Of the Human Visual Pathway

U. Schiefer and W. Hart

Nearly a half of all cortical neurons are devoted to the processing of visual information. The afferent visual pathway from the retina to the primary visual cortex has four neuronal elements (■ Fig. 3.1).

■ First neuron: photoreceptors

■ Second neuron: bipolar cells

■ Third neuron: retinal ganglion cells (and their axonal processes, including the chiasm and optic tracts)

■ Fourth neuron: geniculocalcarine neurons

Primary Visual Pathways

First & second neurons

Third neuron

Retina

Optic nerve, chiasm, optic tract

Fourth neuron

Optic radiations

Visual cortex Area 17/ V,

Surface area: 12 cm

About 60 M -rods

Diameter: 4-5 mm

About 3.2 M cones

About 1.2 M axons

5 M axons

Surface area of the »cerebral retina« is about 30 cm2

500 M neurons

More than 10 M bipolar cells

Fig. 3.1. Schematic diagram of the human visual pathways and their neuronal components. LGB Lateral geniculate body (modified after Krey et al. 1986; see "Further Reading")

Visual Pathway Bipolar Cells
Fig. 3.2. Visual acuity, cone density and differential light sensitivity (DLS) as a function of visual field eccentricity (modified from Coren and Ward 2003, and Trauzettel-Klosinski et al. 1994; see "Further Reading")

First Neuron: Photoreceptors

The retina contains across its outer surface (about 12 cm2) nearly 65 million photoreceptors per eye (see also Chap. 7): about 3.2 million cones and 60 million rods.

The areal density of photoreceptors falls rapidly from the fovea into the retinal periphery (■ Fig. 3.2). Efficient perimetric stimulus presentation considers these factors, using more closely spaced stimuli at the visual field center, with a rapidly decreasing density of stimuli for more peripherally located visual field areas. But even in the most peripheral parts of the retina, there are sufficient numbers of cones to dominate vision under photopic levels of illumination when the rods are completely bleached.

^ Pearl

Most of today's perimeters operate with an adapting background luminance of 3 to 10 cd/m2 in the lower photopic range, and consequently, test the function of cone-initiated vision only.

Second Neuron: Bipolar Cells

In the human retina, "only" 10 million bipolar cells (see also Chap. 7) process the signals arriving from the approximately 65 million photoreceptors. The neural convergence found at this level of retinal circuitry is not homogenous: While the peripheral retinal regions operate with a comparatively sparse population of bipolar cells, the central portions of the retina (foveal and perifoveal macula) process the photoreceptor signals in a 1:1 or cell-for-cell arrangement. In other words, while there is high neural convergence in the retinal periphery, there is a parallel processing of the signals from the densely clustered receptors at the fovea and perifoveal macula.

Third Neuron: Retinal Ganglion Cells

The retinal ganglion cells give rise to axons that are about 75 mm in length (see Chaps. 8 and 12). They join one another at the optic disc to form the optic nerve, being myelinated only in their extraocular course. They pass through the optic chiasm with decussation of more than one half of the fibers to the contralateral side, and pass through the optic tracts to the lateral geniculate body, where they terminate.

Retinal Ganglion Cells and the Optic Nerve

The neuronal signals are concentrated into "merely" 1.2 million ganglion cells (per eye). Their axons form the retinal nerve fiber layer, just deep to the internal limiting membrane. They are characterized by a widely fanned-out shape that skirts the macula, and they then converge at the margin of the optic disc. Their spatial arrangement gives rise to a typical pattern when disease damages associated groups of fibers at the disc margins: The fibers arriving at the temporal sectors of the disc margin arise from cell bodies located either above or below the temporal horizon tal raphe, which they "respect," or do not cross, but form superior and inferior arcuate shapes that converge as they approach the disc.

T| Pearl

Damage to the retinal ganglion cells in the vicinity of the optic disc produces typical arcuate defects in the visual field that do not cross the nasal horizontal meridian, hence the so-called nasal step. The shape of an arcuate scotoma is similar to that of a scimitar; its hilt located at the physiologic blind spot and its tip at the nasal horizontal meridian. The fibers arriving at the disc from the temporal hemiretina (both superior and inferior arcuate groups) cross the borders of the disc at the superior and inferior poles. Bundles of fibers arriving from the nasal hemiretina form a more wedge-like shape with straight sides and the apex located at the nasal disc border.

The ganglion cell axons join one another to exit the eye through the optic disc, which is about 1.5 mm in diameter. In doing so, the fibers from the retinal locations that are closest to the disc rise to the retinal surface and enter the disc at its most central core. Fibers that originate in the retinal periphery, by contrast, course through the innermost portions of the nerve fiber layer, closest to the vitreous body. They exit the eye through the outermost portions of the optic disc, closest to its border (■ Fig. 3.3). Upon exiting the ocular wall through the lamina cribrosa, the axons acquire a myelin sheath, and the diameter of the nerve increases to 4 mm. The fibers are separated into about 300 to 1,000 bundles by connective tissue septae. The optic nerve has a floppy, sinusoidal course within the orbit, which allows the globe to rotate at high speeds and with minimal inertia. The optic nerve has an intraorbital length of 20 to 30 mm, an intracanalicular length of 3 to 8 mm, and an intracranial length of 3 to 16 mm. At the posterior extreme of the optic nerve, the afferent visual pathway acquires a new name, the chiasm, as the optic nerves merge with one another.

T| Pearl

The optic nerve is particularly susceptible to damage by space-occupying lesions within the optic canal. Masses of any kind arising in the canal will compress the nerve, and the rigid walls prevent any escape or decompression of the neural tissues.

Within the retina, the axons arising within the temporal hemiretina follow an arcuate path that skirts the macula above and below the horizontal meridian with the axons respecting (not crossing) the horizontal meridian. As they

Arrangement Fibres Optic Nerve

Fig. 3.3. Schematic diagram of the pattern created by the layering of the ganglion cell axonal nerve fibers in the nerve fiber layer of the retina and the corresponding visual field regions (upper insert) as well as the structures within the optic nerve and the axoplasmic flow parameters (modified from Krey et al. 1986; see "Further Reading")

Fig. 3.3. Schematic diagram of the pattern created by the layering of the ganglion cell axonal nerve fibers in the nerve fiber layer of the retina and the corresponding visual field regions (upper insert) as well as the structures within the optic nerve and the axoplasmic flow parameters (modified from Krey et al. 1986; see "Further Reading")

pass into the optic nerve, the ganglion cell axons acquire a completely different organization, as they separate from one another according to their origins with respect to the vertical meridian. Axons from cells located to the nasal side of the vertical meridian (remember: the vertical meridian through the fovea, not the optic disc) decussate at the chi-asm, while axons from cells located in the temporal hemiretina remain ipsilateral. The tight, septate axon bundles so highly organized in the optic nerve undergo reorganization at this point in the afferent pathway. They intermix in what seems to be a relative disorganized fashion, giving rise to a loss of the topographic organization of the visual field found with diseases of the retina and optic disc. Consequently, the perimetric spatial localization of disease is very poor for lesions of the intracanalicular optic nerve.

Supplements For Diabetics

Supplements For Diabetics

All you need is a proper diet of fresh fruits and vegetables and get plenty of exercise and you'll be fine. Ever heard those words from your doctor? If that's all heshe recommends then you're missing out an important ingredient for health that he's not telling you. Fact is that you can adhere to the strictest diet, watch everything you eat and get the exercise of amarathon runner and still come down with diabetic complications. Diet, exercise and standard drug treatments simply aren't enough to help keep your diabetes under control.

Get My Free Ebook


Responses

  • azzeza
    What is the temporal hemiretina?
    6 years ago
  • karen
    Is the optic nerve afferent pathway?
    6 years ago
  • AMANDA
    What is the function of the nasal hemiretina?
    6 years ago
  • findlay
    How fibres are organized within the optic nerve?
    6 years ago
  • Ruaridh
    What percentage of the axons in your optic nerve crosses at the optic chiasm?
    6 years ago
  • cyril
    What is the human hemeretina?
    6 years ago

Post a comment