The visual (retino-thalamocortical) route and the pupillary light reflex pathway are the two most important pathways that the eye may use when it comes to perception and response to changes in the environment as well as pupillary response in traumatic brain injury. The visual disease may be caused by any lesion in the visual or pupillary reflex pathways. Several relays of information processing effectively transmit from the cornea to the brain.
Function and Structure of the Visual System and Pathway
Cornea
Refracting light, and hence neurological impulses, is the primary function of the cornea, which is located immediately in front of and in close proximity to the optic nerve. There is a fixed focus on the cornea, which accounts for roughly 65 percent of the eye’s total refractive power.
Iris and pupil
Located behind the pupil, the iris is the colorful part of the eye’s front chamber. Phototransduction occurs in the retina, illuminated by light passing through the pupil. The iris serves as a protective shield for the pupil. In response to light and other nearby stimuli, such as pupillary size measurement, the pupil either contracts or expands. Iris muscles that govern the quantity of light entering the retina include the sphincter and dilatation of the pupillary sphincter. The images that appear on the retina are flipped and reversed.
Lens
Lenses and the nearby ciliary muscle concentrate light from varying distances onto the retina. When the ciliary muscle contracts, the zonular fibers connected to the lens relax, resulting in the accommodation reflex. When the lens grows more spherical, it increases axial thickness and dioptric power, bringing the focus back to the retina. To help the eye to focus on closer things, the ciliary muscle gets parasympathetic impulses from the short canaliculus nerve. Lens flattens, and dioptric power is reduced due to increased zonal tension when the ciliary muscle relaxes. As we become older, our capacity to adapt decreases. Presbyopia, or the hardening of the lens, is a prevalent problem among the elderly. The lens can no longer concentrate on things closer to it because of its diminishing capacity to alter its shape.
Retina
For the light to interact with rods and cones in the outer photoreceptor segments, it has to first travel through numerous retina layers to reach the retina. Both of these photoreceptors convert the light signal into a bioelectric signal. Photon absorption activates rods and cones, resulting in a phototransduction cascade and cell hyperpolarization, which reduces the release of glutamate from photoreceptors and so reduces the onset of neuropathic pain. Glucose receptors on bipolar cells transmit signals to ganglion cells, which contain axons that go through the optic nerve in the retinal fiber layer of the retina.
Pupillary Light Reflexes and Pathway
There are many similarities in how the visual and pupillary light reflex pathways work. Still, there is one significant difference: pupillary light reflex fibers terminate in the pretectal nucleus, not the thalamus. The optic chiasm is where the nasally aligned fibers split off and provide the signal to the contralateral pretectal nucleus. In contrast, temporally aligned fibers send it to the ipsilateral pretectal nucleus for processing.
Both Edinger-Westphal nuclei (cranial nerve III) are engaged when each pretectal nucleus projects bilaterally. And synapses with both during pupil evaluation. The efferent limb of the reflex begins by producing action potentials. Neurons in the preganglia deliver signals to the ciliary nerve fibers via axons from these preganglia parasympathetic neurons. This causes the pupillary sphincter muscle to contract when the short ciliary neurons from the ciliary ganglion stimulated. Although only one eye is illuminated, a consensual reaction occurs in the other because the nasal retinal fibers traverse the optic chiasm and reach the contra lateral pretectal nucleus. Each pretectal nucleus’s projection to both Edinger-Westphal nuclei contributes to the contra lateral pupillary reflex. If both Edinger-Westphal nuclei are active, the pupillary constriction occurs on the ipsilateral side, allowing for a bilateral pupillary reflex.
As the light dims during pupillary evaluation, the muscles of the pupillary dilator dilate, causing the pupil to enlarge. The dilator muscle is innervate by sympathetic post ganglionic fibers from the long ciliary nerve.
Blood Supply and Lymphatics
The ophthalmic artery is the first branch of the internal carotid artery that supplies the bulk of the eye’s components with blood. The optic nerve and the ophthalmic artery both reach the orbit of the eye via the optic canal. In addition to the ophthalmic artery, several important arteries branch out from it, including the central retinal artery and the anterior and posterior ciliary arteries. The inferior portion of the optic nerve is puncture by the central retinal artery. To nourish the retina’s inner layers with blood, it exits from the optic disc’s cup. In addition to the optic nerve, the choroid and the outer layers of the retina get blood flow from the posterior ciliary arteries. The anterior ciliary arteries are critical to the iris and ciliary body’s vascular supply.
The central retinal vein, which runs parallel to the central retinal artery at the optic nerve’s head, drains the retina’s venules into the bloodstream. The superior ophthalmic vein, which extends into the cavernous sinus from the cavernous sinus vein, receives blood from the central vein. To get to the cavernous sinus, choroidal veins are connect to vortex veins. Then connected to the superior and inferior ophthalmic veins.
Clinical Significance of the Visual System
Bitemporal hemianopia results from damage to the optic chiasm. These conditions are often cause by a pituitary lesion or an aneurysm in the anterior communicating artery because of their close proximity to the pituitary gland. Toxic damage occurs to the nasal retinal fibers that cross over the optic canal with an optic nerve lesion. This does not influence the ipsilaterally projecting temporal retinal fibers that do not cross but go to the lateral geniculate nucleus.
The temporal half of visual field projected onto the nasal section. Of the retina would be lost since the pictures shown on the rear of the eye are upside-down and backward. It’s common for both eyes to be impact when the optic tract injure. The ipsilateral temporal fibers and the contra lateral nasal fibers. Of each optic tract used to transmit. The visual information from each eye to the brain. The left homonymous hemianopia occurs when the right optic tract is damage, resulting in vision loss in both eyes.