Cardiovascular control 1 Flashcards
Describe the membrane potential when there is no movement of ions
There is no membrane potential
Why do different cells have different shapes for their action potentials
Due to different currents flowing.
Describe the potassium hypothesis
▪ The membrane is more permeable to potassium ions than anything else and these can diffuse down a concentration gradient, carrying positive charge with them. ▪ As they move, the incident chamber becomes increasingly positive relate to the other chamber (as other negative ions cannot move due to the barrier being impermeable to them). ▪ The electrical gradient then directly opposes the concentration gradient and eventually you get a point when the electrical gradient = the concentration gradient and equilibrium is achieved
What is the importance of the Na+/K+ pump
Without it, concentration gradients would collapse, signalling mechanisms and cardiac contraction would fail.
During the upstroke of AP, when the membrane is only permeable to sodium, will the sodium equilibrium potential be reached
No, due to the inactivation of the Na+ channels
Describe the movement of K+ ions at equilibrium
At equilibrium, K+ ions move randomly back and forth
The driving force, the difference between the concentration and the electrical gradient, is zero at equilibrium
Explain how we can calculate resting membrane potential
If the membrane is only permeable to K+ at rest (diastole) then the potential across it will equal the K+ equilibrium potential, (EK)
Equilibrium potential is calculated by solving the Nernst equation for K +
What is the membrane potential better described by
In reality membrane potential is better described by the Goldman-Hodgkin-Katz equation
Takes into account the relative permeabilities of several ions simultaneously
What controls the duration and strength of contraction of the heart
The duration of the action potential.
Describe how the cardiac action potential compares with the nerve action potential
Compared with nerves the cardiac action potential is very long (200-300 ms vs. 2-3 ms)
Why is the duration of the cardiac action potential important
Long, slow contraction is required to produce an effective pump
Describe what is meant by absolute refractory period
Absolute refractory period (ARP) = time during which no action potential can be initiated regardless of stimulus intensity
Describe what is meant by relative refractory period
Relative refractory period = period after ARP where an AP can be elicited but only with stimulus strength larger than normal.
What are refractory period caused by
Refractory periods are caused by Na+ channel inactivation
Na+ channels recover from inactivation as the membrane repolarises
As it repolarises more, more Na+ channels are activated, hence the likelihood of generating an action potential increases (larger depolarisation from the same stimulus).
What is meant by full recovery time
Full recovery time (FRT) = the time at which a normal AP can be elicited with a normal AP.
At what membrane potential does full recovery time occur
Around -85mV.
What is meant by tetany
Series of contractions following a series of stimuli.
What is the importance of the long refractory period seen in cardiac muscle cells
In cardiac muscle, the long refractory period means it is not possible to re-excite the muscle until the process of contraction is well underway hence cardiac muscle cannot be tetanised
it also allows the heart to fill fully before the arrival of the next stimulus.
What are the phases of the action potential in cardiac muscle
Phase 0 - upstroke Phase 1- Early repolarisation Phase 2- Plateau Phase 3- Repolarisation Phase 4- Resting membrane potential.
What is the resting membrane potential determined by
At rest, membrane potential determined by K+. o Large membrane permeability to K+ stabilises membrane potential reducing risk to arrhythmias by requiring a large stimulus to excite the cells.
