Skeletal Muscles Flashcards
name 3 types muscles and where they are located
-skeletal muscle: attached to incompressible skeleton by tendons
-smooth muscle: walls of blood vessels and internal organs including the stomach & intestines
-cardiac muscle: only found in the heart
skeletal muscle
-skeletal muscle is found attached to the skeleton and responsible for the voluntary movement of bones
-skeletal muscle fibres run in parallel tracts and multinucleated and heavily striated
define antagonistic pair of muscles
muscles can only pull, so they work in pairs to move bones around joints.
agonist contracts, antagonist relax
structure of skeletal muscles
made up of large bundles of long cells called muscle fibres.
-the cytoplasm- sarcoplasm
-cell membrane-sarcolemma: folds inwards in places forming transverse tubules (T-tubules) which help to spread electrical impulses through whole sarcoplasm
-endoplasmic reticulum-sarcoplasmic reticulum: stores and releases Ca2+ needed for muscle contraction
-large concentration of mitochondria
ultrastructure of myofibrils
muscle fibres made up of myofibrils. –myofibrils made of two protein filaments-thick-myosin and thin-actin filaments which slide past each other
-made up of repeating short units called sarcomeres
-Z line: marks end of each sarcomere
-I band: light, actin only
-A band: dark, contains both actin and myosin
-H zone- centre of A band, contains only myosin.
where are slow twitch and fast twitch muscle fibres located
slow: sites of sustained contraction, e.g. calf muscle
fast: sites of short term, rapid powerful contractions e.g biceps
slow twitch fibres
-less powerful but over longer period
-adapted to endurance work e.g. marathon
-adapted for aerobic respiration in order to avoid lactic acid build up
-large store of myoglobin-higher affinity for O than haemoglobin in lower partial pressures
-rich supply of blood vessels to deliver oxygen and glucose
-numerous mitochondria for ATP
fast twitch fibres
-powerful contractions over short period
-adapted to intense exercise e.g. weightlifting, sprint
-thicker and more numerous myosin filaments
-high concentration glycogen
-high concentration of enzymes involved in anaerobic respiration which provides ATP rapidly
-store of phosphocreatine, molecule that can rapidly generate ATP from ADP in anaerobic conditions and so provide energy for muscle contraction.
neuromuscular junction
-synapse between motor neuron and sarcolemma of muscle fibre
-nerve impulse causes muscle to contract
-same initial process as cholinergic synapse
-ACh binds to sarcolemma-causes Na+ ion channels to open, depolarising membrane. If threshold temperature is reached - action potential is initiated.
-impulse travels down the sarcolemma and down the T-tubules which allow transmission of action potential into sarcoplasm of muscle cells.
-this depolarisation causes Ca2+ ions to be released which leads to contraction of muscle fibre.
difference between cholinergic synapse and neuromuscular junction
-neuromuscular between motor neuron and muscle only
-excitatory only whereas cholinergic can also be inhibitory
-action potential triggered in sarcolemma and travels down T-tubules compared to postsynaptic membrane
-neuromuscular is end of action potential whereas in cholinergic new action potentialis generated in next neurone
sliding filament theory
-wave of depolarisation travels down T-tubules to sarcoplasmic reticulum, which triggers it to release stored Ca2+ into sarcoplasm.
-influx of Ca2+ triggers muscle contraction
-Ca2+ diffuse into myofibrils, bind to and cause tropomyosin to move exposing binding sites on the actin
-whilst ADP attached to myosin heads, they attach to binding sites on actin, forming actinmyosin cross bridge.
ATP-> ADP + Pi, Pi is released
-myosin head bends pulling actin filament along, ADP is released-power stroke
-a new ATP molecule attaches to the myosin head, changing its shape slighlty, causing it to detach from actin binding site. Z lines move closer together and sarcomere shortens.
-hydrolysis of ATP to ADP and Pi by ATPase enzyme provides energy for myosin head to return to original position, ready to form new cross bridge at next binding site.
-process repeats continually whilst calcium ions remain high & therefore while muscles remain stimulated by nervous system.
role of phosphocreatine
stored in muscle, rapidly provides phosphate to combine with ADP to regenerate ATP when oxygen for aerobic respiration is limited.
creatine can be broken down creatinine and removed by the kidneys- high levels of creatinine may indicate kidney damage.
muscle relaxation
-Ca2+ actively transported back into endoplasmic reticulum.
-reabsorptionof Ca2+ allows tropomyosin to block the actin filaments again
-myosin heads unable to bind to actin filament and contraction stops
evidence to support sliding filament theory
- H zone becomes narrower
- Z lines move closer together- sarcomere shortens
- I band becomes narrower
-A zone remains same width- proves that myosin filaments don’t shorten
