CVS Physiology Flashcards
What is Autorhythmicity?
The heart is capable of generating its own electrical activity and beating in the absence of external stimuli
Where does excitation in the heart originate?
Excitation originates from pacemaker cells in the Sino-atrial node (SA node) located in the upper right atrium close to where the SVC enters.
The rhythm generated from the SA node, controlling heart rate, is known as sinus rhythm.
What gives SA node cells spontaneous pacemaker potential?
SA node cells have an unstable resting membrane potential due to a slow Na influx. Permeability to K does not remain constant between action potentials. The membrane potential gradually depolarises to reach a threshold to generate an action potential within SA node cells.
Describe the ionic mechanism underlying sinoatrial node action potentials
Once the threshold for an action potential has been reached, the rising phase of SA node AP is attributable to the opening of voltage gated Ca2+ channels thus allowing Ca2+ influx.
The falling phase is caused by activation of K+ channels allowing for K+ efflux.
How is electrical excitation spread through the heart?
Excitation is spread from the SA node through the atria via cell-to-cell conduction (gap junctions and desmosomes allow this to happen).
Once the atria have depolarised, excitation is transmitted to the ventricules via the AV node - located at the base of the right atrium. This is the ONLY point of electrical contact between the atria and ventricles.
AV node cells have a slower conduction velocity and delay signals allowing for the atria to complete systole.
From the AV node, conduction proceeds down the bundle of His and through purkinje fibres allowing for rapid spread of conduction within the ventricles.
Describe the ionic mechanisms underlying action potentials in contractile ventricular myocytes
Resting membrane potential in contractile cardiac muscle cells is stable (-90mV).
Upon excitation of the cell, voltage-gated NaCh open and allow for fast Na influx (rising phase 0)
NaCh then close and there is a transient K efflux (phase 1)
Voltage-gated CaCh open allowing influx of Ca maintaining the membrane potential near +30mV for a few hundred milliseconds (PLATEAU PHASE 2 - unique to cardiac muscle cells)
Inactivation of CaCh and activation of KCh causes K efflux and cell repolarisation (phase 3)
Describe parasympathetic innervation of the heart
Innervation is via the vagus nerve. Vagal tone exerts a continuous influence on the SA node under resting conditions - this acts to slow the intrinsic heart rate from 100bpm to 70bpm.
The vagus nerve also supplies the AV node, acting to increase AV nodal delay.
Vagal stimulation acts to hyperpolarise the AP of pacemaker cells, decreasing the slope of the pacemaker potential (increasing the time taken to reach threshold for an AP and therefore decreasing heart rate). This is known as a NEGATIVE CHRONOTROPIC EFFECT.
What is the parasympathetic neurotransmitter and through which receptors does it act on the heart?
ACETYLCHOLINE acting through M2 (MUSCARINIC) receptors
Name a competitive inhibitor of acetylcholine used in severe bradycardia
ATROPINE
Describe sympathetic innervation of the heart
Sympathetic nerves innervate the SA and AV node AND the myocardium. Sympathetic innervation increases heart rate by increasing the slope of the pacemaker potential, decreases AV nodal delay and increases the force of contraction of the heart. This is known as a POSITIVE CHRONOTROPIC EFFECT
What is the sympathetic neurotransmitter and through which receptors does it act on the heart?
NORADRENALINE acting through beta-1 adrenoceptors
Where are baroreceptors located?
Aortic arch and carotid sinuses
