Second-order neurons then pass out of the spinal cord, enter the thoracic ganglia and run superiorly until they reach the superior cervical ganglion, where they again synapse. It originates in the hypothalamus, sending fibers into the cervical spinal cord to synapse in the upper thoracic spinal cord through the brainstem. In the case of vision, mydriasis and elevation of eyelids are the two primary responses. The sympathetic system governs the fight-or-flight response. This is known as the consensual light reflex. One interesting point to note here is that the pretectal nucleus supplies the Edinger-Westphal nucleus bilaterally hence, shining light in one eye causes ipsilateral and contralateral constriction of the pupil. The Edinger-Westphal nucleus (preganglionic parasympathetic) relays to the ciliary ganglion (postganglionic sympathetic) via the oculomotor nerve (CN III), which then directs the pupillary sphincter muscle completing the miotic reflex arc of the pupil. The efferent limb is directed by the oculomotor nerve (CN III). The impulses travel through the optic nerve (CN II), which projects bilaterally to the pretectal nucleus in the midbrain, and then projects to the Edinger-Westphal nucleus. Light stimulates the retinal ganglionic cells. The optic nerve directs the afferent limb of the reflex pathway. The parasympathetic system constricts the pupil to protect itself when light intensity increases to an uncomfortable level by decreasing the amount of light entering the eye. Pupillary Light Reflex Pathway (parasympathetic innervation pathway) The majority of the fibers pass posteriorly to become the genico-calcarine tracts, which have both parietal and temporal loops in the form of the dorsal optic radiation and Meyer's loop and terminate into the cuneus gyrus and lingual gyrus of the primary visual cortex, respectively (Broadmann area number 17). The optic tract then passes posteriorly where most of the axons synapse in the layers of the lateral geniculate body (LGB) of the midbrain, which is a posterolateral extension of the thalamus with a minority passing into the superior colliculus and Edinger-Westphal nuclei these fibers allow for parasympathetic innervation of the pupil, i.e., pupillary constriction. This anomaly helps eye healthcare professionals in the assessment of the site of the lesion along the visual pathway, which produces well-described visual field defects, also known as hemianopias, posterior, or at the chiasm.īeyond the chiasm, the pathway continues as two distinct tracts, each carrying the temporal fibers from the other eye. Here more than half of the nasal fibers from the left eye decussate to join the temporal fibers of the right eye and form the right optic tract and vice versa. This continues till the two optic nerves join together to form the optic chiasm directly behind and above the pituitary stalk. It exits into the middle cranial fossa to form the intracranial part of the optic nerve. It then enters the optic canal, a bone-encased tunnel intended to protect the nerve. The axons exit the orbit through the orbital foramen, simultaneously with the ophthalmic artery and sympathetic fibers. The nerve is surrounded by the dura, which is in continuation of that of the brain, allowing free movement of CSF between the eye and the intracranial vault. These fibers join together at the optic disc and are redirected posteriorly out of the eye to form the orbital part of the optic nerve. It consists of two types of fibers, namely temporal and nasal fibers, which control the nasal and temporal parts of the visual field, respectively. The ganglion cell layer and nerve fiber layer serve as the foundation of the optic nerve the former contains the cell bodies, and the latter contains the axons as they stream across the retina. The photoreceptor layers consist of the rods and cones, which generate action potentials with the help of rhodopsin through photosensitive cycles. ![]() The optic pathway begins in the retina, which is a complex structure made up of ten different layers. The visual system consists of two primary parallel pathways: an optic pathway and a pupillary reflex pathway.
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