Nobel Prize in Physiology or Medicine 2026

How a Photon Fires a Neuron: The Physics of Optogenetics

On 5 October 2026 the Nobel Assembly at Karolinska Institutet awarded the prize to Karl Deisseroth (Stanford University), Peter Hegemann (Humboldt University of Berlin) and Georg Nagel (University of Würzburg), “for their discoveries concerning light-gated ion channels and optogenetics”.

This course follows a single photon from the moment it strikes a molecule to the moment a nerve cell fires, deriving every step rather than asserting it. Level: advanced undergraduate physics or physical chemistry.

A photon becomes a spike across twelve orders of magnitude in time

The first two steps are quantum chemistry inside one molecule; the last three are classical physiology summed over many channels.

  1. ~1 fs

    Photon absorbed

    Retinal jumps from S0 to the excited state S1

  2. ~100s of fs

    Twist through the conical intersection

    All-trans becomes 13-cis, back on the ground state

  3. ~1 ms

    The protein opens its pore

    Strained retinal nudges the helices apart

  4. a few ms

    Cations flow in

    Tens of thousands of channels charge the membrane

  5. ~5 ms

    The neuron fires

    Threshold crossed, about 20 mV above rest

Absorption to a behavioural response (~1 s) spans fifteen orders of magnitude; the five stages above, ending at the spike, span twelve.

Where to start

📰 From Algae to Brain

The story, for any reader: an algal eyespot, a protein that is both sensor and channel, and the leap into neuroscience. No mathematics.

🧭 A Neuron in One Page

For physicists: Nernst potentials, the resting potential, and why channelrhodopsin reverses near 0 mV. One equation, derived.

The three physical steps

1Absorption: retinal as a particle in a boxWhy a conjugated chain behaves like an infinite square well, and why the protein, not the molecule, sets the colour.2Isomerization: falling through a conical intersectionTwo electronic states, a 2 x 2 Hamiltonian, and the funnel that makes the twist fast enough to beat fluorescence.3From channel to spike: the neuron as an RC circuitOne channel carries 3.5 fA. Counting how many must open at once to move the membrane 20 mV.

Then check it yourself

🖥️ Simulations

Four Python simulations that reproduce every number in the lesson.

✏️ Exercises

Six problems from quick estimate to open discussion, with full worked solutions.

📚 Glossary

Fifteen terms, plus the laureates' key papers and the prize sources.

What you should be able to do afterwards

  1. Estimate the absorption wavelength of a conjugated chromophore with the particle-in-a-box model, and explain why the protein shifts it.
  2. Describe photoisomerization with potential energy surfaces, and say why a conical intersection makes it fast and efficient.
  3. Model a neuron's membrane as an RC circuit and estimate how much light-driven current it takes to fire a spike.
  4. Trace one event across fifteen orders of magnitude in time, from femtoseconds to seconds.

Prerequisites: introductory quantum mechanics (particle in a box, energy levels), basic electrostatics and circuits, and a little chemistry of double bonds.

Numerical values in Parts 1 to 3 are approximate teaching estimates, labelled where they are. Text and figures CC BY-SA 4.0; the simulation code is MIT.

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