Messier 81, also known as Bode’s Galaxy, is a spiral galaxy much like our own Milky Way. It is relatively nearby on an intergalactic scale (only a scant 12 million light years away!), and lies in the constellation Ursa Major, which includes the Big Dipper. This massive cloud consists of hundreds of billions of burning stars, each as bright as our sun, and each sending only the tinniest fraction (1 over 10 followed by 50 zeroes) of their luminous output all the way to Earth. In fact, the combined output of these billions of stars only manages to get about a dozen photons* per second into your eye when you look up at the northern sky. But on an exceptionally clear night, if you let your eyes adjust to the darkness, you can just see Messier 81 floating in the bowl of the Big Dipper.
The path of the photon from its fiery birth at the heart of a star to your eye is a relatively uninteresting and lengthy one, but what happens when this photon enters your retina is remarkable. While the single photon alone with its meager energy is unable to effect any measurable change even on a scale as small as a cell, a series of cascading amplification steps allows retinal cells to notice the single photon, and turn this tiny event into an electrochemical pulse detectable by your brain.
The molecule responsible for initiating this response is called rhodopsin. This is a membrane-bound protein packed into top segment of the primary type of photoreceptor cells known as rods. This top segment of the cell is chock full of many layers of protein, arranged in the direction of incident light, so as to have as much chance of detecting a photon as possible. Inside each rhodopsin protein lies a colourful molecule known as retinal, which acts much like chlorophyll in plants and absorbs the incident photon. Once absorbed, the photon excites the retinal, bending it and slightly altering the careful arrangement of the surrounding rhodopsin. This arrangement of excited rhodopsin has a new function—in this short-lived form it is able to activate tens or hundreds of nearby transducin proteins before accepting a new unbent retinal and awaiting another photon. These transducins in turn each activate a single extremely efficient enzyme known as phosphodiesterase (or PDE), which acts as a pair of molecular scissors which rapidly chop an abundant molecule known as cyclic GMP (cGMP) in two. This enzyme is incredibly fast, and cuts up cGMP at a rate near the theoretical maximum for any enzyme diffusing freely in solution.
Nearby, on the outer membrane of the rod cell are many ion channels—small openings in the cell membrane which consistently allow positively charged ions (mostly Na+) to enter the otherwise impermeable membrane. These channels are gated by the presence of cGMP, that is, they open up only when there is enough cGMP nearby. When a photon is absorbed, the nearby million-times amplification of the cascade pathway results in the nearby cGMP concentration dropping significantly below the threshold preferred by the ion channels. With these channels closed, the inward current to the cell is dropped, resulting in a polarization of the cell’s interior with respect to the surroundings. In other words, the cell is now charged, and held at a negative voltage.
As implied by their name, rod cells are long and slender. They extend away along the axis of incoming light in order to present the highest cross-section, and penetrate up towards the synaptic nerves which act to process the light-activated signals and send the signal along to the brain. At their tip, rod cells split into many narrow ribbons with fit intimately with the synaptic cell directly above with an intermediate gap of only 20nm. As the rod cell becomes polarized, it releases glutamate into this gap. Glutamate acts as a neurotransmitter, which tells the next cell in line that a photoreceptor has been activated, and serves to induce another cascading activation cycle the synaptic nerve cell.
Every photon-activated rod cell sends along a message in this way into the ménage of processing nerves which lie below the retina. When enough rods in a certain area have reported, a corresponding signal is sent towards the brain and there interpreted as a very faint twinkle in the night sky.
*Regarding the relevant calculations for the photon flux of Messier 81, the “dozen photons” number is approximate, but well within an educated estimate, excluding supernovae.
M81 IR Photograph Courtesy of NASA.