Type:
Educational Exhibit
Keywords:
Education and training, Education, MR, CT, Neuroradiology brain
Authors:
R. Martinez, S. A. Velásquez, J. Pantoja Yepez, J. A. mora, C. Tramontini; Bogotá/CO
DOI:
10.26044/ecr2019/C-3361
Background
Anatomy of visual pathway.
The visual pathway lies from the globe,
the most anterior structure,
to the occipital cortex,
the most posterior structure.
The optic nerve is formed by the axons of third grade neurons called ganglion cells at the surface of the retina.
It exits the globe posterioly at the optic disc and is divided into four segments: intraocular,
intraorbital (anterior and posterior),
intracanalicular and intracranial Fig. 1 . The intraocular segment is approximately 1 mm in length (disc).
The intraorbital segment is 30 mm long and it is surrounded by pia,
arachnoid,
cerebrospinal fluid and dura mater.
The intracanalicular segment is 6 mm in lengh.
It exits through optic canal,
continues approximately 10 mm and converges with contralateral nerve chiasm in the suprasellar cistern.
In the chiasm occurs a partial decussation of fibers.
[1,2]
The axons from medial portion of each retina cross to join those from the lateral portion of the other retina.
Thus,
the fibers corresponding to the temporal visual fields ocuppy the center of the chiasm,
whereas those for the nasal fields run in the lateral periphery of the chiasm.
Then,
fibers continue as optic tracts,
each carring information about the contralateral visual field.
They curve around the midbrain below the globus pallidus.
Fibers of the optic tract synapse in the lateral geniculate nucleus,
situated at the posterior and inferior aspect of the thalamus.
It is important to know that the superior visual fields project to the lateral portion of the geniculate nucleus,
the inferior fields to the medial portion,
and macular vision to the posterior portion.
[1,2]
From the lateral geniculate nucleus,
the superior efferent axons travel directly in the parietal optic radiations to the primary visual cortex in the medial occipital lobe,
while inferior axons curve anterolaterally,
traverse the posterior portion of the internal capsule,
continue within the temporal lobe and then pass posteriorly to the occipital lobe.
The primary visual cortex receives the axons of the optic radiations and is organized,
with the inferior visual field mapping to the superior Surface of the calcarine fissure,
and the superior field mapping to the inferior Surface of the fissure.
Macular vision projects onto the occipital poles.
Fig. 2 [1,2]
There are different types of pathologies that can affect the visual pathway.
Clinical manifestations depend on where they are located.
According to the visual field evaluation it will be possible to understand where to look for injuries in imaging studies.