Excitable Cells DLA

Question 1

Describe Na+ and K+ movements during the impulse

Answer 1

• Na+ and K+ Movements During the Impulse

– During subthreshold depolarization at the impulse initiation zone, a few voltage-gated Na+ channels (NaV) may open, but depolarization is insufficient to trigger widespread NaV opening. Efflux of K+ via leak channels also electrically offsets the modest entry of Na+. Once the depolarization threshold is reached many of the NaV channels open, and the resulting net influx of positive charge further depolarizes the membrane to open additional NaV channels. A positive feedback cycle thus rapidly ensues, with Na+ cascading inwards along its electrochemical gradient (Vm – ENa). The rising phase of the action potential thus reflects Na+ influx and the membrane depolarizing towards ENa.

– Depolarization also induces the delayed opening of voltage-gated K+ channels. The efflux of K+ ions along their electrochemical gradient eventually repolarizes the membrane, thus yielding the falling phase of the impulse. En route to EK, the membrane may hyperpolarize (below resting Vm), creating an undershoot. Therefore, action potentials initially depolarize towards ENa, then repolarize and hyperpolarize towards EK

Question 2

What are the changes in gNa and gK that underlie action potentials?

Answer 2

• Changes in gNa and gK Underlie Action Potentials

– The Na+ conductance (gNa) curve reflects the transient activity of voltage-gated Na+ channels during the action potential. Note that gNa falls owing to a depolarization- induced inactivation of Na+ channels. The gK curve shows the slower time course of the opening of K+ channels during the action potential. The rising phase of the action potential thus reflects rising gNa. The falling phase of the action potential reflects declining gNa coupled with a rising potassium conductance (gK).

Question 3

What is the ionic basis of action potentials?

Answer 3

Action potential reflects temporally offset Na+ and K+ conductances

– Na+ channels open, then inactivate
– K+ channels wait, then open and inactivate more slowly

Question 4

What are the types of refractory periods?

Answer 4

• Absolute refractory period (ARP)
– during repolarization phase when inactivation gates are closed
– no new AP can be initiated or conducted because NaV channels are blocked (inactivated)

• Relative refractory period (RFP)
– after absolute refractory period
during afterhyperpolarization
– starts when the NaV inactivation gates are open again
– APs initiation is inhibited due to afterhyperpolarization (increased K+ conductance)
– but principally APs can again be initiated since NaV channels have reset (inactivation gates open)