Transcript muscles
Muscle Physiology II Cardiac Muscle • Same mechanism as skeletal • Less calcium stored but longer T-tubules & more released with a single impulse • Impulses travel rapidly from cell to cell so it is self-stimulating Smooth Muscle • Myofibrils are very thin and randomly organized • Use a different protein than troponin • Can be triggered by hormones or even muscle stretching Stimulus Response • An individual fiber’s response to stimulus is called a twitch – when myosin heads link with actin • The delay between stimulation (ACh release) and twitch is called the latent period • This is followed by the contraction & the relaxation periods • myogram Stimulus Response • For a brief moment after stimulus, a fiber is unable to respond to further stimulation –Refractory period • If the next stimulus comes after the refractory period but before relaxation the twitches combine leading to a sustained contraction Fiber Types • 3 types: slow-twitch, fast-twitch, & intermediate • Most muscles have a mixture of all 3 –Postural: mostly slow-twitch –Arms/shoulders: mostly fast-twitch –Legs: even mix of slow and fast-twitch • Strength is directly proportional to diameter Slow-twitch fibers • Thinnest in diameter • Utilize aerobic respiration (oxidative) • Red due to high concentration of myoglobin • Slowest contraction cycle –Prolonged sustained contractions –Resistance to fatigue Fast-twitch Fibers • Thickest in diameter • Utilize anaerobic respiration (Glycolytic) • White due to lack of myoglobin and poor blood supply • Most powerful and rapid contractions –Intense movement but for short duration –Fatigue easily Intermediate Fibers • Like a combination of slow and fasttwitch • Red due to myoglobin • Both oxidative and glycolytic • A middle ground for resistance to fatigue, speed, and power Respiration Aerobic (Oxidative) • Mitochondria ATP • Molecule called myoglobin able to store O2 in the fiber Anaerobic (Glycolytic) • Take over when O2 supply becomes insufficient • Lactic acid produced as a byproduct –Carried to liver for breakdown Lactic Acid Buildup • Lactic acid lowers pH which interferes with the muscles’ ability to respond to stimuli –fatigue • Does not directly cause cramps –Involuntary, sustained contraction –Could be electrolyte imbalance –Likely caused by use during fatigue damage Muscle Fatigue • With prolonged use, a muscle can lose its ability to contract –Decreased blood flow –Ion imbalance –Lactic acid accumulation Exercise • The ratio of slow to fast-twitch fibers has roots in genetics • More fast-twitch fibers is well-suited for weightlifting and sprinting –Short duration & intense • More slow-twitch fibers is well-suited for running and swimming –Endurance is key Heat Production • Respiration creates heat as a by-product • Dispersed throughout the body through bloodstream for release via homeostatic mechanisms Building Muscle Mass Generally, the number of fibers does not change only their characteristics • Aerobic Training – converts fast-twitch to intermediate; more mitochondria, increased blood supply • Strength Training – increases size and strength of fast twitch –Synthesis of actin & myosin Atrophy • Lack of use will cause muscles to deteriorate • Happens naturally with age –Starts around age 30 –Slow, progressive loss of mass –Replaced by connective tissue –Decrease function • Strength • Flexibility • Reflex Muscles after Death • At death, calcium ions are released causing a partial contraction • No new ATP so the muscles can’t relax • Joints become fixed creating the condition known as rigor mortis Muscle Memory • With repeated motion, the nervous system circuitry becomes more efficient at stimulating the muscles • Strength of muscle is first dependent on ability to stimulate, not mass Muscle Problems • Muscles are connected to other muscles by connective tissue called aponeuroses • Muscles are connected to bones by tendons –Tendinitis occurs when the tendons swell causing pain and disrupts the joint function Strains • Overstretching or tear of tendon or muscle –Insufficient warm-up –Poor technique –Untreated minor injuries • Classified like sprains Involuntary, Abnormal Contractions • Spasm: sudden contraction in a single muscle • Tic: twitching in muscles usually under voluntary control • Tremor: rhythmic, purposeless shaking Myasthenia Gravis • Autoimmune –Antibodies block Ach receptors • Damages neuromuscular junction • Muscles weaken and fatigue easily –Can cease to function Muscular Dystrophy • Progressive degeneration of muscle fibers –Protein that reinforces sarcolemma not produced • Duchenne MD caused by mutation on the X • Onset around age 2-5; can’t walk by age 12; cardiac failure at age 20-30 Performance Enhancers Most performance enhancers help build muscle mass by stimulating protein synthesis • Steroids –Mood swings, aggression, baldness, disrupted sexual function, infertility • HGH –Overgrowth of hands, feet, head; enlarged internal organs esp. heart Power • In addition to stored ATP, muscle fibers use creatine for ATP production as well. • This, however, is still not enough for more than a quick burst so cellular respiration must occur Sex Differences • Anatomy is basically the same –Men tend to have more fast-twitch fibers than women • Mass differs –Testosterone hypertrophy –So men have larger fibers and thus more mass and strength