: a) binding site for actin. b) binding site for ATP. 2) Body or arm which forms with the head the cross bridges. 3) Long tail. DR. El-Sawy 4  Physiology Skeletal Muscle Contraction  It is formed of three types of protein molecules: 1) Actin molecules: forming 2 chains which are twisted with each other in a spiral manner. In each molecule, there is a binding site for myosin. 2) Tropomyosin: thin filamentous protein forming 2 chains covering the binding sites of actin during relaxation. 3) Troponin. DR. El-Sawy 5  Physiology Skeletal Muscle Contraction 1) Propagation of muscle action potential and release of Ca++ ions:  Muscle action potential spreads on both sides of the motor end plate along the whole length of muscle fibers and along T-tubules release of Ca++ from the terminal cisterns of sarcoplasmic reticulum into cytoplasm which then diffuses to the region of actin and myosin filaments. 2) Activation of actin by Ca++: a) The released Ca ions combine with a troponin of thin filaments→ movement of tropomyosin laterally away from its position → exposure of binding sites present on actin molecules. b) Cross bridges (heads) from the thick (myosin) filaments combine with the binding sites on the actin. 3) Cross bridge cycling:  It results in the sliding of thin filaments across the thick filaments by the following steps: a) Binding of cross-bridges of myosin with actin. b) Bending of cross-bridges →pulling actin with it towards the center of the sarcomere. c) Detachment of the cross bridges from the actin which needs energy from ATP hydrolysis. d) Return to original position of cross bridges and other cycle starts by binding with another actin molecule and so on.  Cross bridges cycling continues as long as Ca++ binds with troponin leading to sliding of actin over myosin. DR. El-Sawy 6  Physiology Skeletal Muscle Contraction Changes in the sarcomere as a result of muscle contraction: a) Approximation of 2 Z lines. b) Shortening of I band. c) Narrowing of H zone. d) Constant A band. 4) Muscle relaxation:  Active Ca++ uptake by the terminal cisternae of sarcoplasmic reticulum by Ca++ pump (Ca++ ATPase) leading to: a) Release of Ca++ from troponin b) Tropomyosin will cover the binding sites of actin again and the cross- bridge cycling stops. DR. El-Sawy 7