Sense Organs Chapter 38

Chemical Senses

Chemoreceptors are the receptors responsible for smell

and taste.

Because all members of the animal kingdom have

developed a sense of taste and/or smell, chemoreceptors

are believed to be among the first traits that distinguished

animals from other kingdoms

Chemoreceptors can be

Universally located around the body

Annelids, like earthworms

In sensory structures like antennae

Insects and arthropods

Centrally located

Vertebrates

Taste Human taste buds are found on the palate, epiglottus,

pharynx, but primarily on the tongue.

Taste buds begin with a small pore for collecting particles

and chemicals from food products.

Microvilli collect chemicals on the surface of the bud and

pull them into the pore

Inside the pore, receptor proteins for specific chemicals

bind to the product and initiate an action potential.

The impulse is transported to the brain where it interprets

the specific flavor for recognition.

The taste buds provide the brain with relative information

for sweet, salty, bitter, sour, and savory

Smell Olfactory cells in the epithelium of the nasal cavity are

responsible for scents

Each cell bears cilia which contain one of 1000’s of

sensor receptors for aromatic chemicals

The combination of which cilia are triggered by the

chemical creates the unique scent.

Bears, sharks, moths, snakes, hounds, rats, vultures, and T-

Rex’s are so dependent on their sense of smell that a minimum

of 50% of their brain activity is devoted smell sensations alone

Old anecdote: “A pine needle dropped in the forest. The deer

heard it; The eagle saw it; the bear smelled it.”

Smell Smell and taste memories combine into one memory in

the brain

The brain associates smells with taste and taste with

smell so much that the loss of one may trigger a lapse in

the other

How does food taste when you have a cold? How does the kitchen

smell when your tongue is burned?

To heighten the senses, the tongue and nose also send

nerve impulses to each other when one is triggered.

The purpose of a smell and taste sense is to help

organisms distinguish between edible/not edible,

healthy/dangerous.

Vision

Photoreceptors are sensory receptors for light

rays

Eyespots: found on planarians; only able to determine

direction of light

Compound eyes: cornea’s act as lenses in arthropods

to direct light into photoreceptors

A wider range of the visual light spectrum, but not as wide a

depth of field

Camera eyes: found in vertebrates; capable of focusing,

changing shape, and adjusting to light

Human Eye The outer layer of the human eye is the sclera,

which contains the cornea and all eye parts

within

The middle layer, the choroid, contains the blood

vessels

The inner layer, the retina, contains rod and cone

cells, which detect images

The pupil at the front of the eye dilates to control

the amount of light that enters the retina

The cornea and lens control the focusing ability

of the eye

Sight When a rod/cone absorbs light, the pigment rhodopsin

releases an action potential.

When the action potential reaches the brain, it signals

that light was detected at that specific angle in the eye.

Rods are activated by minimal amounts of bright light, but cones

require heavy doses of bright light

Each cone picks up a certain level of red, green, or blue

wavelengths of light

All signals exit the eye through a hole in the back called

the optic chiasm. This is the only area where signals

aren’t received (hence, the “blind spot”)

Hearing The human ear has three divisions: outer, middle, and

inner.

Outer ear

The outer ear contains the pinna (external flap) and the opening to

the auditory canal where sound waves are collected

Middle Ear

The middle ear begins with the tympanic membrane (the eardrum).

The auditory tube is responsible for equalizing air pressure

Inner Ear

Filled with fluid, this section contains nerves which trigger sound

impulses

Balance Mechanoreceptors in the semicircular canals of

the inner ear detect rotational and angular

movement

Tiny hair cells are brushed by the fluid in the

inner ear to one direction or another.

This direction provides the brain with details

about rotational momentum and direction.

A separate series of receptors in the vestibule

monitor gravitational direction in a similar way.