How Vision Works

Among all of our senses, vision may be the most crucial since it provides us with a wealth of data about the outside world. The way how vision works is one of the most fascinating phenomena of human body. The retina, optic chiasm, the optic nerve, lateral geniculate cells in the thalamus, optic tract, and the geniculocalcarine tract which extends to the occipital cortex are all components of the sensation of vision. The retina detects the light that reaches the eye (Murphy & SinghVijjan, 2020). The brain then transforms the light into an image.

Cranial Nerves Associated With Vision

The sense of vision works with the help of following six cranial nerves (Nguyen & Duong, 2021):

1. Optic Nerve: Optic nerve detects the retinal picture and incoming light. This image is then sent from the optic nerve to the cortex (Nguyen & Duong, 2021).
2. Oculomotor Nerve: The majority of the muscles in the eyes are innervated by the oculomotor nerve (Nguyen & Duong, 2021).
3. Trochlear Nerve: The superior oblique muscles is the one that the trochlear nerve supplies with motor innervation (Nguyen & Duong, 2021).
4. Trigeminal Nerve: There are three major branches of the trigeminal nerve. The trigeminal nerve’s ophthalmic branch is the only one that innervates the eye. The trigeminal nerve’s ophthalmic branch innervates the eye’s sensory system. The lacrimation and corneal reflexes’ afferent component is the ocular stem of the trigeminal nerve. The lacrimation and corneal reflex’ efferent portion is carried via the facial nerve (Nguyen & Duong, 2021).
5. Abducens Nerve: lateral rectus muscle is the only eye muscle innervated by the abducens nerve (Nguyen & Duong, 2021).
6. Facial Nerve: By motor innervating the orbicularis oculi muscle, the facial nerve controls eye closure and blinking in the eyes (Nguyen & Duong, 2021).
   The movement, sensory, and physiological structures in the vision are innervated by these cranial nerves.

Anatomy of How Vision Works Related to its Physiology

The outer surface of the eye is kept moist and clear by a tear film, and the intraocular pressure preserves the eye’s shape. The typical vision of the eye depends on this form. The extraocular muscles maintain the alignment of the visual pictures. The pupil size controls how much light enters the eye, the lens and cornea bend light, and the vitreous serves as a transparent optical medium. A neural impulse is created when light strikes the retina, and this impulse travels via the visual path into the visual regions of the cortex cornea, which is in front of the eye, allowing light to reach the lens. The cornea and lens aid in concentrating light rays on the retina. Light is absorbed by the retinal cells, which subsequently transform it into electrochemical signals that are sent to the brain via the optic nerve (Stjernschantz & Astin, 2019). Thus ensuring proper vision.

Signal Transduction in the Eyes

The sensory transmission of the visual system is visual phototransduction. In the rod, cones, and ganglion cells, light is transforms into electrical signals. Four different types of light-sensitive cells—rod and three different types of cones—make up the retina of the eye. Four different kinds of light-sensitive cells use electrochemistry to transform light energy. Finally, there are a number of around 1,000 “discs,” each of which has 80,000 or more pigment molecules on each side. The process of “capturing” visible light into a chemical form is the first step in converting light into a format that the brain can understand (Yao & Kim, 2020). This is possible by the existence of different light “pigment” molecules in these receptors’ outer segments.

In contrast, somatic peripheral sensory nerve receptors are present throughout the body rather than concentrated on a small number of specialized sensory surfaces. The motor neurons in the brain or spinal cord and the sensory neurons in the dorsal root ganglia make up the single-neuron somatic peripheral nervous system (Akinrodoye & Lui, 2020). Thus somatic peripheral sensory nerve receptors differs from visual signal transduction.

Autonomic Activation in Vision

Visual sensory receptors can activate the autonomic nervous system by alerting the body to hazards or lack thereof. Numerous autonomic receptors regulate a variety of autonomic activities in the eye. These include the internal muscles of the eye, which regulate the pupil’s size and the lens’s shape, and the ciliary body’s secretory epithelium, which creates aqueous humor. The lens’s job is to bring a picture into sharp focus for the retina. The lens changes its shape to provide this clear focus on the basis of whether rays of light are traveling from an area seen close up (more light bending necessary) or far off (less light bending required).

The trabecular network becomes tense when the ciliary muscle contracts. This causes the pores to open, allowing the aqueous humor to drain into the Schlemm canal and then return to the systemic circulation. Beta 1 receptors increases the aqueous humor generation upon the upon the stimulation of ciliary body epithelium (Wu, Zhao & Zhang, 2022). The adaptation of the pupils subjected to ambient light and ocular flow in each tissue are two examples of how the autonomic nervous system (ANS) controls the physiological activities of the eye.

Homeostatic Imbalances in Eyes

The homeostatic imbalances are the cell’s failure to maintain equilibrium in its internal environment and can be brought on by a number of factors, including aging and internal or external damage. Homeostatic imbalances in the eyes could lead to conjunctivitis in which the clear membrane that covers the eyelids and eyes is inflamed or infected. Night blindness is a malnutrition-related condition where a deficiency in vitamin B impairs nighttime vision. Color blindness is a result of chemical or physical eye injury. The eye’s lens becomes opaque in Cataracts.

Hemianopia is the loss of vision in half of the visual field following a recent stroke or other types of brain injury. In a detached retina, the retina separates from the layer of support beneath it. Amblyopia is a form of vision impairment that affects only one eye because of the brain and eye malfunctions. Anopia is loss of vision resulting from an eye structural flaw (Garza, 2021). Thus they lead to temporary or permanent visual impairment.

Conclusion

The eye is a complex organ composed of distinctive tissues that have many functions to maintain ongoing visual response. The retina, the optic nerve, the optic tract, the optic chiasm, and the geniculocalcarine that extends to the occipital cortex are all components of the sensation that play a part in how vision works. The lens, cornea, and retinae are the major components of the eye’s front and rear halves, respectively. Numerous autonomic receptors regulate a variety of autonomic activities in the eye.

References

Akinrodoye, M. A., & Lui, F. (2020). Neuroanatomy, somatic nervous system. In StatPearls [Internet]. StatPearls Publishing. PMID: 32310487.
Garza, A., Diaz, G., Hamdan, M., Shetty, A., Hong, B. Y., & Cervantes, J. (2021). Homeostasis and defense at the surface of the eye. The conjunctival microbiota. Current eye research, 46(1), 1-6. DOI: 10.1080/02713683.2020.1788100
Murphy, G., & SinghVijjan, K. (2020). Physiology of Vision. In Fundamentals in Ophthalmic Practice (pp. 35-45). Springer, Cham. PMID: 30860728
Nguyen, J. D., & Duong, H. (2021). Anatomy, head and neck, eye nerves. In StatPearls [Internet]. StatPearls Publishing. PMID: 31751109
Stjernschantz, J., & Astin, M. (2019). Anatomy and physiology of the eye. Physiological aspects of ocular drug therapy. In Biopharmaceutics of ocular drug delivery (pp. 1-25). CRC Press. DOI:10.1201/9780429284755-1
Wu, F., Zhao, Y., & Zhang, H. (2022). Ocular Autonomic Nervous System: An Update from Anatomy to Physiological Functions. Vision, 6(1), 6. https://doi.org/10.3390/vision6010006
Yao, X., & Kim, T. H. (2020). Fast intrinsic optical signal correlates with activation phase of phototransduction in retinal photoreceptors. Experimental Biology and Medicine, 245(13), 1087-1095. doi: 10.1177/1535370220935406.