3.6.1.3 control of heart rate and muscle contraction

stress response in autonomic nervous system

hypothalamus detects threat, pituitary gland releases ACTH, adrenal gland stimulated and adrenaline and cortisol released, liver converts glycogen to glucose, digestion slows down, high bp, hr, breathing speed and sweating

how does sinoatrial node (SAN) control heart rate

san generates impulse causing atria to contract, impulse cannot pass directly to ventricles due to non-conductive tissue in av septum, impulse reaches av node where it is delayed for a fraction of a second to ensure ventricles can fully fill, av node channels impulse down bundle of his then transmitted down septum to apex, impulse spreads up from apex via purkinje fibres causing ventricular systole, blood pushed into pulmonary arteries and aorta

chemoreceptors

sensitive to co2 conc in blood, found in aortic and carotid body

types of muscle tissue

skeletal/striated muscle (tubular), cardiac muscle, smooth muscle (ans, involuntary)

myofibril

tiny muscle fibres that wrap together to form larger bundles of muscle fibres within a muscle cell, mainly composed of 2 types of protein filament (myosin and actin)

sarcoplasm

cytoplasm of fused muscle cells

sarcomere

repeated subunits of myosin and actin filaments, one unit of contraction

transverse tubule

extension of sarcolemma, ensures all myofibrils contract at once

myosin

fibrous, thick filaments, myosin atpase site so can release energy needed for contraction, arranged so heads at opposite ends and tails meet at m-line, head attaches to actin filament

actin

globular, thin chains, 2 chains twist around each other to form filament, ends held at z-line, tropomyosin fits between actin chains, troponin binds to chain at regular intervals

a-band

broad dark zone consisting of myosin filaments interlocking with actin chains

m-line

centre point of a-band

h-zone

only myosin filaments present

i-band

lighter area, consists of only actin filaments and titin

z-line

centre point of i-band, made of titin, between 2 z-lines is the sarcomere

slow twitch fibres

contract slower, less powerful contractions over long periods, found in muscles such as calf, used for endurance, adapted to aerobic respiration, large myoglobin store, rich blood vessels supply, numerous mitochondria

fast twitch fibres

contract faster more powerful contractions in short periods, found in muscles such as biceps, used for intense exercise, thicker + more numerous myosin filaments, high conc of glycogen, high conc of enzymes involved in anaerobic respiration, phosphocreatine store

phosphocreatine

reserve of high energy phosphates in high energy tissue such as skeletal muscle, when o2 is scarce, cell relies on pcr store to produce atp in sarcoplasm, ADP + PCr ←→ATP + Cr

transmission of impulse between motor neurone and muscle

n.transmitter travels along nm junction and attaches to recpetors on sarcolemma, depolarising it by opening ligand gated na+ channels, depolarisation propagated to t-tubules, voltage gated ca2+ channels open so ca2+ ions move down conc gradient to sarcoplasm from sarcoplasmic reticulum and bind to troponin stimulating muscle contraction

effect of ca2+ ions binding to troponin

conformational change occurs so tropomyosin is moved, uncovering myosin binding sites on actin, myosin heads bind and form cross-bridges

power stroke cycle

ADP and Pi attached to myosin head from previous cycle, when cross-bridge formed pi is released, energy stored in head used to pivot and pull head back causing actin to slide past myosin, ADP released as it moves, bond between myosin and actin broken when ATP molecule binds to myosin head, ATP hydrolysed to be reused and head of myosin returns to upright position, if ca2+ ions still present, entire sequence repeated