VERTEBRATE VISUAL SYSTEM

Readings:

            Dowling-Chap. 14, 15, 16 (also see p. 171-180 & 192-202)

            Kandel-Chap. 25, 26, 27

 

I. Phototransduction and Information Processing in the Retina

Fig. 16.1 (Sheperd)

            -Light is electromagnetic radiation of a specific range of wavelengths: 300 nm

             (violet) to 700 nm (red) (visible light).

 

            -Vision begins with phototransduction by specialized receptor cells in the retina of

             the eye.

 

A. Vertebrate Eye Structure:

Fig. 26-1 (Kandel)

Eye is a complex sensory organ which we will only consider in terms of:

            -the light pathway,

            -phototransduction,

            -signal processing within the retina.

 

1-light pathway: light enters the eye by passing sequentially through:

 

            -cornea: the anterior transparent portion of a tough, connective/epithelial tissue that

                         encloses the eye, the cornea contributes to bending light waves as they enter

                         the eye.

 

            -pupil: the opening that allows light to pass into the eye through the lens, the size of

                         the pupil is controlled by constriction of the iris (a pigmented epithelium

                         enclosing smooth muscle).

 

            -lens: a transparent protein jelly enclosed in a tough capsule, the shape of the

                         lens is controlled by ciliary muscles which attach and hold the lens in place.

                         Light is bent to focus an image on the retina and the degree of bending is

                         controlled by changing lens shape (accommodation: for near vision-maximum

                         lens thickness because ciliary muscle contraction relieves tension on lens).

 

Fig. 26.6-Kandel (14.2-Dowling)

            -retina: layer of neurons that lines the posterior wall of the eye, it contains the

                         photoreceptors and several higher order sensory neurons involved in

                         processing the signals generated by the photoreceptors.

 

                        -Note that the photoreceptors point away from the light and are embedded in a

                         darkly pigmented epithelium which keeps light from being reflected inside the

                         eye. 

                                    neuronal elements:

                                    -photoreceptors (rods & cones) ® bipolar cells ® ganglion cells ® CNS

                                    -horizontal cells & amacrine cells mediate lateral interactions between

                                                the above cells.

 

                        -fovea: the portion of the retina containing the highest density of photoreceptor

                                     cells (all cones).  It represents the region of greatest visual acuity and

                                     is also the center of the visual field.

 

                                    -although there are only 1 million optic nerve axons relaying

                                     information from 100 million photoreceptors (1:100), 70,000 optic

                                     nerve axons carry information obtained from only 35,000

                                     photoreceptors in the fovea (2:1).

 

                        -optic disk: is the site at which the axons of the retinal projection neurons

                                     (ganglion cells) exit the eye to form the optic nerve, hence there are

                                     no photoreceptors in this area (it is the so-called blindspot, this region

                                     of visual field is seen by other eye in animals with binocular vision).

 

 

B. Vertbrate Phototransduction

-Phototransduction is the conversion of light energy to a change in membrane potential of the photoreceptor cell.

 

Fig. 26-2 & Table 28-1, Kandel (7.12-Dowling)

1-rods and cones: vertebrates may possess two classes of photoreceptors.

            -both have relatively similar structural organization:

                        -outer segment that mediates actual phototransduction.

                        -synaptic terminal which is the site of neurotransmitter secretion (postsynaptic

                                     cells are the bipolar cells and horizontal cells).

 

            -In general:

                        -rods mediate high sensitivity, low acuity vision (such as night vision), are

                                     absent from the fovea in animals possessing cones.

                        -cones mediate low sensitivity, high acuity vision as well as color vision

                                     (predominantly localized to the fovea).

 

            -Physiology (both rods and cones):

Box 26-1, Kandel (7.13 & 7.15-Dowling)

                        -in the dark, photoreceptors are depolarized by an inward nonselective cation

                                     current in the outer segment that is maintained by cyclic GMP (cGMP)

                                     binding to these cation channels (cGMP-gated channels).

 

                                    -an outward K+ current in the inner segment opposes the depolarizing

                                     effect of the inward current and an active Na/K pump in the inner

                                     segment maintains the intracellular concentrations of Na+ and K+.

 

                                    -in the dark, the cell is depolarized by the inward current and

                                     continuously releases neurotransmitter (glutamate) onto postsynaptic

                                     cells (bipolar cells, horizontal cells).

 

                        -in response to light, phototransduction shuts the cyclic GMP-gated channels

                                     by breaking down cGMP, the photoreceptors are repolarized by the

                                     K+ current, and neurotransmitter secretion ceases.

 

 

2-Phototransduction (main players):

Fig. 26-3, Kandel (8.5-8.7-Dowling)

            -outer segment membranes contain:

 

                        -light absorbing photopigment called rhodopsin (consists of a protein (opsin)

                         and and organic molecule, retinal (derived from vitamin A).  Color vision is

                         due to expression of different types of opsins which shift the spectral

                         sensitivity for light absorption and, thus, light excitation.

 

                                    -note this opsin molecule is structurally homologous to seven

                                     transmembrane receptors coupled to heterotrimeric G proteins.

 

Fig. 26-4, Kandel (7.17-Dowling)

                        -G-protein called transducin (a heterotrimeric G-protein) is activated by

                         rhodopsin:

draw:

 

 

                        -cyclic GMP phosphodiesterase (PDE) which breaks down cyclic GMP to

                         GMP when activated by binding to GTP-alpha G protein subunit.

draw:

 

 

                        -guanylate cyclase (GC) which produces cyclic GMP.

draw:

 

 

                        -cyclic GMP-gated nonselective cation channel in the plasma membrane.

draw:

 

 

            -Mechanism (note signal amplification that occurs at each stage):

 

                        1-retinal absorbs a photon, changes conformation, activating rhodopsin.

 

                        2-activated rhodopsin binds and activates transducin, exchanging GTP for GDP

                                     on transducin (one rhodopsin activates 500 transducins).

 

                        3-α-GTP subunit of transducin binds and activates cGMP phosphodiesterase

                                     (cGMP PE) (one transducin activates one cGMP PE).

 

                        4-cGMP PE breaks down cGMP, lowering [cGMP] and closing cGMP-gated

                                     channels (one cGMP PE breaks down about 1 million cGMP

                                     molecules, closing hundreds of channels).

 

                        5-closing cGMP-gated channels allows membrane potential to hyperpolarize

                                     and shuts off Ca++ influx and neurotransmitter secretion at the

                                     synaptic terminal.

 

            -Termination:

                        -α subunit hydrolyzes GTP to GDP, ending activation of cGMP-dependent

                                     PDE.

                        -guanylate cyclase replenishes cGMP, cGMP binds and opens channels.

                        -many more aspects to this relatively simplistic scheme.

 

 

C. Signal processing in the retina

back to

Fig. 26-6 Kandel (14.2-Dowling)

            -Action potentials in the ganglion cells represent the response of the retina to visual stimulation that is communicated to the CNS.  Therefore the response properties of the ganglion cells are of primary interest.

 

            -Importantly, ganglion cells do not behave like photoreceptors which essentially respond in proportion to intensity of light impinging upon them.  Instead, ganglion cells are responsive to differences in light intensity within their receptive fields and thus represent the beginning stage of image recognition.

 

In this regard:

            -Uniform illumination of the retina is a very poor stimulus for ganglion cells which usually exhibit a tonic rate of action potential production in the dark.  Instead a small spot of light (0.2 mm in diameter) is a very effective stimulus if it falls within the receptive field of a particular ganglion cell.  However such a stimulus can be either excitatory or inhibitory for the ganglion cell.

 

Fig. 26-7, Kandel (14.4 Dowling)

1-ganglion cell receptive fields exhibit a center/surround antagonistic organization, each

             cell having either of two basic patterns:

 

            -on-center/inhibitory surround receptive field:

                        -these cells are stimulated when a spot of light falls on the center of their

                         receptive field.  Stimulation is maximal when the light completely covers the

                         central (excitatory) portion of the receptive field.

 

                        -when the spot of light falls on the surround, tonic activity is inhibited.

                         Maximal inhibition is attained by a ring of light which fills the surround

                         portion of the receptive field.

 

            -off-center/excitatory surround receptive field:

                        -these cells are inhibited when a spot of light falls on the center of their

                         receptive field.  Inhibition is maximal when the light completely covers the

                         central portion of the receptive field.

 

                        -when the spot of light falls on the surround, the cells are stimulated.

                         Maximal stimulation is attained by a ring of light which fills the surround

                         portion of the receptive field.

 

 

2-Retinal circuitry mediating ganglion cell center/surround receptive fields (for cone photoreceptor input).

 

Box 26-3 Kandel (14.9 Dowling)

            -bipolar cells are the primary source of stimulation for ganglion cells.  These cells do

             not generate action potentials, as in photoreceptors, neurotransmitter secretion is

             graded in proportion to membrane potential.

 

            -bipolar cells receive synaptic contacts from the cones but also exhibit center/surround

             receptive fields:

 

                        -on-center/off-surround bipolar cells are stimulated by light stimulation of

                         the cones, 

 

                        -glutamate released from the cone inhibits the bipolar cell (via G-protein

                         coupled receptor causing either activation of K+ channels in some or closing

                         nonselective cation channels in others).  Thus, the bipolar cell depolarizes

                         when light shuts off cone neurotransmitter secretion, stimulating bipolar cell

                         secretion (glutamate) onto on-center ganglion cells and stimulating them.

 

 

                        -off center bipolar cells are inhibited by light stimulation of cones,

 

                        -glutamate released from the cone excites the bipolar cell (via ligand-gated

                         channel (AMPA-type).  Thus, the bipolar cell hyperpolarizes when light shuts

                         off cone neurotransmitter secretion, reducing bipolar cell stimulation of

                         the off-center ganglion cells.

 

            -horizontal cells (also non-spiking) mediate ganglion cell/bipolar

             cell receptive field surround effects (lateral effects).

 

                        -horizontal cells transfer information from distant cones to "center" bipolar

                         cells by inhibiting cone (and bipolar cell) neurotransmitter secretion (via

                         neurotransmitter glycine).

 

                                    -this pathway is a little confusing, example shown is for "on-center"

                                     bipolar cell:

 

                                                            -a horizontal cell receives excitatory (glutamate) input

                                                             from cones in the "surround" area of the bipolar cells

                                                             receptive field, and forms inhibitory (glycine) synapses

                                                             with cones in the "center" area of the bipolar cell

                                                             receptive field.

 

                                    when light on the surround cone-

                                                            -surround cones are hyperpolarized by the light,
                                                                       inhibiting secretion and horizontal cell

                                                                         stimulation.

                                                            -horizontal cell hyperpolarizes, inhibiting its secretion.

                                                            -inhibition of the "on-center" cones is relieved, allowing

                                                                         these cones to depolarize (they are in the dark)

                                                                         and inhibit "on-center" bipolar cells, in turn,

                                                                         reducing bipolar cell stimulation of

                                                                         "on-center" ganglion cells (inhibition).

 

            (Kandel and Dowling texts disagree on the role of horizontal cell to bipolar cell

             synapses!)

 

                        -some types of amacrine cells transfer information transfer information from

                         distant bipolar cells to ganglion cells.  Amacrine cells contribute to temporal

                         response properties of the ganglion cells (phasic vs. tonic responses to

                         stimulation).

 

 

3. Other classifications of ganglion cells exist and are based on response properties and size of receptives fields (both on-center & off-center types in each class):

                        M cells (or X cells) have large receptive fields and tend to respond transiently to sustained stimulation
                        and project to the magnocellular layer of the LGN (see below).  These neurons are thought to be mainly
                         involved in motion detection.

 

                        P cells (or Y cells) have much smaller receptive fields, are color sensitive, and project to the parvocellular
                         layers of the LGN (see below).  These neurons are thought to be involved in mediating high visual acuity.

 

 

 

II. Central Visual Pathways

Fig. 27.1 Kandel (16.1 Dowling)

A. Visual field is the image of that area of space falling on the retina of both eyes when held in fixed position.

 

Fig. 29-3, Kandel

            -Recall the lens inverts the image of the visual field on the retina.

 

            -in binocular animals (not animals with two eyes!) there is considerable overlap of

             visual space falling on the two retinas.  Binocular animals control their eye

             movements to maintain a the same visual field center on the fovea of each eye.

 

            -Thus, the visual field contains:

                        -a binocular zone seen by both eyes,

                        -monocular zones seen only by the eye on that side, these zones fall on the

                                     nasal portion of each retina.

 

            -Ganglion cell central projections:

                        -axons of ganglion cells on the lateral (temporal) portion of the retina project

                                     to the same side of the brain.

                        -axons of ganglion cells on the nasal portion of the brain cross over at the

                                     optic chiasma to project to opposite side of the brain.

 

                        -thus, ganglion cells from both eyes representing the same regions of visual

                                     field project together (left brain sees right visual field, right brain sees

                                     left visual field).

 

 

B. Ganglion cell axons project via the optic nerve (cranial nerve II) to four subcortical locations in the CNS (mammalian):

Fig. 27-4

            -projections to the pretectal area (junction of the midbrain & thalamus) mediate

                         pupil reflexes (control the diameter of the pupil and amount of light

                         entering the eye).

 

            -projections to superior colliculus (midbrain) provide sensory information used to

                         direct eye/heads movements towards a stimulus.

 

            -most ganglion cells project to the lateral geniculate nucleus (LGN) located in the

                         thalamus.  These cells provide sensory information for conscious visual

                         perception.

 

            -most recently (not shown), a special group of ganglion cells project to a hypothalamic

                         nucleus (the suprachiasmatic nucleus, located just above the optic chiasma).

                         These ganglion cells are themselves light sensitive and transmit light:dark

                         information to the circadian clock neurons found in this nucleus.

 

 

C. Organization of lateral geniculate nucleus (LGN).

 

Fig. 27-6 Kandel (16.2 & 16.4 Dowling)

            -LGN contains interneurons organized into distinct layers, the numbers of layers

                         varies from one species to the next (6 in humans)

 

            -Input from each eye is segregated, each layer receives input from only one eye

                         (monocular input).

 

            -Layers are also distinguished by LGN cell type:

                        -the two ventral most layers (magnocellular layers) contain large cell bodies

                                     and receive input from "M-type" ganglion cells.

                        -the four more dorsal layers (parvocellular layers) contain smaller cells and

                                     receive input from "P-type" ganglion cells.

 

            -The spatial relationships of ganglion cells in the retina is preserved in their

                         projections to the LGN, thus LGN neurons also form topographic maps of

                         visual field.

 

            -LGN neurons exhibit center/surround receptive fields and response properties similar

                         to that of the ganglion cells that innervate them.

 

            -LGN neurons project to the primary visual cortex in a highly ordered fashion.

 

 

D. Primary visual cortex:

            -LGN neurons project to Brodman's area 17 in the occipital lobe of the cerebral

             cortex, also called primary visual cortex or striate cortex.

Fig. 27-9

            -the topographic map of visual space in the LGN is projected onto the primary visual

                         cortex, such that visual cortex contains a map of contralateral visual space,

                         with representation of the occupying the largest portion of visual cortex.

 

 

-Gray matter of visual cortex is organized into layers, each layer containing one or more    specific cell types:

Fig. 27-10 Kandel (16.5 & 16.6 Dowling)

                        -pyramidal cells are projection neurons and are found in layers 3, 4, 5 and 6.

 

                        -several types of stellate cells function as local interneurons.

 

                        -LGN neurons synapse with stellate neurons in layer 4 of the visual cortex.

                                     Note that P and M type LGN cells have slightly different projections.

 

-Response properties of cortical neurons:

David Hubel and Torsten Wiesel pioneered studies of the properties and organization of neurons in the visual cortex throughout the 1960s and 1970s (received Nobel prize in 1981).

 

            1-Receptive field properties of cortical cells were found to be substantially different

             from those ganglion cell and LGN neurons-exception is the stellate cells of layer 4

             which still exhibited circular center/surround receptive fields.

 

Fig. 27-11 Kandel

            -this figure illustrates how the receptive field and its properties can be mapped.

 

            -Two main classes of cortical neuron receptive fields were found:

Fig. 27-12 Kandel (15.3, 15.4 & 15.9 Dowling)

                        1-simple cells exhibit rectangular receptive fields with center/surround

                         characteristics, the optimal stimulus is a bar of light which covers the "on"

                         portion of the receptive field.  Several variations of this pattern are observed.

 

                                    -one feature is orientation selectivity (the angle of the light bar).

 

                                    -the size of the receptive field is about 6 degrees of visual field.

 

                                    -simple cell receptive fields are attributed to input from several adjacent

                                     and identical "on/off" ganglion cell receptive fields (LGN and stellate,

                                     also).

 

figure 27-13 Kandel (15.5, 15.6 & 15.10 Dowling)

                        2-complex cells exhibit much larger receptive fields which lack definite

                         center/surround antagonism.  The optimal stimulus is a wide bar of light

                         which covers about half the field, better yet is a bar of light moving across

                         the field in a specific direction.

 

                                    -like simple cells, complex cells are highly sensitive to orientation of

                                     the light bar.

 

                                    -complex cell receptive fields are attributed to input from several

                                     simple cells with adjacent receptive fields and identical orientation

                                     preference.

 

 

-Higher order organizational features of primary visual cortex:

            -cortex is functionally organized into alternating columns in which cortical neurons

             preferentially respond to input from one eye or the other (ocular dominance

             columns), stimulus orientation (orientation columns), and color perception (blobs).