Special Senses – Anatomy of the Eye
Special Senses – Anatomy of the Eye
Topics Covered
Introduction
Accessory structures of the eye
Anatomy of the eyeball
Introduction
Senses
• Sensory receptors – Detects changes in body’s internal and external environments
• Two types – General and special senses
General sense
· Have simple neural pathways
· Touch, temperature, pain, chemical and pressure detection, and body perception
· Does not have a specialized organ but comes from almost all over the body
Special sense
· Have complex pathways
· Includes taste (gustation), smell (olfaction), vision, hearing (audition) and equilibrium.
· Information from special senses is carried in special somatic afferents and special visceral afferents.
Vision and Light
• Vision (sight) is perception of light emitted or reflected from objects in the environment
• Visible light is electromagnetic radiation with wavelengths from 400 to 750nm
• Light must cause a photochemical reaction in order to produce a nerve signal that is sent to the occipital lobes of the brain for processing
• Radiation below 400 nm has so much energy it kills cells (Ultraviolet)
• Radiation above 750 nm has too little energy to cause photochemical reaction (it only warms the tissue) (Infrared)
Accesory structures of the eye
(Link to figure 14.13- Structure of the Eye)
• Lateral and medial commissure – upper and lower lids meet
• Lacrimal caruncle – Red fleshy globe-like nodule in the medial commissure that contains sebaceous and sweat glands
• Palpebrae – eyelids
• Palpebral fissure – Space between eyelids
• Palpebral conjunctiva
o Thin mucus membrane that lines the inside of the eyelids.
o Secretes mucus to reduce friction and moisten the eyeball surface
• Ocular conjunctiva
o Covers the sclera (white part of the eye). Reflects over the anterior surface of eyeball.
• Levator Palpebrae Superioris Muscle – lifts upper eyelids
• Cornea- the transparent part of the coat of the eyeball that covers the iris and pupil and admits light to the interior
• Tarsal Plates- are two comparatively thick, elongated plates of dense connective tissue, and contributes to eyelids form and support.
Lacrimal Apparatus
Lacrimal fluid or tears flowing across eyeball helps wash away foreign particles, help with diffusion of O2 & CO2 and contain bactericidal enzyme called lysozyme.
Extrinsic Eyes Muscles
• 6 muscles inserting on external surface of eyeball 4 rectus muscles move eye up, down, left & right superior & inferior oblique more complicated
• Innervated by cranial nerves III, IV and VI
• CN III (Oculomotor): Superior rectus, medial rectus, inferior rectus, and inferior oblique
• CN IV (Trochlear): Superior oblique
• CN VI (Abducens): Lateral rectus
• The Trochlea of superior oblique is a pulley structure in the eye. The tendon of the superior oblique muscle passes through it
Anatomy of the eye
(Linked to Figure 14.15)
The Tunics of the Eyeball
tunica fibrosa – fibrous layer
• sclera and cornea
• provides protection
tunica vasculosa – vascular layer
• choroid, ciliary body & iris
• provides nourishment
tunica interna – internal layer
• retina and optic nerve
• converts light to nerve signal and sends it to the occipital lobes of the brain
Tunica fibrosa
• Sclera
o White part of the eye
o Covers the eyeball EXCEPT at the cornea
o Covered by the ocular conjunctiva
o Resists punctures and protects the eye
o Helps maintain pressure in the eye to keep its shape and keep the retina adhered to the back of the eye
o Attachment point of extrinsic muscles of the eye
• Cornea
o Clear, curved portion covering the pupil
o Light enters the eye through the cornea
o Most responsible for bending of light to focus towards the back of the eye
o Consists of many layers of densely packed collagen fibers
o Dense and fibrous nature provides protection
Tunica vasculosa
• Choroid
o Most posterior portion of this tunica
o Contains melanin that absorbs light to prevent reflection back into eyeball, which can cause blurred vision
o Also provides nourishment to the retina
• Ciliary body
o Contains ciliary process and ciliary muscle
o Connected to the lens by the suspensory ligaments
o Helps in adjusting the shape of the lens for near and far vision
o Produces aqueous humor
• Iris
o Pigmented portion around the central aperture --the pupil
o Constrictor and dialator muscle of the iris change the diameter of the pupil to regulate the amount of light striking the retina
Tunica interna
• Retina and optic nerve
o Contains 2 layers
o Outer pigmented layer prevents light scattering and absorbs light
o Inner neural layer contains 2 types of photoreceptors (Figure 14.16)
§ rods – determine motion and general shape of objects in dim light; black-and-white vision
§ cones – color vision and visual acuity; used in bright light
o Anterior margin of the retina where the choroid is exposed is the
Ora serrata.
· Optic nerve exits the eyeball at the optic disc, so the optic disc lacks photoreceptors and, hence, is called the blind spot.
· Lateral to the optic disc is an area of high cone density called the
Macula lutea.
- In the center of the macula lutea is a small depression called the fovea centralis, which is the area of sharpest vision because of the abundance of cones.
Additional Structures
• Pupil- a contractile hole located in the center of the iris of the eye that allows light to enter the retina
• Lens – fine-tunes bending of light to focus the image on the retina
o Divides the eyeball into two cavities:
o Anterior cavity – Area between the lens and the cornea
o Posterior cavity – Area between the lens and the retina
• Anterior cavity
o Contains a watery fluid called aqueous humor
o Aqueous humor helps to maintain the intraocular pressure and supply nutrients to the lens and the cornea
o Divided into an anterior and posterior chamber by the iris
• Posterior cavity
o Contains a jelly-like substance called vitreous humor
o Vitreous humor holds the retina against the choroid layer and prevents the eyeball from collapsing