BIOL 164 Human Biology Ch 6 Muscular System Muscle (ssue has 4 important proper(es: Ch. 6 The Muscular System The Muscular System 1. Excitability it can respond to s(muli 2. ContracBlity it can shorten 3. Extensibility it can stretch 4. ElasBcity it can return to original length a;er shortening or stretching Review of 3 types of muscle Bssue 1. Cardiac muscle striated (striped), involuntarily (subconsciously) controlled Found in walls of heart Pumps blood 2. Smooth muscle not striated, involuntarily controlled Found in walls of hollow organs Moves eggs, sperm, urine, food, hairs, etc. 3. Skeletal muscle striated, voluntarily controlled Usually agached to bones (via tendons) The major focus of this chapter Many func(ons (see next slide)
Skeletal muscles = organs made up mainly of skeletal muscle (ssue, but also CTs, nerves, and blood vessels The cells of skeletal muscle (ssue can be very long (up to 1 foot!) and thus are o;en called muscle fibers or myofibers Func(ons: 1. Produce skeletal movement 2. Maintain posture and body posi(on 3. Support sok Bssues E.g. floor of pelvic cavity 4. Guard entrances and exits E.g. esophagus, external anal sphincter 5. Produce heat to help maintain body temperature 6. Store nutrient (protein) reserves Skeletal muscles work in pairs or groups Skeletal muscles cause movement (ac(ons) by contrac(ng/shortening, bringing the inser6on closer to the origin Antagonists are muscles that perform opposite ac(ons When one is contrac(ng, the other is relaxing E.g. biceps and triceps in flexion and extension (respec(vely) of forearm Synergists are muscles that work together to perform the same ac(on E.g. the 4 quadriceps in extension of lower leg Some examples of skeletal muscles FYI only do not memorize!
OrganizaBon of skeletal muscle part 1 Whole muscle Cell Myofilaments = special long contractile proteins actin and myosin Myofibril = bundle of myofilaments Bundle of cells OrganizaBon of skeletal muscle part 2 (smaller view) OrganizaBon of skeletal muscle part 3 (smallest view) (thin filament) (thick filament)
The sliding filament model Sarcomeres = (ny, repea(ng func(onal units of myofibrils Thousands per myofibril arranged end to end The model: The sarcomere shortens as thin and thick filaments slide past one another Sarcomeres shorten myofibrils shorten muscle cells shorten whole muscle shortens! Muscle contracbon 1: at rest ATP has agached to myosin heads split into ADP and Pi releasing energy to cock myosin head into a high energy posi(on Ac(n myosin binding sites are covered Muscle contracbon 2: the sbmulus AP from motor neuron arrives at neuromuscular junc(on (synapse) ACh released, new AP generated along muscle cell membrane Ca2+ ions released from sarcoplasmic re(culum a specialized type of smooth ER)
Muscle contracbon 3: cross bridge formabon/ auachment Muscle contracbon 4: the power stroke power stroke ( uncocking ) Muscle contracbon 5: cross bridge detachment
Muscle contracbon 6: myosin reacbvabon Repeat steps 3 5 (as long as [Ca 2+ ] and [ATP] high enough) shortening sarcomeres and thus en9re muscle crea9ng a pulling force (tension) on the a;ached bone ProperBes of skeletal muscle fiber contracbon All or none all sarcomeres in a given muscle fiber contract together maximally when fiber is s(mulated I.e., muscle fiber is either on (producing tension) or off (relaxed) The more frequently a muscle fiber is s(mulated, the stronger the contrac(on (see below) Motor units and recruitment Motor unit one motor neuron and all of the muscle fibers that it innervates Size of motor unit inversely related to precision of control (e.g. eye muscles vs. leg muscles) Recruitment = s(mula(ng more motor units For stronger contrac(on of whole muscle
Muscle tone = low level of tension in skeletal muscle present at all (mes (but NOT enough tension to produce movement), causing healthy muscle to feel solid and firm to the touch even at rest Some motor units are ac(ve at any given point in (me Which motor units are ac(ve varies constantly Func(ons of muscle tone: Stabilizes bones and joints Maintains body posi(on (posture) Allows more rapid ac(va(on of whole muscle (i.e, accelerates recruitment) when needed energy usage when muscles are at rest (i.e., res(ng metabolic rate) Lookin good! Energy sources for muscle contracbon Maximal intensity (e.g. sprinting) Submaximal intensity (e.g. jogging) (anaerobic) Longer duration (minutes to hours) ATP generated aerobically in mitochondria from glucose, fats, or proteins Rate of oxygen delivery meets demand Very efficient, and no acid buildup Slow vs. fast twitch muscle fibers Note: the figure at right is labeled wrong slow twitch fibers are smaller and darker while fast twitch fibers are larger and paler Par(cular mix of fiber types gene(cally determined and varies among people