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.
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.
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.
They transmit an impulse to bipolar cells (the second-order neurons),
which reach out to the ganglion (multipolar) cells (the third-order neurons) located here, in the retina.
The optic nerve can be subdivided into four main parts:
1. The intraocular part is located within the eyeball.
2. The intraorbital part spreads from the posterior pole of the eyeball to the intraorbital opening of the optic canal.
3. The intracanalicular part is located within the optic canal.
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.
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.
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.
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.
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.
The optic tract transmits information to the subcortical visual centers (fourth-order neurons): the lateral geniculate body,
the superior colliculus,
and the pulvinar nuclei.
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)
fibers pass into the primary visual cortex in Brodmann area 17, forming the 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).
These are the cuneus (cuneus)
and the 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.
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),
- 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.
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.
- 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
- Tectobulbar tract
- tractus tectobulbaris
- Tectopontine tract
- tractus tectopontinus
- Tectospinal tract
- tractus tectospinalis
- Superior colliculi
- colliculi superiores
- Lingual gyrus
- gyrus lingualis
- Optic radiation
- radiatio optica