8 Electrical Activity of the Heart Flashcards
Q: Set the scene for the potassium hypothesis. What’s the chemical concentration gradient? potential difference?
A: 2 chambers separated by impermeable membrane
2 different concentrations of KCl added to each compartment (high conc. on one side & low conc. on the other)
Add measuring device-> can measure potential across membrane
Chemical concentration gradient but since barrier is impermeable there is no potential difference between the chambers
Q: What happens when you take the potassium hypothesis set up and make the membrane permeable to potassium only? (3) What happens if this is left for longer?
A: 1. K ions diffuse down their concentration gradient carrying positive charge with them
- Cl ions are still separated by impermeable barrier so there is a concentration gradient for Cl ions but no movement of Cl from one chamber to another
- net result- small electrical gradient forms in opp direction to potassium movement
Q: What happens when you take the potassium hypothesis set up and make the membrane permeable to potassium only and the system is left for a long time? (4) What happens when things stop changing? (2)
A: K ions diffuse down their concentration gradient carrying positive charge with them
Positive charge accumulates in the right hand compartment and an electrical gradient builds up
Electrical gradient opposes the movement of K
When the electrical gradient exactly balances the chemical gradient, equilibrium is achieved and there is no further net movement of ions
- At equilibrium, K ions more randomly back and forth
- The driving force, the difference between the concentration and the electrical gradient, is zero at equilibrium
Q: What does the resting membrane depend on? How can we predict what a potential will be across a semi-permeable membrane? What will the membrane potential change depending on?
A: flow of K out of cells
Nernst equation
………..RT……..[K+]o
E(k)= —- x ln——-
………..zF………[K+]i
relative permeabilities of the membrane to various ions
Q: What happens if a membrane is only permeable to K at rest? Calculation? What maintains the K concentration?
What would the membrane potential value be in this situation?
A: (diastole)
the potential across it will equal the K equilibrium potential, E(K)
Equilibrium potential is calculated by solving the Nernst equation for K
Na/K ATPase
-80mV
Q: Membrane permability switches from K to Na only. What will the membrane potential equal? Value? How was this calculated?
A: (during the upstroke of the action potential)
then the potential across it will equal the Na equilibrium potential, E(Na)
+66mV
We can predict what a potential will be across a semi-permeable membrane when the membrane is permeable to Na also using the Nernst equation.
Q: When is a cell permeable to K+? to Na+?
A: diastole
during the upstroke of the action potential
Q: In reality, membrane potential is better described by? why?
A: Goldman-Hodgkin-Katz equation (not nernst)
Takes into account relative permeabilities of ions
Q: What causes an upstroke in the membrane potential in an action potential? goes towards? What’s this followed by?
How long does an AP last in a nerve cell? how long is a cardiac one? Why is there a difference?
A: permeability to Na+ (goes towards Na eqm potential)
increase in K permeability -> cell repolarises
1-2 miliseconds
several hundred milliseconds (much longer)
Duration of action potential controls the duration of contraction of the heart
Long, slow contraction is required to produce an effective pump
Q: How are nerve and cardiac cells action potential similar? 2 subtypes.
A: have refractory periods
Absolute refractory period (ARP) = time during which no action potential can be initiated regardless of stimulus intensity
Relative refractory period = period after ARP where an AP can be elicited but only with stimulus strength larger than normal.
Q: Why do refractory periods exist? When does it end?
A: Occur as a result of Na channel inactivation
Na channels recover from inactivation when the membrane is repolarised (more negative the membrane gets, the more Na channels that are available for activation)
Q: What’s the benefit of the refractory period in cardiac cells? (2) How does this compare to skeletal muscle? (3)
A: In cardiac muscle it is not possible to re-excite the muscle until the process of contraction is well underway hence cardiac muscle cannot be tetanized
heart muscle takes a while to be restimulated
In skeletal muscle repolarization occurs very early in the contraction phase making re-stimulation and summation of contraction possible
=> allow the production an unfused tetanus (raises membrane potential reached)
=> if in even faster succession get a fused tetanus (no relaxation)
Q: What are the 5 phases of a cardiac muscle cell action potential? Draw graph.
A: Phase 0 = upstroke Phase 1 = early repolarisation Phase 2 = plateau Phase 3 = repolarization Phase 4 = resting membrane potential (diastole)
Q: Phases 4 and 0 of the cardiac action potential. What determines each one?
A: Phase 4 = resting membrane potential (diastole)
Phase 0 = upstroke
Resting membrane potential determined by K flowing out of cells
Upstroke determined by large increase in permeability of membrane to Na
Q: Phase 2 of the cardiac action potential. What causes it? Why does this happen?
Explain the shape.
A: Phase 2 = plateau
Ca influx through L type channel= required to trigger Ca release from intracellular stores = essential for contraction
get plateau because gradual activation of K currents (K moving outward) that balance, then overcome, inward flow of Ca
