Week 6 Lecture 2: Muscle

• efferent neurons of the somatic nervous system are called motor oneurons
• a neuromuscular junction is the synapse between a motor neuron and a muscle fiber
• axon terminals of the motor neuron are called terminal boutons - they store and release acetylcholine
• motor end plate is a specialized muscle membrane
• 2 anatomy parts of the neuromuscular junction - terminal bouton and motor end plate
• all motor neurons release acetylcholine
• all synapses at the neuromuscular junction are excitatory
• motor neuron is activated by action potentials which are propagated to the terminal bouton - the resulting depolarization causes calcium channels in boutons to open allowing calcium to enter and trigger the release of acetylcholine by exocytosis - acetylcholine then binds to nicotinic cholinergic receptors at the motor end plate causing cation channels to open - this allows sodium to open and a depolarization called an end-plate potential
• end-plate potentials usually generate action potential which triggers the contraction of the muscle fiber
• the cross bridge cycle is how muscles generate force
• crossbridge cycle - oscillating back and forth motion of myosin cross-bridges which is powered by ATP hydrolysis
• crossbridge cycle - binding of myosin to actin, power stroke: myosin head moves propelling thin filament toward center of muscle, rigor, unbinding of myosin and actin: thick and thin filaments detach, cocking of myosin head: myosin head returns to initial position
• high energy form of myosin has a high affinity for actin the low energy form has a low affinity for actin
• shortening of sarcomere is what happens during muscle contraction
• sarcomeres are made up of actin and myosin
• tropomyosin is a molecules that blocks myosin binding sites in muscles at rest
• troponin is made up of 3 proteins - one attaches to actin, tropomyosin and a site where calcium ions can bind
• excitation-contraction coupling - action potential in sarcolemma causes contraction
• excitation - contraction coupling - acetyl choline released from motor neuron and binds to receptors on motor end plate, action potential propagates along sarcolemma, action potential triggers calcium release, calcium binds to troponin exposing myosin-binding sites, crossbridge cycle begins, calcium transported back and tropomyosin blocks myosin-binding sites
• a contraction ends with calcium leaves the binding sites and is removed from the cytosol back into the sarcoplasmic reticulum
• twitch contraction - contraction produced in a muscle fiber in response to a single action potential - it is all or nothing event
• phases of twitch - latent period: delay that occurs between action potential in muscle cell and start of contraction, contraction phase: muscular tension peaks, relaxation phase: longest of 3 phases end of contraction
• summation - muscle is stimulated repetitively such that action potentials arrive before twitches are completed, this yields a force greater than a single twitch
• tetanus - summation peak
• incomplete tetanus (unfused) - small oscillations with brief periods of relaxation between peaks
• complete tetanus (fused) - myosin-binding sites on actin are continuously exposed