Unit One: Introduction to Physiology: The Cell and General Physiology

Unit Ten: The Nervous System: B. Special SensesChapter 50: The Eye: II. Receptor and Neural Function of the RetinaGuyton and Hall, Textbook of Medical Physiology, 12th editionAnatomy and Physiology of the Retina Layers of the Retina-functional components arranged in layers from the outside to the inside

Pigmented layerLayer of rods and conesOuter nuclear layer containing the cell bodies of therods and conesOuter plexiform layerInner nuclear layerInner plexiform layerGanglionic layerLayer of optic nerve fibersInner limiting membraneAnatomy and Physiology of the Retina Layers of the Retina

Fig. 50.1 Layers of the retina Anatomy and Physiology of the Retina Fovea- minute area in the center of the retina(1 sq mm) capable of acute vision; containsonly cones

Rods and Cones- the major functional segmentsof either a rod or cone are:

The outer segmentThe inner segmentThe nucleus The synaptic body Anatomy and Physiology of the Retina

Fig. 50.3 Schematic drawing of the functional parts of the rods and conesAnatomy and Physiology of the Retina Rods and Cones

Light sensitive photochemicals are found in theouter segment

In rods, it is rhodopsin

In cones, it is one of three color pigments which function exactly like rhodopsinAnatomy and Physiology of the Retina Rods and Cones

In the outer segments of both rods and cones arelarge numbers of discs (as many as 1000 per rod orcone)

Pigments are conjugated proteins incorporated into the membranes of the discs

Inner segment contains the usual organelles andcytoplasm

Anatomy and Physiology of the Retina Rods and Cones

Synaptic body connects with the neuronal cells,the horizontal and bipolar cells

Pigment Layer of the Retina

Melanin prevents light refraction throughout the eyeball

b.Stores large quantities of vitamin A

Anatomy and Physiology of the Retina Pigment Layer of the Retina

Vitamin A is an important precursor of the photosensitive chemicals of rods and cones

Anatomy and Physiology of the Retina

Fig. 50.4 Membranuous structures of t he outer segments of a rod and coneAnatomy and Physiology of the Retina Blood Supply of the Retina

Central retinal artery enters with the optic nerve

Branches to supply the entire retinal surface

Outermost layer is adherent to the choroid whichis also a highly vascular areaPhotochemistry of Vision Rhodopsin-Retinal Visual Cycle

Fig. 50.5 Rhodopsin-retinal visual cycle in the rodPhotochemistry of Vision Rhodopsin-Retinal Visual Cycle-The Decompositionby Light Energy

When light energy is absorbed by rhodopsin, therhodopsin begins to decompose;

The cause of this is photoactivation of electrons inthe retinal portion of rhodopsin, which convertscis into a trans form and cannot bind to the activesite on the protein.

c.This leads to unstable intermediates

Photochemistry of Vision Reformation of Rhodopsin

First step is re-convert to cis form of retinal

Requires energy and is catalyzed by retinal isomerase

Once formed it binds to the protein and is stable

Photochemistry of Vision Role of Vitamin A

Second pathway converts the trans-retinal totrans-retinol (one form of vitamin A)

The trans-retinol is then converted to cis-retinal

Vitamin A is present in the pigment layer of theretina and in the cytoplasm of rods

d.Excess retinal is converted to vitamin A

Photochemistry of Vision Excitation of the Rod When Rhodopsin is Activatedby Light

The rod receptor potential is hyperpolarizing, not depolarizing

When rhodopsin decomposes, it decreases therod membrane conductance for sodium ionsin the outer segment of the rod

This causes hyperpolarization of the entire rodmembranePhotochemistry of Vision Fig. 50.6 Movement of sodium and potassium ions through the inner and outer segments of the rod

Photochemistry of Vision Fig. 50.7 Phototransduction in the outer segment of the photoreceptor membrane

Photochemistry of Vision Duration of the Receptor Potential and Log Relationof the Receptor Potential to Light Intensity

Receptor potential occurs in 0.3 seconds and lasts for about 1 second in the rods

In the cones it occurs four times as fast

Receptor potential is approx. proportional to the logarithm of the light intensity which allows theeye to discriminate light intensities through a rangemany thousand times as great as would be otherwisePhotochemistry of Vision Mechanism by Which Rhodopsin Decomposition Decreases Membrane Sodium Conductance(Excitation Cascade)

Photon activates an electron in the cis-retinal portionof rhodopsin and leads to the formation of metarhodopsin

Activated rhodopsin acts as an enzyme to activatemany molecules of transducin

Activated transducin activates many mcles of phosphodiesterasePhotochemistry of Vision Mechanism by Which Rhodopsin Decomposition Decreases Membrane Sodium Conductance(Excitation Cascade)

Activated phosphodiesterase hydrolyzes cGMP whichallows the sodium channels to close

e.Within a second, rhopdopsin kinase inactivatesmetarhodopsin and reversion back to the normalstate with open sodium channelsPhotochemistry of Vision Photochemistry of Color Vision by the Cones

Only one of three types of color pigments is presentin each of the different cones

Color pigments are blue, green, and red sensitive pigments

Photochemistry of Vision Fig. 50.8 Light absorption by the pigment of the rods and the three color receptive cones

Photochemistry of Vision Automatic Regulation of Retinal Sensitivity

Light Adaptation- in bright light the concentrations of photosensitive chemicals arereduced

Dark Adaptation- in darkness, the retinal andopsins are converted back into the light sensitive pigmentsPhotochemistry of Vision Fig. 50.9 Dark adaptation, demonstrating he relation of cone adaptation to rod adaptation

Photochemistry of Vision Other Mechanisms of Light and Dark Adaptation

Change in pupillary size

Neural adaptationColor Vision Tricolor Mechanism of Color Detection

Spectral sensitivities of the three types of cones

Interpretation of color in the Nervous SystemFig. 50.10 Demonstration of the degree of stimulation of the different color sensitive cones by monochromatic lights of four colors: blue, green, yellow, and orange

Color Vision Perception of White Light- equal stimulation of the red, green, and blue cones gives the sensation of seeing white

Color Blindness- when a single group of cones ismissing, the person is unable to distinguish some colors from others

Red-greenBlue weaknessNeural Function of the Retina Fig. 50.12 Neural organization of the retina; peripheral area to the left, foveal area to the right

Neural Function of the Retina Neural Circuitry of the Retina

Photoreceptors transmit signals to the outer plexiform layer where they synapse with bipolar cells and horizaontal cells

Horizontal cells which transmit signals horizontally in the outer plexiform layer from the rods and cones to bipolar cells

Bipolar cells which transmit signals vertically to the inner plexiform layer, where they synapse with ganglion cells and amacrine cells Neural Function of the Retina Neural Circuitry of the Retina

Amacrine cells transmit signals either directly from bipolar cells to ganglion cells or horizontally from axons of the bipolar cells to dendrites of the ganglion cells or other amacrine cells

Ganglion cells which transmit output signals from the retina through the optic nerve into the brain

Neural Function of the Retina Visual Pathway from the Cones to the Ganglion Cells Functions Differently from the Rod Pathway

(Fig. 50.12) Visual pathway from the fovea has three neurons in a direct pathway: cones, bipolar cells, and ganglion cells

For rod vision there are four neurons in the direct pathway: rods, bipolar cells, amacrine cells, and ganglion cells

Neural Function of the Retina Neurotransmitters

Rods and cones release glutamateAmacrine cells release: GABA, glucine, dopamine,acetylcholine, and indolamine; all of which are inhibitory

Transmission of Most Signals Occurs in the Retinal Neurons by Electrtonic Conduction, Notby Aps- direct flow of electric current in the neuronal cytoplasm and nerve axons from the point of excitation all the way to the output synapsesNeural Function of the Retina Lateral Inhibition- enhances visual contrast and is a function of the horizontal cells

Fig. 50.13 Excitation and inhibition of a retinal area caused by a beam of light

Neural Function of the Retina Excitation and Inhibition- two sets of bipolar cells provide opposing and inhibitory signals in the visual pathway

Depolarizing bipolar cells

Hyperpolarizing bipolar cells

Neural Function of the Retina Amacrine Cells and Their Functions- 30 typesidentified and the functions of 6 have beencharacterized

Part of the direct pathway for rod visionResponds strongly at the onsetResponds to changes in illuminationMovement of a spot across the retina

Neural Function of the Retina Ganglion Cells and Optic Nerve Fibers

100 million rods, 3 million cones, and 1.6 millionganglion cells (60 rods and 2 cones converge on an individual ganglion cell)Central fovea has 35,000 cones and no rods Greater sensitivity of the peripheral retina to weak lightd.Rods are 30-300x more sensitive to light than cones; 200 rods converge on a fiber in the periphery

Neural Function of the Retina Excitation of the Ganglion Cells

Spontaneous continuous APs in the ganglion cellsTransmission of changes in light intensity- the off-on response

Fig. 50.14 Responses of a ganglion to lightNeural Function of the Retina Transmission of Signals Depicting Contrasts in the Visual Scene: The Role of Lateral Inhibition

Fig. 50.15

Neural Function of the Retina Transmission of Color Signals by the Ganglion Cells

Single ganglion may be stimulated by several cones or by only a fewSome cells may be stimulated by one type but inhibited by anotherImportance of color contrast mechanisms is that the retina itself begins to differentiate colors