Module 2 | Source: A Brain-Mind Odyssey, UC Berkeley, Ch. 5
Tags: neurons, action potential, ion channels, ion pumps, membrane potential, resting potential, depolarization, hyperpolarization, voltage-gated channels, myelin, saltatory conduction, axon hillock, refractory period, nodes of Ranvier
Neurons generate electrical signals by controlling the flow of ions across their membranes. A resting neuron maintains a voltage of about –65 mV through ion pumps and selective channels. When threshold is reached, voltage-gated sodium channels open to produce an action potential that propagates along the axon, sped dramatically by myelin insulation and saltatory conduction.
Diffusion
The movement of particles from regions of higher concentration to lower concentration, driven by thermal agitation. Ions in solution distribute themselves uniformly over the available volume.
Ion channels
Membrane proteins that allow specific ions to pass through when open. Ions move by diffusion, from more concentrated to less concentrated.
Ion pumps
Proteins that use energy (ATP) to move specific ions against their concentration gradient. The sodium-potassium pump moves 3 Na+ out and 2 K+ in per molecule of ATP consumed.
ATP (adenosine triphosphate)
The cell's primary energy currency. Composed of an adenine base, ribose sugar, and three phosphate groups. Energy is stored in the phosphorus-oxygen bonds.
Membrane potential
The voltage difference across the cell membrane, resulting from unequal distribution of charged ions on either side. Represents stored potential energy.
Resting membrane potential
The membrane potential when a neuron is not sending a signal. Typically around –65 mV (inside negative relative to outside).
Depolarization
A shift in membrane potential toward a less negative (more positive) value. Caused by influx of positive ions such as Na+ or Ca++ through ion channels.
Hyperpolarization
A shift in membrane potential toward a more negative value, increasing the charge separation across the membrane. Caused by outflow of K+ or influx of Cl–.
Action potential
A rapid, transient reversal of membrane voltage that travels along a neuron's axon. Produced by the sequential opening and closing of voltage-gated Na+ and K+ channels.
Voltage-gated ion channels
Sodium and potassium channels that open and close in response to changes in membrane voltage rather than to a chemical signal.
Axon hillock
The region where the axon emerges from the cell body (soma). Has the highest density of voltage-gated Na+ and K+ channels, making it the site where action potentials are initiated.
Refractory period
The brief interval after an action potential during which voltage-gated channels cannot be re-opened. Ensures the action potential travels in one direction only (hillock to terminus).
Myelin
A fatty insulating sheath covering many axons. Composed of roughly 70% lipid bilayer membrane, with more than 25% of that lipid being cholesterol.
Oligodendrocytes
Glial cells that form myelin in the central nervous system (brain and spinal cord).
Schwann cells
Glial cells that form myelin in the peripheral nervous system.
Nodes of Ranvier
Small gaps in the myelin sheath along a myelinated axon, where voltage-gated Na+ channels are concentrated and action potentials are regenerated.
Saltatory conduction
The "leaping" propagation of an action potential from one node of Ranvier to the next in a myelinated axon. From the Latin saltare, to leap. Greatly increases conduction speed.
Four major ions for neural function: Na+, K+, Cl–, Ca++.
Outside the neuron: Na+, Cl–, and Ca++ are more concentrated.
Inside the neuron: K+ is more concentrated.
The sodium-potassium pump actively maintains these gradients, using 1 ATP to export 3 Na+ and import 2 K+.
The brain accounts for roughly 25% of total basal energy consumption (about 360 kilocalories per day).
Around 60% of that brain energy (about 220 kcal/day) is spent running sodium-potassium pumps.
Because ions carry charge, unequal concentrations create a voltage across the membrane.
At rest (no signal being sent), this voltage sits at approximately –65 mV, with the inside of the cell negative relative to the outside.
Opening Na+ or Ca++ channels lets positive ions flow in, making the inside less negative: depolarization.
Opening K+ channels (K+ flows out) or Cl– channels (Cl– flows in) makes the inside more negative: hyperpolarization.
At rest, voltage-gated Na+ channels are closed.
If depolarization reaches the threshold of about –50 mV, voltage-gated Na+ channels open and Na+ rushes in.
The membrane voltage rapidly climbs to about +30 mV.
At +30 mV, Na+ channels close and voltage-gated K+ channels open.
K+ flows out, repolarising the membrane.
The voltage briefly overshoots to about –70 mV before returning to the resting –65 mV.
The entire cycle takes about 1 millisecond.
Voltage-gated channels are distributed along the full length of the axon.
When Na+ enters at one point, it drifts to adjacent regions, depolarising them past threshold, which opens Na+ channels there. This cascading process carries the signal down the axon, like a stadium wave.
Action potentials start at the axon hillock (highest density of voltage-gated Na+ channels) and travel toward the axon terminals.
They do not travel backward because the channels behind the advancing signal are in their refractory period and cannot reopen for 1–2 milliseconds.
Myelin wraps around the axon in segments, with small gaps (nodes of Ranvier) between segments.
Under myelin, ions cannot cross the membrane. The current from incoming Na+ at one node travels through the axon interior to the next node, where it triggers a fresh action potential.
This node-to-node jumping (saltatory conduction) dramatically increases speed:
Unmyelinated axon: less than 10 m/s (several miles per hour).
Myelinated axon: up to 100 m/s (more than 200 miles per hour).
Sodium-potassium pump stoichiometry: 1 ATP → 3 Na+ out, 2 K+ in
Resting membrane potential: approximately –65 mV
Action potential threshold: approximately –50 mV
Action potential peak: approximately +30 mV
Undershoot before recovery: approximately –70 mV
Action potential duration: ~1 millisecond
⚠️ Know the ion distribution: Na+, Cl–, Ca++ concentrated outside; K+ concentrated inside. This is fundamental to understanding every electrical event in neurons.
⚠️ The sodium-potassium pump ratio (3 Na+ out, 2 K+ in per ATP) is a classic exam detail.
⚠️ Be able to walk through the action potential sequence: resting → threshold (–50 mV) → Na+ channels open → peak (+30 mV) → Na+ channels close, K+ channels open → repolarization → undershoot (–70 mV) → return to rest (–65 mV).
⚠️ The refractory period explains unidirectional propagation, not just a timing detail. Expect questions on why action potentials only travel one way.
⚠️ Saltatory conduction vs. continuous conduction: know why myelin speeds things up and what happens at nodes of Ranvier.
⚠️ Oligodendrocytes = CNS myelin; Schwann cells = PNS myelin. A common mix-up.
Q: What is the resting membrane potential of a typical neuron, and what maintains it?
A: Approximately –65 mV. It is maintained primarily by the sodium-potassium pump (which exports 3 Na+ and imports 2 K+ per ATP) and by selective ion channel permeability.
Q: At what voltage do voltage-gated Na+ channels open, and what happens when they do?
A: They open at approximately –50 mV (threshold). Na+ rushes into the cell down its concentration gradient, rapidly depolarising the membrane toward +30 mV.
Q: Why does the action potential travel in only one direction along the axon?
A: Two reasons. First, there are very few voltage-gated Na+ channels in the soma direction from the hillock. Second, the refractory period of channels that have just fired prevents the signal from propagating backward along the axon.
Q: What is saltatory conduction and why is it faster than continuous conduction?
A: Saltatory conduction is the propagation of an action potential by jumping from one node of Ranvier to the next in a myelinated axon. It is faster because the electrical signal passes rapidly through the myelinated segments as a current rather than regenerating at every point along the membrane.
Q: Which glial cells produce myelin in the CNS versus the PNS?
A: Oligodendrocytes produce myelin in the central nervous system. Schwann cells produce myelin in the peripheral nervous system.
neuron signalling, ion channels, ion pumps, sodium-potassium pump, Na+/K+-ATPase, ATP, diffusion, concentration gradient, membrane potential, resting potential, depolarization, hyperpolarization, threshold, action potential, voltage-gated sodium channels, voltage-gated potassium channels, axon hillock, refractory period, myelin sheath, oligodendrocytes, Schwann cells, nodes of Ranvier, saltatory conduction, nerve impulse, Hodgkin and Huxley, conduction velocity, myelinated vs unmyelinated