Control Of Cardiac Function Flashcards
What causes the pacemaker potential (funny (If) current) ?
Sodium moves into the cell and a small amount of potassium
Why do pacemaker cells spontaneously depolarise?
Because they have a pacemaker current (funny current)
How is the funny channel unusual?
Funny channel opens when membrane is hyperpolarised (more -Ve)
When is the funny channel open?
When the membrane is hyperpolarised (more -Ve)
When the funny channel is open sodium moves into the cell, this causes what to happen to the membrane?
Starts to Depolarise
As the membrane depolarises the inward rectifier current is reduced how does this affect the membrane?
Funny current dominates increasing depolarisation
There are 2 types of calcium channels what are these known as?
L-type (long lasting) T-type (transient)
The opening of T-type calcium channels has what effect on the membrane and how?
Increased depolarisation caused by influx of calcium into the cell
When the funny channel is open what enters the cell?
Sodium
What does the inward rectifier current do?
Allows potassium to leave the cell
What causes repolarisation of the membrane?
Acetylcholine activates voltage gated potassium Channels; efflux of potassium re-polarises membrane
What causes the membrane to become hyperpolarised?
Calcium channels inactivate
How is the pacemaker potential initiated?
- funny current (If) - sodium enters (depolarisation)
- Inward rectifier (Ik) reduced - less potassium leaves -funny current dominates
- t-type calcium channel opens - Inc depolarisation - calcium influx
- L-type calcium channels open at threshold - Huge calcium influx - depolarisation (phase 0)
- Repolarisation - Ach activated voltage gated potassium channels open - potassium efflux repolarises membrane
Is the response time to ANS stimulation quicker for vagal stimulation or for sympathetic stimulation?
Vagal stimulation is fastest
Cardiac response to vagal stimulation is rapid. How fast is the change in the heart rate seen?
3 seconds
Cardiac response to sympathetic stimulation is slow. How long does it take for the heart rate to change?
About 13 seconds
Why is the cardiac response to vagal stimulation faster than for sympathetic stimulation?
Because of the 2nd messenger system - (ACh - direct coupling of MAChr to G protein)
- ACh hydrolyses rapidly
What is the sequence of events in the 2nd messenger system when ACh binds to a receptor during parasympathetic stimulation of the cardiac system?
- Ach Binds to receptor
- G protein moves across membrane
- potassium channels open - hyperpolarisation
- calcium channels close - prevents pacemaker potential - HR slows
Why is the cardiac response to sympathetic slower than that for parasympatheic stimulation?
slower because more steps required for activation.
- Activation of adenylate cyclase, converts ATP to cAMP,
- cAMP activates protein kinase
- protein kinase then phosphorylates the funny channel and t-type calcium channel
why does it take longer for heart rate to return to baseline after sympathetic stimulation of the cardiac sycle than parasympathetic stimulation?
Noradrenaline metabolism and reuptake is slower than hydrolysis of acetylcholine.
Which ANS system is responsible for beat to beat changes in heart rate ?
Parasympathetic nervous system
