The special senses are involved in external sensation and perception mediated by sensory organs. Special receptors in these organs receive stimuli and convert them into electrical impulses that are transmitted via cranial nerves to the central nervous system (CNS) for interpretation. The sensory information may be decoded as sight, sound, taste, or touch. Photoreceptors in the eyes detect and transduce light stimulus into neural signals relayed through the optic nerve to the brain for processing. This paper describes the cranial nerve involved in vision, the anatomy of the sense organ, signal transduction, autonomic activation, and physiological consequences of homeostatic imbalances in this sense organ.
Innervation by cranial nerves helps control sense organs and relay sensory information to the brain. The optic nerve (ON) is dedicated to vision; it transmits image impulses received by the retina to the visual cortex – the lateral geniculate nucleus – for processing (Armstrong & Cubbidge, 2019). It is the second of a system of cranial nerves (Cranial Nerve II or CN II) that leaves the eye at the posterior region of the eyeball. Retinal axons of the ganglion cells make up the ON (Armstrong & Cubbidge, 2019). Unlike other cranial nerves, the ON exits the cranial cavity.
Anatomy and Physiology
Vision is a function of the eyes, which occur in protective bony orbits encased by cranial bones. Externally, eyelids with lashes shield the eyeball surface from abrasive materials (Armstrong & Cubbidge, 2019). The conjunctiva protects the sclera (white layer), while the lacrimal gland found in the lateral nasal edges secrete tears that cleans the eyeball. Six extraocular muscles, including superior, medial, inferior, and lateral oblique, contract to control eye movement within their sockets (Armstrong & Cubbidge, 2019). The eyeball itself is spherical, comprising three layers: the outer fibrous tissue, vascular (middle) structure, and inner neural tunic. These tissues are involved in light stimuli sensation within the visual field.
The fibrous tunic comprises the sclera (white part) and the anterior cornea. The latter structure is transparent, allowing light to pass into the eye. The vascular tunic comprises three structures: a vascularized structure called the choroid that nourishes the eyeball, the ciliary body joined to the lens and the iris that controls the opening and closing of the pupil in response to visual information (Armstrong & Cubbidge, 2019). The central part of the eye (pupil) can dilate or constrict to control the amount of light entering the eye. The neural tunic or retina is the inmost structure that senses visual stimuli. It comprises rods and cones, which produce neurochemicals into the synaptic layer upon stimulation (Chen et al., 2019). Bipolar cells link these photoreceptors (retina) to the retinal ganglion cell whose axons form the ON that transmits the electrical impulse generated to the brain. The eye consists of the anterior and posterior cavities that contain the watery aqueous humor and viscous vitreous humor, respectively. These fluids are important in maintaining the spherical shape of the eyeball.
The signal-transduction pathway for vision involves two types of photoreceptors: rods and cones. These specialized cells contain photo-pigments that perceive light stimuli reaching the retina. Rods comprise rhodopsin that is sensitive to faint light, while cones contain opsins, seven-transmembrane (7TM) domain receptors that detect brilliant colors – red, green, and blue (Martemyanov & Sampath, 2017). When a light photon hits the retina, 11-cis-retinal change (photoisomerization) occurs in the hydrocarbon constituting the opsins. The resulting all-trans-retinal molecules activate the G proteins in the retina. According to Armstrong and Cubbidge (2019), the breakdown of cyclic guanosine monophosphate (cGMP) polarizes the photoreceptor membrane, generating a potential that causes neurochemical release into the outermost retinal layer. The nerve impulse produced is transmitted via the ON to the brain for interpretation.
In contrast, signal transduction in the somatic peripheral sensory nerve involves somatosensory receptors. Cutaneous neurons in the skin detect various stimuli, including touch, irritation, and cold or heat, and generate an electrical signal. Multiple transduction mechanisms are involved, with touch sensitivity involving the fast ion-channel pathway mediated by intra- and extra-cellular proteins (Martín-Alguacil et al., 2016). Unlike photoreceptors that involve a single pathway (11-cis-retinal change), multiple transducer systems regulated by sodium and acid-sensitive ion channels are implicated in somatic signaling. Upon activation, neurons in the skin produce neurotransmitters that result in a nerve impulse through the G-protein mechanism. Unlike in photo-sensation, where only cGMP is involved in membrane polarization, the somatic sensory nerve involves ATP, cAMP, and cGMP molecules in signal transmission (Martín-Alguacil et al., 2016). Myelinated and non-myelinated afferent nerve fibers that innervate the dermis transmit the electrical impulse to the CNS for interpretation.
Autonomic Activation and Downstream Effects
The autonomic nervous system (ANS) affects ocular functioning and parts of the eye. This mechanism occurs through parasympathetic and sympathetic innervation of the ciliary and ganglia cells (Martemyanov & Sampath, 2017). Ganglion neurons from the ciliary muscles terminate in the iris, and thus, can constrict the pupil in response to bright light. Neural receptors in the superior cervical ganglion end in pupil dilator muscles, causing autonomic regulation of pupil enlargement in low light (Martemyanov & Sampath, 2017). Additionally, ocular blood flow into different parts of the eye is under ANS control. Autonomic vasodilatory fibers innervate the vascular system in the mammalian ON, ciliary body, and iris. Vitreous and aqueous humor flow into and out of the ocular cavities are also under autonomic regulation. A downstream effect is an increase in intraocular pressure and blood flow to the episcleral vessels. Bright light reaching the retinal wall triggers a parasympathetic response via the oculomotor nerve (Martín-Alguacil et al., 2016). As a result, the postganglionic fibers are stimulated, causing iris contraction and pupil constriction.
The maintenance of optimal intraocular pressure (IOP) is under homeostatic control. When the IOP is high, the Schlemm’s canal (SC) cells and trabecular mesh (TM) reduce aqueous humor flow from the anterior cavity, restoring normal pressure levels (Chen et al., 2019). The distension or distortion of these cells triggers the sensation of elevated IOP. Consequently, an extra-cellular matrix turnover mechanism is initiated (a component of IOP homeostasis) to correct the outflow resistance resulting from high ocular pressure. Glaucoma is a physiological consequence of elevated IOP whose diagnosis involves optical disc cupping (Chen et al., 2019). Maintaining IOP within acceptable limits can reverse the visual loss resulting from this condition. The pressure imbalance diminishes TM/SC cells functioning, resulting in stiffness in the eyes.
Vision is a special sense involving photoreceptors that transduce visual stimuli into a nerve impulse transmitted by the ON to the brain for interpretation. Its complex anatomy is consistent with the physiological functions of the different parts of the eye. Signal transduction in this organ involves photoreceptors (rods and cones) and photo-pigments (rhodopsin and opsins). Autonomic regulation involves a parasympathetic response that contracts the iris, causing the pupil to narrow in bright light. Optimal intraocular pressure is maintained throughout life, but prolonged IOP homeostatic imbalance leads to glaucoma.
Armstrong, R. A., & Cubbidge, R. C. (2019). The eye and vision: An overview. In V. R. Preedy & R. R. Watson (Eds.), Handbook of Nutrition, Diet, and the Eye (2nd ed.). Academic Press.
Chen, W., Chen, Z., Xiang, Y., Deng, C., Zhang, H., & Wang, J. (2019). Simultaneous influence of sympathetic autonomic stress on Schlemm’s canal, intraocular pressure and ocular circulation. Scintific Reports, 9(20060), 1-9. Web.
Martemyanov, K. A., & Sampath, A. P. (2017). The transduction cascade in retinal ON bipolar cells: Signal processing and disease. Annual Review of Vision Science, 3, 25-51. Web.
Martín-Alguacil, N., de Gaspar, I., Schober, J. M., & Pfaff, D. W. (2016). Somatosensation. In D. W. Pfaff & N. D. Volkow (Eds.), Neuroscience in the 21st Century (863–902). Springer.