Visual System Circuitry

Csilla Egri, KIN 306, Spring 2012

Cuteness. One of the many functions of the visual system

Outline

Retinal circuitry “Surround” receptive fields

Visual pathways Projections to thalamus and cortex

Lesions in visual pathway

2

Retinal circuitry: review of cell types

3

rods and cones synapse on bipolar cells and horizontal cells

horizontal cells make lateral inhibitory synapses with surrounding bipolar cells or photoreceptors

bipolar cells make synaptic connections with ganglion cells and amacrine cells

amacrine cells transmit signals from bipolar cells to ganglion cells or to other amacrine cells

ganglion cells transmit action potentials to the brain via the optic nerve

B&L Figure 8-7

Retinal circuitry: key features4

2 types of bipolar cells On center: hyperpolarized

by glutamate Off center: depolarized by

glutamate Bipolar and horizontal cells

play a role in lateral inhibition Important for increasing

visual contrast Set up “surround”

arrangement of ganglion cell receptive fields

B&L Figure 8-7

Receptive fields5

Photoreceptor receptive fields include retinal area that, when stimulated by light, results in hyperpolarization of individual photoreceptor Small and circular

Ganglion cell receptive field size determined by ganglion cell type degree of convergence of photoreceptors and bipolar cells

and field type by retinal circuitry (lateral inhibition) On-center/off-surround Off-center/on-surround

Where in the retina is there is there a high degree of convergence?

Receptive fields6

On-center/off-surround Light shines on center of

ganglion cell receptive field ganglion cell increases AP firing

Light on surround region decreased AP firing

Off-center/on-surround Light on center decreased

AP firing Light on surround

increased AP firing

B&L Figure 8-8

Neural circuits of retinal receptive fields

7

centresurround surroundGanglion cell

receptive field

P PP

B B

G G

H H

_ _

On-center bipolar and

ganglion cells

Off-center bipolar and

ganglion cells

Neural Circuits of RetinalReceptive Fields

8

Light stimulus on center: ↓ glu release from central

photoreceptor ↓ inhibition of on-center

bipolar cell depolarization ↑ NT release on-

center ganglion cell excited

less glu available to excite off-centre bipolar cell hyperpolarization

↓NT release off-center ganglion cell inhibited

light

Neural Circuits of Retinal Receptive Fields

9

light light

Light stimulus on surround: ↓ glu release from surround

photoreceptor ↓ excitation of horizontal cells

↓ inhibitory NT released ↓ inhibition of central

photoreceptor ↑ glu released

↑ glu hyperpolarizes on-center bipolar cell and depolarizes off-center bipolar cell

On-center ganglion cell inhibited, off-center ganglion cell excited

Retinal receptive fields: outcome

10

Surround arrangement and lateral inhibition allows ganglion cells to respond best to contrast borders in a visual scene Ex. Reading dark letters against a white background Respond only weakly to diffuse illumination

B&L Figure 8-8

Ganglion cell types and projections

11

Lateral geniculate nucleus

Visual pathway

12

Light from binocular zone strikes retina in both eyes

Monocular zone only strikes retina on same side as light

The right visual field is projected to the ___________________ and ___________________ hemiretina

The optic nerves segregate and carry information from ______________________

Each ___________________ crosses at the optic chiasm

The optic tracts carry information from ______________________ to the brain

Right temporal hemiretin

a

Left temporal

hemiretina

Left/right nasal

hemiretina

Optic nerves

Optic tracts

B&L Figure 8-9

Left visual field Right visual field

Visual pathway13

Major projections to the lateral geniculate nucleus in the thalamus, but also to:

Hypothalamus Regulation of

circadian rhythm Pretectum between the

thalamus and midbrain Pupillary light reflex

Superior colliculus in the _________________

Reflex movements of head and eyes towards stimulus

Right and left visual fields project to contralateral hemispheres of the visual (striate) cortex

Lateral geniculatenucleus

14

LGN transmits info to 1º visual cortex (area 17) Gates transmission of signal to cortex

Divided into 6 nuclear layers: 2 magnocellular layers (layers 1-2)

Input from M ganglion cells concerned with location and movement of

visual image (neurons respond to brightness)

4 parvocellular layers (layers 3-6) Input from P ganglion cells Concerned with color and form of image

(cells respond to color contrast) each layer receives input from only one eye

(maintains retinotopic organization)

Kandel Figure 27-6

Primary visual cortex15

LGN neurons representing each eye project to primary visual cortex

Retinotopic map for monocular and binocular visual fields maintained

B&L Figure 8-10

Extrastriate Cortex16

Thalamus projects to layer 4 of primary visual cortex (Broadmann’s area 17, or V1), info processed and sent to diffuse locations in the extrastriate cortex Broadmann’s area 18 (V2) –

analysis of visual meaning Dorsal stream (MT) – spatial

recognition Perception, analysis of visual

scene Ventral stream (V4) object

recognition Action, guided movement and

spatial characteristics of the environment

Monkey brain

Lesions in visual pathway17

Kandel Figure 27-20

Level of lesion can be determined by specific deficit in the visual field

1.Right optic nerve Loss of vision in right

eye1.Optic chiasm

Loss of vision in temporal visual field of both eyes

1.Right optic tract Loss of vision in left

visual field of both eyes

Objectives

After this lecture you should be able to: Describe the components of retinal circuitry

Relate these connections to synaptic transmissions in center surround receptive fields

Trace the pathway from the retina to the primary visual cortex

Describe the structure and function of the lateral geniculate nucleus and its projections List the major functions of projections to the extrastriate

cortex Determine the level of a lesion in the visual pathway

based on a specific deficit in the visual field or visa versa

18

19

1. Axons from the __________________ hemiretina cross at the optic chiasm

2. For the following schematic diagram of the cells of the retina, name each of the cells. Explain how the firing rate of cell (c) is affected by light shining on the surround if this arrangement represents an off centre-on surround receptive field. Include in your answer a description of the events that occur at each synapse involved.

Test your knowledge

a)

b)

c)

d)