Action Potential Propagation and Myelination

1. Local current flow and electrotonic conduction

Action-potential propagation works by local current flow. At the active (depolarized) site, entering Na⁺ makes the inside positive relative to neighboring resting membrane, driving current passively along the axoplasm and back through the membrane to depolarize the region ahead. This passive, decremental spread — electrotonic conduction — brings the adjacent membrane to threshold, opening voltage-gated Na⁺ channels and regenerating a full action potential. Because the signal is reborn at each point, it does not decay with distance.

2. Cable properties: length and time constants

An axon behaves like an electrical cable, and its cable properties set how far and fast electrotonic current spreads. The length (space) constant (λ) is the distance over which a voltage change decays to about 37% of its original value; larger λ means current spreads farther. The time constant (τ) is how quickly the membrane voltage changes (τ = membrane resistance × membrane capacitance); smaller τ means faster charging and faster propagation. Both arise from internal resistance (axoplasm, lowered by larger axon diameter) and membrane resistance (leak, raised by myelin).

3. Myelin and saltatory conduction

Myelin is a lipid-rich, tightly wrapped sheath that insulates the axon, raising membrane resistance and lowering capacitance. In the peripheral nervous system it is made by Schwann cells (one cell wraps one segment of one axon); in the central nervous system by oligodendrocytes (one cell wraps segments of several axons). Myelin is interrupted at gaps called nodes of Ranvier, where voltage-gated Na⁺ channels are densely concentrated. Because current spreads under the myelin with little leak, the action potential regenerates only at nodes, appearing to jump node to node — saltatory conduction (Latin saltare, "to leap"). This raises conduction velocity and cuts ion-pumping cost, since far less membrane is depolarized per unit length.

How it works

1. An action potential at one site creates a local positive charge inside.
2. Local current spreads passively (electrotonically) to the next region.
3. The next region depolarizes to threshold and fires its own action potential.
4. The refractory period keeps the signal moving only forward.
5. Myelin raises membrane resistance so current spreads farther, letting the spike leap between nodes of Ranvier.
6. Large diameter lowers internal resistance, further increasing velocity; the safety factor keeps each node reliable.

Common confusions

Memory aids

"Fatter or wrapped goes faster." A larger axon diameter (lower internal resistance) or a thicker myelin wrap (higher membrane resistance) both let current spread farther and make the action potential travel faster — and myelin makes it leap from node to node.

Quick review

Topic Recap

Action potentials propagate by local current flow that passively depolarizes (electrotonic conduction) the adjacent membrane to threshold, where a fresh action potential is regenerated. Cable properties — the length and time constants, set by internal resistance (diameter) and membrane resistance (myelin) — determine how far and fast that current spreads. Myelin from Schwann cells (PNS) and oligodendrocytes (CNS) makes propagation saltatory at the nodes of Ranvier, greatly increasing conduction velocity and energy efficiency, while demyelination compromises the safety factor and slows or blocks the signal.

Knowledge Check

1. Why does an action potential not decay as it travels down an axon?
2. How does myelin increase conduction velocity?
3. Which cells make myelin in the PNS and CNS, and how do they differ?
4. What are the length and time constants, and how do axon diameter and myelin affect them?
5. Why does demyelination slow or block conduction?

Answers and Rationales

1. Each patch regenerates a full all-or-none action potential from the passively spread current, restoring amplitude at every point. Rationale: propagation is active regeneration, not passive relay.
2. Myelin raises membrane resistance and lowers capacitance, so current spreads farther and the spike leaps between nodes, needing fewer regenerations. Rationale: better spread plus node skipping equals speed.
3. Schwann cells (PNS) each wrap one segment of one axon; oligodendrocytes (CNS) wrap segments of several axons. Rationale: the myelin source differs by nervous-system division.
4. λ = √(r_m/r_i) is how far voltage spreads before decaying; τ = r_m·c_m is how fast the membrane charges. Large diameter lowers r_i (larger λ); myelin raises r_m and lowers c_m (larger λ, smaller τ). Rationale: both improve passive spread and speed.
5. Loss of myelin raises capacitance and lowers membrane resistance, so current leaks out before reaching the next node, dropping the safety factor below threshold. Rationale: without insulation, passive spread fails.