Flashcards in deck_651714 Deck (26):
Define the cardiac resting membrane potential and how is the membrane potential achieved?
The potential inside a cardiac cell relative to the extracellular solution. The difference between the two is achieved by the selective permeability of the membrane to different ions, by way of channel proteins.
How do you measure the resting membrane potential?
MEasure it as if the the outside of the cell is zero volts and give the inner potential in relation to that value
What is the cardiac resting membrane potential value?
What helps to achieve the resting membrane potential value and how is it achieved?
the K+ equilibrium potential of -80mv as the cell membrane off mycocardial cells is mostly permeable to potassium ions -- The ions move out of the cells down their concentration gradient
How are electrical events in myocardial cells generated?
Due to pacemaker cells which generate an electrical event, causing and action potential which spreads over the myocardial cells.
Describe the changes in the membrane potential in ventricular cells
-- In diastole, the MP is close to the EP of K+ (~90mV)-- Depolarisation occurs due to spread of electrical activity from pacemaker cells. This opens fast voltage gated Na channels causing depolarisation towards EP of Na+-- Brief repolarisation (K+ outflow) occurs (MP=0mV)-- Ca channels open due to depolarisation, and the inward flow of Ca maintains depolarisation. -- Ca influx causes the release of more Ca from intracellular stores which causes contraction. -- After ~280ms, Ca channels close and K+ efflux returns the membrane potential to resting.
Describe the changes in membrane potential in pacemaker cells
-- Slow, gradual depolarisation occurs due to open Na channels. -- Once threshold has been reached, Ca2+ channels are stimulated and they open, giving a slow depolarisation. -- Ca2+ channels close, and the cell repolarises due to K+ efflux
What is the spontaneous gradual depolarisation of pacemaker cells known as?What causes this depolarisation?
pacemaker or ‘funny’ current Slow Na+ channels
How is calcium removed from the cardiac cells once depolarisation has taken place?
Can be taken up by the SR and mitochondria
What type of channels are involved with action potentials?
Voltage-gated ion channels
What is the force generated in a cell proportional to?
The Ca2+ concentration
What does the force on contraction depend on?
The balance between the rate of Ca2+ entry into the cytoplasm and its rate of removal.
Give some difference between contraction of skeletal muscle and cardiac muscle
The contraction in cardiac cells is much longer than in skeletal muscleSkeletal muscle is stimulated by conduction excitation rather than by cells themselves in cardiac muscle
How is the heart rate controlled?
By the rate of depolarisation of the pacemaker potential.
What would increase the heart rate?
Noradrenaline binding to B1 adrenoceptors would cause the pacemaker potential to happen more quickly, which would increase the heart rate. Is controlled by the sympathetic nervous system
What would decrease the heart rate?
Acetylecholine binding to nicotinic cholinergenic recptors would decrease the slope of the pacemaker potential, therefore decreasing the heart rate.
What is hyperkalaemia?What affect will it have on the membrane potential?
When the extracellular concentration of potassium rises above a critical valueIt will make the membrane potential less negative/more positive
What is hypokalaemia?What affect will it have on the membrane potential?
When the extracellular concentration of potassium falls below a critical valueIt will make the membrane potential more negative/less positive
What is the normal range of values for the extracellular concentration of potassium?
3.5 - 5.5 mmol per litre
What is the equilibrium potential?
The hypothetical membrane potential which would develop if it were the only ion that could cross the membrane
The ease with which ions can cross the membrane
What conducts excitation in the heart?
Purkinje fibres conduct excitation through the ventricular myocardium--
Where are pacemaker cells found?
SANAVNBundle of His -- natural rate is slower than SAN so are overridden (may be important if there is a conduction block
Give some characteristics of pacemaker cells
Have a slower AP due to it being a timing signalDo not have fast Na channelsUpstroke = due to Ca channelsDownstroke = due to K channels
When does the pacemaker potential occur?
Between the action potentials