VISUAL CORTEX

 

The visual cortex consists of the primary visual area (Brodmann's area 17) and the secondary visual area (Brodmann’s area 18 & 19).

Primary Visual Area (Area 17):

The primary visual area occupies largely the medial aspect of the occipital lobe.

It is present around and in the calcarine sulcus, with extensions into the cuneus and lingual gyrus.

Posteriorly it extends into the occipital pole, limited by the sulcus lunatus.

A small portion of variable size extends onto the posterolateral aspect of the pole.

Anteriorly the area extends forward above the calcarine sulcus as far as the parieto-occipital sulcus; below the calcarine sulcus, it extends forward a little farther.

On sectioning a fresh brain, the primary visual cortex can be recognized by its thinness and the characteristic presence of a white line or stria (of Gennari) in the gray matter (hence the term striate cortex or area striata). The white line is formed in the fourth layer of the cortex by the presence of myelinated fibers from the optic radiation and association fibers.

In the posterior part of the calcarine sulcus, the stria (lines) appear above and below the sulcus. But in the posterior part they are seen in the part of the cortex below the sulcus.

The striate cortex is bound by the cuneate sulcus above and the collateral sulcus below.

The area of the striate cortex is about 3000mm².

The shape is of an elongated ovoid, the narrow end being close to the splenium of the corpus callosum.

The cerebral cortex (except the allocortex of the hippocampal formation, or archipallium) has a laminar arrangement of neurons and their processes.

The general structure of the visual cortex resembles the primary sensory cortex in having six layers.

The layers of the visual cortex are as follows:

I. Plexiform lamina: external layer; dense interviewing of neurites (axons & dendrites) or neuropil, derived from intrinsic cortical neurons (mostly stellate cells) and pyramidal cells. There are also association fibers from other parts of the cortex.

II. External granular lamina: Has large number of nuclei visible (hence, granular). Contains bodies of neurons.

III. Pyramidal lamina: Contains “pyramidal” nucleus. Stellate interneurons with processes oriented vertically (fusiform cells) and horizontally (‘basket cells’).

The layers I-III are also called “SUPRAGRANULAR LAYERS”.

IV. Internal granular lamina: It is thin and contains stellate interneurons and few pyramidal cells. There is condensation of horizontal processes (known as the EXTERNAL BAND OF BAILLARGER). This layer is subdivided into IVA, IVB, IVC. This layer has the most densely arranged cells of any cortical area. This area is also most wide among all other cortical areas.

V. Ganglionic lamina: Contains stellate and pyramidal cells. This layer has the solitary neurons of Meynert, which pyramidal neurons and the largest seen in any cortical area. The cell bodies project to the superior colliculus and possibly to ocular motor nuclei. Interspersed between the neuronal cell bodies is a dense neutropil of dendrites and axons, extending to other levels of the cortex.

VI. Multiform lamina: Consists of small neurons mostly of ‘granular’ or stellate interneuron type with few pyramidal cells. Some of the latter (Martinotti neurons) send a long centrifugal axon into the plexiform layer and vertically arranged dendrites ramify into deeper layers of the cortex.

Layers V and VI are also called the “INFRAGRANULAR LAYERS”.

The striate cortex is the main receptor area for the optic radiation coming from the lateral geniculate nucleus, but other SECONDARY VISUAL AREAS, such as the surrounding peristriate and parastriate zones (area 18 & 19 of Brodman) also receive such fibers directly or through connections from the striate cortex (area 17). The cells in laminae II and III especially project to the secondary visual area.

The primary visual cortex receives the optic radiation fibers and the right half of the field of vision is represented in the visual cortex of the left cerebral hemisphere. The cells of lamina V project to the superior colliculus; those of lamina VI, to the lateral geniculate body.

The superior retinal quadrants (inferior field of vision) pass to the superior wall of the calcarine sulcus, while the inferior retinal quadrants (superior field of vision) pass to the inferior wall of the calcarine sulcus.

The macula lutea, which is the central area of the retina and the area for most perfect vision, is represented on the cortex in the posterior part of area 17 and accounts for one-third of the visual cortex. The peripheral parts of the retina are represented more anteriorly.

PHOTORECEPTORS; RETINAL GANGLION CELLS

 

The retina and the optic nerve form the proximal part of the visual pathway.

The visual pathway is made up of the retina, optic nerves (ON), optic chiasma, optic tracts, lateral geniculate nucleus (LGN), optic radiations and visual cortex.

Some other areas of the cortex are also associated with vision such as the frontal eye fields.

The retina is composed of three superimposed neurons that establish a connection with each other. The outer-most neuron is the photoreceptor. The second neuron, the bipolar cell, is in the nuclear layer. The third or internal neuron is the retinal ganglion cell (RGC).

The cell bodies (soma) of the RGCs are located in the ganglion cell layer (GCL), between the retinal nerve fibre layer (NFL) and the inner plexiform layer.

Their axons form the retinal NFL and synapse with neurons in the LGN of the thalamus.

There are up to seven layers of RGC cells in the central retina or fovea (60–80 μm thickness) and a few as one cell layer in the peripheral retina (10–20 μm).

There are between 500.000 and 1.2 million RGCs per retina and approximately 100 rods and 4–6 cones per RGC.

The axons form criss-crossed bundles which are separated and ensheathed by glial cells. The bundles leave the eye to form the optic nerve (ON). Upon existing through the lamina cribrosa, the axons become myelinated with oligodendrocytes.

The ganglion cell complex (GCC) represents the combination of three layers: the NFL, GCL and inner plexiform layer. These layers contain, respectively, the axons, the cell bodies and the dendrites of the ganglion cells.

A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction.

The photoreceptors convert light (visible electromagnetic radiation) into signals that can stimulate biological processes.

There are currently three known types of photoreceptor cells in mammalian eyes: rods, cones, and intrinsically photosensitive retinal ganglion cells.

Rods primarily mediate scotopic vision (dim conditions) whereas cones primarily mediate photopic vision (bright conditions).

A third class of mammalian photoreceptor cell was discovered during the 1990s, known as the intrinsically photosensitive retinal ganglion cell. These cells are do not contribute to visual impulses directly, but have a role in circadian rhythm and pupillary reflex.

The photoreceptors contain certain proteins to enable phototransduction.

The membranous photoreceptor protein opsin contains a pigment molecule called retinal. In rod cells, these together are called rhodopsin.

In cone cells, there are different types of opsins that combine with retinal to form pigments called photopsins. Three different classes of photopsins in the cones react to different ranges of light frequency, a differentiation that allows the visual system to calculate color.

The function of the photoreceptor cell is to convert the light information of the photon into a form of information communicable to the nervous system and readily usable to the organism. This conversion is called signal transduction.

The opsin found in the intrinsically photosensitive ganglion cells of the retina is called melanopsin. These cells are involved in various reflexive responses of the brain and body to the presence of (day)light, such as the regulation of circadian rhythms, pupillary reflex and other non-visual responses to light. Melanopsin functionally resembles invertebrate opsins.

The distribution of rods and cones (and classes thereof) in the retina is called the retinal mosaic. Each human retina has approximately 6 million cones and 120 million rods.

The number and ratio of rods to cones varies among species, dependent on whether an animal is primarily diurnal or nocturnal.

 

OPTIC NERVE HEAD; OPTIC DISK

 

• The optic disk or the Optic Nerve Head (ONH) is the distal portion of the optic nerve.

• It extends from the retinal surface to the beginning of the myelinated portion of the optic nerve posterior to the lamina cribrosa (LC).
• The ONH can be regarded as a round or oval plughole containing a sieve-like sheet known as the LC through which the RGC axons pass through.
• There are no rods or cones overlying the optic disc, and so it corresponds to the blind spot in each eye.
• The LC inserts and anchors itself into the parapapillary scleral connective tissue which provides substantial support to counteract IOP.
• The LC comprises of multiple ‘‘plates’’ of connective tissue which includes several types of interstitial collagen, proteoglycans and elastin.
• The posterior or scleral portion of the LC contains significantly higher amounts of collagen compared to the anterior or choroidal part of the LC. However, the anterior portion of the LC contains large numbers of astrocyte processes rich in glial fibrillary acidic protein (GFAP). These processes surround the bundles of axons passing through the LC.
• The fibrous astrocyte processes contain bundles of intermediate filaments and form the borders of the canals in the LC. The intermediate filaments provide tensile strength to the canals.
• The canals vary in diameter from 25-250µm. These canals are further subdivided into smaller compartments measuring 2.5-5.0µm by webs of smaller astrocytic processes.
• The LC region can be regarded as a specialized extracellular matrix with fenestrated sheets of connective tissue and occasional elastic fibers. The extracellular matrix components of the LC, such as collagen types I-VI, laminin and fibronectin, have characteristics which are different from those in the sclera. This could be a significant biomechanical factor in the development of GOA.
• The retinal nerve fibers pass through the LC, become aggregated to form the optic nerve behind the eyeball and travel towards the brain.
• The retinal nerve fibers from all over the retina converge towards the ONH.
• Thus, nearly 1-2 million afferent nerve fibers pass out through the LC posteriorly and form the optic nerve.

• The central retinal artery and vein also occupy this space.
• The optic disc is placed 3 to 4 mm to the nasal side of the fovea. It is a vertical oval, with average dimensions of 1.76mm horizontally by 1.92mm vertically.
• The center of the ONH has a depression of variable size, known as the optic cup.