Photoreceptors

This article explains the workings of the outermost layer of the retina which includes the retinal pigment epithelium, the photoreceptors (including rods and cones) and the bipolar cells. A basic explanation is provided about how rods and cones convert photons of light into electrochemical signals and transport those electrochemical signals to the next layer of the retina, the bipolar cells.

Retina showing rods, cones, bipolar cells and retinal pigment epithelium

After entering the eye through the pupil, light makes contact with the light sensitive, half a millimeter thick layer of tissue (the retina) which lines the inside of the eye ball. This light then has to pass all the way through the retina before striking and activating the photoreceptive rods and cones.

schematic diagram of eye showing nerve cells and neurons

Rods and cones are specialised types of neuron (nerve cells) found only in the retina. It is the rods and cones that process particles of light (photons) using electrochemical means to convert those photons into electrical signals.

These electrical signals (or ‘neural impulses’) provide the vital information used by our brain’s visual system to form an accurate representation of the visual world, giving us visual perception.

  • So what components form the outermost layer of the retina?

Retinal Pigmented Epithelium (RPE)

We start with the retinal pigmented epithelium (RPE), located adjacent to the membrane (Bruch’s membrane) which separates the retina from the blood supply provided by the choroid.

retinal pigment epithelium roda and cones under electron microscope

The RPE performs several vital functions on behalf of the photoreceptors including transporting nutrients to the rods and cones to keep them healthy and working properly.

In addition the RPE absorbs stray photons of light and prevents their reflection back to the photo receptors, which would otherwise overload those photoreceptors and make any images appear blurred.

The RPE also ‘phagocytoses’ (or consumes) cellular material shed by the photo receptors as the photo receptors go about the important process of cell renewal.

Photoreceptors- rods and cones

Transduction is the process by which energy is converted from one form to another. With the process of vision, transduction occurs when the photoreceptors convert light energy, in the form of photons, into both chemical and electrical energy.

Photoreceptors, bipolar and ganglion cells

This converted light energy is then transmitted to the brain for visual processing via the optic nerve in the form of electrical signals better known as “neural impulses”.

Retina of eye showing photoreceptive layer, ganglion, bipolar cells and optic nerve

  •  How do these specialised neurons transmit and receive information? How do photoreceptors convert light energy into neural impulses?

In order to communicate, rods and cones need to transmit information both within themselves and from themselves to adjoining bipolar cells. The process of transmitting information at an intra and inter cellular level employs electrical signals (‘neural impulses’) as well as chemical messengers.

The conversion of light energy into neural impulses starts with the light-sensitive pigments located on the disks of rods and cones. Rods of the retina are responsible for our vision at low levels of light whilst cones are responsible for our vision at higher levels of light and for our color vision.

Rods and cones showing disks, synaptic endings and nuceli

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To understand how light energy is converted into neural impulses we firstly need to have an understanding of what happens when light does not strike the light sensitive pigments of the disks.

The ‘dark response’, when no light strikes the disks, results in ‘depolarization’

Depolarization is the process by which the inside of each photoreceptor becomes less negatively charged in relation to the outside.

This uneven distribution in electrical energy, which can be described as an ‘electrical potential difference’ across the membrane of each cell wall, is usually referred to as a ‘membrane potential’.The ‘membrane potential’ is critical to the photoreceptor’s ability to communicate and transmit electrochemical signals to adjoining cells.

The value of the membrane potential, ie the charge across the membrane wall of the photoreceptor, is a measurement of the internal electrical charge compared to the external electrical charge. (ie the ‘difference’ in electrical charge)

Inside of photoreceptor negatively charged and outside positively charged across membrane

In the dark with no photons of light being absorbed by the membranous disks, and following a complicated cascade of events, large numbers of  ‘messenger’ molecules called cyclic guanosine monophosphates (cGMP’s) ….

cGMP 's released when photoreceptor in darkness

….are manufactured inside the outer segment of each photoreceptor.

gated sodium channels open in outer segment of photoreceptor

These cyclic GMP’s bind themselves to voltage gated sodium channels also located in the outer segment. This binding of cyclic GMP’s to voltage gated channels allows those channels to open, with the result that positively charged sodium (Na+) ions flood into the photoreceptor from outside.

sodium channel opens and sodium ions enter cells of rod

Whereas cyclic GMP messenger molecules provide the chemical signal allowing sodium voltage gated channels to open and close, the actual flow of Na+ ions is regulated by the difference in the electrical charge across the membrane wall (the ‘membrane potential’) – hence the name “voltage gated ion channel”.

After positively charged sodium ions enter the outer segment of the photoreceptors through the voltage gated channels, the inside of the photoreceptor becomes less negatively charged relative to the outside.

voltage gated sodium channels open to allow sodium ions to cross the membrane wall

The actual difference in electrical charge between the outside in comparison with the inside can be measured by inserting one micro electrode through the membrane wall into the cytoplasm (material inside a cell) of the photoreceptor and another micro electrode into the liquid outside the photoreceptor. The resulting voltage is measured in millivolts.

membrane potential of a depolarized photoreceptor measured in millivolts

This typical membrane potential of -30mV’s (shown above) represents the voltage of a depolarized photoreceptor where the photoreceptor provides an excitatory response to a lack of light. This excitatory response (explained below) results in the release of large numbers of chemical messengers called ‘neurotransmitters’.

(Photoreceptors behave very differently when depolarized compared to other types of neuron located elsewhere in the central nervous system. When other types neurons become depolarized, they become inhibited rather than excited.  When this happens the release of neurotransmitters is severely curtailed.)

With no light stimulus a depolarized photo receptor is able to maintain a constant voltage of -30mV’s.

Dark current of photoreceptor is a constant -30mV

The membrane potential is maintained at a constant -30 mV by the regulation of the flow of ions in and out of the photoreceptor across the membrane wall.

After entering the outer segment through voltage gated Na+ channels….

voltage gated sodium channels

…the sodium ions diffuse into the inner segment and then out into the intercellular space through the  Sodium/Potassium pump.

photoreceptor showing flow of dark current and function of sodium and potassium pump

The Naions then enter the photoreceptor once again through the voltage gated Na+channels. This circular motion of Naions is called the ‘Dark Current’ and is a vital mechanism in keeping the photoreceptor depolarized.

The Na/K pump not only exports sodium ions, but also imports potassium ions at the same time. The Na/K pump is not voltage gated; it is an active pump that relies on harnessing the energy of molecules of Adenosine triphosphate (ATP) through a process of hydrolysis.(ie by reacting with water)

The Na/K pump always transports three sodium ions outside the photoreceptor for every two potassium ions it pumps inside.

sodium potassium pump exports three sodium ions for every two potassium ions imported

The negatively charged membrane potential is also maintained by the leakage of  K+ ions out of the cell through the cell membrane and ‘down the gradient’. ‘Down the gradient’ means that the potassium ions leak out of the membrane from a higher internal concentration to a lower external concentration of potassium ions.

Ion move down the gradient through potassium leakage channels in photorteceptor

So the ‘Dark Current’, combining the action of voltage gated sodium channels, the sodium/potassium pump and potassium leak channels is what maintains the photoreceptor in a constant state of depolarization.

As previously mentioned, a depolarized photoreceptor is a neuron in an increased state of excitability. With the photoreceptor in an ‘excitatory’ state, (involving the operation of voltage gated N+channels along with N+/Kpumps and Kleak channels) and following a cascade of other electrochemical events, voltage gated calcium ( Ca2+) channels now open allowing positively charged calcium ions to enter the photoreceptor from the intercellular space outside.

calcium ions enter photoreceptor and neurotransmitters are released form vesicles

This influx of calcium ions leads to the release of a large number of neurotransmitters (chemical signals) stored in ‘vesicles’ at the synaptic ending. In photo receptors the neurotransmitters that are released are molecules of glutamate.

It is when the photoreceptors receive no light, and when depolarized, that the number of open Ca2+ channels in the synaptic terminal is high and glutamate is released in large concentrations.

Molecules of glutamate are released into the synaptic cleft, that 10 nanometer gap which separates the synaptic terminal (ending) of the photo receptor from the dendrites of the adjoining bipolar cell.

Glutamate diffuses across the synaptic cleft and binds onto ligand gated ion channels located in the dendrites of the adjoining bipolar cell.

dark response of photoreceptor releases glutamate acrsoo synaptic gap

The binding of glutamate onto ligand gated ion channels of adjoining bipolar cells permits the opening of the ligand gated ion channels, allowing the entry of sodium ions into the bipolar cells.

“Ligands” (in the form of glutamate in bipolar cells) are signaling molecules that give instructions for ligand gated ion channels to open.

glutamate attaches itself to ligand gated ion channels of bipolar cells

  • So what effect do large concentrations of glutamate released by a photoreceptor have on adjoining bipolar cells?

Alternative outcomes are possible depending on whether the bipolar cell is an ON center or an OFF center type.

An OFF center type bipolar cell that synapses with a depolarized photoreceptor will produce an excitatory response by itself depolarizing and releasing large quantities of glutamate across the synaptic cleft to the adjoining ganglion cell.

An ON center type of bipolar cell becomes inhibited when it synapses with a depolarized photoreceptor. It becomes hyperpolarized (explanation below of ‘hyperpolarization’) and the amount of glutamate released to a synapsing ganglion cell is reduced.

Hyperpolarization of on center bipolar cell and transmitter release decreased

 Light Response and Hyperpolarization

When stimulated by light the photoreceptors become ‘hyperpolarized’ meaning that the inside of the photoreceptor becomes even more negatively charged than it was when in a depolarized state, relative to the outside of the cell.

Becoming more negatively charged is not so difficult when you consider that the inside of the photoreceptor is naturally more negatively charged relative to the outside of the cell. This results from the continued presence of larger negatively charged molecules called “anions” such as Chloride (Cl –  ) inside the photoreceptor. These anions remain in situ and do not exit the photoreceptor through any of the channels.

  • So how does a photoreceptor become hyperpolarized?

As photons are absorbed by the photoreceptive pigments of the disks they come into contact with proteins located in the membrane walls.  These light sensitive proteins are from the G Protein-coupled receptor (GPCR) family and are called “opsins”. Rhodopsin is the protein present in rods, while photopsins are present in cones.

These proteins initiate a cascade of events that lead to a reduction in the concentration of cyclic guanosine monophosphate (cGMP) ‘messenger’ molecules. This means that fewer of these molecules are available to bind onto voltage gated sodium channels found in outer segment of the photo receptor.

If fewer cGMP molecules are available to bind onto sodium channels many of those voltage gated channels now close. As a result the ‘Dark Current’ is no longer able to flow.

The closure of voltage gated sodium channels is accompanied by the continued influx of potassium ions and efflux of sodium ions through the Na+/K+pump and the continued leakage of potassium ions out through the membrane.

Following a cascade of events voltage-gated calcium ion (Ca2+) channels now close.

The closure of voltage gated calcium channels leads to a decrease in the concentration of intracellular calcium ions with the result that glutmate release is considerably reduced. This is because the presence of calcium ions is required if the glutamate-containing vesicles are to release their contents.

The response of the photoreceptor to different levels of illumination is graded. If the illumination of the photoreceptor is intense, with many photons of light being absorbed the photoreceptor, the membrane potential reaches about -65 mV before becoming less negatively charged (more depolarized). In such conditions the Ca2+ channels all but shut, severely decreasing the release of glutamate.

A very dull illumination will lead to the membrane potential becoming a little less negative, resulting in some Ca2+ channels shutting and only slightly reducing the levels of glutamate release.

So these graded responses or ‘graded potentials’, based on the intensity of illumination, do not come in just one size; instead they come in a wide range of slightly different sizes, or gradations.

The light response of a photoreceptor can be summarized as follows:Light Response with cone hyperpolarizing and glutamate release decreased

  • So why do photoreceptors work in the opposite way in which other neurons work? Why do photoreceptors become both excited and depolarized when there is no light and become inhibited and hyperpolarized when there is light.

The short answer is that our visual system would very quickly become overloaded and cease to operate effectively if every time photoreceptors absorb photons of light of the correct frequency they become excitatory and depolarized.

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