Lecture 3: Electrical Activity of the Heart

Question 1

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

Answer 1

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.

Question 2

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

Answer 2

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

Question 3

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

Answer 3

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.

Question 4

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

Answer 4

ION CHANNELS

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

Question 5

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

Answer 5

Na and Ca

Question 6

draw the action potential within a cardiac contractile myocyte

Answer 6

draw it.

Question 7

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

Answer 7

entry of Na through voltage gated channels

Question 8

what occurs in phase 1 of the cardiac A.P

Answer 8

early re-polarisation due to efflux of K+

Question 9

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

Answer 9

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

Question 10

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

Answer 10

repolarisation with efflux of K+

Question 11

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

Answer 11

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

Question 12

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

Answer 12

true

Question 13

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

Answer 13

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

Question 14

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

Answer 14

cardiac muscle can’t be tetanised

= heart can refill before the next contraction

Question 15

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

Answer 15

pacemaker cells

in the sinoatrial node and the atrioventricular node

Question 16

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

Answer 16

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

Question 17

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

Answer 17

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

Question 18

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

Answer 18

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

Question 19

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

Answer 19

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

Question 20

what is the rate set by the SA node called?

Answer 20

the sinus rhythm

Question 21

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

Answer 21

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

Question 22

describe the shape and location of the SA node

Answer 22

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

Question 23

describe the shape and location of the AV Node

Answer 23

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

Question 24

where does the bundle of His run?

Answer 24

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

Question 25

what are purkinje fibres?

Answer 25

final extension of the R and L arms of the bundle of HIs

network of sub-endocardial conducting fibres

Question 26

what are some histological features of purkinje fibres?

Answer 26

very large diameter

pale central area - glycogen

Question 27

how is the cardiac action potential propagated?

Answer 27

through gap junctions

Question 28

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

Answer 28

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.

Question 29

fast action potentials are typical of

Answer 29

cardiac myocytes

Question 30

slow action potentials are typical of

Answer 30

pacemaker cells