5 - Cardiovascular control 1 Flashcards
The potassium hypothesis - 2 chambers, left has high [K+], right has low [K+]. The membrane is impermeable. What happens?
No movement even though concentration gradient exists.
Potential = 0mV
The potassium hypothesis - 2 chambers, left has high [Cl-], right has low [Cl-]. The membrane is permeable to K+. What happens?
No movement as membrane isn’t permeable to Cl-.
Potential = 0mV
The potassium hypothesis - 2 chambers, left has high [K+], right has low [K+].
The membrane is permeable to K+. What happens?
K+ moves from left to right down conc. gradient.
+e charge builds up on right side - electrical gradient stops further entry of K+.
Potential = -58mV
When is equilibrium achieved in a membrane?
When the concentration gradient matches the electrical gradient. No net movement of ions.
What does membrane potential depend on?
Potassium moving OUT of the cells.
What equation do you use to work out the potential across a membrane, if it is ONLY permeable to 1 thing?
Nernst equation
E(K)= RT/zF x ln([K] outside / [K] inside)
At rest, if the membrane is only permeable to K+, what is the membrane potential?
Membrane potential equals equilibrium potential for K+ (-80mV)
What maintains [K+]?
Na+/K+ ATPase
If the membrane is ONLY permeable to Na+, what is the membrane potential?
+66mV
What equation is used to calculate resting membrane potential?
Goldman-Hodgkin-Katz equation. Takes into account permeability to different ions.
E = -61log (inside added together)/(outside added together)
Are cardiac action potentials longer or shorter in time than nerve action potentials? Why?
Longer. IT controls duration of contraction of the heart. Long, slow contractions makes the heart more effective.
What is the absolute refractory period? (ARP)
Time when no action potential can be generated regardless of stimulus intensity. Due to sodium channel inactivation.
What is the relative refractory peroid? (RRP)
Time after ARP where an AP can be generated IF the stimulus strength is larger than normal.
Due to sodium channel inactivation
Draw a cardiac action potential. What are the phases?
Drawing. Phase 0 - Upstroke Phase 1 - Early repolarisation Phase 2 - Plateau Phase 3 - Repolarisation Phase 4 - Resting membrane potential
Why can APs be generated during the RRP? (At the end)
During repolarisation, gradually more sodium channels become available.
Why are ARPs and RRPs necessary in the heart?
So the heart can fill/empty with blood before the next contraction occurs.
The muscle cannot be re-exicited too soon so cardiac muscle cannot become tetanised.
When do different ion channels open during cardiac action potential?
Na+ open. Na+ enters cell.
Ca+ open. Ca+ enters
Various K+ open. Ka+ leaves cell.
I(K1) responsible for repolarising cell.
Why do different parts of the heart have different action potential shapes?
Different parts have different ion channel expression in their cell membranes.
Can the heart beat without a nervous stimulation?
Yes - it can beat independently. The nervous supply from the autonomic nervous system is there only to modify the intrinsic heart beat.
Draw the membrane potential graph for the SA node cells. Why is there no resting potential?
Drawing
Upstroke is caused by Ca2+ influx - NOT sodium as there is litte Na+ influx.
There is no I(K1) channel so no resting membrane established.
L-type and T-type calcium channels - which activate at more negative potentials?
T-type
What transmitters are released from the sympathetic and parasympathetic nervous system that influences heart rate?
Parasympathetic: acetylcholine
Sympathetic: noradrenaline
What does increased sympathetic stimulation to the SAN do?
More noradrenaline = depolarisation is quicker.
HR increases
What does increased parasympathetic stimulation to the SAN do?
More acetylcholine = depolarisation is slower.
HR decreases
