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Biology 219 – Human Physiology Clemens Muscle Physiology 2 Text: Ch. 12 E. Activation of Contraction in Skeletal Muscle 1. Neuromuscular Junction - synapse between motor neuron and muscle fiber motor neuron AP → ACh release → synaptic transmission → EPP → muscle AP 2. Muscle Action Potential - formed along the sarcolemma, similar to axon membrane 3. Excitation-Contraction Coupling “Calcium is the key.” a) T tubules - conduct APs deep into the muscle fiber - DHP receptors function as voltage sensors, activated by the muscle AP b) sarcoplasmic reticulum (SR) - stores Ca2+ and releases it to myofilaments; - ryanodine receptors (RyR) in the SR membrane are Ca2+ channels RyR of the SR interact with DHP receptors of the T tubule Ca2+ released through RyR channels activates muscle contraction - Ca2+ pumps in the SR membrane actively transport Ca2+ back into the SR Sequence of Events in Excitation-Contraction Coupling 1) Muscle AP travels down the T-tubules. 2) DHP receptors on the T-tubule are activated. 3) Activated DHP receptor opens RyR Ca2+ channels. 4) Ca2+ diffuses out of the SR into the cytosol and to the thin filaments. 5) Ca2+ binds to troponin, which moves tropomyosin off the myosin binding site. 6) Myosin binds to actin → sarcomere contracts (crossbridge cycle) When APs stop, Ca2+ is pumped back into SR → sarcomeres relax F. Control of Whole Muscle Contraction motor unit = one motor neuron + all the muscle fibers it stimulates small motor units (1:10) → fine control, less force per unit large motor units (1:2000) → coarse control, greater force per unit recruitment - increase in number of active motor units - increased CNS stimulation activates more motor neurons with higher thresholds ↑ motor unit recruitment → more muscle fibers activated → ↑ force of contraction H. Length-Tension Relationship sarcomere length = 2.0 - 2.2 µm (“resting length”) - maximal number of crossbridges → maximal tension length > 2.2 µm - overlap decreases → tension decreases length < 2.0 µm - interference between filaments → tension decreases Tension (% ) G. Muscle Twitch and Summation 1. Twitch - single contraction relaxation cycle, evoked by a single muscle AP a. latent period b. contraction phase c. relaxation phase 2. Summation - two or more closely spaced APs → ↑ force 3. Tetanus - high frequency of APs → maximum, sustained force Sarcomere Length (µm) - skeletal muscle mostly operates at the top part of the L-T curve - cardiac muscle operates at shorter than optimal length, so ↑ stretch → ↑ force Biology 219 – Human Physiology Clemens I. Muscle Metabolism 1. Functions of ATP in muscle contraction: detaches the actin-myosin crossbridges energizes the myosin head powers the Ca2+ pump in the SR 2. Sources of ATP a. intracellular ATP stores b. phosphocreatine + ADP → creatine + ATP c. glycolytic (anaerobic) metabolism glycogen → glucose-6-P→ lactic acid, yields 3 ATP per glucose unit d. oxidative (aerobic) metabolism glycogen → glucose-6-P → CO2 + H2O, yields ~32 ATP per glucose triglycerides → fatty acids → CO2 + H2O, yields >100 ATP per fatty acid 3. Muscle fiber types a. slow-twitch oxidative (SO) (“red muscle”) - slow myosin ATPase, small diameter → low power - abundant mitochondria and myoglobin - high triglyceride content - highly aerobic → slow fatiguing b. fast-twitch oxidative-glycolytic (FOG) - fast myosin ATPase, intermediate diameter, intermediate power - can utilize both aerobic and anaerobic metabolism c. fast-twitch glycolytic (FG) (“white muscle”) - fast myosin ATPase, large diameter → high power - high glycogen content - mostly anaerobic → fast fatiguing J. Smooth Muscle Physiology 1. General Features - no sarcomeres, oblique arrangement of thick and thin filaments - contraction results from crossbridge formation between thick and thin filaments - contraction activated by action potentials, graded potentials, or chemical signals - spontaneous contraction (pacemaker activity) in some smooth muscle cells Activation of Contraction in Smooth Muscle 1) Depolarization of smooth muscle cell (AP or GP) 2) Ca2+ enters cytosol from ECF (through voltage-gated Ca2+ channels) and/or from sarcoplasmic reticulum (through RyR Ca2+ channels) 3) Ca2+ binds to calmodulin → Ca-calmodulin 4) Ca-calmodulin complex activates myosin light chain kinase (MLCK) 5) MLCK catalyzes phosphorylation of myosin light chain 6) phosphorylation of myosin light chain → crossbridge formation → contraction When myosin light chain is dephosphorylated, crossbridge activity stops → relaxation 2. Types of Smooth Muscle a. single-unit smooth muscle - extensive connections between cells via gap junctions - fewer innervation points (via varicosities of autonomic motor neurons) b. multi-unit smooth muscle - multiple innervation points, little or no connection between cells