Conduction Flashcards
What is autorhythmicity of the heart?
the heart is capable of beating regularly without external factors
What drives the sinus rhythm of the heart?
SA node
Do pacemaker cells of the sinoatrial node have a stable membrane potential? what does this allow for?
no
generation of an action potential
What changes the membrane potential?
movement of ions
What is depolarization?
the membrane potential becomes less negative or even positive
What is hyperpolarization?
the membrane becomes more negative
What is the resting potential of a pacemaker cell?
- 70 mV
What drives the passive movement of an ion through an ion channel?
the electrochemical gradient
What is pacemaker potential?
rising depolarisation to the threshold
What causes pacemaker potential?
funny currents that allow slow influx of sodium
decreased K+ Efflux
Na+ and K+ influx
transient Ca+ influx
Are voltage-gated Na+ channels hyperpolarising or depolarising?
depolarising
Are voltage-gated K+ channels hyperpolarising or depolarising?
hyperpolarizing
Which direction does a) sodium b) calcium b) potassium tend to flow?
a) in to cell
b) in to cell
c) out of cell
What causes the rising phase of depolarisation in a pacemaker cell?
activation of long lasting, L type Ca+ channels causes calcium influx
What causes repolarisation in a pacemaker cell?
inactivation of L type Ca+ channels
activation of K+ channels causing K+ efflux
At what membrane potential, do sodium channels become inactive and potassium channels open in a pacemaker cell?
+ 40 mV
Which type of transport in a pacemaker cell gets the membrane potential from
a) resting to threshold?
b) threshold to +40?
a) mechanic sodium channels
b) voltage-activated sodium channels
Is sodium or potassium conductance earlier on, greater and more short-lived?
sodium
What are action potentials?
brief electrical signals in which the polarity of the nerve cell membrane is momentarily reversed
What is the magnitude and velocity of an action potential determined by?
it is constant for a given axon
all or none, either there or not
What activates voltage-gated channels?
depolarization
Do voltage-activated calcium channels activate with positive or negative reinforcement?
positive: opening of a few channels stimulates others
Do voltage-activated potassium channels activate with positive or negative reinforcement?
negative: outward movement of potassium repolarizes the cell which turns off the channels
What is the refractory period?
when a voltage-activated sodium channel enters an inactive state during maintained depolarisation, it requires to be repolarized before another AP can occur
What does the refractory period do?
it limits the frequency at which any nerve fibre can fire an action potential
What is the relationship between diameter of axon and resistance to current flow?
the bigger the diameter of axon, the less resistance to current flow
How does conduction spread across the atria?
cell to cell via gap junctions
What happens to conduction in the AV node? what does this allow?
delayed
allows atrial systole (contraction) to precede ventricular systole
What allows for rapid spread of action potential to the ventricles?
The Bundle of His and its branches and the network of Purkinje fibers
What is the resting potential for a cardiac myocyte?
- 90 mV
What causes phase 0 of a cardiac myocyte action potential?
fast Na+ influx causes depolarisation
What causes phase 1 of a cardiac myocyte action potential?
Closure of Na+ channels and Transient K+ efflux
What causes phase 3 of a cardiac myocyte action potential?
Closure of Ca++ channels and K+ efflux
What is phase 4 of a cardiac myocyte action potential?
resting potential
What causes the plateau in cardiac myocyte action potential?
Influx of Ca++ through L-type Ca++ channels
How does the vagus nerve slow the heart rate?
Using AcH acting through muscarinic m2 receptors
What competes with AcH to inhibit the vagus nerve from slowing HR?
atropine
What effect does an agent that slows the heart rate have?
negative chronotropic effect
What does a negative chronotropic effect mean for
a) slope of pacemaker potential
b) frequency of AP?
a) decreases
b) decreases
What effect does an agent that speeds up the heart rate have?
positive chronotropic effect
What does a positive chronotropic effect mean for
a) slope of pacemaker potential
b) frequency of AP?
a) increases
b) increases
What does sympathetic supply have on
a) pacemaker cell K+ efflux?
b) pacemaker cell Na+ and Ca++ influx?
a) decrease
b) increase
What does parasympathetic supply have on
a) pacemaker cell K+ efflux?
b) pacemaker cell Na+ and Ca++ influx?
a) increase
b) decrease
