Factors Affecting Skeletal Muscle Contraction PDF

Factors Affecting Skeletal Muscle Contraction Dr. Hader I. Sakr Associate professor, Medical Physiology  ‫بسم هللا الرمحن الرحمي‬ ‫اَّلل‬ ‫اَّلل ۗ َو َُّ‬ ‫﴿ َوات َّ ُقوا َّ َ‬ ‫اَّلل ۖ َوي ُ َع ِّل ُم ُ ُُك َُّ‬ ‫َش ٍء عَ ِّل ممي﴾‬ ‫ك َْ‬...

Factors Affecting Skeletal Muscle Contraction Dr. Hader I. Sakr Associate professor, Medical Physiology  ‫بسم هللا الرمحن الرحمي‬ ‫اَّلل‬ ‫اَّلل ۗ َو َُّ‬ ‫﴿ َوات َّ ُقوا َّ َ‬ ‫اَّلل ۖ َوي ُ َع ِّل ُم ُ ُُك َُّ‬ ‫َش ٍء عَ ِّل ممي﴾‬ ‫ك َْ‬ ‫ِّب ُ ِّ‬ Learning objectives: Type of muscle fibers. Stimulus Factors. Length-tension relationship. Load-Velocity Relationship. Muscle fatigue. Type of muscle fibers  Type of muscle fibers Red muscle fibers Pale muscle fibers Type Small muscle fibers (I) Large muscle fibers (IIb) Innervation Small slowly conducting motor Large rapidly conducting motor neurons. neurons. Blood supply Extensive capillaries less blood supply Energy source Fatty acid oxidation Glucose oxidation to pyruvate (oxidative fibers) (glycolytic fibers) Mitochondrial volume More Less Myoglobin content High Low ATPase activity Low High Contractile mechanism Slow Fast Resistance to fatigue High Low Importance Tenses a part of the body Moves part of the body  Type of muscle fibers - Most muscles of the body contain both types of muscle fibers as gastrocnemius. - Muscles adapted for long posture-marinating contractions e.g. back muscles & soleus muscle are composed mainly of slow fibers. - Muscles specialized for fine skilled movements e.g. extra-ocular muscles & some hand muscles are composed mainly of fast fibers. - Muscle groups with a high percentage of fast fibers exert more force and a greater velocity than those with predominant slow fibers. - Aging is associated with a loss of muscle mass. Type of muscle fibers Stimulus Factors  Stimulus Factors a- Strength of the stimulus: - Increasing the strength of stimulus will increase the number of activated fibers [recruitment] with gradual increase in whole muscle response. - Maximal stimulus activates all muscle fibers. - Supra maximal stimulus would not give further response as each fiber responds maximally according to all or none law.  Stimulus Factors b- The frequency of muscle stimulation: 1. Low frequency (2 / sec): Leads to separate twitches as this allows enough time for the muscle to complete relaxation. - Treppe "The Stair Case Phenomenon”: It refers to the progressive increase in the magnitude of separate twitch contraction of skeletal muscle to a plateau value during repetitive stimulation after a period of rest. - This phenomenon is explained by the persistent elevated levels of [Ca2+] in the cytoplasm.  Stimulus Factors 2. Intermediate frequency (5 / Sec): Leads to Clonus as the stimuli falls during the relaxation time  Stimulus Factors 3. High frequency (10 / Sec): Leads to tetanus. As the stimuli falls during the contraction time. - During a complete tetanus, the tension developed is about 4 times (4X) that developed by the individual twitch contractions. - When repeatedly stimulating the muscle, the level of [Ca+2] in the myofibrils remains continuously above the level required for full activation of the contractile process i.e. continuous cycling of the cross-bridges.  Stimulus Factors Comments:- - Mechanical contractions can fuse to form tetanus, while electrical response (APs) never fuses. - Muscles in the body contract by tetanic contractions and not by SMT. - The tetanic contractions result in high and maintained mechanical response which enables the muscle to do useful work. Length-tension relationship  Length-tension relationship - Measures tension developed during isometric contractions when the muscle is set to fixed lengths (preload).  Length-tension relationship - Preload is the load that a muscle experiences before the onset of contraction. - Passive tension is the tension developed by stretching the muscle to different lengths. - Total tension is the tension developed when the muscle is stimulated to contract at different lengths. - Active tension is the difference between total tension and passive tension  Length-tension relationship - There is a relationship between the initial muscle fiber length [preload] and the active tension developed during its isometric contraction. - This relation is best described by Starling’s Law, which states that “within physiological limits, the more the initial length of the muscle fibers, the more is its force of contraction.” Length-tension relationship Length-tension relationship Length-tension relationship  Length-tension relationship 1. When the muscle fiber length is set at a sarcomere length = 2.2 μm (in vivo resting muscle length) → maximal force is obtained. - At this length, the overlap between thick and thin filaments is optimal, since every cross-bridge from the thick filament is opposite an actin molecule. 2. Sarcomere length ˂ 2.2 μm → decrease in force development. - The ends of the two action filaments overlap each other, in addition to overlapping the myosin filaments, making it more difficult for the muscle to develop force. 3. At sarcomere length ˃ 2.2 μm, the overlap between thick and thin filaments is decreased. Thus, some cross-bridges do not have actin filaments to combine with → decrease in the force development. Load-Velocity Relationship  Load-Velocity Relationship - In isotonic contraction, for the muscle to shorten, it must lift a weight, called afterload. - Afterload is a load that is encountered by the muscle only after it starts to contract. Load-Velocity Relationship  Load-Velocity Relationship - Increasing the afterload has the following effects: 1. The velocity of shortening decreases because each cross-bridge cycle takes longer. 2. The amount of shortening decreases as the muscle spends more time in the isometric component of the contraction. 3. The maximal velocity of shortening (Vmax) occurs when there is no external load (zero load). 4. The velocity of shortening is zero (V0) when the afterload is maximum (Pmax). At this point, the contraction will become isometric. N.B. Vmax is theoretical, because load cannot be zero. Muscle Fatigue  Muscle Fatigue - Definition: a state results in decreasing the strength of contraction, prolongs its duration, and relaxation becomes incomplete [contracture] due to rapid repeated stimulation. - Causes of fatigue: 1. Gradual inactivation of Ca+2 channels. 2. Exhaustion of the vesicles and decreased ACh at NMJ (main cause in vitro). 3. Down-regulation of NM receptors. 4. Decreased ATP, Glycogen, CP and other muscle enzymes. 5. Accumulation of lactate → pain (main cause in vivo).  Muscle Fatigue - Effects of fatigue: 1. Progressive decrease in the amplitude of contraction. 2. Progressive prolongation in the durations (LP, contraction, relaxation, and recovery times = slower contraction). 3. May lead to muscle contracture (incomplete relaxation) due to depletion of ATP.  Conclusion Factors affecting skeletal muscle contraction are: 1. Type of muscle fibers: I, IIa, IIb. 2. Stimulus factor: strength and frequency (SMT, clonus or tetanus). 3. Preload (length-tension curve) → Starling’s law (Direct within limits). 4. Afterload (load-velocity curve) → Inverse proportion. 5. Fatigue. References Guyton and Hall, 13th edition. Unit II(6); 81-2. Ganong’s review of medical physiology 25th ed. Section I(5); 107-10. Thank You