Anatomy, Head and Neck, Eye Fovea

Article Author:
Ibraheem Rehman
Article Editor:
Tayyeba Ali
Updated:
9/19/2018 11:55:33 AM
PubMed Link:
Anatomy, Head and Neck, Eye Fovea

Introduction

Vision is perhaps the most useful of the senses for humans. More than 50% of the sensory receptors in the human body are located in the eyes, and a significant portion of the cerebral cortex is devoted to interpreting visual information. The eyes are responsible for detecting visible light, which ranges from 400 to 700 nanometers in wavelength. Objects can absorb and reflect different wavelengths of light. They appear to be they reflect. An object appears white if it reflects all wavelengths of light and it appears black if it absorbs all wavelengths of light. The fovea is an important part of the eye that contributes to eyesight.

Structure and Function

The fovea centralis is located in the center of the macula lutea, a small, flat spot located exactly in the center of the posterior portion of the retina. The fovea is a small depression in the center of the macula lute and contains only cones. The macula lutea is about 5.5 mm in diameter, while the fovea is 0.35 mm in diameter. Furthermore, the fovea has about 50 cone cell per 100 micrometers squared and has an elliptical shape horizontally. It is the spot of highest visual acuity, or resolution, in the eye. This is because it is the most concentrated with cone cells and layers of ganglion and bipolar cells, which scatter light to some degree, do not cover the cones in the fovea and are instead pushed to the periphery of the fovea centralis. Resolution or sharpness in vision is the shortest distance between 2 points in which the 2 points can still be distinguished as 2 separate dots. The main reason a person moves their head and eyes to focus on something is that they are placing the image on the fovea centralis. In fact, even while reading this sentence, the reader is placing these words on your fovea.

Rods are completely absent from the fovea centralis and are more abundant on the periphery of the retina. Rod vision is more sensitive than cone vision, and for this reason, one can see a faint object better if they look slightly to one side rather than looking directly at it. One real-world example of this strategy is demonstrated by how pilots see other aircraft while flying at night. Pilots are taught to routinely survey their surroundings and be aware of other aircraft in the sky. At night, they are taught to look slightly to the side of the area they are examining to detect the lights on other faraway aircrafts better.

Embryology

In general, eye formation in the human embryo begins at approximately 3 weeks into embryonic development and continues through the tenth week. Cells from both the ectodermal and the mesodermal tissues contribute to its development. Specifically, the eye is derived from the neuroepithelium, the surface ectoderm, and the extracellular mesenchyme which is made up of both the neural crest and mesoderm. Neuroepithelium forms the retina and therefore plays a role in the formation of the fovea.

Blood Supply and Lymphatics

Although the fovea is located within the retina, it is not supplied by the central retinal artery or any of its branches. It is instead supplied with blood by the choroid, a vascular layer of the eye that contains connective tissue and lies between the retina and the sclera. The choroid also supplies oxygen to the outermost layers of the retina.

The central 500 micrometers of the fovea contains no retinal capillaries and therefore, must be supplied by the capillaries of the choroid.

Nerves

After light reaches the cones in the fovea and other parts of the eye, signals are conveyed to the brain through the optic nerve. The retina converts light energy into electrical energy, and the corresponding information is transmitted to the brain through the optic nerve. The optic nerve is relatively close to the fovea, but at that point, there are no cones. This results in a blind spot.

Muscles

The 6 extrinsic eye muscles that move each eye are the superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique, and inferior oblique. These muscles are important for moving the eyes in order to place an image on the fovea and get maximum resolution.

Clinical Significance

The eyeball is a complex structure with many small organs that are essential for vision and movement of the eye. An injury to any part of the eye can result in discomfort, reduced vision, and in some cases blindness. The eye is a source of injury, infection, and multiple medical diseases. It is important for all health practitioners to be able to identify and describe the anatomical features of the eye. Multiple diseases can directly or indirectly affect the fovea, thus reducing or erasing perfect vision.

An eye-related disease is age-related macular disease, which is also known as macular degeneration (AMD). AMD is a degenerative disorder of the retina that affects the macula lutea. Individuals with AMD retain their peripheral vision but lose their ability to see straight ahead. Despite the loss of photoreceptors within the fovea, the retina has an innate ability to use the remaining photoreceptors in the most effective manner possible. The retina creates a pseudo fovea, which is a fixation point that stabilizes vision. AMD is the most common cause of blindness for people older than the age of 75. It afflicts over 13 million Americans, and currently, there is no effective treatment.

Most of the time, macular degeneration, which results in the death of photoreceptors cells in the fovea, occurs in older individuals. However, it is possible for younger individuals to suffer from macular degeneration is possible. Stargardt disease is the most common form of juvenile macular degeneration. Stargardt disease is almost always inherited in an autosomal recessive manner. The most common cause of Stargardt disease is a mutation in the ABCA4 gene that is responsible for creating proteins that clear away vitamin A byproducts inside photoreceptors; as these byproducts accumulate, central vision decreases. The progressive vision loss from this disease also results from the death of photoreceptor cells in the macula. Stargardt disease usually destroys central vision but preserves side vision. The vision loss is not correctable with prescription eyeglasses, contact lenses, or refractive surgery.