Question

Figure for Ca2+ dynamics. Draw a model with arrows and #’s in the order

“The scheme of ionic currents within a cardiac cycle for a mammalian pacemaker cell”

The scheme of ionic currents within a cardiac cycle for a mammalian pacemaker cell (i.e., SA node) is generally described with the background [Ca2+]i continually increasing and decreasing. Starting in diastolic depolarization with a slow release of Ca2+ by ryanodine receptors (RyR), from the SR, leads to a rise in [Ca2+]i . The SERCA pumps Ca2+ back into the SR and the NCX removes [Ca2+]i in exchange for Na+ ions across the plasma membrane of the cell. The influx of Na+ ions can lead to a depolarization of the plasma membrane. Thus, opening low voltage-gated T-type Ca2+ channels (VCa) (Hüser et al. 2000) and potentially voltagegated Na+ channels. The influx of Ca2+ acts on the RyR to cause the ER to dump Ca2+ which results in a calcium induced inhibition of the RyR. Until the [Ca2+]i is reduced by the SERCA and NCX, the RyR stay inhibited but will start leaking Ca2+ as [Ca2+]i returns to a low level to then repeat the cycle (Subramani and Subbanna, 2006). In the mammalian heart, the pace making sinus node cells do not contain a K+ current (IK1) which is thought to be one reason the pacing cells do not show a resting membrane potential (Opthof, 2007).

Answer 1

Action potential in the cardiac muscle regulated by voltage gated Na+ channel, voltage gated Ca2+ channel and K+ channel - slow delayed rectifier and rapid delayed rectifier

the action potential in the cardiomyocyte occure in phases - phase 0(rapid depolarisation phase due to the opening of Na+ channel), phase 1 (partial rpolarization due to Na+ inactivation and opening of rapid K+ channel), phase 2 ( plateau due to the opening of Ca2+ channel ), phase 3(opening of slow K+ channel) and phase 4 is the resting phase . So the action potential terminates