Comparative Physiology - Photoreception

Flashcards about Photoreception based on lecture notes.

Ommatidium

Functional units or optic units forming compound eyes in insects and mollusks.

Retinular cells with rhabdomers

The light-capturing and processing part of the ommatidium, containing photoreceptors.

Photoreceptors (receptor protein)

Proteins within rhabdomeres that capture light, typically numbering eight, organized into outer and inner photoreceptors.

Drosophila eyes

The Drosophila compound eye is formed from approximately 800 ommatidial units, each comprising 6 outer (R1–6) and 2 inner photoreceptors (R7 and R8).

Photopigments

Visual pigments largely determine the sensitivity of photoreceptors in Drosophila eyes.

Detection of Polarized Light

In insects and mollusks, the ability to detect the angle of incoming light, aiding in orientation and navigation.

Camera Eyes (Cephalopods)

Eyes similar to vertebrate eyes but evolved independently, seen in squid, cuttlefish, and octopus.

advantage of insect's compounded eyes

The main advantage are the larger visual field and sensibility to quick movements due to the fact that movement is controlled by the camera

Fovea

The point with highest visual activity and the highest number of cones.

Pigment Epithelium

Layer in the retina providing high turnover rate of photoreceptor discs to sustain visual processing.

RGCs (Retinal Ganglion Cells)

Retinal ganglion cells whose axons merge to form the optic nerve, sending processed information to higher brain regions.

Müller Cells

Glial cells in the vertebrate retina that act as optic fibers, guiding light through the inverted retina to reach photoreceptors.

Human Cones

Opsins associated with 11-cis-retinale; three classes in humans (red, blue, green) that give us the capability to sample three different wavelengths.

Opsins

Transmembrane proteins (Rhodopsins, conopsins) in photoreceptor membranes made of a protein and retinal; they change conformation upon light interaction.

Transducin

Trimeric G-protein activated by rhodopsin that activates an effector protein called phosphodiesterase (PDE).

Phosphodiesterase (PDE)

Enzymes that hydrolyze cyclic GMP (cGMP) into GMP, leading to the closure of Na+ channels and hyperpolarization.

Vertebrate photoreceptors under light

The current that is generated by the photoreceptor is actually reduced, because the membrane is Hyperpolarized with saturation at -65 mV

Phototransduction cascade

Trimeric G-protein, which is called Transducin (because it transduce light)

Enzyme rate

Enzyme rate (Kcat/KM) = 108 M-1s-1 (mammals)

Retina Structure (cellular connections retina organization)

Specialized system where light is detected by photoreceptors connected to bipolar cells, connected to ganglion cells.

Receptor Fields

Regions of the retina where the action of light alters the firing of a neuron.

High sensitivity receptor fields Cell 1

Cell 1 has a high sensitivity, low convergence (one photoreceptor connected to one bipolar cell, connected to one retina ganglion cell).

OFF- Bipolar cells

Cells that do whatever the photoreceptor does(sign-conserving synapses).

ON- Bipolar cells

Invert the signal (Respond in opposite ways to the glutamate released by the photoreceptors because they express different glutamate receptors.)

ganglion cells (RGC)

Ganglion cells fire in all lighting conditions, but it is the relative firing rate that encodes information about light.

Trick of the retina in encoding of the boundaries of the objects

Mediated by the organization of the receptor field into center and surrounds

if light hits the surround

In ON center connection, the surround acts as OFF

Receptors that are placed in the surround

Are only indirectly connected to the bipolar cells.

Effect on photoreceptor upon light stimulation

Horizontal cells are not inhibitory, so a hyperpolarization in the horizontal cell would lead to depolarization in the center photoreceptor, and an inverting sign.

Firing Rate System using 2 different types of Receptor Field:

ON-centre/OFF-surround and OFF-centre/ON-surround

Antagonistic response between the centre and the periphery

It's very important for discriminating objects in the environment observed by the visual field

Explanation of inverting sign mechanism by GABA (professor becomes a receptor that binds inhibitor neurotransmitters(ionotropic receptors))

Allows passage of ions because there is a membrane potential difference and at the end a change in the cell potential 🡪 Hyperpolarization

Area connections; either direct (centre) or indirect (horizontal cells)

The way in how we define a receptor field

Duplex retina

High-sensitive rods and low-sensitive cones

importance of respond to differences of illumination

Because detecting changes in the illumination is more important than detecting information about absolute illumination.

Range of the visual system

Our visual system can operate across 14 orders of magnitude because of the presence of these two types of photoreceptors (rods and cones)

Roads and Cones are more sensitive

Rods is encoded by one gene and then we have other three genes encoding rhodopsin for cones, in total we have four genes

Scotopic vision

Under dim-light conditions, when our cones are not active: in fact, night vision is more or less black and white.

Mesopic vision

Involves both roads and cones, and it's in the middle of luminance magnitude (moonlight).

Photopic Vision

Basically using only cones during daylight.

CILIARY VS RHABDOMERIC

Short reminder 🡪 we saw the very first day when we started this discussion that there was an ancestor photoreceptor that at some point split, between vertebrates and invertebrates.

Rhabdomeric photoreceptor

Response to light is by depolarization

Ciliary photoreceptors CONS

Evolution overcame this by developing two types of receptors (one highly sensitive, the rod, one with low sensitivity, the cone) 🡪 duplex retina

Rhabdomeric photoreceptors PROS:

Higher amplification of the signal (via Ca2+ sparks) compared to ciliary photoreceptors 🡪 large dynamic range

Calcium amplification importance

This higher calcium amplification provides the opportunity to really inspect light changes

Ciliary photoreceptors come to dominate the chordates

Probably the first developed receptor was a high sensitivity rod, which supplemented the low sensitivity cone-like photoreceptors of invertebrates and allowed the principal eyes of chordates to cover the full intensity range