Chapter 8Special Senses

The SensesThe Senses

Special senses Smell Taste SIGHT Hearing

Equilibrium

Lady website

The Eye and VisionThe Eye and Vision

Of all the sensory receptors in the body

70% are found in the eyes

Optic tracts

Carry information from the eyes to the brain

Consists of over a million nerve fibers per eye

The Eye and VisionThe Eye and Vision Protection for the eye

Only 1/6th of the eye’s surface can be seen Remaining portion of the eye

Enclosed in a bony orbit Cushioned by a layer of fat

Accessory Structures Eyelids Conjuctiva Extrinsic eye muscles Lacrimal apparatus

Accessory Structures of the EyeAccessory Structures of the Eye

• Eyelids

• Meet at the medial & lateral corners of the eye = (Medial & lateral canthus)

Nictitating membrane

Eyelashes

Accessory Structures of the EyeAccessory Structures of the Eye

Accessory Structures of the EyeAccessory Structures of the Eye

Conjunctiva Delicate membrane

Lines the eyelids & covers part of the outer surface of the eyeball

Connects to the surface of the eye

Secretes mucus to lubricate the eye

Conjuctivitis?

Accessory Structures of the EyeAccessory Structures of the Eye

Conjunctivitis

PINKEYE Common condition

Inflammation of the conjunctiva Clear membrane that covers the white part of the eye & lines

the inner surface of the eyelids

No danger to the eye or your vision

Causes Bacteria, viruses, allergies (hay fever), irritants

Accessory Structures of the EyeAccessory Structures of the Eye

Lacrimal apparatus Consists of the lacrimal glands and ducts drain lacrimal

secretions into the nasal cavity Lacrimal glands

Continually release tears Tears

Antibodies Lysozyme

Enzyme that destroys bacteria Clean, protect and moisten the eye surface

Accessory Structures of the EyeAccessory Structures of the Eye

Extrinsic (external) eye muscles

6 muscles attached to the outer surface of each eye

Produce eye movements

Internal Structures: The EyeballInternal Structures: The Eyeball

Eyeball

Hollow sphere

Its wall is composed of three tunics (layers) Sclera or fibrous tunic

“White of the eye” Outermost layer

Choroid or vascular tunic Middle layer

Sensory tunic Innermost layer Retina

Interior is filled with fluids called humors

Internal Structures: The EyeballInternal Structures: The Eyeball

The Fibrous TunicThe Fibrous Tunic Sclera

White connective tissue layer “White of the eye”

Cornea Transparent “window”- central anterior portion Allows for light to pass through Repairs itself easily Only human tissue that can be transplanted from

one person to another without the fear of rejection No blood vessels beyond the reach of the

immune system

Choroid LayerChoroid Layer Middle layer Blood-rich nutritive tunic Pigment prevents light from scattering Modified anteriorly into two structures

Cilliary body – smooth muscle to which the lens is attached

Iris Pigmented layer that gives eye color Pupil – rounded opening in the iris through

which light passes

Sensory Tunic (Retina)Sensory Tunic (Retina)

Contains receptor cells (photoreceptors) Rods Cones

Signals pass from photoreceptors and leave the retina toward the brain through the optic nerve

Photoreceptors Distributed over the entire retina, except

where the optic nerve leaves the eyeball

Optic disc or blind spot*

Neurons of the RetinaNeurons of the Retina

Neurons of the Retina & VisionNeurons of the Retina & Vision

Rods

Most are found towards the edges of the retina

Allow dim light vision and peripheral vision

Test fact of rod location by looking “off-center” at objects at night…stars/glow-in-the-dark objects on ceiling

Perception is in all gray tones

Neurons of the Retina & VisionNeurons of the Retina & Vision

Cones Densest in the center of the retina Fovea centralis

Area of the retina with only cones Greatest visual acuity

3 types of cones respond to a particular wavelength of light Blue light Green light Both green & red light

** Color blindness lack of one to all cone types

Color-Sensitive Cones

LensLens Focuses light entering the eye on the

retina

Flexible biconvex crystal-like structure

Held in place by a suspensory ligament attached to the ciliary body

Divides the eye into 2 segments Anterior (aqueous) segment

Posterior (vitreous) segment

LensLens

Internal Eye Chamber FluidsInternal Eye Chamber Fluids

Anterior (aqueous) Segment

Contains a watery fluid Aqueous humor

Found in chamber between the lens and cornea

Similar to blood plasma

Helps maintain intraocular pressure

Provides nutrients for the lens and cornea

Reabsorbed into venous blood

Blocked drainage = Glaucoma

Internal Eye Chamber FluidsInternal Eye Chamber Fluids

Posterior (Vitreous) Segment

Vitreous Fluid

Gel-like substance behind the lens

Keeps the eye from collapsing

Lasts a lifetime and is not replaced

Lens AccommodationLens Accommodation

Light must be focused to a point on the retina for optimal vision

Resting eye is set for distance vision (over 20 ft away)

The lens must change shape to focus for closer objects

Vision

Emmetropia Correct focus

Myopia nearsightedness Focus of light in front of retina Eyeball too long or lens too strong Distant objects are blurry

Hyperopia farsightedness Focus of light beyond the retina Short eyeball or lazy lens Near objects are blurry.

Emmetropia

Hyperopia