Action Potentials in Contractile Cells

Leaky K+ Channel

Permeable to K+ at rest. K+ efflux contributes to a resting potential of -90mV

When do “Ordinary” voltage-gated Na+ channels (VGSC) open

At threshold

What do “Ordinary” voltage-gated Na+ channels (VGSC) allow for?

rapid Na+ influx that makes up the rising phase

How does “Ordinary” voltage-gated Na+ channels (VGSC) close?

Time-dependent Inactivation

When do “Transient” voltage-gated K+ channels (T-VGPC) open?

At the peak of the AP (+30mV)

What do “Transient” voltage-gated K+ channels (T-VGPC) allow?

a fast, limited efflux of K+ & Brief, small repolarization (as the membrane becomes slightly less positive)

“Slow” L-type Ca2+ channels

Voltage-gated

What is the result of “Slow” L-type Ca2+ channels?

slow, inward diffusion of Ca2+

What do the “Slow” L-type Ca2+ channels get amplified in?

EC coupling

What is the role of “Slow” L-type Ca2+ channels in excitation-contraction (EC) coupling?

To increase cytosolic Ca2+ for contraction, which allow Ca2+ in for the CB cycle to happen

How do “Ordinary” voltage-gated K+ channels (O-VGPC) open?

slowly, & have a delayed activation.

What does “Ordinary” voltage-gated K+ channels (O-VGPC) lead to?

K+ efflux that creates significant membrane repolarization.

What is the resting membrane potential of contractile cells? Phase 1

-90mV

Which channels are open at the resting membrane potential (-90 mV) in contractile cells? Phase 1

Leaky K+ Channel

At what threshold potential does the membrane get depolarized? Phase 2

-70mV

What are the two main ways contractile cells reach threshold potential? Phase 2

Gap junctions & direct stimulation from electrical conduction system (Pacemaker cells)

What happens when the membrane reaches threshold in contractile cells? Phase 3

“Ordinary” voltage-gated Na+ channels (VGSC)

What is the result of VGSC opening in contractile cells? Phase 3

rapid depolarization of membrane caused by a large influx of Na+ ions

What happens to “Ordinary” voltage-gated Na+ channels (VGSC) at the peak of the action potential? Phase 4

It becomes inactive

Which channels open at the peak of the action potential to cause quick and slight repolarization? Phase 4

“Transient” voltage-gated K+ channels (T-VGPC)

What doe highly depolarized membrane cause to open in contractile cells? Phase 5

“Slow” L-type Ca2+ channels

What is the result of “Slow” L-type Ca2+ channels opening? Phase 5

slow & sustained influx of Ca2+

What phase is created by the sustained influx of Ca²⁺? Phase 5

Plateau phase

Which channels are closing during the plateau phase? Phase 5

“Transient” voltage-gated K+ channels (T-VGPC) & Leaky K+ Channel

What important physiological process occurs during the plateau phase? Phase 5

Cross-Bridge cycle

What causes the rapid falling phase of the action potential in contractile cells? Phase 6

inactivation “Slow” L-type Ca2+ channels & delayed activation “Ordinary” voltage-gated K+ channels (O-VGPC)

What is the result of Inactivation and delayed activation? Phase 6

The membrane will begin to repolarize

As the membrane approaches resting potential, what happens to “Ordinary” voltage-gated K+ channels (O-VGPC)? Phase 7

It closed

Which channels open as the membrane returns to resting potential? Phase 7

Leaky K+ Channel