Chapter 8 - The Senses
Chapter 8 - The Senses
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Chapter 8 - The Senses
Our bodies are composed of many different
sensory receptors that allow us to respond to
stimuli such as touch, pressure, temperature,
light, and pain.
Why are senses, such as pain, a good thing?
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Chapter 8 - The Senses
Two main classifications of sense organs:
1.) General Sense Organ - Sense stimuli such as
pain and touch and are widely distributed
throughout the body
2.) Special Sense Organ - Complex organ with
large amounts of specialized receptors
- Eye, ear, tongue
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Chapter 8 - The Senses
Receptors are often classified by the type of
stimuli they detect and whether they are
encapsulated or unencapsulated
sort of capsule or are "free" or "naked"
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Chapter 8 - The Senses
Photoreceptors are stimulated by light. Located in
the eye.
Chemoreceptors are activated by chemical stimuli
such as taste and smell
Pain receptors respond to physical damage or injury
Thermoreceptors respond to change in temperature
Mechanoreceptors detect
mechanical changes in
position or in some way
deform the capsule that
surrounds them
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Chapter 8 - The Senses
How is a stimulus converted into a sensation?
The receptors change the stimulus into a nerve
impulse
The nerve impulse that gets transmitted via the
nervous system to the brain
The brain decides which sensation is actually
percieved
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Chapter 8 - The Senses
When you look at person, you only see a small
part of the whole eye
Three layers of tissue form the eyeball:
1.) Sclera
2.) Choroid
3.) Retina
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Chapter 8 - The Senses
Outer layer consists of tough fibrous tissue
There are two parts to the front surface of the sclera
1.) One layer makes up the "white" of the eye
2.) The other layer is called the cornea
The cornea is the transparent layer that protects the eye
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Chapter 8 - The Senses
The cornea appears to have color because it lies over
the iris
The iris is the colored part of your eye
It is actually a muscle!
It has some fibers arranged like spokes on a wheel
When they contract the pupils dilate (get bigger)
Other fibers are circular
When they contract the pupils constrict (get smaller)
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Chapter 8 - The Senses
There is a mucous membrane called the conjunctiva
lines the eyelids and covers the sclera in front
The conjunctiva is kept moist by tears formed in the
lacrimal gland (tear ducts)
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Chapter 8 - The Senses
the pupil
Because this is actually a hole, optometrists can
use a bright light and actually see the back of
your eye!
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Chapter 8 - The Senses
The lens of the eye lies directly behind the pupil
It is held in place by a ligament attached to the
ciliary muscle
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Chapter 8 - The Senses
When we look at distant objects, the ciliary muscle is
relaxed, and lens only has a slightly curved shape
To focus on near objects, the ciliary muscle must contract
causing the lens to curve even more
As we grow older we tend to lose the ability to focus on
called presbyopia
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Chapter 8 - The Senses
Long-time exposure to ultraviolet (UV) radiation in sunlight
may cause the lens to become hard, lose its transparency,
and become "milky" in appearance
This condition is called cataract. An artificial lens can be
surgically implanted
This is why you should wear UV blocking sunglasses
Modern contacts often contain slight UV filters
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Chapter 8 - The Senses
The retina is the innermost layer of the eyeball and
contains microscopic photoreceptor cells
There are two main types of photoreceptor cells:
1.) Rods
2.) Cones
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Chapter 8 - The Senses
Rods can detect very dim light but cannot determine color
- They only sense light intensity
- Rods are responsible for our night vision
Cones require fairly bright light to be activated, but can
sense color
- Cones are responsible for your daytime vision
- There are three kinds of cones; each sensitive to
different colors
- Red, green, blue
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Chapter 8 - The Senses
A specific area of the retina contains the greatest
concentration of cones of any area of the retina
We call this area the fovea centralis
In good light, greater visual acuity (sharpness) can be
obtained if we look directly at an object and focus the
image on the fovea
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Chapter 8 - The Senses
There are fluids that fill the inside of the eyeball.
They help maintain the normal shape of the eye
Aqueous humor is the watery fluid in front of the lens
Vitreous humor is the jellylike fluid behind the lens
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Chapter 8 - The Senses
Aqueous humor is constantly being formed, drained, and
replaced. If drainage is blocked the internal pressure
increases
This can lead to blindness and is called glaucoma
blowing high pressure air at your eye and measuring the
deflection of your eye
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Chapter 8 - The Senses
The rods and cones convert light stimuli into an electrical
signal that is transmitted via the optic nerve to the brain
No rods or cones are present in the area of the retina
where the optic nerve fibers exits the eye
This results in a "blind spot" known as the optic disc
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Chapter 8 - The Senses
What causes vision problems?
To have good vision a clear image must be focused on
the retina
In a normal eye, light rays enter the eye and are
focused into a clear, upside-down image on the retina
The brain can fix the upside-down image, but cannot
fix an image that is not sharply focused
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Chapter 8 - The Senses
Myopia (Nearsightedness)
This occurs in if eyes are elongated.
This causes more distant objects to look fuzzy.
This can be corrected using a concave lense
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Chapter 8 - The Senses
Hyperopia (Farsightedness)
The image focuses behind the retina rather than on it.
This occurs if eyes are shorter than normal.
This causes closer objects to look fuzzy.
This can be corrected using a convex lense
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Chapter 8 - The Senses
Astimatism
Astigmatism is an eye disorder in which the cornea or the
lens is abnormally curved, creating vision problems
Causes both near and far objects to be blurred
It is very common to have mild astigmastism
objects
If more severe glasses, or hard contact lenses may be
needed
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Chapter 8 - The Senses
The Ear
The ear does more than just hear things, it also
functions as a sense organ of equilibrium and balance
The sensory neurons within the ear are
mechanoreceptors, meaning they respond to physical
forces
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Chapter 8 - The Senses
The external ear is only one part of the ear, the most
crucial parts are inside the temporal bone
The ear is divided into three areas:
1.) External Ear
2.) Middle Ear
3.) Inner (Internal) Ear
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Chapter 8 - The Senses
The External Ear
The external ear has two parts:
1.) The auricle (or pinna)
- The appendage on the side of the head which
surrounds the external auditory canal
2.) External Auditory Canal
- A tube that extends into the temporal bone and
ends at the tympanic membrane (ear drum)
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Chapter 8 - The Senses
The tympanic membrane is the partition that divides the
external and middle ear
The auditory canal contains short hairs and ceruminous
glands that produce a waxy substance called cerumen (ear
wax)
Excess ear wax can impair hearing because it blocks
incoming sound waves
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Chapter 8 - The Senses
The Middle Ear
The middle ear houses three very small bones
The bones collectively are called the ossicles
The bone names describe their shapes
The malleus (hammer), incus (anvil), stapes (stirrup)
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Chapter 8 - The Senses
The "handle" of the malleus attaches to the inside of
incus
The incus attaches to the stapes, which presses against
a membrane that covers a small opening called the oval
window
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Chapter 8 - The Senses
Movement of the stapes against the oval window
causes movement of fluid in the inner ear
The oval window separates the middle ear form the
inner ear
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Chapter 8 - The Senses
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Chapter 8 - The Senses
The Inner Ear
The inner ear consists of three spaces in the temporal
bone
These spaces are collectively known as the bony labyrinth
This space is filled with a watery fluid called perilymph
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Chapter 8 - The Senses
The bony labyrinth is divided into three parts:
1.) Vestibule
2.) Semicircular Canals
3.) Cochlea
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Chapter 8 - The Senses
The specialized mechanoreceptors for balance and
equilibrium are located in the three semicircular canals
and the vestibule
The sensory cells have hair-like extensions that are
suspended in a thicker fluid called endolymph
When your head moves, the endolymph flows causing
the hairs to bend, which is detected by the
mechanoreceptors
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Chapter 8 - The Senses
The organ of hearing, is inside the snail-shaped cochlea and
is called the organ of Corti
There is a tube that runs through the organ of Corti called
the cochlear duct
The duct is filled with endolymph and specialized hair cells
that generate nerve impulses when they are bent
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Chapter 8 - The Senses
Remember the endolymph is vibrating because
soundwaves vibrated the ear drum, which vibrated
the ossicles, which vibrated the oval window, which
caused the endolymph to flow
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Chapter 8 - The Senses
Taste Receptors
The chemoreceptors that generate nervous impulses
in the sense of taste are called taste buds
About 10,000 taste buds are found on structures on
the tongue called papillae
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Chapter 8 - The Senses
We can taste four primary sensations
1.) Sweet
2.) Sour
3.) Bitter
4.) Salty
Most flavors result from a combination of taste
bud and olfactory receptor stimulation
When you are sick you tend to not be able to taste
foods as well
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Chapter 8 - The Senses
The Smell Receptors
Chemoreceptors are responsible for our sense of smell,
we often call these olfactory receptors
- They are located in the upper part of the nasal cavity
Each olfactory cell has specialized cilia that sense
different chemicals to generate a nervous impulse
- To detect chemicals they must be dissolved in the
watery mucus that lines the nasal cavity
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Chapter 8 - The Senses
Olfactory receptors are extremely sensitive
- You can smell chemicals that are as lowly
concentrated in the air as 1 part per billion (ppb)
Even though they are highly sensitive, they easily
adapt and lose their ability to respond to smells
- This is why you "get used" to smells after a
short time
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