cardiac conduuction

Question 1

refractory period

Answer 1

more polarised the cell - smaller stimulus needed to cause an AP
longer in cardiac tissue than in skeletal muscle - muscle inert
means contraction well underway before the heart can be restimulated
allowing the heart to fill fully before contraction
cannot get tetanic contractions

Question 2

explain membrane potential and changes in ionic permeability for ventricles

Answer 2

Upstroke caused by influx of Na cells by Na channels opening - very rapid
small repolarisation some increase in Pk - opening of channel gives rise to Pto (transient outward current)
also get increase in Pca - important for cardiac contraction, Ca channels still open during upstroke
specialised Ik1 - large conductance K rapidly leaves the cell, this stabilises the resting membrane potential

Question 3

explain membrane potential and changes in ionic permeability of SAN

Answer 3

SAN has different channels so different activation
small Na influx causes diastole potential - not stable because on Pki
upstroke is caused by Ca influx through T and L type Ca channels
slower depolarisation because there are fewer channels
increase in Pk = repolarisation

Question 4

SAN

Answer 4

is the intrinsic pacemaker - allows the heart to be autonomous
the SAN is where the superior and inferior vena cava meet the R atrium
this can be modulated by the nervous and hormonal systems

Question 5

describe the cardiac conduction pathways

Answer 5

SAN produces electrical impulse - modulated by the autonomic nervous system
travels through internodal fibres across atria
held in the AVN (bottle neck) - insulates the superior ventricular myocardium from the excitation
travel down bundle of his through septum to apex
contraction spread through base of heart, up the ventricles via the purkinje fibres
stimulate ventricular contraction and increase pressure
the AP is spread from cell to cell

Question 6

describe how the AP is spread from cell to cell

Answer 6

the AP is spread from cell to cell
GAP junctions made of connexons provide a low resistance pathway for AP
depolarisation is spread to the adjacent cell
if it reaches threshold in this cell an AP is generated and it is depolarised
this is passed on
if it doesn’t reach threshold the depolarisation spreads down the cells and dwindles.