Saltatory Conduction

What saltatory conduction actually is

A nerve impulse is an electrical event — the action potential — that must be carried from one end of an axon to the other without fading. The problem is physics: the axoplasm is a poor wire, and the membrane is a leaky capacitor. If a neuron simply let current dribble down a bare fiber, the signal would decay within a millimeter. Nature solves this by regenerating the spike over and over, but regeneration is slow. Saltatory conduction is the elegant compromise: insulate most of the axon so current spreads passively and fast, and regenerate the spike only at brief exposed gaps. The action potential therefore seems to jump — saltare, to leap — from gap to gap.

The insulation is myelin, a spiral of 50 to 100 compacted glial-cell membranes wrapped around the axon. The exposed gaps are the nodes of Ranvier, each only about 1 micrometer wide, separated by myelinated internodes of roughly 0.2 to 2 mm. At the nodes the axon membrane is studded with voltage-gated sodium channels at densities exceeding 1,000 per square micrometer; under the myelin, the internodal membrane carries almost none. The action potential is thus a series of nodal explosions linked by near-instant passive current spread.

The cable physics: why insulation makes it fast

Treat the axon as an electrical cable. Two passive properties govern how far and how fast a local depolarization spreads before it must be regenerated:

• Membrane resistance (R_m). Myelin stacks dozens of lipid bilayers in series, multiplying resistance and blocking ion leak. High R_m means the current that enters at a node stays inside the axon and travels farther down the core.
• Membrane capacitance (C_m). Capacitance falls in inverse proportion to insulator thickness. Thick myelin slashes C_m so little charge is wasted charging the membrane to threshold; the same current depolarizes the next node sooner.

The product of these properties sets the length constant (λ), the distance over which a passive signal falls to 37% of its strength. Myelination raises λ so that a single node's depolarization easily reaches the next node well above threshold, even with a safety factor of 5 to 7 to spare. Conduction velocity scales roughly linearly with diameter in myelinated fibers (about 6 m/s per micrometer of outer diameter by Hursh's rule) but only with the square root of diameter in unmyelinated ones — which is why evolution chose insulation over sheer girth to make vertebrate nerves fast without making them impossibly thick.

Step by step along the fiber

1. An action potential fires at node n, driving the inside of the membrane to about +30 mV as sodium floods in.
2. That local positivity creates a longitudinal current that flows down the low-resistance axoplasm beneath the insulating internode, losing almost nothing to leak.
3. The current reaches node n+1 and depolarizes its membrane past threshold (~−55 mV) within microseconds.
4. The dense Nav1.6 channels at node n+1 snap open, regenerating a full-amplitude spike. The signal is reborn, not merely relayed.
5. Meanwhile node n enters its refractory period, its sodium channels inactivated, which prevents the impulse from running backward and enforces one-way travel.
6. The cycle repeats node to node. Active regeneration consumes time only at the nodes; the internodal hops are nearly instantaneous, so the impulse appears to skip ahead.

The energy savings are real: because the membrane is excited only at the nodes, far fewer Na⁺ and K⁺ ions cross per impulse, so the ATP-driven Na/K pump has much less work to restore the gradients. A myelinated fiber can fire repeatedly on a fraction of the metabolic cost of a bare fiber of equal speed.

Myelinated vs unmyelinated conduction

The same membrane biophysics produces wildly different behavior depending on insulation. The table contrasts saltatory conduction in a large myelinated fiber with continuous conduction in an unmyelinated one.

Property	Myelinated fiber (saltatory)	Unmyelinated fiber (continuous)
Conduction mode	Spike leaps node to node	Spike regenerated at every patch
Typical velocity	70–120 m/s (Aα); 5–30 m/s (smaller)	0.5–2 m/s (C fibers)
Na⁺ channel location	Concentrated at nodes (>1,000/µm²)	Spread evenly along membrane
Energy per impulse	Low — ions cross only at nodes	High — ions cross everywhere
Space needed for speed	Thin axon + thick sheath	Requires very large diameter (e.g. squid giant axon)
Example fibers	Motor neurons, proprioception, Aδ sharp pain	Dull/burning pain, postganglionic autonomic, olfactory
Disease vulnerability	Demyelination (MS, GBS, CMT)	Small-fiber neuropathy (diabetes, amyloid)

The trade-off is starkly illustrated by the squid giant axon, which an invertebrate evolved to roughly 1 mm wide just to reach ~25 m/s for its escape reflex. A myelinated mammalian fiber hits that speed at one-fortieth the diameter — the difference between a fire hose and a thread.

Clinical correlations and disease

Because saltatory conduction depends on intact insulation and correctly positioned nodal channels, it fails in characteristic ways when either is disturbed. These are among the most common reasons a neurologist orders nerve conduction studies.

• Multiple sclerosis. Autoimmune attack on central myelin exposes the internode; leaked current may fail to reach the next node, slowing conduction velocity, dispersing the signal, or blocking it outright. Heat worsens block (Uhthoff's phenomenon) because warming shortens the action potential. See multiple sclerosis.
• Guillain-Barre syndrome. Acute inflammatory demyelination of peripheral nerves, often after infection, causes ascending weakness. Because Schwann cells remyelinate well, many patients recover substantially — unlike central demyelination.
• Charcot-Marie-Tooth disease. Inherited mutations (e.g. PMP22 duplication) produce abnormally thin myelin and slow conduction from childhood, giving the classic high arches and distal wasting.
• Diabetic and toxic neuropathies. Metabolic injury and small-fiber loss slow conduction and blunt vibration and proprioceptive sensing, raising fall and ulcer risk.
• Channelopathies. Mutations that mislocate or dysregulate nodal Nav1.6 channels can block conduction even when the myelin looks structurally normal on imaging.

Diagnostically, the leap mechanism is exactly what the clinic measures. Nerve conduction studies report velocity in m/s and look for conduction block and temporal dispersion — hallmarks of demyelination — versus the reduced amplitude that signals axon loss. Visual evoked potentials detect optic-nerve demyelination by timing how long the signal takes to climb the visual pathway, often the earliest objective sign of MS.

Development, remyelination, and why repair is hard

Myelination is not present at birth; it proceeds for years and is why infant reflexes are slow and why the corticospinal tract is not fully myelinated until adolescence. In the periphery, each Schwann cell myelinates one internode of one axon and can remyelinate after injury — but the new internodes are shorter, so conduction recovers but never to baseline speed. In the central nervous system a single oligodendrocyte myelinates up to 40 to 60 internodes across multiple axons; when it dies, remyelination by resident progenitor cells is slow and often incomplete, which is why MS lesions accumulate. Restoring saltatory conduction — by promoting remyelination or by pharmacologically blocking the potassium channels that "short out" demyelinated axons, as the MS drug dalfampridine does — is an active therapeutic frontier.

This is educational content, not medical advice. If you have neurological symptoms, consult a qualified clinician.