Lecture 3: Electrical Activity of the Heart Flashcards Preview

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Flashcards in Lecture 3: Electrical Activity of the Heart Deck (30):

What is the net charge of the intracellular environment? briefly state the contributions to this charge

net -ve charge
contributing factors:
- membrane more permeable to K (electrical gradient)
- Na/K ATPase pump ensuring Na is far from concentration gradient
- membrane not permeable to large -ve proteins that are trapped intracellularly.


What is the direction of K and Na ions into and out of the cell and via which gradients?

K+ moves into the cell down its ELECTRICAL gradient
K+ moves out of the cell down its CONCENTRATION gradient
Na+ moves into the cell down its CONCENTRATION gradient AND ELECTRICAL gradient


What mechanism ensures the cells resting membrane potential doesn't reach equilibrium?

equilibrium is reached when the 2 opposing forces (electrical and concentration gradient exactly balance)
the Na/K ATPase pump ensures the Na concentration gradient is far from equilibrium.


What are the two types of ion channels that allow the selecting movement of ions into the cardiac myocyte?

1. Receptor Operated - open when receptor bind
2. Voltage Gated - gated by memb. potential.


What ions generate the action potential within the cardiac contractile myocyte?

Na and Ca


draw the action potential within a cardiac contractile myocyte

draw it.


what occurs in phase 0 of the cardiac myocyte A.P

entry of Na through voltage gated channels


what occurs in phase 1 of the cardiac A.P

early re-polarisation due to efflux of K+


what occurs in phase 2 of the cardiac myocyte A.P

plateau due to entry of Ca++ through voltage gated channels


what occurs in phase 3 of the cardiac myocyte A.P

repolarisation with efflux of K+


what occurs in phase 4 of the cardiac myocyte A.P

restoration; ionic re-distribution with exchange of Na+ for K+


True or False: cells are in absolute refractory sate during most of the action potential?



What phase do the Na channels undergo refractory, when do they reactivate?

Na channels rapidly inactivated in phase 0 and do not reactive until memb potential becomes more -ve than -65mV


what special feature of cardiac muscle results from the lengthened refractory period coupled with a prolonged action potential?

cardiac muscle can't be tetanised
= heart can refill before the next contraction


which cells generate spontaneous action potential's to stimulate contraction, and where are they located?

pacemaker cells
in the sinoatrial node and the atrioventricular node


what are four ways in which the pacemaker action potential differs from the and cardiac myocyte action potential?

- resting membrane potential is unstable
- rate of depolarisation is slower
- amplitude of action potential lower
- plateau phase is shorter


what are the four main ionic basis for the pacemaker action potential?

1. increased permeability of sodium so that RMP tends towards threshold
2. NO voltage gated Na channels so no sudden upswing with depol
3. depot results from the entry of calcium through VO channels
4. threshold for depot more +ve due to dependence on VO channels


why does the SA node set the pace for the heart?

different autorhthimc tissues have different rates of depol to threshold, cells with a faster decay to threshold will reach threshold sooner = opening of VO channels and initiation of A.P
SA node has the FASTEST rate of decay

other autorythmic tissue unable to assume own naturally slower rate cause already activated by AP in SA node


list in order, the fastest to the slowest rate of decay of the autorhytmic tissue

SA node > AV node > Bundle of HIs > purkinje fibres


what is the rate set by the SA node called?

the sinus rhythm


how is heart rate controlled? incl. the description of neural and hormonal input on the SA node.

SA node depolarises independent of any external drive = autorythmic

neural and hormonal input can modify --> both symp and parasymp nerve fibre supply SA node
Simp Increases permeability of Na = faster decay
parasymp increases perm of K = hyperpolerisation = slower decay


describe the shape and location of the SA node

lateral wall of the right atrium, at the junction of the cranial vena cava
small mass of nodal myocytes


describe the shape and location of the AV Node

club shaped mass of nodal myocytes located at the junction of the intertribal septum


where does the bundle of His run?

runs from AV node to the ventricles
divides into R and L branches
R crus - runs to apex, major branches are the right ventricle papillary muscles, to the right septomarginal trabecular and ouster wall of ventricle


what are purkinje fibres?

final extension of the R and L arms of the bundle of HIs
network of sub-endocardial conducting fibres


what are some histological features of purkinje fibres?

very large diameter
pale central area - glycogen


how is the cardiac action potential propagated?

through gap junctions


what are the three things conviction velocity of a myocyte action potential is dependent on?

1. shape of the action potential - the upswing generates greater local currents
2. diameter of the muscle fibres
3. disease states ie/ changes in plasma electrolyte levels change in ionic conductance.


fast action potentials are typical of

cardiac myocytes


slow action potentials are typical of

pacemaker cells