Anatomy & Physiology: Special Senses

Anatomy & Physiology: Special Senses

I.  The Eye & Vision

A.  Accessory Structures of the Eye

1.  Orbital Cavity

a.  Frontal, maxilla, ethmoid, ethmoid, sphenoid, lacrimal, & palatine bones.

b.  Optic foramen – opening in posterior wall through which the optic nerve and ophthalmic nerve pass.

c.  Superior orbital foramen – opening through which other nerves and arteries pass.

d.  Fat pad – cushions & absorbs mechanical blows to the eye.

2.  Eyebrow – ridge of frontal bone that is covered by coarse hair. Provides shade and prevents perspiration from dripping into the eye.

3.  Eyelids – thin, skin covered folds that protect the eye from foreign objects and helps keep eyes moist. The blinking reflex occurs every 3 – 7 seconds.

4.  Conjunctiva – transparent mucous membrane that coves the inside of the eyelid and the white of the eyeball. Functions to keep the eye moist and prevent objects from going behind the eyeball.

5.  lacrimal Apparatus

a.  Lacrimal gland – located on the superior, lateral area of the eye.

b.  Lacrimal secretion (tears) – dilute saline, mucus, & enzyme solution that keeps eye moist, pH balanced, and free of germs.

c.  Blinking washes tears across the eyes.

d.  Nasolacrimal duct – drains excessive tears from the eye into the nasal cavity.

6.  Extrinsic Eye Muscles – 6 muscles that control the movement of the eyes

a.  superior rectus

b.  inferior rectus

c.  lateral rectus

d.  medial rectus

e.  inferior oblique

f.  superior oblique

B.  Structure of the Eyeball (three layers: Vascular Tunic, Fibrous Tunic, Nervous Tunic)

1.  Fibrous Tunic – outermost layer of the eye

a.  Sclera (White of the eye) – white collagenous fibers that compose 5/6 of the eye. Very tough & functions in protection & holding the eye together.

b.  Cornea – anterior, transparent 1/5 of the eyeball. Contains no blood vessels and is continuous with the sclera. Functions as a window to allow light to enter the eye and to bend most of the light entering the eye.

2.  Vascular Tunic – middle layer of the eye that has many blood vessels

a.  Choroid - Black to brown tissue that lines the back 5/6 of the eye. Functions to supply nutrients and blood to the inner tissues of the eye and t absorb excess light.

b.  Ciliary body – thickened ring of tissue that encircles the lens.

1)  Ciliary Muscles – muscles that control the shape of the lens.

2)  Suspensory ligament – connects the ciliary muscles to the lens.

c.  Lens – biconvex, transparent, flexible structure that is able to change its shape to allow for precise focusing.

d.  Iris – colored part of the eye. Composed of circular and radial arranged smooth muscles that function to regulate the amount of light that enters the eye from its center opening (pupil).

1)  Iris dilates to let in more light and constricts to decrease the amount of light.

2)  Colors are brown, blue, green, or gray depending upon the amount of brown pigment.

e.  Aqueous Humor – clear fluid between the cornea and the ciliary body. It maintains a constant pressure of fluid to maintain the shape of the cornea and eyeball.

3.  Nervous (Sensory) Tunic – innermost layer of the eye

a.  Retina – layer that contains visual receptor cells.

1)  rods – responsible for black & white vision. Stimulated in dim light.

2)  Cones – responsible for color vision. Stimulated in bright light.

a)  concentrated near the center of the retina

b)  three colors: red, blue, & green.

b.  Macula lutea (yellow spot) – central region of the retina. Small depression in the middle called the Fovea Centralis, which is where most of the light is focused and most of the cones are concentrated.

c.  Optic disc (blind spot) – located just medial of the macula lutea, this area contains NO photoreceptors and is where the optic nerve exits the eye.

d.  Vitreous Humor – clear, jelly-like fluid that functions to support the shape of the eyeball and to hold the retina against the surface of the eye ball.

C.  Vision Physiology

1.  Refraction & lenses

a.  Refraction is the bending of light as it passes through a transparent object.

b.  A lens is a transparent object that is curved on one or both sides such that light is bent as it passes through.

1)  convex lens – cause light waves to converge

2)  concave lens – cause light waves to diverge

c.  Focal point – the point at which convergent light rays meet.

d.  The thicker the lens, the more the light rays will be bent.

e.  Real image – the image formed by a convex lens will be upside down and backwards.

2.  Visual focusing

a.  As light reaches the eyeball the cornea refracts it towards the lens. Most of the refraction of the light occurs at the cornea.

b.  Light passes through the lens and is focused on the retina.

1)  Distant objects require less focusing, thus the lens is stretched by the ciliary muscles into a thinner shape.

2)  Near objects need more focusing thus the ciliary body relaxes and allows the lens to recoil back to its shorter, fatter shape that refracts the light more. (accommodation)

c.  Emmetropic vision is normal vision and requires NO accommodation or change in lens shape (usually 20 feet.)

3.  Refraction problems

a.  Myopia (nearsightedness) – results from an eyeball that is too long. Objects close up can be seen, but far away objects are focused in front of the retina. Correction involves concave lenses.

b.  Hyperopia (farsightedness) – results from too short of a eyeball. Objects can be seen far away, but near objects focus beyond the retina. Correction involves convex lenses.

c.  Astigmatism – unequal curvature of the lens that causes unequal focusing of objects back on the retina. Correction involves special cylindrically ground lenses.

d.  Presbyopia (farsightedness due to age) – elasticity of the lens decreases with age (usually starting around age 45) and accommodation decreases. Correction involves bifocal glasses that have increased convex lenses at the lower half for reading and close up work.

D.  Disorders of the Eyes

1.  Cataracts – thickening and hardening of the lens that causes it to cloud over and eventually blindness. Due to age (normally), UV light, smoking, diabetes. Lens replacement surgery is only cure.

2.  Glaucoma – a buildup of aqueous humor in the anterior chamber of the eye that eventually can cause blindness. Due to decreased drainage of the aqueous humor past the age of 40. If detected early on, it can be corrected using eye drops that increase drainage.

3.  Colorblindness – sex-linked hereditary condition that results in the absence of one or more types of cones.

a.  Monochromates – black & white only

b.  Dichromates – blue-yellow or red-green