Physiology of The Muscle Week 5 Dr. Walid Daoud A. Professor

Skeletal Muscle Morphology: Morphology: -Begin and end in fibrous tendons and made up of muscle fibers arranged in parallel bundles between tendinous endings. -Each muscle fiber is a multinucleated unit made up of many parallel myofibrils. -Each myofibril contains interdigitating myofilaments: thick (Myosin) and thin (Actin) in the sarcotubular system.

Skeletal Muscle Sarcomere: -Functional unit of the muscle. -Extend between 2 transverse protein sheets called Z lines. -Thick filaments in the middle of sarcomere. -Thin filaments arranged on both sides of sarcomere with one end attached to Z line and the other overlap with thick filaments.

Myofilaments Contractile muscle proteins: myosin & actin. 1-Thick (myosin) filaments: -Each myosin molecule has 2 heads attached to double helix tail. -Each myosin molecule has 2 heads attached to double helix tail. -Myosin head contains -Myosin head contains. Actin-binding site.. Actin-binding site.. ATP-binding site.. ATP-binding site.. Catalytic site for ATP hydrolysis.. Catalytic site for ATP hydrolysis. -Myosin head protrude out of thick filaments -Myosin head protrude out of thick filaments cross-bridges that contact actin molecules. cross-bridges that contact actin molecules.

Myofilaments 2- Thin (actin) filaments: Formed of 3 muscle proteins: Formed of 3 muscle proteins:. Actin: double helix form with active site to combine with myosin cross bridges.. Actin: double helix form with active site to combine with myosin cross bridges.. Tropomyosin: strands cover the active sites on actin molecules during rest.. Tropomyosin: strands cover the active sites on actin molecules during rest.. Troponin: 3 subunits: troponin I, T and C with strong affinity for actin, tropomyosin and Ca, respectively.. Troponin: 3 subunits: troponin I, T and C with strong affinity for actin, tropomyosin and Ca, respectively.

Sarco-tubular System 1- T-Tubules: Network of transverse tubules surround each myofibril. They are invaginations of muscle fiber membrane to carry wave of the action potential to cell interior. 2- Sarcoplasmic reticulum (SR): Network of longitudinal channels between T- Tubules surrounding & parallel to myofibrils. It is site of Ca storage & release during muscle Contraction. Expanded ends (terminal cistern).

Changes following Skeletal Muscle Stimulation 1- Electrical Changes. 2- Excitability Changes. 3- Mechanical Changes. 4- Metabolic Changes.

Electrical Changes Similar to those in nerve fiber but: - RMP in the muscle fiber is -90 mv. - Action potential lasts 2-4 msec & precedes muscle contraction by 2 msec. Single muscle fiber obeys All or None Rule. Simple muscle twitch: Single action potential causes brief contraction followed by relaxation. It starts 2 msec after start of depolarization.

Excitability Changes Skeletal muscle fiber, like nerve fiber, is refractory to re-stimulation during the action potential. Thus, as the muscle begins to contract, it has regained its excitability.

Mechanical Changes Excitation-Contraction Coupling Steps that action potential initiates muscle contraction 1-Release of Ca. 2-Activation of muscle proteins. 3-Generation of tension by cross bridge cycling: a-Binding. a-Binding. b-Bending. b-Bending. c-Detachment. c-Detachment. d-Return. d-Return. 4-Relaxation: Ca pump.

Types of Skeletal Muscle Contraction 1-Isotonic contraction:. Muscle shortens.. Muscle shortens.. Tension is constant.. Tension is constant.. Work done by muscle = Weight x Distance. Work done by muscle = Weight x Distance 2-Isometric contraction:. Muscle length remains constant.. Muscle length remains constant.. Tension is increased.. Tension is increased.. Work done by muscle = Zero.. Work done by muscle = Zero. If person lifts heavy weight, contraction starts isometrically then completes isotonically. If person lifts heavy weight, contraction starts isometrically then completes isotonically.

Factors Affecting Muscle Contraction 1-Types of muscle fibers: Rapid (pale) muscle fibers Slow (red) muscle fibers -Rapid contraction and relaxation. -Poor in myoglobin. -Easily fatigued. -Adapted for fine skilled Movements: hand and extraocular muscles. -Slow contraction and relaxation. -Rich in myoglobin. -Not easily fatigued. -Adapted for prolonged weight bearing contractions: soleus muscle.

Factors Affecting Muscle Contraction 2-Types of load: a-Preload: before start of contraction: a-Preload: before start of contraction: The more the initial length of muscle the more will be the tension in isometric contraction (within limits). The more the initial length of muscle the more will be the tension in isometric contraction (within limits). b-Afterload: after contraction started. b-Afterload: after contraction started. The more the afterload, the less will be the velocity of contraction. The more the afterload, the less will be the velocity of contraction.

Factors Affecting Muscle Contraction 3-Stimulus factors: a-Stimulus strength: a-Stimulus strength: The more the strength, the more fibers are stimulated, the more force of contraction. The more the strength, the more fibers are stimulated, the more force of contraction. b-Stimulus frequency: b-Stimulus frequency: Low frequency: separate muscle twitch. Low frequency: separate muscle twitch. Medium frequency: clones. Medium frequency: clones. High frequency tetanus: maintained High frequency tetanus: maintained contraction with no relaxation. contraction with no relaxation.

Factors Affecting Muscle Contraction 4-Fatigue: Repeated stimulation of muscle results in muscle fatigue due to: Repeated stimulation of muscle results in muscle fatigue due to: - Depletion of ATP, creatine phosphate & glycogen. - Depletion of ATP, creatine phosphate & glycogen. - Consumption of acetyl choline in motor end plate. - Consumption of acetyl choline in motor end plate. - Accumulation of metabolites: lactic acid. - Accumulation of metabolites: lactic acid. - Decreased oxygen & nutrient supply. - Decreased oxygen & nutrient supply.

Metabolic Changes in Muscle Muscular contraction requires energy and has been called a machine for converting chemical energy into mechanical work. Sources of energy: - Adenosine triphosphate (ATP). - Creatine phosphate (CP). - Glycogen or glucose. - Lipids: free fatty acids.

Steps of Metabolic Changes 1- One mol ATP is hydrolyzed to provide energy for muscle contraction. 2- Resynthesis of ATP from ADP and CP. 3- At rest during light exercise muscles utilize free fatty acids. free fatty acids. 4- As exercise intensity increases, muscles utilize glucose if O2 supply is sufficient. 5- If O2 supply is not sufficient (anaerobic glycolysis), glucose will give lactic acid.

Oxygen Debt It is extra O2 consumption after exercise is over to remove excess lactate, replenish ATP and CP stores. This O2 consumed is proportionate to the extent to which the energy demands during exercise exceeded the capacity of the aerobic synthesis of energy stores. O2 debt is 6 times the basal O2 consumption.