The SensesChapter 35.4

Sensory Receptors

• Sensory receptors react to specific stimuli

– Stimuli include light, sound, motion, chemicals, pressure, and changes in temperature

Sensory Receptors

• Each sensory receptor reacts to a specific stimulus by sending impulses to other neurons, and eventually to the central nervous system (CNS)

Most sensory receptors are found in sense organs

ex. eyes, inner ear, nose, mouth, and skin

Categories of Sensory Receptors

• There are five major categories of sensory receptors:

1. pain receptors

2. thermoreceptors

3. mechanoreceptors

4. chemoreceptors

5. photoreceptors

Categories of Sensory Receptors

• Pain receptors– Found all over the body EXCEPT in the brain

Respond to chemicals released by damaged cells

• Thermoreceptors– Located in the skin, body core, and hypothalamus

Detect variations in temperature

• Mechanoreceptors– Found in the skin, skeletal muscles, and inner ear

Sensitive to touch, pressure, stretching of muscles, sound, and motion

Categories of Sensory Receptors• Chemoreceptors

– Located in the nose and taste budsSensitive to chemicals in the external environment

• Photoreceptors– Located in the eyes

Sensitive to light

Vision

• Light enters through the cornea (a tough transparent layer of cells)cornea helps focus light

• Light passes through a chamber filled with a liquid called aqueous humor

• At the back of the chamber is the irisiris is the colored part of eye

Vision

• In the middle of the iris is the pupil (small opening) Regulates the amount of light that enters the

eye

• Behind the iris is the lens Changes shape to help adjust the eye’s

focus to see near or distant objects– controlled by muscles

Vision

• Behind the lens is a chamber filled with vitreous humor (a transparent, jelly-like fluid)

• Light is focused on retina– Photoreceptors found here– Converts light to nerve impulses which are

then carried to the CNS

The Retina

• Two types of photoreceptors found on the retina:– Rods

very sensitive to light but not to colors

– Cones

not as sensitive to light but responds to light of different colors, producing color vision

Retina• Cones are concentrated in

the foveafovea = site of sharpest

vision

• A blind spot is a spot on the retina where no photoreceptors existblind spot = where optic

nerve passes through back of eye

Choroid

Retina

Blood vessels

Optic nerve

Fovea

Vitreous humor

Sclera

Ligaments

Iris

Pupil

Cornea

Aqueous humor

Lens

Muscle

Section 35-4

Figure 35-14 The Eye

Question

• If a tree falls in the woods and no one is there to hear, does it make a sound?

Auditory canal

Tympanum Round window Eustachian tube

Bone

Cochlea

Cochlear nerve

Semicircular canals

Oval window

StirrupAnvilHammer

Section 35-4

Figure 35-15 The Ear

Hearing• Sound is vibrations in the air

– The ear distinguishes the pitch and loudness of the vibrations

• Vibrations enter the ear through the auditory canal– These vibrations hit the tympanum (eardrum)

Hearing

• The vibrations picked up by the tympanum are transferred to three boneshammer, anvil, stirrup

• These are the smallest bones in the body

• The stirrup transmits the vibrations to the oval window

• When the oval window vibrates, it creates pressure waves in the cochlea

Cochlea

• The cochlea is fluid-filled and lined with tiny hairs– The pressure waves

produced by the oval window push the hairs back and forth

– This hair movement produces a nerve impulse that is transmitted through the cochlear nerve to the CNS

Balance

• Semicircular canals, and two tiny sacs behind them, monitor the position of your body, especially your head, in relation to gravity

• Each canal and sac is filled with fluid and hair– As your head changes position, the fluid moves causing the

hair to bend– The CNS receives a nerve impulse to determine body motion

and position

Smell and Taste

• The senses of smell and taste are a result of chemoreceptors sending nerve impulses in response to chemicals in the environment

– Smell ability of chemoreceptors lining the nasal passageway to respond to chemicals

– Taste ability of taste buds to detect chemicals• Taste buds are broken into categories: salty, bitter, sweet,

and sour• The tongue’s sensitivity to each varies by region

Touch

• It is a unique sense in that the nerves are not found in one particular region of the bodyall regions of skin are sensitive to touch– It responds to temperature, pressure, and pain

• Different body parts have different concentrations of nervesmore touch receptors found on fingers, toes, and face

Cerebral cortex

Nasal cavity

Taste bud

Smell sensory area

Tastesensory area

Thalamus

Olfactory(smell) bulb

Olfactorynerve

Smell receptor

Taste pore

Taste receptor

Sensorynerve fibers

Section 35-4

The Senses of Smell and Taste