Lecture 3: Electrical Activity of the Heart Flashcards
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?
ION CHANNELS
- Receptor Operated - open when receptor bind
- 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?
true
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?
- increased permeability of sodium so that RMP tends towards threshold
- NO voltage gated Na channels so no sudden upswing with depol
- depot results from the entry of calcium through VO channels
- 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?
- shape of the action potential - the upswing generates greater local currents
- diameter of the muscle fibres
- 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
