The Visual System

The hypothalamus | Main Anatomy Index | Central visual pathways

• Almost 1/2 of our cerebral cortex is devoted to the processing of visual information.
• Compared to most mammals, humans have sharper vision (acuity), a better sense of depth (binocularity) and superior colour sensitivity (trichomacy).

Click here for the Eye and Orbit page (First Year Gross Anatomy, ANAT 1006)

Click here for a diagram of the eye.

• This is an air-fluid interface and is the main site of refraction.

• This is constricted by the sphincter pupillae muscle (parasympathetic).
• It is dilated by the dilator pupillae muscle (sympathetic).

• The ciliary muscles attach to the suspensory ligaments.
• They contract during accommodation to allow the lens to return to a more spherical shape.

• This is naturally elastic and it is contained within a fibrous capsule.
• It is flattened by the tension of the zonular ligaments at the equator.
• During accommodation, the anterior part of the lens bulges forward.

• The lens has a slight yellow-colour and is involved in ultraviolet absorption.

• This constitutes about 70% of the volume of the eye.
• It maintains the shape of the eye and also the clarity of the light path.

• This contains the sensory epithelium of the eye.

• This is the vascular net that supplies the photoreceptors.

• This is the white protective coat of the eyeball.
• It is continuous with the dural sheath of the optic nerve.

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• The retina has a two-part structure.
• The outer portion, adjacent to the choroid, is the retinal pigment epithelium.
• The inner portion, adjacent to the vitreous humour, is the neural retina.

• Under normal conditions there is no space between the pigment epithelium and the neural retina in the adult.
• However, mechanical connections between the two are not very strong.
• Under certain circumstances this potential space opens causing retinal detachment.
• Retinal receptors are metabolically dependent on pigment epithelial cells, so detached areas stop working.

Click here for a diagram of the retinal layers. 

• In naming these layers, the term nuclear refers to cell bodies and the term plexiform to synaptic zones.

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• Simplified pathways of the retinal neurons (starting peripherally):

• Axons of the ganglion cells leave the eye as the optic nerve (CN I).
• The processes of the amacrine cells spread laterally and interconnect bipolar cells, ganglion cells, and other amacrine cells.

• Rods and cones are the two different types of vertebrate photoreceptor.
• Each consists of an outer segment, an inner segment, a cell body, and a synaptic terminal.

• Photons traversing the outer segment must pass through hundreds or thousands of sheets of membrane, each full of visual pigment molecules.
• The outer segment is the site of visual transduction.
• Photons absorbed here cause a receptor potential that spreads to the rest of the cell.
• Note that the photosensitive portion is located in the part of the neural retina that is farthest removed from the incoming light.

• This part contains, among other organelles, a very prominent collection of mitochondria.
• These are thought to supply the energy necessary for processes associated with transduction and for the synthesis of visual pigments.

• The outer segment of a rod is relatively long and cylindrical.
• They are specialised for night vision.
• These cells contain rhodopsin, and are maximally sensitive to the blue-green part of the visible spectrum.
• They can be activated by a single photon.

• Rods become saturated (fully hyperpolarised) by light levels brighter than those at dusk.
• To increase sensitivity, 10-100 rods converge their input onto a single bipolar cell (via spherules).
• The trade-off is lower acuity.

• These are specialised for day vision and they are activated by light levels that are brighter than those at dusk.
• Cones are concentrated at the fovea.
• They have a low degree of convergence onto bipolar cells (via pedicles) and they provide high acuity.
• Thus, acuity is highest at the centre of our visual field.

• Humans have 3 types of cone pigment that are maximally sensitive to either red, green or blue.
• Perception of colour is based on a comparison between the sensitivity to a spot of light by adjacent cones.
• A minimum of 2 photopigments is required for colour perception.

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• This layer contains the cell bodies of the ganglion cells, whose dendrites ramify in the inner plexiform layer and whose axons leave the eye as the optic nerve.
• This cell layer is considerably thinner than either the outer or the inner nuclear layer.
• This reflects the fact that there are about 5 million cones and about 100 million rods but only about 1 million ganglion cells.

• There is a good deal of convergence involved in retinal processing.

• There are several ganglion cell types:

• These comprise about 80% of all ganglion cells.
• They receive input from cone bipolars.
• These have small dendritic trees (i.e., small receptive fields) and slowly conducting nerves.

• They project to the parvocellular layers of the dorsal lateral genicular nucleus (LGNd).
• These cells serve:

1. Colour vision;
2. Acuity;
3. And feature detection.

• These comprise about 10% of all ganglion cells.

• They receive input from both rod and cone bipolar cells.
• They have large receptive fields and rapid conduction axons that project to the superior colliculus and magnocellular layers of the LGNd.

• These cells serve:

1. Movement and flicker detection;
2. Stereopsis;
3. Analysis of form;
4. And night vision.

• These comprise of about 10% of all ganglion cells.
• They receive input from both rod and cone bipolars.

• They project to the koniocellular laminae of the LGNd and also to the superior colliculus, pretectum, and hypothalamus.

• These cells serve subconscious aspects of visual processing:

1. Circadian rhythms;
2. And pupillary reflexes.

• These reduce the response of the cone photoreceptors surrounding a spot of light.
• This increases the contrast.

• These relay rod bipolar input to the ganglion cells.

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• There are no photoreceptors, interneurons, or ganglion cells at the optic disk.
• It is here that the axons of ganglion cells leave the eye to form the optic nerve.

• Since there are no photoreceptors here, we are blind to any object whose image falls on this part of the retina.
• We usually have no awareness of this blind spot, however, as our nervous system simply "fills it in."

• This begins near the lateral edge of the optic disk.
• The macula lutea (L. yellow spot) is a circular portion of the retina, about 1 cm in diameter.
• Many cells here contain a yellow pigment (hence its name).

• This is in the centre of the macula.
• It is a depression about 1.5 mm in diameter and is particularly rich in cones.

• In the central part of the fovea is a pit, only about 350 um across, which contains only elongated cones (no rods) and is directly in line with the visual axis.
• This is specialised for vision of the highest acuity.

• All the neurons and capillaries that are present elsewhere are collected around the edges of the fovea.

• The packing density of cones decreases sharply outside the fovea.

• That of the rods increases, reaching a maximum just outside the macula.

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