|CHAPTER||15||The Afferent (Sensory) System|
The images arising in each eye separately are appreciated as a single mental impression in the visual cortex. The vision achieved by the coordinated use of both eyes is known as binocular vision. The image of an object is built up of two separate halves with the object divided vertically. The image of the right half of the object is formed on the nasal retina of the right eye and the temporal retina of left eye which, in turn, is perceived by the left half of the visual cortex. Similarly, the image of left half of the object is formed on the temporal retina of the right eye and the nasal retina of left eye which, in turn, is perceived by the right half of the visual cortex. A composite picture of the whole object is built, therefore, by the activity of striate areas on both side of the visual cortex and the higher visual centers in the adjacent parastriate and peristriate areas.
▃Anatomy of Visual Pathway (AN30.5)
Visual sensations are perceived by rods and cones and conducted to the visual cortex through three sets of neurons:
•First-order neurons are bipolar cells of inner nuclear layer with their axons in the inner plexiform layer (IPL).
•Second-order neurons are ganglion cells in the retina, with their processes, which pass into the nerve fiber layer (NFL) and optic nerve to the lateral geniculate body (LGB).
•Third-order neurons transmit impulses through optic radiations to the visual center in occipital lobe.
Therefore, the visual pathway extends from retina to visual (occipital) cortex and consists of:
Retina – optic nerves – optic chiasma – optic tracts – lateral geniculate bodies – optic radiations – visual cortex (Fig. 15.1).
■Arrangement of Nerve Fibers
Nerve fibers are arranged at the following places:
2.Within optic nerve head (ONH).
3.In optic nerve.
5.In optic tract.
7.In optic radiations.
•Temporal half of retina projects the nasal field of vision. Fibers from the temporal half of the retina enter the chiasma and pass into optic tract of same side.
•Nasal half of retina projects the temporal field of vision. Fibers from the nasal half of each retina decussate in chiasma and pass into optic tract of opposite side.
The retina is also divided into superior and inferior halves by a horizontal meridian (raphe) that passes from the fovea to the temporal periphery. The superior half projects the inferior field of vision and the inferior half projects the superior field of vision. Fibers do not cross horizontal meridian.
Fibers from macula pass straight to optic nerve head (ONH), forming papillomacular bundle. Fibers from nasal retina also pass straight to ONH. Fibers arising temporal to the macula follow an arcuate path, around papillomacular bundle, to reach ONH (Fig. 15.2).
In optic nerve head and anterior part of optic nerve, the fibers from the macular lesion enter the nerve in a triangular form, with the apex toward the center of the nerve. In general, fibers from the peripheral retina enter the peripheral optic nerve, while the central retinal fibers enter the central part of the nerve (Fig. 15.3).
In Optic Nerve
The nerve fibers enter the chiasma. The papillomacular bundle from the macular region becomes more centrally arranged in the posterior part of the nerve (Fig. 15.4). Otherwise, the fibers from retina maintain the relative position up to chiasma, where the nasal fibers decussate.
Fibers from nasal retina (nasal macular and peripheral fibers) decussate in the chiasma and pass to the opposite optic tract, while the fibers from temporal retina (temporal macular and peripheral fibers) pass uncrossed to the optic tract of same side. Fibers from inferonasal retina cross the chiasma low and anteriorly. After crossing, these fibers loop forward into the optic nerve of opposite side (Knee of Wilbrand) before turning back into the optic tract. So, these fibers may be affected by lesions in the posterior part of the optic nerve. Superonasal fibers traverse the chiasma high and posteriorly.
In Optic Tract and LGB
Optic tracts arise at posterior aspects of chiasma and extend posteriorly around cerebral peduncles. Optic tracts terminate in the LGBs. Optic tract contains both visual fibers (uncrossed temporal fibers of same side + crossed nasal fibers of opposite side) and pupillomotor fibers. Visual fibers terminate in LGB, while the pupillomotor fibers leave optic tract anterior to LGB, project through superior colliculus, and terminate in pretectal nuclei.
Visual Fibers in Optic Tract and LGB
Macular fibers occupy the central part of optic tract and LGB. Inferior peripheral retinal fibers (uncrossed inferotemporal [IT] and crossed inferonasal [IN]) lie on the lateral side in optic tract. So, these occupy lateral end of LGB. Superior peripheral retinal fibers (uncrossed superotemporal [ST] and crossed superonasal [SN]) lie on the medial side in optic tract. So, these occupy medial end of LGB. In LGB, the fibers are arranged in a laminated pattern. Crossed and uncrossed fibers pass to alternating laminae in LGB. The corresponding third-order neurons originate in LGB as optic radiations (Fig. 15.5).
Visual fields and retina have an inverted and reversed relationship, that is
•Inferotemporal (IT) retina projects superonasal field.
•Superotemporal (ST) retina projects inferonasal field.
•Superonasal (SN) retina projects inferotemporal field.
•Inferonasal (IN) retina projects superotemporal field.
In Optic Radiations
These are third-order neurons which extend from LGB to striate cortex located on medial aspect of occipital lobe above and below the calcarine fissure. Visual fibers in optic radiations run behind the motor fibers in internal capsule (like other sensory tracts) and separate thereafter.
Inferior (ventral) fibers from LGB course into temporal lobe (temporal radiations). These fibers first loop forward into temporal lobe (called Meyer’s loop) and then turn backward to the lower portion of visual cortex. Inferior “macular” fibers do not course as far anteriorly in the temporal lobe.
Superior (dorsal) fibers from LGB run backward in a direct course through parietal lobe to occipital cortex (parietal radiations).
In Occipital Cortex
Visual center (striate cortex) is located on the medial aspect of the occipital lobe, above and below the calcarine fissure. The part above calcarine fissure represents upper corresponding quadrants of both retinae (i.e., lower visual fields), while the part below calcarine fissure represents lower quadrants of both retinae (i.e., upper fields) (Fig. 15.6). The anterior most part of occipital cortex (striate cortex) subserves extreme nasal fibers, that is, temporal extremity of visual fields of contralateral eye which is perceived monocularly (i.e., temporal crescent).
The anterior part (posterior to anterior most part) represents peripheral fibers. The posterior tip represents macular fibers, that is, central macular vision.
▃Blood Supply of Visual Pathway
Visual pathway is supplied by the ophthalmic artery which is a branch of internal carotid artery and Circle of Willis. Blood supply of different parts of visual pathway is listed in Table 15.1 and depicted in Fig. 15.7.
Table 15.1 Blood supply of parts of visual pathway
Part of visual pathway
(Intraorbital, intracanalicular, and intracranial)
Branches of ophthalmic artery
Circle of Willis
Anterior choroidal artery (branch of middle cerebral artery)
Lateral geniculate body
Anterior choroidal and posterior cerebral arteries
Branches of the MCA and PCA
Primary visual cortex
Calcarine branch of the PCA
Abbreviations: MCA, middle cerebral artery; PCA, posterior cerebral artery.