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Summary
The visual pathway describes the anatomical pathway responsible for the conversion of light energy into electrical action potentials. which are then sent to the brain. It consists of the retina, optic nerves, optic chiasm, optic tracts, lateral geniculate bodies, optic radiations, and visual cortex.
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The entire visual pathway can be regarded as a part of the CNS, as embryologically the retina is an outgrowth of the forebrain, and the optic nerve is white matter (meaning, it has no power to regenerate) It is also covered with pia, arachnoid, and dura mater like other parts of the CNS. We have covered retinal anatomy in detail in our Anatomy and Physiology section.
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Optic Nerve
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The optic nerve may be divided into 4 parts:
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- Intraocular (1mm)
- Intraorbital (25mm)
- Intracanalicular (5mm)
- Intracranial (10mm)
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Intraocular portion (1mm)
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This includes the optic disc the optic nerve within the sclera.
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The optic disc, often referred to as the optic nerve head, is composed of retinal ganglion cell axons. It is known as the blind spot, due to the absence of rods and cones. Lying about 3mm nasal to the macula lutea and slightly above the posterior pole of the eyeball, the optic disc measures around 1.5mm in diameter, with slightly raised edges and a central depression.
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The intraocular portion is supplied by the branches of the circle of Zinn in the sclera, around the optic nerve. This circle receives supply from the short posterior arteries.
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,Orbital portion (25mm)
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The retinal ganglion cell axons are unmyelinated at the optic disc, and at the periphery of the disc are seen to be covered by the internal limiting membrane of the retina. As the optic nerve exits the eye, it traverses through the lamina cribrosa (a mesh like structure which occupies the hole in the sclera through which the optic nerve leaves the eye), and acquires myelin sheaths. This results in an increase of diameter to around 3-4mm.
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At 25mm, this part of the optic nerve is the longest section. It is surrounded by dura, arachnoid, and pia mater, which extend into the eyeball and blend with the sclera.
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The ciliary ganglion lies between the lateral border of the optic nerve and the lateral rectus muscle. Posteriorly, the nasociliary nerve and ophthalmic artery move forward and pass medially, crossing above the optic nerve. At the same place, the inferior division of the oculomotor nerve crosses the optic nerve below it. Around 12mm behind the eyeball, the inferomedial surface of the dural sheath is pierced by the central artery and vein of the retina.
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The orbital portion receives its blood supply from the pial plexus, which receives supply from branches of the ophthalmic artery. The extraneural part of the central artery of the retina also contributes a few branches.
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,Intracanalicular portion (5mm)
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The optic canal lies within the lesser wing of the sphenoid bone. The optic nerve is fixed in this location, as the dural sheath fuses with the periorbita lining the canal. The subarachnoid space around the nerve is also continuous with intracranial subarachnoid space. The opthalmic artery and it’s accompanying postganglionic sympahetic nerves pass through the canal on the inferolateral border of the optic nerve.
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The intracanalicular portion is supplied by branches from the pial plexus.
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,Intracranial portion (10mm)
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The optic nerve after leaving the canal, passes backwards, upwards and medially within the subarachnoid space to reach the optic chiasm.
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The intracranial portion receives blood supply from the pial plexus.
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Optic chiasm
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The optic chiasm is situated at the junction of the anterior wall and floor of the third ventricle. Anteriorly it is continuous with the optic nerves, and posteriorly with the optic tracts. Blood supply to the chiasm is from the small branches from the arterial plexus in the pia mater (pial plexus). This is supplied by branches from the internal carotids, superior hypophyseal branch of internal carotid, posterior communicating artery, anterior cerebral artery, and anterior communicating artery.
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Anterior relations: The anterior cerebral arteries and anterior communicating artery
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Posterior relations: The tuber cinereum
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Lateral relations: Internal carotid artery
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Superior relations: Lamina terminalis (anterior wall of the third ventricle) and cavity of the third ventricle
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Inferior relations: Diaphragma sellae and hypophysis cerebri
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Optic tracts
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Emerge from the posterior aspect of the chiasma. Each tract passes posterolaterally between the tuber cinereum medially, and the anterior perforated substance laterally. The tract then winds around the lateral aspect of the upper part of the cerebral peduncle, where it adheres to the midbrain.
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Most of the nerves of the optic tract end in the lateral geniculate body and are concerned with conscious visual sensation. There is a smaller medial root of the optic tract with an unknown function. 10% of fibres leave the optic tract before entering the lateral geniculate body, and traverse the superior brachium below the pulvinar to reach the superior colliculus and pretectal nucleus. These fibres are responsible for the pupillary light reflex.
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The optic tract is supplied by pial arteries which receive supply from the anterior choroidal artery and posterior communicating artery.
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Lateral geniculate bodies
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This is a small swelling on the undersurface of the pulvinar of the thalamus. The lateral aspect receives the lateral root of the optic tract. The main lateral geniculate nucleus consists of 6 curved layers of cells (arranged like a stack of hats, one on top of the other). These cells are each separated by optic nerve fibres, which are the axons of the retinal ganglion cells that come from the temporal half of the ipsilateral retina and the nasal half of the contralateral retina (which have crossed at the chiasm). The nerve fibres that decussate in the midline end in layers 1,4 and 6, whilst the temporal fibres that do not cross terminate in layers 2,3 and 5.
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The lateral geniculate body is supplied by the anterior choroidal branch of the middle cerebral artery, the thalamogeniculate branches of the posterior cerebral artery, and the lateral choroidal arteries.
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Optic Radiations
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These are formed from nerve fibres that originate in the laminae of the lateral geniculate bodies (see above). The fibres from the lateral part of the lateral geniculate body receives impulses from the inferior retinal fields (superior visual fields) and fan out around the temporal horn of the lateral ventricle before swinging posteriorly. The fibres associated with the peripheral retina swing the most whilst macular fibres have a small loop.
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These fibres from the inferior retina are known as Meyer’s loop. They continue to travel posteriorly in the retrolentiform part of the internal capsule, lateral to the temporal and occipital horns of the lateral ventricle, and terminate on the inferior lip of the calcarine fissure.
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The fibres which originate from the medial portions of the lateral geniculate body receive impulses from the superior retina (inferior visual field) turn directly and posteriorly to accompany the other fibres in the retrolentiform internal capsule, this time terminating in the superior lip of the calcarine fissure.
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The anterior portion of the optic radiation is supplied by the anterior choroidal branch of the internal carotid, while the posterior by the middle cerebral artery and the posterior cerebral artery.
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Visual cortex
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,Primary visual area (Brodmann’s area 17)
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The primary visual area constitutes the walls of the deep calcarine sulcus on the medial surface of the hemisphere, and extends onto the cortex above and below the sulcus. It extends posteriorly to the posterior pole, and a small portion extends onto the posterolateral aspect of the pole. Anteriorly, the area extends forwards up to the parieto-occipital sulcus above the calcarine sulcus, and a little further below the sulcus.
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The primary visual cortex receives afferent fibres from the lateral geniculate body via the optic radiation. Fibres from the temporal ipsilateral retina and nasal contralateral retina are received. The right eye is represented in the left cerebral hemisphere and the left eye in the right. Additionally, the superior retinal quadrants (inferior visual field) pass to the superior wall of the calcarine sulcus, whilst the inferior retinal quadrants (superior visual field) pass to the inferior wall of the calcarine sulcus. The macula lutea, the central area of the retina, is represented on the posterior part of area 17 in the occipital cortex, whilst the peripheral retina is represented more anteriorly.
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The primary visual cortex is also known as the striate cortex because of white lines (stria) in the gray matter, this is from myelinated fibres from the optic radiation
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,Secondary visual area (Brodmann’s area 18 and 19)
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The secondary visual areas surround the primary visual area. These receive afferent fibres from area 17 and other cortical areas and the thalamus. Their function is to relate the visual information from area 17 to past visual experiences (enabling us to recognise what we see) Area 18 also may integrate the two halves of the visual fields through commissral fibre that cross the midline. It might be involved in sensory motor eye coordiation and also linked to the descending pathways to cranial nerve nuclei controlling extaocular muscles, involved with smooth pursuit.
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The visual cortex is supplied by the posterior cerebral artery, but the anterior end of the calcarine sulcus may have some contribution from the middle cerebral artery.
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References
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- Salmon, John F., and Jack J. Kanski. Kanski’s Clinical Ophthalmology: A Systematic Approach. Ninth Edition, Elsevier, 2020.
- Snell, Richard S., and Michael A. Lemp. Clinical Anatomy of the Eye: Snell/Clinical Anatomy of the Eye. Blackwell Science Ltd,., 1997 https://doi.org/10.1002/9781118690987
- Covington, Benjamin P., and Yasir Al Khalili. ‘Neuroanatomy, Nucleus Lateral Geniculate’. StatPearls, StatPearls Publishing, 2022. PubMed, http://www.ncbi.nlm.nih.gov/books/NBK541137/
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