4.5 Nerve Cell Conduction & Electrochemical Gradients

The nerve cell (neuron) is the MCAT's clearest real-world application of this entire chapter: its axon behaves as an electrical cable governed by the same resistance and capacitance concepts from earlier sections, while its resting and action potentials are generated by ion concentration gradients working exactly like the concentration cells and batteries covered previously. Cardiac muscle uses the same physics, which is why the surface ECG can be read as a circuit problem in a conductive body volume.

The Axon as an Electrical Cable

An axon can be modeled as a long, thin electrical cable with two key properties acting simultaneously:

• Axial (internal) resistance — the cytoplasm inside the axon (axoplasm) resists the longitudinal flow of current down its length, just like the internal resistance of any conducting wire (R = ρL/A). Larger axon diameter lowers axial resistance and speeds passive current spread.
• Membrane resistance and capacitance — the phospholipid bilayer acts as both a leaky resistor (ions can cross through embedded channels) and a capacitor, since it is an insulating layer separating two conductive fluids (intracellular and extracellular) that carry opposite net charge — precisely the parallel-plate arrangement where C ∝ A/d.

Because the membrane behaves as a capacitor, it must be charged (ions must physically accumulate on each side) before the membrane voltage can change — this takes real time, and it is the single biggest obstacle to fast signal propagation along an unmyelinated axon. The local circuit of an action potential is current flowing in through open channels, along the axoplasm, and out through adjacent membrane — Kirchhoff's junction rule still holds at every point along the cable.

Myelin Sheath, Schwann Cells & Insulation of the Axon

The myelin sheath is a fatty, electrically insulating layer wrapped in concentric layers around the axon. In the peripheral nervous system, myelin is produced by Schwann cells, glial cells that each wrap around and myelinate one segment of a single axon (a single Schwann cell does not myelinate multiple different axons). In the central nervous system, oligodendrocytes provide myelin, and one oligodendrocyte can myelinate segments of multiple axons — a detail sometimes tested in biology-flavored Chem/Phys passages.

Recall that a parallel-plate capacitor's capacitance is inversely proportional to the distance separating its plates: C is proportional to 1/d. Wrapping additional layers of myelin around the axon membrane effectively increases the separation distance between the intracellular and extracellular conductive fluids, so myelin decreases membrane capacitance. At the same time, the fatty, ion-impermeable myelin layers block ions from leaking across the membrane, which increases membrane resistance.

This combination — high resistance, low capacitance — is exactly what a well-designed electrical cable needs to transmit a signal over distance with minimal loss: current entering the axon at one point spreads passively (electrotonically) down the inside of the cable with very little leaking out through the myelinated membrane, and because the membrane's capacitance is low, very little charge is needed to change the local membrane voltage. Both effects let electrical signals travel down myelinated internodes far faster than an unmyelinated membrane could support.

Nodes of Ranvier and Saltatory Conduction

Myelin does not cover the axon in one continuous unbroken sheath — it is wrapped in discrete segments called internodes, separated by small unmyelinated gaps called the Nodes of Ranvier, where the axon membrane is directly exposed to extracellular fluid. Voltage-gated Na⁺ channels are densely concentrated at the nodes, and it is only at these nodes that the action potential is actively regenerated (fresh Na⁺ influx, fresh depolarization).

Because the high-resistance, low-capacitance myelinated internodes let depolarizing current spread passively and almost instantaneously to the next node, the action potential appears to "jump" from node to node rather than propagating continuously point-by-point along the membrane. This is called saltatory conduction (from the Latin saltare, to leap), and it dramatically increases conduction velocity: unmyelinated axons conduct impulses at roughly 1 m/s, while myelinated axons of similar diameter conduct at roughly 100–120 m/s — a speed increase on the order of 100-fold.

Worked example: Compare how long it takes a nerve impulse to travel 1 m along each type of axon.

• Unmyelinated axon at 1 m/s: t = d/v = 1 m ÷ 1 m/s = 1 s
• Myelinated axon at 100 m/s: t = d/v = 1 m ÷ 100 m/s = 0.01 s (10 ms)

That 100-fold difference in travel time is why myelinated pathways dominate reflex arcs and any circuit where speed is critical, while thin unmyelinated fibers (common in pain and autonomic pathways) tolerate slower conduction.

Demyelinating conditions illustrate the underlying physics directly: when myelin is damaged, the affected membrane segment regains higher capacitance and lower resistance, so depolarizing current leaks out and decays before it can reach the next node with enough strength to trigger regeneration — conduction slows or fails entirely, even though the underlying ion channels and Na⁺/K⁺ gradients themselves may be largely intact.

Electrochemical Gradients, Nernst Potentials & the Action Potential

The previous section introduced the concentration cell — a device that generates voltage purely from an ion concentration difference across a barrier. The neuron's resting membrane potential is generated by exactly this mechanism, with the axon membrane and its selective channels playing the role of the barrier.

Na⁺ is far more concentrated outside the cell than inside; K⁺ is far more concentrated inside the cell than outside. These gradients are actively established and continuously maintained by the Na⁺/K⁺-ATPase pump, which spends cellular energy (ATP) to pump 3 Na⁺ out of the cell for every 2 K⁺ pumped in — actively "recharging" the concentration gradients the way an external power source recharges a secondary battery during electrolysis. The pump is also slightly electrogenic (net +1 charge out per cycle), contributing a few millivolts to the inside-negative potential.

For a membrane permeable to only one ion, the Nernst equilibrium potential is the transmembrane voltage at which the chemical driving force (concentration gradient) and electrical driving force exactly cancel for that ion. Qualitatively:

• K⁺ wants to leave the cell chemically; the inside-negative voltage that stops net K⁺ flux is strongly negative (EK often near −90 mV)
• Na⁺ wants to enter the cell chemically; the voltage that stops net Na⁺ flux is positive (ENa often near +60 mV)
• Cl⁻ and other ions have their own equilibrium potentials depending on concentration ratios and charge

A real resting neuron is permeable to more than one ion, so the resting potential (roughly −70 mV) is a weighted compromise — closer to EK than to ENa because resting K⁺ permeability is much higher. This multi-ion weighting is the idea behind the Goldman–Hodgkin–Katz equation; you rarely compute it on the MCAT, but you must know that raising Na⁺ permeability pulls membrane voltage toward ENa (depolarization), while raising K⁺ permeability pulls it toward EK (repolarization or hyperpolarization).

Action potential ion flow (circuit discharge and recharge):

1. Local depolarization to threshold opens voltage-gated Na⁺ channels
2. Na⁺ rushes in down its electrochemical gradient — membrane potential races toward ENa (the upstroke)
3. Na⁺ channels inactivate; voltage-gated K⁺ channels open
4. K⁺ exits, restoring a negative interior (repolarization), sometimes overshooting toward EK (afterhyperpolarization)
5. Na⁺/K⁺-ATPase and leak channels restore gradients over longer timescales so the axon is ready to fire again

In this way the entire chapter unites: axial and membrane R and C determine how fast a signal spreads passively between nodes; ion concentration gradients acting as a biological concentration cell supply the voltage discharged and recharged at every node; and Faraday/current ideas (I = dQ/dt through channels) set how quickly membrane capacitance can be recharged.

ECG/EKG as a Circuit Analog (Integration)

Cardiac myocytes also maintain resting potentials and fire action potentials, but they are electrically coupled through gap junctions so depolarization spreads as a coordinated wave across the atria and ventricles. That wave is a moving set of current sources and sinks in a three-dimensional volume conductor — the torso filled with ionic fluid.

Surface ECG/EKG electrodes do not clamp a single membrane. They measure potential differences between skin locations, exactly as a voltmeter measures ΔV between two points on a resistive network. Lead geometry selects different projections of the heart's electrical vector (P wave, QRS complex, T wave as sequential atrial depolarization, ventricular depolarization, and ventricular repolarization patterns). Flatline means those coordinated sources have stopped producing organized currents — not that body resistance became infinite.

Integration checklist for passages:

• Neuron/axon → cable (R, C), myelin (↑R_m, ↓C_m), saltatory conduction, Nernst/Goldman potentials
• Concentration cell → resting Vm from ion gradients + selective permeability
• Battery recharge → Na⁺/K⁺-ATPase spending ATP
• Capacitor discharge → rapid Na⁺ influx changing membrane voltage
• Body ECG → volume conduction + surface voltmeter leads
• Demyelination → leaky, high-C membrane → failed node-to-node current spread

If a Chem/Phys passage mixes a circuit diagram with a neuron or ECG figure, map each biological structure onto one of these circuit elements before diving into arithmetic.

Common MCAT Traps

• Thinking myelin speeds conduction only by "insulating like plastic wrap" without linking to ↑ resistance and ↓ capacitance.
• Placing voltage-gated Na⁺ channels uniformly under myelin. Regeneration is at nodes.
• Equating resting potential with EK exactly. Rest is near EK but pulled slightly toward other ions.
• Claiming the Na⁺/K⁺ pump fires the action potential spike. The pump maintains gradients; the spike is passive channel flow down those gradients.
• Treating ECG voltage as a single-cell membrane potential rather than a body-surface potential difference from volume currents.