(XXVI.) Recruitment and Summation in Skeletal Muscle Dep. of Physiology, Fac. of Medicine, MU, 2016 © Jana Svačinová •Myography – method of recording of the muscle contraction •Motor unit: a group of muscle fibers innervated by a single a - motoneuron • •Muscle twitch – elementary mechanical response to a single stimulus (action potential) • •Types of muscle fibers: •S (slow) – slowly get tired, used in long-term performance, many mitochondria, well vascularized, a lot of myoglobin •F (fast) – fast contraction, quickly get tired, a lot of glycogen, a little myoglobin t (s) F (mN) Muscle twitch stimulation Strength of contraction Contraction of the skeletal muscle Morphology of the skeletal muscle fiber sarcomere Z disc M disc actin filament I band H zone A band myosin filament myosin heads Motor end-plate mitochondria nicotinic receptors Plasma membrane myelin Schwann cell cytoplasm Axon of the a-motoneuron Terminal button Vesicle of acetylcholine Muscle fiber Excitation – contraction coupling Excitation •Action potential (AP) spreads on axon from alfa-motoneuron to neuro-moto end-plate •Release of acetylcholine from vesicles to synaptic cleft •Binding of acetylcholine with the nicotinic receptors placed on post-synaptic membrane •Opening of Na+ channels (connected with acetylcholine receptors) and intake of Na + •Local depolarization of the membrane •Opening of voltage gaited channels for Na + •Formation of action potential AP Sarcoplasmic reticulum Sarcolemma cytoplasm Ca2+ Sarcoplasmic reticulum DHPR RYR1 T-tubule Contraction •Spreading of action potential (AP) across fiber and into transversal tubule (T-tubule) •Dihydropyridine receptors (DHPR) in the membrane changes its conformation •Interaction of DHPR with ryanodine receptors (RYR1) in the membrane of sarcoplasmic reticules •Opening of calcium channels in the sarcoplasmic reticulum and intake of Ca2+ into cytoplasm •Binding of Ca2+ with troponin C •Binding of myosin heads on actin •If enough of Ca2+ and ATP in cytoplasm, myosin shifts along actin ® contraction of muscle • Contraction ends with decrease od Ca2+ concentration in the cytoplasm (Ca2+ is pumped by Ca-ATPase into the reticulum) Rigor mortis – caused by ATP deficit ® formation of strong link between actin and myosin AP Sarcoplasmic reticulum sarcolemma cytoplasm Ca2+ Sarcoplasmic reticulum DHPR RYR1 T-tubule Excitation – contraction coupling Recruitment of skeletal muscle Increasing of the number of simultaneously activated motor units I – intensity of stimulation IP – threshold intensity of stimulation – first fibers started their contraction Imax – maximal intensity of stimulation – all motor units are activated I < IP I = IP IP < I < Imax IP < I < Imax IP < I < Imax I = Imax I > Imax t (s) F (mN) stimulation Force of contraction I (mA) F (mN) IP Imax Stimulus duration Stimulus intensity 2 x rheobase rheobase chronaxia As the strength of the applied current increases, the time required to stimulate the membrane decreases (and vice versa) to maintain a constant effect Rheobase: The smallest stimulus leading to contraction (infinite stimulus duration) Chronaxia: stimulus duration necessary for a contraction in case of two rheobases Dependence of contraction formation on the stimulus duration and strength Summation is due to repetitive activation prior to full relaxation (higher frequency of stimulation, higher force of contraction) Principle: The higher the frequency of the stimulus, the higher concentration of calcium in the cytoplasm ® increase of the contraction force If the next stimulus arrives before the contraction is completed, both mechanical responses fuse Superposition – if the fused contraction if double peaked Summation – if the new contraction occurs during crescent, resulting double contraction has a single peak Series of stimuli Incomplete tetanic contraction – cumulative superposition Smooth tetanic contraction – exerted by a train of stimuli during ascending phase superposition summation F (mN) t (s) Incomplete tetanic contraction Smooth tetanic contraction F (mN) t (s) Summation of skeletal muscle Heterometric autoregulation (Frank-Starling): Increase of the heart filling leads to stronger contraction of the heart Principles: 1) the relative position of actin and myosin during different stretch of muscle 2) Fiber stretching increases sensitivity of troponin to calcium Homeometric autoregulation: Increasing heart rate leads to muscle contraction increase Principle: Increase of ratio Intracellular/Extracellular calcium concentration Low heart filling High heart filling Extremal muscle stretch t Force of contraction Bowditch (Staircase) phenomenon Autoregulation of the cardiac muscle Homeometric autoregulation is analogous to the summation of the skeletal muscle. Cardiac muscle can not get into tetanic contraction because of long refractory phase. Cardiac muscle Skeletal, cardiac and smooth muscle – action potential and contraction Skeletal muscle Smooth muscle Action potential (AP): approx. 250 ms Contraction: approx. 250 ms 0 200 100 300 400 Time from AP onset (ms) AP: approx. 5 ms Contraction: approx. 20 ms AP: approx. 50 ms Contraction: approx. 1000 Fluctuating resting membrane potential Long refractory time AP duration depends on heart rate Duration of the electro-mechanical latency and contraction depends on the fiber type (F or S) spike