What is the optic pathway?

The optical pathwayalso known as visual pathway either visual systemis the transmission network of nerve impulses from the retina to the brain.

The optic pathway is made up of photoreceptor cells (rods and cones), the neurons of the retina and the optic nerves that carry information to the visual area of ​​the brain, located in the cortex of the occipital lobe (visual cortex).

Optical Pathway Overview

In the rods and cones, excitable cells located in the retina, light strikes and produces a chemical reaction that transforms the light stimulus in a nerve impulse (a process known as sensory transduction).

The nerve impulse generated in the photoreceptor cells is transmitted through synapses to the so-called bipolar neurons and you're at ganglion neurons.

The axons of the ganglion neurons form the fibers of the optic nerve and leave the retina towards the lateral geniculate nucleus (NGL), within the thalamus.

The neurons of the lateral geniculate nucleus are the ones that finally transmit the nervous impulse to the visual cortex through optic radiation (geniculostriate route).

Therefore, photoreceptor cells and 4 synapses between neurons intervene in the optic pathway until the visual stimulus reaches the cerebral cortex.

  1. cones and rods: are highly specialized modified neurons. These cells are often referred to as photoreceptors. The cones stand out in daytime vision (photopic vision) while the rods act in scotopic vision (poor light, do not recognize details or colors).
  2. Neurons or bipolar cells: connect photoreceptor cells with ganglion neurons.
  3. Neurons or ganglion cells: its axons form the optic nerve
  4. Neurons of the lateral geniculate nucleus: here the visual stimulus is regulated and transmitted to the visual cortex through optic radiation.
  5. Neurons of the visual cortex

It should be clarified that for some authors the photoreceptor cells are not neurons and they consider that the optic pathway actually originates in bipolar neurons.

Other authors consider that photoreceptor cells are indeed a type of neuron and that this is where the optic pathway begins. Where there is no discrepancy is that the optic nerve originates from ganglion cells.

Optic pathway structures

Scheme of the optic pathway
Scheme of the optic pathway

The most prominent structures of the optic pathway are:

  1. anterior optic pathway: are the structures of the optic pathway anterior to the lateral geniculate nucleus:
    • Retina
    • Optical disc
    • optic fascicles
    • Optic nerve
    • Optic chiasma
    • optic tract
  2. lateral geniculate nucleus (NGL)
  3. posterior optic pathway: structures posterior to the lateral geniculate nucleus:
    • optical radiation
    • visual cortex

1.- Anterior optic pathway

Retina

The retina it is where the functional organization of the optic pathway begins. On its surface are the cones and rodsthe photoreceptor cells that transform the light stimulus into a nerve impulse.

The retina is divided into two hemiretinae, the nasal or internal hemiretina, the half inside the fovea (closest to the nose), and the temporal or external hemiretina, located outside the fovea. The nasal hemiretina captures the external visual field.

Simplified organization of retinal cells
Simplified organization of retinal cells

Optical disc

The optic disc, papilla, or blind spot, is an area approximately 1.5 x 2.5 mm located in the center of the retina. It is called the blind spot because it is an area that lacks photoreceptor cells and, therefore, is insensitive to light. This is where the axons of the ganglion cells exit.

optic fascicles

The axons of the ganglion cells that exit through the optic disc leave the eyeball grouped in fascicles. Little by little they increase in thickness due to the myelination of the nerve fibers until they form the optic nerve.

Optic nerve

The optic nerveeither 2nd cranial nerve, measures approximately 4 cm on average and is responsible for transmitting the nerve impulse from the retina to the brain. There are two optic nerves, one from each eye. Each optic nerve is made up of two fiber bundles, those originating from the nasal hemiretina and those originating from the temporal hemiretina.

Approximately 90% of the axons of the optic nerve send their signals to the lateral geniculate nucleus to transmit the signal to the cerebral cortex in charge of image generation.

Of the rest of the axons, there is a group that sends the information to the superior cuniculus, in the midbrain, and assists in the motor control of the eye. Another group of axons are grouped in the retinohypothalamic tract and go to the pretectum or pretectal area, in the hypothalamus, where control over the pupillary reflex is exerted.

Optic chiasma

Bottom view of the human brain. deep dissection.
Bottom view of the human brain. In the upper central part you can see the optic chiasm.

It is a structure located at the base of the hypothalamus, in front of the Turkish chair. Here takes place a crossing of the fibers of the nasal hemiretina of each eye. Fibers from the nasal hemiretina of the left optic nerve pass to the ribbon of the right optic nerve and vice versa.

optic tract

Two nerve fibers, one right and one left, depart from the posterior chiasm, and each one is called the optic tract (also known as the optic tract).

Each optic tract contains visual fibers from the eye on the same side and those that had crossed from the opposite side at the chiasm. They also contain motor fibers for the eye on the same side (there are no crossing motor fibers).

On reaching the cerebral peduncle, the optic tract divides into several branches, and the visual fibers continue to the lateral geniculate nucleus. A small fraction of the fibers pass directly to the pretectum (or pretectal nucleus), which is involved in the consensual and photomotor light reflex.

Another small part of the optic tract starts towards the superior colliculus, related to the control of the ciliary and pupillary muscles through its connection with the ophthalmic ciliary ganglion from which the short ciliary nerves that innervate both muscles depart. Other axons go to the suprachiasmatic nuclei of the hypothalamus and mediate the response of the endocrine system and heart rate to light stimuli.

2.- Lateral geniculate nucleus

The lateral geniculate nucleus (NGL), or underlying external nucleus, is located in the thalamus, which is why it is also known as the optic thalamus. It is the primary processing center for visual information that arrives directly from the retina via ganglion cell axons.

The ipsilateral visual fibers (from the eye on the same side) separate from the heterolateral ones (which had crossed at the chiasm) and terminate in different layers of the lateral geniculate nucleus.

The neurons of the lateral geniculate nucleus send their axons through the optic radiations to the primary visual cortex located in the occipital lobe. It also receives many feedback neuron connections from the primary visual cortex. In the LGN there is information processing, it is not a mere connection of the stimuli received from the periphery to the cerebral cortex.

In the lateral geniculate nucleus, three types of neurons can be differentiated, each one related to a specific visual perception:

  1. M cells (magnocellular): are large cells located in layers 1 and 2 of the LNG. They receive stimuli from the rods of the retina and are related to the perception of movement, visual depth and small differences in brightness. They are the most numerous, up to 95% of all LNG cells.
  2. P cells (parvocellular): they are small cells and are located in layers 3, 4, 5 and 6 of the LNG. They receive the information collected by the red and green cones and are related to the perception of color and shape. They represent up to 5% of the total cells of the LNG.
  3. K cells (koniocellular or interlaminar): very small cells that receive information from the blue cones. They are located between each layer M and P. Their number is much lower than the rest.

3.- Posterior optical pathway

optical radiation

Bundles of nerve fibers known as the posterior geniculate nucleus originate from each layer of the posterior geniculate nucleus. Gratiollette optic radiations, formed by the axons of the LGN neurons. It is also known as the geniculocalcarine tract, geniculostriate pathway, or, a more recent name, posterior thalamic pathway.

These fibers divide into two, the superior and inferior division, and conduct the nerve impulse to the visual cortex.

visual cortex

The visual cortex or cortex refers to the areas of the cerebral cortex named V1 (known as primary visual cortex or striate cortex), V2, V3, V4 and V5. V1 is anatomically equivalent to BA17 (Brodmann Area 17), is a koniocortex-type cortex (sensory) and is located in and around the calcarine fissure in the occipital lobe. From a functional point of view it is the largest system identified in the human brain.

The V1 cortex is the one that receives information directly from the lateral geniculate nucleus through optic radiation. The V1 cortex passes the information to the V2, V3, V4, and V5 zones that collectively form what is known as the extrastriate visual cortex. Each of these zones is related to a specific visual interpretation that increases in complexity from V1 to v5.

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