Transcript muscles
Somatic nervous system • Signals from CNS are sent to skeletal muscles. Final result is a muscle contraction. • Motor neuron starts in CNS and its axon ends at a muscle cell. Alpha motor neuron Alpha motor neurons branch into several terminals (can be over 1000), each contacting a separate muscle cell. Nerve meets muscle Axon of motor neuron Action potential of motor neuron Terminal button Voltage-gated calcium channels Action potential propagation in muscle fiber Voltage-gated Na+ channel Acetycholinesterase acetylcholine Motor end plate Organization of cells Sarcomere Myofibril Muscle cell Sarcomere – the unit of contraction, made of thin (actin) and thick (myosin) filaments Myofibril sarcomere Z band sarcomere Z band Z band A band Contraction of filaments Before Contraction Z Z After Contraction Z Z Length of sarcomere shortens with contraction but filament length is unchanged Myosin Actin Myosin Actin • Tropomyosin normally covers the myosin binding site on actin • When calcium binds with troponin, it pulls tropomyosin away from the binding sites troponin tropomyosin myosin actin myosin binding site blocked Calcium myosin actin cross-sectional view Figure 8.2 (3) Page 259 Sarcomere Myosin Actin Actin Myosin Muscle Contraction • Signal from motor neuron causes action potential in muscle cell • Calcium ions released (from sarcoplasmic reticulum) • Actin and myosin filaments slide relative to each other Myosin cross bridge BINDING Myosin cross bridge binds to actin POWER STROKE Cross bridge bends, pulling thin filament. DETACHMENT Cross bridge detaches and returns to original shape- *ATP required* BINDING to next actin molecule; repeat Figure 8.13 Page 267 Myosin needs ATP to change shape Energized Resting Binding Detachment Bending (power stroke) Myosin has a binding site for ATPase Rigor complex Signal coming to muscle motor neuron Sarcoplasmic reticulum (Ca+2 storage) T tubule From action potential to contraction • Calcium is the link – Acetylcholine released at the neuromuscular junction - action potential on muscle fiber – Action potential down “T tubule” to sarcoplasmic reticulum at muscle fibers – Calcium released from the SR to muscle fibers Pathway review Terminal button T tubule Action potential AcetylcholineAcetylcholine gated cation channel (Na+ moves in) Tropomyosin Actin A calcium pump in SR allows muscle to relax Troponin Cross-bridge binding Myosin cross bridge Muscles contain groups of motor units Units are recruited during motor activity Muscle force depends on # muscle fibers contracting Motor unit = motor neuron + muscle fibers it innervates The number of muscle fibers varies among different motor units. – muscles can have many small units or a few large units – Asynchronous recruitment of motor units delays or prevents muscle fatigue. Tension and frequency of stimulation twitch - brief contraction resulting from 1 action pot’l tetanus - twitch summation from sustained Ca+2 Muscle length and force Fast and slow twitch muscle cells Differences in time when maximum tension is reached Slow twitch (Type I) - have myoglobin, many mitochondria, oxidative Fast twitch (Type IIa) - myoglobin, mitochondria, oxidative & glycol. “Very” Fast twitch (Type IIb) - use glycolysis, split ATP quickly Fast and slow twitch muscle cells Oxidative - resistant to fatigue, high rate of O2 transfer from blood, recruited 1st Glycolytic - more prone to fatigue b/c less ATP produced, harder to recruit Endurance vs. Bursts of power • People are born with certain ratio of slow vs. fast twitch fibers – usually an even mix in most skeletal muscles Sensation at muscle Spindle muscle fibers (deep within muscle) sense stretch, and Golgi tendon organs (in tendons) sense tension. Intrafusal (spindle) muscle fibers Knee spinal reflex Muscle spindle Extensor muscle motor neuron Patellar tendon Primary types of contraction • Isometric contraction - muscle tension is not enough to move load. Muscle doesn’t shorten. • Isotonic contraction – Concentric – muscle shortens to lift a load. – Eccentric - shortened muscle has controlled lengthening. slowly lowering the weight Exercising your muscles Endurance training type IIb type IIa more mitochondria, glycogen, vascularization Exercising your muscles Strength training hypertrophy of type II fibers Hypertrophy: how muscles get bigger Muscle cells have satellite cells nearby that respond to muscle injury and wear Why are muscles sore after lifting? Hypertrophy: how muscles get bigger Satellite cells: – activated at microtears – add nucleus to muscle cell – more myofibrils made – cell wider Muscle hypertrophy vs. hyperplasia Hypertrophy Hyperplasia How can muscle recruitment change with exercise? • The CNS can become trained to provide more force (apparent in early training) – Better inhibition of antagonistic muscles – Improved recruitment of different muscles over a movement to gain power ATP sources at muscles 3a 1 2 3b What does creatine do? What are muscle cramps? • When muscle fibers contract without our control it is a muscle spasm or cramp • Due to motor neurons being hyperexcited, often b/c of a shift in body fluids or ion levels (dehydration, low Ca, Mg, K) or vigorous activity Smooth muscle • Smooth muscle cells are small and unstriated – No sarcomeres – Smooth muscle cells contract when Ca+2 enters – Myosin cross bridges are phosphorylated and bind to actin • Is there a point where being muscular negatively affects our bodies? • What is largest muscle in the body and why • Why do our eyes twitch sometimes, seemingly for no reason. Smooth muscle Smooth muscle Striated muscle • Multiunit - similar to skeletal motor units • Single unit - gap jxns b/w muscle cells. Many cells contract as a unit. (uterus, intestine, bladder) Cardiac muscle Pacemaker muscle cells - action potential gradually depolarizes, then repolarizes Contraction spreads from pacemaker through gap jxns Spontaneous action potential Pacemaker cell Gap junctions Action potential spread to other cells