BIOL325 - Action Potential Propagation

AP signal pathway

Dendrites → soma → trigger zone → axon → axon terminals.

Absolute refractory period

No second AP possible; Na+ channels are open or inactivated.

Relative refractory period

Second AP possible but requires stronger depolarization.

Why is relative refractory threshold harder?

Some Na+ channels remain unavailable and K+ conductance stays high.

Absolute refractory timing in lecture

Rising phase and first third of falling phase.

Relative refractory timing in lecture

Later falling phase and afterhyperpolarization.

Why does AP normally travel one way?

Recently activated membrane behind spike is refractory.

Membrane resistance (rm)

Resistance to current leaking through the membrane.

Axial resistance (ra)

Resistance to current flowing lengthwise inside an axon.

Length constant (λ)

Distance over which passive voltage falls to about 37% of initial size.

How does larger axon diameter affect speed?

Lowers axial resistance and increases conduction speed.

How does myelin affect membrane resistance?

Raises membrane resistance and reduces current leakage.

How does myelin affect conduction speed?

Allows depolarizing current to spread farther between nodes.

Myelin

Insulating glial membrane wrapping around axons.

Myelinated fibers

Axons wrapped in myelin with gaps at nodes of Ranvier.

Node of Ranvier (neurofibril node)

Gap in myelin rich in voltage-gated channels.

Contiguous conduction

AP regenerated along each successive region of unmyelinated axon.

Saltatory conduction

AP regenerated at nodes while current travels under myelin.

Where are VG channels concentrated in myelinated axons?

At nodes of Ranvier.

Why is saltatory conduction advantageous?

Faster signaling with less ion movement and pumping demand.

Which conducts faster: myelinated or unmyelinated?

Myelinated axon of comparable diameter.

Which glia myelinate CNS vs. PNS?

Oligodendrocytes in CNS; Schwann cells in PNS.