Pupillary light reflex and visual pathway

1. Stimuli from the right visual field projects on the left retinal field of both eyes and vice versa; stimuli from the upper visual fields project upon the lower retinal fields of both eyes and vice versa
   1. axons from ganglion cells form the optic nerves, chiasm, and tracts; retinotopic organization is preserved within the optic nerves
   2. in the optic chiasm, temporal retinal fibers occupy the lateral chiasm and enter the ipsilateral optic tract while nasal retinal fibers cross in the central chiasm to enter the contralateral optic tract making all visual sensory input from the right visual field entering the left optic tract; a few of the optic tract fibers enter the midbrain pretectal area as afferent fibers for the pupillary reflex but most terminate on the lateral geniculate body; the lateral geniculate body is not involved in the pupillary light reflex
      1. the blood supply to the optic chiasm consists of branches from the ICA, Acomm, Pcomm, and ACA (NOTE: the anterior choroidal does not supply the optic chiasm according to most texts)
      2. NOTE: transection of CN 3 does not interrupt the afferent limb of the pupillary reflex
   3. as the optic tract approaches the lateral geniculate body it internally rotates 90 degrees so the superior retinal fibers project onto the medial prortion of the lateral geniculate and the inferior retinal fibers project onto the lateral portion of the lateral geniculate and the macular fibers occupy an intermediate position
      1. uncrossed fibers of the optic tract terminate on layers 2,3 and 5 of the lateral geniculate while crossed fibers terminate on layers 1,4, and 6
         1. Remember: ‘un’crossed as 2 more letters than ‘crossed’ and terminates beginning on layer 2 of the lateral geniculate
      2. the lateral geniculate body, like the retina, has concentric receptive fields while the visual cortex has rectangular receptive fields
   4. most lateral geniculate neurons project their axons to the primary visual cortex over the optic radiations; those representing inferior retinal fields project forward sublenticularly into the temporal lobe where they form Meyer’s loop as they turn laterally above the inferior horn of the lateral ventricle and then posteriorly to form the inferior portion of the optic radiations; those axons representing upper retinal fields project directly laterally retrolenticularly where they pass anterior to the atrium of the lateral ventricle before turning posteriorly to form the upper portion of the optic radiations; macular fibers occupy an intermediate position in the optic radiations
   5. the primary visual cortex (striate cortex, Brodmann’s area 17) is mostly represented on the deep calcarine fissure on the medial surface of the occipial lobe; it is retinotopically organized with upper retinal fibers ending in the upper lip (cuneate gyri) of the calcarine fissure and lower retinal fibers ending in its lower lip (lingual gyri) (NOTE: cuneate and lingual gyri are separated by the calcarine sulcus); the peripheral retina is represented most anteriorly while the macula is more posteriorly represented at the occipital pole
      1. primary visual cortex lesions are characterized by macular sparing while damage to the macula produces a severe loss of visual acuity
      2. depth peception requires a convergence of images from both eyes, this first occurs in the striate cortex which receives input from layer 4 of area 17
   6. visual association cortex (Brodmann’s areas 18 and 19) surround the primary visual cortex and receives input from both the primary and visual cortex and pulvinar
2. light reflex
   1. light hits the rods and cones, they project to the bipolar neurons which project to the ganglion cells which enter the optic chiasm
      1. rods and cones are sensitive to wavelengths from 400-700 nm
   2. optic tract then carries light impulses in between the lateral and medial geniculate bodies to the pretectal nucleus
      1. the pretectal nuclei communicate with each other through the posterior commissure
   3. the pretectal nucleus sends a pretectal-oculomotor tract (through the periaqueductal grey) to both Edinger Westphal nuclei
   4. Edinger Westphal nuclei then send fibers that form CN 3 through the red nucleus and into the ciliary ganglia; NOTE: these fibers are parasympathetic – CN 3 does not have any sympathetic fibers nor does the ciliary ganglia although the ciliary ganglion does have sympathetic fibers that run through it without synapsing
   5. fibers from the ciliary ganglia innervate the circular muscle of the iris causing constriction of the pupil
      1. postganglionic sympathetic fibers from the carotid plexus also innervate the ciliary muscles (called Tiedmann’s nerve via the nasociliary and long ciliary nerves)