SPECIAL SENSES

the human body is very sensitive to conditions in both its internal and external environment

the nervous system collects information about its environment with a wide variety of receptors located throughout the body:

Chemoreceptors: sensitive to chemicals.

Baroreceptors: sensitive to pressure,

Osmoreceptors: sensitive to water levels

Mechanoreceptors: sensitive to vibrations, Photoreceptors: sensitive to light

SENSORY RECEPTORS: are highly modified ends of sensory neurons, often located in specialized sensory organs.

SENSORY ADAPTATION: is a condition where a sensory receptor becomes conditioned to a stimulus; the neuron doesn’t reach an action potential even though the stimulus is still present.

Special Sense Organs: 1.The Eyes

I. STRUCTURE

structures on the external portion that are not used in the process of vision include:

Eye Lid and Lashes: used for protection and elimination of foreign particles

Extrinsic Muscles: control the movement of the eye in the oribital cavity.

the interior of the eye has many specialized structures that allow the transmission of visual information to the Occipital Lobe of the Brain.

a. SCLERA:

is the outer white layer of the eye; attachment site of the extrinsic muscles, gives shape to the eyeball.

b. CHOROID LAYER:

inside the sclera, contains blood vessels supplying the eye with nutrients and oxygen, contains a dark pigment making the eye appear dark inside.

c. RETINA: is the innermost layer of the

eye containing photoreceptors cells. There are two types of photoreceptor cells:

i) CONES: specialized for color vision and sharp detail; activated by bright light; less numerous than rods .

ii) RODS: specialized for vision in dim light; used to discriminate between shades of grey, used to see shapes and movement.

d. FOVEA : an area at the back of the retina which has a high concentration of cones and no rods; used for viewing fine details.

e. Blind Spot

the area at the back of the eye containing no rods or cones, the location of the optic nerve

f. OPTIC NERVE:

the nerve that carries visual information to the occipital lobe of the brain.

g. Cornea

transparent covering of the front of the eye, bends light towards the pupil.

Continuous with the sclera

h. AQUEOUS HUMOR:

fluid behind the cornea, supplies the cornea with nutrients.

I. Iris

made of colored muscle that controls the size of the pupil depending on light availability; the darker it is the larger the pupil is.

j. PUPIL:

the opening into the eyeball, it is black in color because of the pigmentation in the choroid layer

k. Lens located behind the iris, focuses the image on the retina; the shape of the lens is controlled by cilliary muscle

l. VITREOUS HUMOR:

a cloudy, jelly like material that helps maintain the shape of the eye,fill the eyeball.

II. HOW WE SEE ?

particles of light travel in various wavelengths; light enters the eye through the pupil and passes into the lens

the lens changes shape to focus the light on the photoreceptors in the retina

In dim light: RODS, contain a chemical

called RHODOPSIN, made from a protein and Vitamin A

when a light wave strikes a rod cell, the chemical rhodopsin changes form causing an action potential to occur;

the transmission is carried through the optic nerve and to the Occipital Lobe of the Brain.

The brain then interprets the information and you see.

In bright light: Rhodopsin breaks down

faster than it can be restored, creating a need for Cone Cells.

there are three types of CONE cells each sensitive to a different color of light (Red, Blue and Green)

we see different colors because of different combinations of cones that are stimulated

a similar chemical reaction occurs in Cones as in Rods when light of a particular wave length strikes them.

III. EYE REFLEXES

1. LIGHT REFLEX:

the opening and closing of the iris to accommodate different light intensities

2. ACCOMMODATION REFLEX:

changes in shape of the lens to accommodate the distance at which objects are viewed.

distant objects; the lens becomes thinner.

near objects; the lens thickens

3. ADAPTATION REFLEX:

involves the shifting of the reception of light from rods to cones or from cones to rods.

IV. EYE DISORDERS:

Near Sightedness: the image does not focus on

the retina it focuses in front so distant objects aren’t clearly seen

caused by too long of an eyeball or a lens that can’t flatten

corrected by wearing a CONCAVE lens

Far Sightedness: the image focuses behind

the retina so near objects are out of focus

caused by the eyeball being to short or the lens being to stiff.

corrected by wearing a CONVEX Lens

Astigmatism: caused by an uneven

curvature of the cornea or lens

creates blurred vision, head aches and red eyelids

Corrected by wearing an irregular shaped lens