Anatomy of sight
Perception of the external environment has allowed animals to interact with their surroundings and each other. One of the mechanisms by which this interaction occurs is visual perception. The visual pathway involves an intricate complex of structures, including the eyes, optic nerves and tracts, nuclei of the thalamus and midbrain, and grey matter areas of the occipital lobes. Each of these segments of the visual pathway has warranted their own articles. As a result, this article will focus on the general overview of the visual pathway, as well as the parts of the eye directly associated with vision. Furthermore, several aetiologies of loss of vision, as well as other abnormalities of the visual pathway, will also be discussed.
Anatomy of the Eyes
The eyes are paired slightly asymmetrical spherical structures encased within the bony orbit. Their primary function is to facilitate the detection and conversion of light into nerve impulses. These nerve impulses are subsequently interpreted by the brain as images. There are some areas of the eye that are directly associated with vision, while others provide nutrition and structural support.
Iris, Pupil and Sclera
Direct visualization of an eye reveals three primary structures: the pupil, the iris and the sclera. The pupil is the black aperture in the center that allows light to enter the eye. Since the light rays that enter the eyes are absorbed by pigments in the retina or adjacent tissue, it appears black.
The iris is the pigmented circular muscle, which gives us the color of our eyes. It is an adjustable pigmented diaphragm that divides the anterior segment into an anterior chamber (between the cornea and the iris) and a posterior chamber (between the iris and the anterior surface of the lens). The muscles of the iris are arranged as circular and longitudinal fibers. These are the sphincter pupillae and dilator pupillae, respectively. By the action of these muscles, the iris is able to alter the diameter of the pupil (central aperture of the eye) in response to the intensity of light.
The outer wall of the eye is called the sclera. The sclera is a collagenous white supporting wall of the eye that is continuous with the cornea anteriorly.
The cornea is a transparent external surface that covers both the pupil and the iris. The cornea is the first layer that light encounters; it serves as the first and most powerful lens of the visual pathway. This is a clear, avascular, convex anterior continuation of the outer coating of the eye. It is continuous with the sclera (opaque outer coating of the eye) at the corneoscleral junction, also known as the limbus. Five distinct layers of the cornea are visible on cross-section. From superficial to deep these are:
- corneal epithelium
- Bowman’s layer (anterior limiting lamina)
- stroma (substantia propria)
- Descemet’s layer (posterior limiting lamina)
The eye is divided into three chambers. The anterior chamber lies between the cornea and the iris. The cornea forms the anterior border of the anterior chamber of the eye, which is a subsection of the anterior segment. For completion, the anterior segment is bordered posteriorly by the iris, lens and ciliary body.
The posterior chamber is located between the iris and the lens. Both the anterior and posterior chambers are filled with aqueous humor, a watery solution high in nutrients to support the avascular cornea and lens.
The vitreous chamber extends back from the lens to the retina and is filled with the thick, gel-like vitreous humor.
Another significant point of refraction occurs at the lens. This is an encapsulated structure that is biconvex in shape. It is initially a transparent body that becomes opaque as individuals age (senile cataracts). The lens has three major regions that could be compared to the structure of a mango. The outer capsule of the lens is analogous to the skin of the mango, the cortex could be compared to the fleshy substance, and the nucleus would be homologous to the seed of the mango.
The lens is suspended by ligamentous attachments of the ciliary body (apparatus) in aqueous humour (produced by the ciliary body). From superficial to deep, the supraciliary layer, smooth ciliary muscles, stroma and bilaminar epithelium form the four layers of the ciliary body. The structure can be further subdivided into the pars plicata (circumscribes the iris) and the pars plana (adjacent to the junction of the ciliary body and retina – the ora serrata).
All the structures previously mentioned are concerned with modifying light in order to focus the image on the retina. Once at the retina, light is then detected and transduced into nerve impulses. The retina is comprised of ten cellular layers that work sequentially in order to transduce light energy. From superficial to deep (i.e. from the outer layer of choroid to the layer adjacent to the vitreous humour), these layers are:
Inner limiting membrane
Nerve fibre layer
Ganglion cell layer
Inner plexiform layer
Inner nuclear layer
Outer plexiform layer
Outer nuclear layer
Outer limiting membrane
Retinal pigmented layer
The retina coats the inner surface of the globe and extends circumferentially to the ora serrata. There are several distinct features of the retina that can be observed with direct ophthalmoscopy. These include the vascular arcade emanating from the optic disc (optic nerve) as well as the macula, which is inferotemporal to the disc. The macula is responsible for central vision, and contains the fovea which is responsible for the ultimate central vision. It is the most sensitive part of the retina. It should be noted that in these areas, the retina does not have its typical ten layer appearance.
The optic nerve (CN II) is formed from the axons of the retinal ganglion cells (layer three). Each CN II contains fibers of the nasal and temporal fields of the respective eye that leave the globe via the optic disc. The fibers leave the posterior pole of the eye and meet medially in the chiasmatic groove (anterior to the sella turcica of the middle cranial fossa). In the groove, fibers of the nasal field of each eye decussate to form an “x shaped” structure called the optic chiasm. After crossing over, the fibers from the nasal field travel with the fibers of the temporal fields of the contralateral eye. Each subsequently forms their respective optic tracts.
While the six extraocular muscles do not assist in the perception of light, they facilitate the movement of the ocular globe, allowing it to capture light being emitted from various structures. Four of the muscles are arranged roughly at right angles to each other; these are the superior, inferior, medial and lateral recti. They are responsible for deviating the globe in their named directions.
Two additional muscles known as the superior oblique and inferior oblique muscles turn the globe up and out, and down and in, respectively. The muscles work collaboratively or independently to alter the gaze in a variety of directions.
Light rays of the visible spectrum travel in a vacuum or through media (water, transparent solids, etc.) as parallel waves. In order for waves to be focused on the retina so that they can be transmitted to the brain, the waves must be refracted. Refraction refers to the slowing of waves as they transition from one medium to the other, as a result of reduction in the speed at which they travel. The cornea provides the first point of refraction for light waves reflecting from an object and entering the eyes. The aqueous humour also provides some amount of refraction before the rays make contact with the lens, which provides the greatest degree of refraction in the eyes. The refractive index of the lens can be changed with alternating tension of the ciliary muscles depending on the distance of the object from the observer.
It should also be noted that the amount of light that enters the light is regulated by the action of the iridial muscles (muscles of the iris), which is under autonomic control. Simultaneous relaxation of the annular sphincter pupillae and contraction of the radial dilator pupillae result in increased pupillary diameter. The converse results in a reduction in the pupillary diameter. These processes not only regulate the amount of light entering the globe, but it also aids in the accommodation process to enhance vision.
After passing through the vitreous humour (which also offers some amount of refraction), light passes through all ten layers of the retina to stimulate the pigmented cells of the retinal pigmented epithelium, which rests on Buch’s membrane (deep to the choroid). Impulses generated from stimulating these cells then pass to the proximal dendrites of the bipolar rods and cones in the photoreceptor layer. The outer and inner segments of the rods and cones are separated from their cell bodies and distal axons by the outer limiting membrane; and are found in the outer nuclear layer. The proximal axons of bipolar and horizontal cells of the inner nuclear layer interact with distal axons of rods and cones at the outer plexiform layer. Amacrine cells of the inner nuclear layer and ganglion cells of the ganglion cell layer interact at the inner plexiform layer. The distal axons of the ganglion cells coalesce and form the nerve fibers of the optic nerve in the nerve fibre layer, which are separated from the vitreous cavity by the inner limiting membrane.
Axons from the ganglion cells leave the retina via the optic disc and form the optic nerve. Each nerve contains fibers arising from both the temporal and nasal fields of each respective eye. In the chiasmatic sulcus in the middle cranial fossa, the nasal fibers from each eye decussate in the optic chiasm to join the temporal fibers of the contralateral eye to form the respective optic tracts. The optic tracts will then travel posteriorly along the inferior surface of the brain to give fibers to the lateral geniculate bodies of the thalamus and finally, to the optic fields (primary visual cortex) of the occipital lobe (Brodmann area 17). The primary visual cortex (commonly referred to as V1 (visual 1) is located on the medial surface of the occipital lobe, surrounding the calcarine sulcus.