sensory organ: eye-3

The Conjunctiva

Contraction of the levator palpebrae muscle opens the eyes whereas contraction of the orbicularis oculi closes the eyes. The blinking of the eye lid muscles protects the eye when it is threatened by foreign objects.

Functions of the Eyelids

• It covers the eye and shields it from debris, other foreign objects, and too much sun exposure.
• Tears are dispersed throughout the cornea by the reflexive blinking of the lid muscles, which also prevents drying.
• The sebaceous and sweat glands of the eyelid produce secretions that keep the eye moist.

Eyelashes

They are attached as a double or triple row of hairs on the boarder of each eyelid.

Functions of the Eyelashes

• Eyelashes trap the dust and other foreign objects from reaching the eye.
• Meibomian glands in the inner margins of the eyelid produce sebum that lubricates the lids.

Conjunctiva

The conjunctiva is a thin transparent mucous membrane. The palpebral conjunctiva lines the inner surface of the eyelids and the bulbar conjunctiva covers the anterior sclera of the eye but not the cornea. When the eye is closed, a slit like space occurs between the conjunctiva-covered eyeball and eyelids. This so-called conjunctival sac is where a contact lens lies and eye medications are often administered into its inferior recess.

Functions of the Conjunctiva

• It produces lubricating mucus that prevents the eyes from drying.
• It protects the eye from foreign objects.

Lacrimal Apparatus

The lacrimal apparatus is a group of structures that consists of:

• Lacrimal glands,
• Excretory lacrimal ducts,
• Lacrimal puncta,
• Lacrimal canals,
• Lacrimal sac,
• Nasolacrimal duct.

The lacrimal glands lies within the orbit above the lateral end of the eye and are about the size and shape of an almond. They produce lacrimal fluid (tears) which contains mucus, antibodies and lysozyme, an enzyme that kills certain bacteria. The lacrimal fluid drains into 6-12 excretory lacrimal ducts that empty tears onto the surface of the conjunctiva of the upper lid. Blinking spreads the tears downward and across the eyeball to the medial canthus. From here the tears enter in two small openings called lacrimal puncta (singular punctum). Tears then pass into two ducts, the lacrimal canals, which lead into the lacrimal sac and then into the nasolacrimal duct. The nasolacrimal duct empties into the nasal cavity at the inferior nasal meatus.

Functions of the Lacrimal Apparatus

• Fluids produced by lacrimal glands protect the eye from harmful bacteria.
• Tears moisten the surface of eye, lubricate the eyelids.
• Tears wash away dust particles and the foreign objects.

Movement of each eyeball is accomplished by six muscles. They are:

• Superior rectus,
• Inferior rectus,
• Lateral rectus,
• Medial rectus,
• Superior oblique,
• Inferior oblique.

Functions of the Extrinsic Eye Muscles

• These muscles help to maintain the shape of the eyeball.
• They together hold the eye in the bony orbit.
• These muscles move the eyeball in different directions i.e. superiorly, inferiorly, laterally and medially.

Physiology of Vision (Function of the Eye)

Light is the stimulus that results in vision. The eye, like camera, forms a real inverted image on the retina. To understand how the eye forms clear images of the objects on the retina, we must examine three processes:

• The refraction of light,
• Accommodation,
• Constriction of the pupil.

The Refraction of light: An important characteristic of light is that it can be refracted (bent). As light passes from air to some other more dense substances such as glass or water, it bends at the junction between the two substances. This bending is called the refraction. This principle is used in the eye to focus light on the retina.

Light enters the eye through the pupil and is refracted or bent so that it is focused on the retina. Refraction occurs as light passes through the cornea, the aqueous humor, the lens, and the vitreous humor on its way to the retina. The cornea is responsible for most light refraction (about 75%) in the eye.

Accommodation: The convex lens becomes more curved in order to focus on near objects. This increase in the curvature of the lens for the near vision is called accommodation. Due to the accommodation, the light rays are refracted (bent) to focus on the retina. It is a process that increases the refractory power of the lens so that diverging light rays are bent more sharply.

 This is accomplished as the ciliary muscles contract. This action pulls the ciliary body anteriorly and inward toward the pupil and releases tensions in the suspensory ligaments. The elastic lens no longer stretched and recoils providing the shorter focal length needed to focus image of a close object on the retina. Contraction of the ciliary muscles is controlled mainly by the parasympathetic fibers of the oculomotor nerves (CN-III).

Constriction of the Pupil: Constriction of the pupil is a narrowing of the diameter of the hole through which light enters the eye due to the contraction of the circular muscles of the iris. The circular muscles of iris reduce the size of the pupil toward 2mm. This pupillary reflex occurs simultaneously with accommodation. This prevents the light rays from entering the eye through the periphery of the lens and thus prevents the blurred vision.

As the light enters the eye, the photoreceptor cells (rods and cones) are stimulated. They respond to a light stimulus by producing a nervous impulse. The rod and cone photoreceptor cells synapse with neurons in the bipolar and ganglionic cell layers of the retina.

The axons of all retinal ganglionic cells in one eye exit the eyeball at the optic disc and form the optic nerve on that side. The optic nerve leaves the eye and passes through the optic chiasma where they are connected to each other. Ganglion cell axons from the nasal side retina cross through the optic chiasma and project to the opposite side of the brain. Ganglion cell axons from the temporal side retina pass through the optic nerves and project to the brain on the same side of the body without crossing.

Beyond the optic chiasma, the route of the ganglionic axons is called the optic tract. Most of the optic tract axons terminate in the lateral geniculate nucleus of the thalamus. Some axons do not terminate in the thalamus but separate from the optic tract to terminate in the superior colliculi. Superior colliculi is the center for reflexes initiated by visual stimuli. Neurons of the lateral geniculate ganglion form the fibers of the visual radiations, which project to the visual cortex (primary visual area) in the occipital lobe.

Neurons of the visual cortex integrate the messages coming from the retina into a single message, translate that message into a mental image, and then transfer that image to other parts of the brain where it is evaluated.

In essence, the photoreceptor cells in the eye transform light rays into nerve impulses when they reach the eye. The optic nerve emerges from the eyeball after nerve impulses exit the retina and stretch posteriorly to the optic disc. The optic nerve travels via a number of channels after exiting the eye, including the lateral geniculate body, optic tract, and optic chiasma. The optic nerve then continues on to the occipital lobe's visual cortex. The process of "seeing" in this region of the brain involves the visual interpretation of the nervous impulses produced by light stimuli in the rods and cones of the retina.