mechanics

Question 1

what shape are cardiac myocites

Answer 1

rod, 100um long, 200nm diameter t-tubules

Question 2

cardiac myocites: requirement

Answer 2

must have Ca2+ entering cells from outside to contract

Question 3

cardiac myocites: T-tubules spacing and function

Answer 3

cell surface invaginated by T-tubules; spaced so lies alongside each Z-line of every myofibril; carry surface depolarisation deep into cell

Question 4

cardiac myocites: sarcoplasmic reticulum function

Answer 4

stores Ca2+

Question 5

cardiac myocites: composition

Answer 5

myofibrils and mitochondria very high; sarcoplasmic reticulum, nucleus

Question 6

cardiac myocites: method of Ca2+ release in excitation-contraction coupling

Answer 6

many proteins along surface of T-tubule and cell membrane → depolarisation sensed by L-type Ca2+ channel → channel opens → Ca2+ influx down conc. gradient → some activates microfilaments; most binds to SR RyRCa2+ release channel → causes conformational change so opens → Ca2+ released

Question 7

fate of Ca2+ released from SR

Answer 7

bind to troponin for contraction; during relaxation period pumped back into SR via Ca2+ ATPase (so all released by SR reabsorbed) or removed from cell through Na+/Ca2+ exchanger (uses conc. gradient - antiporter facilitated diffusion, so all entered via L-type Ca2+ channel exists); resuls in same [Ca2+] as start

Question 8

why doesn’t skeletal muscle require Ca2+ entering cells from outside

Answer 8

once depolarised, L-type Ca2+ channel activates but there is a direct mechanical linkage to SR Ca2+ release channel

Question 9

force produced vs Ca2+ influx

Answer 9

force myocardial cell produces dependent on amount of Ca2+ influx, so complexly related to amount of Ca2+ in cytoplasm

Question 10

what does a force transducer do

Answer 10

measures force produced

Question 11

length-tension relation: passive and active force

Answer 11

when cardiac muscle stretched, force baseline (passive force) increases continuously and active force increases (up to point)

Question 12

total force

Answer 12

passive force + active force

Question 13

cardiac vs. skeletal

Answer 13

cardiac more resistant to stretch and less complient (due to properties of ECM and cytoskeleton); can’t overstretch cardiac tissue as contained in pericardium sac - only works in ascending limb of length-tension relationsip

Question 14

forms of muscle contraction

Answer 14

isometric and isotonic

Question 15

isometric contraction: define and effect on ventricular pressure

Answer 15

muscle fibres don’t change length; pressure increases in both ventricles (initial contractile event when all valves closed so against blood)

Question 16

isotonic contraction: define and effect on blood flow

Answer 16

muscle fibres do change length (when pressure reaches certain level, valves open so fibres shorten and blood injected)