Download Contraction in Skeletal Muscle - Human Physiology - Lecture Notes and more Study notes Human Physiology in PDF only on Docsity! Te ns io n (% ) Muscle Physiology 2 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 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 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 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 Docsity.com
