Special Senses -The Eye

Bony Orbits

• The eye, the organ of vision, consists of the eyeball and optic nerve.
• The bony orbit houses the eye and its accessory visual structures.
• These are bilateral bony cavities within the facial skeleton.
• The orbit is shaped like a pyramid, with the base directed anteriorly and the apex pointing posteriorly and medially.
• The bony orbit has four walls: superior, inferior, lateral, and medial.
• The apex of the orbit contains the optic canal, which transmits the optic nerve (CN II).
• The orbit has two fissures (slit-like openings): the superior and inferior orbital fissures.
• These fissures are used by nerves and vessels to pass into and from the orbit.

Accessory Structures of the Eye

• Accessory structures include: eyebrows, eyelashes, eyelids, extrinsic muscles that move the eyeballs, and the lacrimal apparatus.
• Eyelids and eyelashes protect the eyeballs from foreign objects and direct sunlight.
• The lacrimal apparatus is the tear system (“lacrima” = tear).
• It includes glands, ducts, and canals that produce and drain lacrimal fluid (tears) from the orbit into the nasal cavity.
• The lacrimal gland is located in the upper lateral corner of the orbit, and the direction of tear flow is towards the medial corner, where tears are collected and delivered into the nasolacrimal duct, which opens to the floor of the nasal cavity.
• Tears are watery solutions containing salts, some mucus, immunoglobulins, lactoferrin (milk-based protein), and a bacteria-killing enzyme called lysozyme.
• Tears clean, lubricate, and moisten parts of the eyeball exposed to air to prevent drying.
• Tears are usually cleared away by evaporation or by passing into the nasal cavity as fast as they are produced.
• If an irritating substance contacts the eye, lacrimal glands are stimulated to oversecrete, and tears accumulate as a protective mechanism.
• Parasympathetic nervous system stimulation of the lacrimal glands produces excessive tears that may spill over the edges of the eyelids and overfill the nasal cavity with fluid, explaining why crying produces runny noses.

Extraocular Muscles

• Six extrinsic eye muscles move the eyeball: left, right, up, down, and diagonally.
• Three pairs of cranial nerves are dedicated to the extraocular muscles (CN III, IV, and VI).
• Neurons in the brainstem and cerebellum coordinate and synchronize eye movements.
• The specific muscles and their actions are:
  • Superior Rectus: Elevates, adducts, and medially rotates the eyeball.
  • Medial Rectus: Adducts the eyeball.
  • Lateral Rectus: Abducts the eyeball.
  • Inferior Rectus: Depresses, abducts, and laterally rotates the eyeball.
  • Inferior Oblique: Abducts, elevates, and laterally rotates the eyeball.
  • Superior Oblique: Abducts, depresses, and medially rotates the eyeball.

Eyeball Structure

• The eyeball is made up of structures designed to support and protect the photoreceptor cells, gather, focus, and process light into images.
• The adult eyeball measures approximately 2.5 cm in diameter and is divided into three layers:

Layers of the Eyeball

• Fibrous Layer (Fibrous tunic): external layer consisting of two parts; cornea & sclera
• Vascular Layer (Vascular tunic): Middle layer consisting of three parts; choroid, ciliary body & iris
• Retina: Internal layer consisting of a pigmented layer and a neural layer

Fibrous Tunic

• Outer coat consisting of the anterior cornea and posterior sclera.
• The cornea is a transparent fibrous coat that covers the colored iris. It is curved and helps focus light rays onto the retina.
• The sclera, the white of the eye, is a coat of dense connective tissue that covers the entire eyeball except the cornea.
• It provides shape to the eyeball, makes it more rigid, and protects its inner parts.
• An epithelial layer called the conjunctiva covers the sclera but not the cornea and lines the inner surface of the eyelids.
• Inflammation of the conjunctiva is called conjunctivitis.

Vascular Tunic

• Also referred to as ‘uvea,’ it is the middle layer of the eyeball.
• Composed of the choroid, ciliary body, and iris.

Choroid

• Thin membrane lining most of the internal surface of the sclera.
• Contains many blood vessels.
• Contains melanocytes that produce the pigment melanin, which causes this layer to appear dark brown in color.
• Melanin absorbs stray light rays, preventing reflection and scattering of light within the eyeball.

Ciliary Body

• Located at the front of the eye.
• Consists of the ciliary processes, folds on the inner surface of the body whose capillaries secrete a fluid called aqueous humor, and the ciliary muscle - a smooth muscle that alters the shape of the lens for viewing objects up close or at a distance.

Lens

• Transparent structure that focuses light rays onto the retina.
• Constructed of many layers of elastic protein fibers.
• Zonular fibers or suspensory ligaments attach the lens to a ciliary muscle and hold the lens in position.

Iris

• The colored part of the eyeball.
• Includes both circular and radial smooth muscle fibers (sphincter & dilator pupillae muscles, accordingly).
• The hole in the center of the iris, through which light enters the eyeball, is the pupil.
• The sphincter and dilator pupillae muscles regulate the amount of light entering the pupil by decreasing or increasing the size of the pupil, accordingly.
• Attached to processes of the ciliary body are the suspensory ligaments (or zonular fibers) that stretch the lens to focus the image of objects based on how close/far they are.
• The contraction of the ciliary muscle causes the lens to assume a more rounded shape.
• This is because the lens is suspended by fine ligaments called zonules which attach to the ciliary body.

Accommodation

• When the ciliary muscle contracts, the anchoring point of the zonules move inward, relaxing tension on zonules and natural elasticity of the lens causing it to take on a more spherical shape.
• This is how our focus shifts.

Iris Muscles

As mentioned, iris contains muscles that regulate the amount of light entering the pupil:

1. The sphincter pupillae reduces the pupil size.
2. The dilator pupillae increases the pupil size.

• Since these are smooth muscles, they are innervated by the autonomic nervous system

Retina

• Third layer and inner coat of the eyeball - lines the posterior three-quarters of the eyeball and is the beginning of the visual pathway.
• It has 2 layers:
  • Neural Layer: multilayered outgrowth of the brain, includes 3 distinct layers of retinal neurons -
  • the photoreceptor layers
  • the bipolar cell layer
  • the ganglion cell layer
  • Pigmented Layer: Sheet of melanin-containing epithelial cells located between the choroid and the neural part of the retina -
  • Melanin in the pigmental layer helps to absorb stray light rays

Photoreceptors

• Specialized cells that are found within the neural layer of the retina.
• Begin the process by which light rays are ultimately converted to nerve impulses - two types
  • Rods: Allow us to see shades of grey & dim light, such as moon light.
  • Cones: Stimulated by brighter light giving rise to a highly acute, colour vision - several types of cones, and colour vision results from the stimulation of various combinations of different types of cones.
• Complete loss of cone vision causes a person to become legally blind - people who lose rod vision mainly have difficulty seeing in dim light and thus cannot drive at night (night blindness).
• Absence of deficiency of one of the 3 types of cones from retina cannot distinguish some colours (colour-blindness).
• Cones are most densely concentrated in the fovea centralis - a small depression in the centre of the macula lutea, or yellow spot, in the exact centre of the retina - area of highest visual acuity or resolution (sharpness of vision) because of its high concentration of cones.
• Rods are absent from the fovea centralis and macula lutea and increase in numbers toward the periphery of the retina.
• From photoreceptors, information flows to bipolar cells and then from bipolar cells to the ganglion cells
• The axons of ganglion cells extend posteriorly to a small area of the retina called the optic disc (blind spot), where they all exit as the optic nerve (CN II)
• Because the optic disc contains no rods or cones, we cannot see an image that strikes the blind spot

Ophthalmoscopic View

• Ophthalmoscopy is often done as part of a physical eye exam - it visualizes the posterior part or the fundus of the eye, which includes the retina, optic disc, choroid and blood vessels
• The retina is the only portion of the CNS visible from the exterior
• The macula of the retina is a spot containing cone cells - located on a direct pass of light through the eyeball
• The fovea is located in the center of the macula - highest visibility
• The fundus of the eye is only location in the body where the vasculature can be visualised
• Located on the medial side (nasal side) of the fundus is the blind spot or the optic disc - where the axons of ganglionic cells form the optic nerve, which exits the eyeball

Eye Segments

The lens divides the interior of the eyeball into 2 cavities or segments:

• Anterior segment: located anterior to the lens
  • Lies between the lens and the cornea
  • Filled with aqueous humour, a watery fluid similar to cerebrospinal fluid
  • Blood vessels within the ciliary body secrete these fluids - drains into the scleral venous sinus located where the sclera and cornea meet
  • Aqueous humor helps maintain the shape of the eye & nourishes the lens and corneum - neither of which have blood vessels
• Posterior segment: located posterior to the lens
  • Larger than the anterior - also called the vitreous chamber because it contains a clear, jellylike substance called the vitreous body
  • This body helps prevent the eyeball from collapsing & holds the retina flush against the choroid
• Pressure in the eye, called intraocular pressure, is produced mainly by the aqueous humour with a smaller contribution from the vitreous body
• Intraocular pressure maintains the shape of the eyeball & keeps the retina smoothly pressed against the choroid, so the retina is well nourished & forms clear image
• Normal intraocular pressure (about 16 mm Hg) is maintained by a balance between production & drainage of the aqueous humour

Visual Pathway

After stimulation by light, the rods & cones trigger electrical signals that create a cascade of events associated with the visual pathway

• Because of the crossing at the optic chiasm, the right side of the brain receives signals from both eyes for interpretation of visual sensations from the left side of an object, and the left side of the brain receives signals from both eyes for interpretation of visual sensations from the right side of an object

Steps

1. Retina
   • Photoreceptors & neurons process the stimulus from incoming light - rod/cones trigger electrical signals in bipolar cells, transmitting signals to ganglion cells - ganglion cells depolarize and generate nerve impulses
2. Optic Nerve
   • Axons of retinal ganglion cells form optic nerves & exit eye
3. Optic Chiasm
   • Optic nerve axons from the medial region of the retina cross at the optic chiasm; axons from the lateral region of the retina remain uncrossed
4. Optic tract
   • Contains axons from both eyes - axons project to either the thalamus or midbrain
5. Thalamus
   • The majority of optic tract axons project to the lateral geniculate nucleus in the thalamus
6. Midbrain
   • Fibers projected here are important for pupillary reflexes & accommodation & for reflective movements of extraocular eye muscles
7. Projection Fibers (Optic Radiation)
   • Projection tract functioning to transmit visual input from the retina, optic nerve, and optic tract to the primary visual cortex in the occipital lobe
8. Primary Visual Cortex
   • The primary visual cortex is located in the occipital lobe. It receives processed information from the thalamus for vision