Case 2 Physiology Flashcards
What changes occur to ion channels in the phases of action potential generation in sinoatrial node pacemaker cells? (3 phases)
Phase 0: Ca2+ channels open
Phase 3: Ca2+ channels close, K+ channels open
Phase 4: HCN channels open, Ca2+ channels begin to reopen
What changes occur to membrane potential in the phases of action potential generation in sinoatrial node pacemaker cells? (3 phases)
Phase 0: membrane potential increases/depolarisation
Phase 3: membrane potential decreases/hyperpolarisation
Phase 4: membrane potential increases slowly/prepotential (funny current)
What major movements of ions occur in the phases of action potential generation in sinoatrial node pacemaker cells? (3 phases)
Phase 0: Ca2+ in
Phase 3: K+ out
Phase 4: Na+ in
Complete the sentences. Sinoatrial HCN channels are X-dependant and activated by Y. They permit the slow influx of ions in phase 4, mostly Z ions, to generate a prepotential.
X = cAMP Y = hyperpolarisation Z = Na+
What is the most important difference between Purkinje fibres and ventricular myocytes?
Purkinje fibres are intrinsic action potential firers (initiate depolarisation on their own through prepotential generation) whereas ventricular myocytes are not.
What changes occur to ion channels in the phases of action potentials in ventricular myocytes? (5 phases)
Phase 0: Na+ channels open
Phase 1: Na+ channels close, fast K+ channels open
Phase 2: Fast K+ channels close, Ca2+ channels (slow) open
Phase 3: Ca2+ channels close, slow K+ channels open
Phase 4: No changes (resting potential)
(Think ‘nakcak’ for order of channel opening)
Which is the slowest of the 4 major ion channels involved in ventricular myocyte action potentials?
Ca2+ channels
What major movements of ions occur in the phases of action potentials in ventricular myocytes? (5 phases)
Phase 0: Na+ in Phase 1: K+ out Phase 2: Ca2+ in Phase 3: K+ out Phase 4: No major changes (resting potential)
Which phase of the ventricular myocyte action potential is described as a ‘plateau’ and why?
Phase 2, there is (almost) an equilibrium of positive charge (K+, Ca2+) entering and leaving the cell (Cl- is also involved)
Which ion enables the functioning of the Na+/K+ ATPase pump to prevent cardiac arrhythmias?
Mg2+
An excess of which ion can cause increased depolarisation in phase 0, leading to more excitable myocytes?
Na+
An excess of which ion causes increased force of contraction and possible systolic cardiac arrest?
Ca2+
An excess of which ion disrupts phase 1 of the myocyte action potential, leading to possible diastolic cardiac arrest?
K+
How does acetylcholine lead to decreased cardiac output in terms of ion movement? (2 ways)
- K(Ach) channels activated, increased K+ efflux in phase 3, greater hyperpolarisation, more Na+ needed to reach threshold, decreased heart rate
- Ca2+ channels inhibited, decreased heart rate and contractility
How do the catecholamines lead to increased cardiac output in terms of ion movement?
Activate (phosphorylate) Ca2+ channels in the sarcoplasmic reticulum and myocyte cell surface membrane (especially in SAN), increasing contractility, conduction speed and heart rate.
What is the heterometric control mechanism of the heart?
Starling’s law – force of contraction is proportional to the initial diastolic fibre length (EDV, end diastolic volume)
What effect does underfilling of the heart have on contractility? Explain why.
According to Starling’s law, the overlap of actin and myosin is not optimal. Therefore, contractile ability is reduced.
What effect does overfilling of the heart, e.g. in heart failure, have on contractility? Explain why.
According to Starling’s law, actin and myosin become physically separated, cross-bridges cannot be formed and contractile ability is reduced.
A relatively high (but within normal range) end diastolic volume increases the contractility of cardiac muscle. True or false? Explain why.
True. According to Starling’s law, slightly increased distance between actin and myosin allows for more tension when cross-bridges form, leading to a more forceful contraction.
What is meant by cardiac homeometric control?
Extrinsic mechanisms to regulate cardiac output, independent of muscle fibre length changes (Starling’s law). This includes parasympathetic and sympathetic stimulation.
Complete the sentence. Catecholamines are X inotropes, Y dromotropes and Z chronotropes.
X = positive, Y = positive, Z = positive
How do catecholamines increase the speed of diastole? (5 steps)
- Bind to ß1 receptors
- Activate cAMP
- Deactivate phospholamban
- Upregulation of Ca2+ ATPase pumps in sarcoplasmic reticulum
- Faster Ca2+ uptake into sarcoplasmic reticulum following systole
What effect does sympathetic activity have on cytosolic Ca2+ concentration in cardiac myocytes? Explain how.
Increase. Voltage-gated Ca2+ channels open, releasing Ca2+ from the sarcoplasmic reticulum.
Complete the sentences. In exercise, blood pressure increases. This is detected by X and leads to a compensatory response (involving process Y). However, this is over-ridden by Z as higher blood pressure is needed to meet the metabolic need of contracting skeletal muscle.
X = Baroreceptors Y = Vasodilation Z = Central nervous system (Sympathetic NS)
What causes increased venous return to the heart in exercise?
Greater pumping of skeletal muscle to squeeze blood through veins back to the right atrium.
