2 The Action Potential

This lecture has three goals: to describe the steps of an action potential, to explain the key properties of action potentials, and to discuss how changes in action potentials affect the brain and behavior.

Anatomy of a neuron

A neuron receives signals on its dendrites, the branching extensions around the cell body. The soma (cell body) contains the nucleus. From the soma runs a single long axon, which is wrapped along its length in myelin sheaths. The axon ends in axon terminals, which pass the message on to other neurons. A message always travels in one direction through the cell: dendrites, then soma, then axon, then axon terminals.

A second diagram of the same parts, used in the review questions, adds two more labels. It marks each segment of myelin as a Schwann cell, and it marks the short gaps of bare axon between neighboring segments as nodes of Ranvier.

A little history

The lecture’s short history of the action potential introduces three physiologists: Alan Lloyd Hodgkin, Andrew Fielding Huxley and John Eccles.

The lecture then shows a squid and, beside it, an axon dissected out of a squid lying in a dish of fluid next to a fine glass pipette. Next comes a microscope image of an axon with recording electrodes placed on it, and a graph titled “Hodgkin–Huxley” that plots an action potential against time in milliseconds. In that graph a brief pulse of stimulating current, lasting under a millisecond, is followed by the membrane voltage rising from about mV to a peak near mV at around 2 ms. The voltage then falls back and settles at or slightly below its starting level by about 5 ms.

The action potential: it’s just ions moving around

At rest, the inside of a neuron is negative relative to the outside. Its resting potential is about mV. An animation in the lecture shows this along an axon, with a row of + charges on the outside of the membrane and a row of − charges on the inside.

What changes the voltage is the movement of two ions across the membrane:

• At rest, sodium (Na) is concentrated outside the cell. You put salt on the outside of your food.
• At rest, potassium (K) is concentrated inside the cell. Bananas are inside a peel.

When a neuron is activated, these two ions temporarily trade places: Na flows into the cell and K flows out. The lecture’s figure here shows the sodium–potassium pump, a protein in the cell membrane that carries Na and K ions across it.

The action potential plays out in five steps, over a few milliseconds:

1. Stimulus. Something pushes the membrane potential up from its resting level of mV. If the push does not carry the potential up to the threshold of mV, the potential just sinks back to rest. The graph shows these small bumps as “failed initiations.”
2. Depolarization. Once the potential reaches threshold, Na ions rush into the cell and the potential shoots up past 0 mV to a peak of about to mV (the two graphs in the lecture differ).
3. Repolarization. K ions flow out of the cell and the potential falls back down.
4. Hyperpolarization. The potential overshoots, dipping below the resting level for a moment.
5. Resting state. The potential returns to mV, ready for the next action potential.

A video from LabXchange (Amgen Foundation and Harvard) shows the action potential traveling down the axon, with voltmeters placed along the axon that read mV at rest.

Properties of action potentials

The lecture lists five key properties.

1. Information is communicated when a neuron fires. The action potential travels from the cell body along the axon. The lecture illustrates this with an action potential moving along a myelinated and along an unmyelinated axon (both are drawn in the section on myelin below).
2. Firing is a result of depolarization and repolarization. An action potential is the rise and fall of the membrane potential traced above: Na in, then K out.
3. All-or-nothing law. A neuron either fires or it doesn’t. On the graph, a stimulus that stays below threshold produces only a failed initiation, and a stimulus that reaches threshold produces the full action potential.
4. Action potentials propagate over long distances. A giraffe makes the point. A motor neuron sits in the giraffe’s spinal cord, and its axon runs all the way down the leg to an axon terminal in the foot. A second panel shows that the long axon also carries signals the other way: molecules released by the target cell (1, 2) are picked up at the axon terminal (3) and carried back up the whole length of the axon to the cell body (4) and into its nucleus (5).
5. The magnitude of a response is the frequency of firing. Every action potential has the same size, so a neuron signals a stronger stimulus by firing more often. A weak stimulus raises the firing rate a little while it is on; a strong stimulus makes the neuron fire much faster. When the stimulus turns off, firing drops back to its occasional background rate.

Tetrodotoxin: puffer fish poison

Tetrodotoxin, the poison of the puffer fish, works as a sodium channel blocker: it blocks the channels in the membrane through which Na enters the neuron. Without Na coming in, a neuron cannot depolarize, so it cannot fire. Tetrodotoxin causes full-body paralysis and loss of sensation, and it is more toxic than cyanide. Even so, it has increasing use in clinical and research applications.

Myelin

Myelin is electrical insulation for the axon, and it speeds up the rate of transmission. With the myelin sheath intact, the message moves quickly, at about 400 km/h, hopping from one gap in the myelin to the next like a kangaroo. On an unmyelinated axon the action potential has to spread continuously along the whole membrane, which is slower.

The class then did an activity, “The Human Axon-ipede: The Activity Literally Nobody Asked For,” followed by “The Human Axon-ipede Part 2: The Sequel Literally Nobody Asked For,” both built around the diagram of the myelinated and unmyelinated axon.

When myelin is missing or damaged

What happens if you don’t make enough myelin, or your myelin is damaged?

Check yourself

The first three questions review the previous lecture.