What was that sound?
The action potential as a story about ions and a leaky bucket, and why the click you heard is a hundred times smaller than the voltage swing inside the cell.
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When you brushed the cockroach leg, the speaker clicked. Each click was one neuron firing one action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. Related: Membrane potentialMembrane potential The voltage across a cell's membrane, about −70 millivolts at rest, set by ion concentrations and which channels are open., Ion channelIon channel A protein pore in the membrane that lets specific ions through, often only when the voltage or a chemical tells it to open., SpikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide., Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing.. Glossary entry: a swing of about a hundred millivolts across the cell’s membrane, lasting about a millisecond, that travels down the nerve fiber toward the body. What your pin recorded was not that hundred-millivolt swing. It was its shadow in the salty fluid outside the cell, a blip a hundred to a thousand times smaller. Let’s take that apart, starting with the membrane at rest.
A cell at rest is a charged battery
A neuron is a bag of salty water inside a bag of different salty water. Inside: lots of potassium, little sodium. Outside: the reverse. The membrane between them is mostly impermeable, but it is peppered with ion channelsIon channel A protein pore in the membrane that lets specific ions through, often only when the voltage or a chemical tells it to open. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane., Hodgkin–Huxley modelHodgkin–Huxley model The 1952 set of four differential equations for the squid axon that reproduces the action potential from voltage-dependent sodium and potassium channels.. Glossary entry, protein pores that let one kind of ion through. At rest, mostly potassium channels are open. Potassium leaks out, down its concentration gradient, carrying positive charge with it, until the inside is negative enough to pull it back. The balance point is around −70 millivolts inside relative to outside. That is the resting potentialResting potential The steady membrane voltage of a neuron that is not firing, typically −60 to −70 millivolts. Related: Membrane potentialMembrane potential The voltage across a cell's membrane, about −70 millivolts at rest, set by ion concentrations and which channels are open.. Glossary entry.
Think of the membrane as a leaky bucket with a pump. The pump (a protein that burns energy to push sodium out and potassium in) keeps the concentrations different. The leak (the open potassium channels) sets the voltage.
The spike is two doors opening in sequence
Some of the channels are voltage-gated: they open when the membrane voltage crosses a threshold. Push the inside a little less negative, to about −55 millivolts, and sodium channels open. Sodium rushes in, because there is more of it outside and the inside is negative. The inside goes positive, fast, to about +40 millivolts. That is the upstroke, and it takes a fraction of a millisecond.
Two things then happen. The sodium channels close on their own after about a millisecond (they inactivate), and the slower potassium channels open. Potassium rushes out. The inside swings back negative, overshoots a little, and settles. The whole event is one to two milliseconds long, and it is all-or-nothing: either the threshold is crossed and the full swing happens, or it is not and nothing happens.Hodgkin and Huxley worked this out in 1952 on the squid’s giant axon with glass electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry and a lot of algebra. Their four equations are still the model, and in Phase 4 you will implement them and watch this happen in a simulation you wrote.
While the sodium channels are inactivated, the cell cannot fire again. That is the refractory periodRefractory period The millisecond or so after a spike during which the neuron cannot fire again because its sodium channels are inactivated; it caps firing rates at a few hundred per second. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane., Ion channelIon channel A protein pore in the membrane that lets specific ions through, often only when the voltage or a chemical tells it to open.. Glossary entry, and it puts a ceiling on firing rate of a few hundred spikesSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry per second.
A spike inside the cell is about 100 mV. What did the pin in the cockroach leg, sitting outside the nerve, see?
Around 100 microvolts, give or take a factor of a few. The pin was outside the cell, so it saw the currents flowing through the fluid around the axon, not the voltage across the membrane. Those currents make a small, brief field that falls off quickly with distance.
Outside the cell you hear the shadow
During the spikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry, current flows into the axon at the active spot and back out along the neighbouring membrane. That loop of current runs through the salty fluid outside, and current through a resistive fluid makes a voltage. A pin nearby sees a brief blip, usually biphasic (down then up, or up then down depending on where it sits), of tens to hundreds of microvolts, that lasts about as long as the spikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry. That is the extracellularExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. Related: SpikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide., Local field potential (LFP)Local field potential (LFP) The slow, summed electrical activity of many neurons near an electrode, below a few hundred hertz, as opposed to individual spikes., Intracortical recordingIntracortical recording Recording from tiny electrodes inserted into the cortex itself, close enough to hear individual neurons.. Glossary entry spikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry, and it is what almost every neural recording system in the world records, because getting inside a cell without killing it is hard and does not scale.
Move the pin a few hundred micrometres away and the blip shrinks to nothing. This is why an electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry hears only the few neurons closest to it, why a recording site can pick up two or three cells at once with different waveform shapes (Figure 1 has three), and why spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. Related: SpikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide., NeuropixelsNeuropixels Silicon probes with hundreds to thousands of recording sites along a thin shank, the current standard for large-scale animal recording.. Glossary entry exists.
Why it clicked
Your amplifier turned a 100 µV blip into a volt or so, and the speaker turned a one-millisecond voltage pulse into a one-millisecond pressure pulse, which the ear hears as a click. Brushing the leg’s hairs bent mechanoreceptors, which opened channels, which pushed their neurons over threshold, which sent spikesSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry up the leg’s nerve at a hundred or more per second. You heard sensation being encoded as a rate.
Deep dive Why a millisecond, and why a hundred millivolts? 5 min
The hundred millivolts comes from the concentration ratios. For an ion with a ten-to-one ratio across the membrane, the voltage at which diffusion and electrical pull balance is about 60 mV (the Nernst potential). Potassium’s ratio gives about −90 mV; sodium’s gives about +60 mV. The resting cell sits near potassium’s value because potassium channels dominate at rest; the spiking cell heads toward sodium’s value because sodium channels dominate for a moment. The spikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry is the membrane switching which ion it listens to.
The millisecond comes from channel kinetics: how fast the sodium gates open, how fast they inactivate, how fast the potassium gates follow. Those rates depend on temperature, which is why a cold cockroach leg is quieter and slower. Mammalian neurons at 37 °C fire spikesSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry about half a millisecond wide.
Deep dive The membrane as a circuit 4 min
Electrical engineers see the membrane as a capacitor (the thin insulating lipid bilayer, about 1 microfarad per square centimetre) in parallel with several variable resistors (the channel populations), each in series with a battery (that ion’s Nernst potential). The cell’s voltage is the capacitor’s voltage. A spikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry is the resistors changing value in sequence. Every neuron model, from Hodgkin–HuxleyHodgkin–Huxley model The 1952 set of four differential equations for the squid axon that reproduces the action potential from voltage-dependent sodium and potassium channels. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Leaky integrate-and-fire neuronLeaky integrate-and-fire neuron The simplest useful neuron model: a leaky capacitor that fires and resets when it reaches a threshold. . Glossary entry down to the leaky integrate-and-fire toy, is a version of this circuit with more or fewer of the resistors allowed to change.
Explain what this page was about to your roommate in three sentences. No jargon they would not know.
We heard clicks because brushing leg hairs pushed sensory neurons past about minus 55 millivolts, opening sodium channels that drove the inside toward plus 40 millivolts for about 1 to 2 milliseconds. We recorded only the shadow of that 100 millivolt swing, since our pin sat outside the cells and sensed tens to hundreds of microvolts through the surrounding fluid. We count those brief blips as spikes traveling toward the body at more than 100 per second, with a refractory pause capping each cell at a few hundred per second.
Yours does not have to match. Check that the three ideas are there.