CN II: Optic nerve

CN II: Optic nerveAnatomy and function of the optic nerve. Visual system.
~ 8 min

In this note, we are going to discuss the anatomy and function of the optic nerve, which is part of the visual system.

It transmits visual impulses from the retina to the brain. According to the generally accepted classification of functional components, it contains special somatic afferent fibers (SSA). That means that it is a sensory nerve.

Optic nerve (n. opticus)
Optic nerve  (n. opticus)

The optic nerve (CN II) is formed by axons of retinal ganglion cells, which radially converge to the optic disc to form the actual nerve.

Retina (retina)
Retina (retina)
Retinal ganglion cells
Retinal ganglion cells
Optic disc (discus n. optici)
Optic disc (discus n. optici)

The retina’s photoreceptor cells are capable of visual phototransduction. That means they can convert the energy of light into a nerve impulse. The two classic photoreceptor cells are rods and cones.

Rods and cones
Rods and cones

They transmit an impulse to bipolar cells (the second-order neurons),

Bipolar cells
Bipolar cells

which reach out to the ganglion (multipolar) cells (the third-order neurons) located here, in the retina.

Ganglion cells
Ganglion cells

The optic nerve can be subdivided into four main parts:

1. The intraocular part is located within the eyeball.

Intraocular part (pars intraocularis)
Intraocular part (pars intraocularis)

2. The intraorbital part spreads from the posterior pole of the eyeball to the intraorbital opening of the optic canal.

Intraorbital part (pars orbitalis)
Intraorbital part (pars orbitalis)

3. The intracanalicular part is located within the optic canal.

Intracanalicular part (pars intracanalicularis)
Intracanalicular part (pars intracanalicularis)

4. And the intracranial part extends from the internal orifice of the optic canal to the optic chiasm, which is a site of decussation of the right and left nerves.

Intracranial part (pars intracranialis)
Intracranial part (pars intracranialis)

At the optic chiasm, the nerve fibers originating from the temporal sides of the retina remain on the side of the particular eye, and the fibers that originate from the nasal fields cross over and enter the contralateral optic tract, which is the intracranial continuation of the optic nerve.

Optic chiasm (chiasma opticum)
Optic chiasm (chiasma opticum)

So, for example, the right optic tract contains fibers from the temporal side of the retina of the right eye and fibers from the nasal part of the retina of the left eye.

Optic tract (tractus opticus)
Optic tract (tractus opticus)

And we also need to take into consideration that the light falls on the retina from the opposite visual field. Therefore, visual input from the left visual field travels in the right optic tract, and from the right visual field – in the left optic tract.

Optic tract (tractus opticus)
Optic tract (tractus opticus)

Some axons from each eye leave the optic chiasm and head superiorly into the supraoptic nucleus of the hypothalamus, taking part in the regulation of circadian rhythms.

Supraoptic nucleus of the hypothalamus (nucleus supraopticus)
Supraoptic nucleus of the hypothalamus (nucleus supraopticus)

The optic tract transmits information to the subcortical visual centers (fourth-order neurons): the lateral geniculate body,

Lateral geniculate body (corpus geniculatum laterale)
Lateral geniculate body (corpus geniculatum laterale)

the superior colliculus,

Superior colliculi (colliculus superior)
Superior colliculi (colliculus superior)

and the pulvinar nuclei.

Pulvinar of the thalamus (pulvinar)
Pulvinar of the thalamus (pulvinar)

A small part of the axons leaves each tract and terminates in the pretectal nucleus to participate in the pupillary light reflex.

From the lateral geniculate body (corpus geniculatum laterale)

Lateral geniculate body (corpus geniculatum laterale)
Lateral geniculate body (corpus geniculatum laterale)
Lateral geniculate body (corpus geniculatum laterale)
Lateral geniculate body (corpus geniculatum laterale)

fibers pass into the primary visual cortex in Brodmann area 17, forming the optic radiation (radiatio optica).

Optic radiation (radiatio optica)
Optic radiation (radiatio optica)
Optic radiation (radiatio optica)
Optic radiation (radiatio optica)
Optic radiation (radiatio optica)
Optic radiation (radiatio optica)

The visual cortex corresponds to two gyri located on the internal surface of the occipital lobe around the calcarine sulcus (sulcus calcarinus).

Calcarine sulcus (sulcus calcarinus)
Calcarine sulcus (sulcus calcarinus)
Calcarine sulcus (sulcus calcarinus)
Calcarine sulcus (sulcus calcarinus)

These are the cuneus (cuneus)

Cuneus (cuneus)
Cuneus (cuneus)

and the lingual gyrus (gyrus lingualis).

Lingual gyrus (gyrus lingualis)
Lingual gyrus (gyrus lingualis)

The superior colliculi (colliculi superiores) regulate rapid eye movements between fixation points (the so-called saccadic movements) in response to visual and auditory stimuli.

Superior colliculi (colliculus superior)
Superior colliculi (colliculus superior)

The superior colliculi consist of layers of gray and white matter, which are usually divided into superficial and deep layers.

  • The superficial layers respond to visual stimuli by receiving sensory information from the retina and transmitting it to the cortex via the lateral geniculate body and the pulvinar. From the deep layers, descending pathways responsible for the movement of the head and eyes begin. These are the tectospinal tract (tractus tectospinalis),
Superior colliculi (tractus tectospinalis)
Superior colliculi (tractus tectospinalis)
  • tectobulbar tract (tractus tectobulbaris)
  • tecto-pontine tract (tractus tectopontinus)

The pulvinar (pulvinar), unlike the two previous subcortical visual centers, is not directly connected with the optic tract.

Pulvinar (pulvinar)
Pulvinar (pulvinar)
Pulvinar (pulvinar)
Pulvinar (pulvinar)

This is where the fibers from the superficial layers of the superior colliculi end. From the pulvinar, the nerve impulse goes mainly to the associative or secondary visual cortex, which is located on the basal and lateral surfaces of the occipital lobe, as well as to the basal nuclei and limbic system.

Occipital lobe (lobus occipitalis)
Occipital lobe (lobus occipitalis)
Dictionary

Optic nerve

Optic nerve
nervus opticus
Calcarine sulcus
sulcus calcarinus
Oculomotor nerve
nervus oculomotorius
Nucleus of the oculomotor nerve
nucleus n. oculomotorii
Accessory nucleus of the oculomotor nerve
nucleus oculomotorius accessorius
Superior branch
ramus superior
Inferior branch
ramus inferior
Oculomotor root
radix oculomotorius
Ciliary ganglion
ganglion ciliare
Rectus muscle of the eye
m. rectus oculi
Pulvinar
pulvinar
Tectobulbar tract
tractus tectobulbaris
Tectopontine tract
tractus tectopontinus
Tectospinal tract
tractus tectospinalis
Superior colliculi
colliculi superiores
Lingual gyrus
gyrus lingualis
Cuneus
cuneus
Optic radiation
radiatio optica
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