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BIO 103 CHAPTER 29
The Senses
Learning Targets
Sensory Reception
29.1
29.2
29.3
Describe the essential roles of sensory receptors. Explain how electromagnetic receptors help
the hammerhead shark perceive its world.
Define sensory transduction, receptor potential, and sensory adaptation, and provide examples
of each.
Describe the five general categories of sensory receptors found in animals, and provide examples of each.
Hearing and Balance
29.4
29.5
29.6
List the structures of the ear in the sequence in which they participate in
hearing. Describe the possible causes of hearing loss. Explain how the cochlea distinguishes
sounds of different pitch.
Explain how body position and movement are sensed in the inner ear.
Explain what causes motion sickness and what can be done to prevent it.
Vision
29.7
29.8
29.8
29.9
29.10
Compare the structures and functions of the eye cups of planarians, the
compound eyes of insects and crustaceans, and the single-lens eyes of humans.
Describe the parts of the human eye and their functions.
Compare the mechanisms used to focus the eyes of a squid and a human.
Explain the causes and symptoms of myopia, hyperopia, presbyopia, and
astigmatism.
Compare the structures, functions, distributions, and densities of rods and
cones. Explain how light is perceived in the retina.
Taste and Smell
29.11
29.12
29.13
Explain how odor and taste receptors function.
Describe the characteristics of human “supertasters,” and their potential implications for health.
Describe the role of the central nervous system in sensory perception.
Key Terms
aqueous humor
astigmatism
auditory canal
basilar membrane
chemoreceptor
choroid
cochlea
compound eye
cones
conjuctiva
cornea
eardrum
electromagnetic receptor
Eustachian tube
eye cup
farsightedness
fovea
hair cells
inner ear
iris
lens
mechanoreceptor
nearsightedness
middle ear
organ of Corti
outer ear
pain receptor
photopsin
photoreceptor
pinna
pupil
receptor potential
retina
rhodopsin
rods
sclera
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semicircular canals
sensory adaptation
sensory receptor
sensory transduction
stretch receptor
single-lens eye
thermoreceptor
visual acuity
vitreous humor
Lecture Outline
I. Introduction
A. Bats use echolocation to detect their environment.
B. High-pitched sounds are
1. produced in their larynx and
2. emitted from their mouths and noses.
C. Their brains process the time delay and spatial arrangement of the echoes to determine the size,
shape, location, speed, and direction of objects in their environment.
D. Marine mammals
1. include dolphins, killer whales, and sperm whales,
2. produce ultrasonic clicking sounds in their nasal passages,
3. focus the sound by bouncing it off of skull bones and an oil-filled structure in their
forehead, and
4. receive the echo in a narrow window of bone behind the jaw.
E. Echolocation has also been observed in some species of cave-dwelling birds and forest-dwelling
shrews.
II. Sensory Reception
A. 29.1 Sensory organs share a common cellular basis
1. All animal senses originate in sensory receptors, specialized cells or neurons that are tuned to the
a. conditions of the external world and
b. the internal organs.
2. All sensory receptors
a. trigger an action potential and
b. send information to the central nervous system.
c. Sensation depends on the part of the brain that receives the action potential.
B. 29.2 Sensory receptors convert stimulus energy to action potentials
1. Sensory receptors detect stimuli.
2. All stimuli represent forms of energy.
3. In a process called sensory transduction, receptors
a. detect one type of signal (the stimulus) and
b. convert the signal to another type, an electrical signal.
4. When a sensory receptor cell in a taste bud detects sugar molecules,
a. sugar molecules enter the taste bud,
b. sugar molecules bind to sweet receptors, specific protein molecules embedded in a taste receptor cell membrane, and
c. the binding triggers a signal transduction pathway that causes some ion channels in the membrane to close and others to open.
d. These changes in the flow of ions create a graded change in membrane potential called a receptor potential.
5. The stronger the stimulus,
a. the more neurotransmitter released by the receptor cell and
b. the more frequently the sensory neuron transmits action potentials to the brain.
6. Repeated stimuli may lead to sensory adaptation, the tendency of some sensory
receptors to become less sensitive when they are stimulated repeatedly.
C. 29.3 Specialized sensory receptors detect five categories of stimuli
1. There are five categories of sensory receptors.
a. Pain receptors detect dangerous stimuli including high heat and pressure.
b. Thermoreceptors detect heat or cold.
c. Mechanoreceptors respond to
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i. mechanical energy,
ii. touch,
iii. pressure, and
iv. sound.
d. Chemoreceptors
i. include sensory receptors in our nose and taste buds and
ii. respond to chemicals.
e. Electromagnetic receptors respond to
i. electricity,
ii. magnetism, and
iii. light (sensed by photoreceptors).
III. Hearing and Balance
A. 29.4 The ear converts air pressure waves to action potentials that are perceived as sound
1. The human ear channels sound waves
a. from the outer ear with a flap-like pinna,
b. down the auditory canal,
c. to the eardrum, which separates the outer ear from the middle ear,
d. to a chain of bones in the middle ear (malleus, incus, and stapes), and
e. to the fluid in the coiled cochlea in the inner ear.
f. The Eustachian tube connects the pharynx to the middle ear, permitting pressure equalization.
2. Pressure waves transmitted to the fluid of the cochlea
a. bend hair cells in the organ of Corti against the basilar membrane and
b. trigger nerve signals to the brain.
3. Louder sounds generate more action potentials.
4. Various pitches stimulate different regions of the organ of Corti.
5. Deafness is the loss of hearing.
6. Deafness can be caused by the inability to detect sounds resulting from
a. middle-ear infections,
b. a ruptured eardrum, or
c. stiffening of the middle-ear bones.
7. Deafness
a. can also result from damage to sensory receptors or neurons and
b. is often progressive and permanent.
B. 29.5 The inner ear houses our organs of balance
1. Three organs in the inner ear detect body position and movement. These include
a. three semicircular canals and
b. two chambers, the utricle and the saccule.
c. All three of these structures operate on the same principle: the bending of hairs on hair cells.
2. The three semicircular canals detect changes in the head’s rotation or angular
movement.
3. The utricle and saccule detect the position of the head with respect to gravity.
C. 29.6 CONNECTION: What causes motion sickness?
1. Motion sickness may be caused by conflicting signals between the
a. inner ear and
b. eyes.
2. Motion sickness can be a severe problem for astronauts.
3. Motion sickness may be reduced by
a. closings the eyes,
b. limiting head movements,
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c. focusing on a stable horizon,
d. sedatives such as dramamine or bonine, or
e. long-lasting, drug-containing skin patches.
IV. Vision
A. 29.7 EVOLUTION CONNECTION: Several types of eyes have evolved independently among animals
1. The ability to detect light plays a central role in the lives of nearly all animals.
2. All animal light detectors are based on cells called photoreceptors that contain pigment molecules
that absorb light.
3. Most invertebrate eyes include some kind of light-detecting organ.
4. One of the simplest organs is the eye cup,
a. used by planarians,
b. which senses light intensity and direction.
5. Two major types of image-forming eyes have evolved in the invertebrates.
a. Compound eyes of insects
i. consist of up to several thousand light-detectors called ommatidia,
ii. function as acute motion detectors, and
iii. usually provide excellent color vision.
b. In single-lens eyes
i. light enters the front center of the eye through a small opening, the pupil,
controlled by an iris,
ii. passes through a single disklike lens, and
iii. is focused onto the retina, which consists of many photoreceptor cells.
iv. The center of focus is the fovea, where photoreceptor cells are highly
concentrated.
v. Single-lens eyes
(I) evolved independently in the vertebrates but
(II) are similar in structure.
B. 29.8 Humans have single-lens eyes that focus by changing position or shape
1. The outer surface of the human eyeball is a tough, whitish layer of connective tissue called the
sclera.
a. At the front of the eye, the sclera becomes the transparent cornea, which
i. lets light into the eye and
ii. also helps focus light.
b. The sclera surrounds a pigmented layer called the choroid. The anterior choroid forms the iris,
which gives the eye its color.
2. The lens and ciliary body divide the eye into two fluid-filled chambers.
a. The large chamber behind the lens is filled with a jellylike vitreous humor.
b. The smaller chamber in front of the lens contains the thinner aqueous humor.
c. These humors
i. help maintain the shape of the eyeball and
ii. circulate nutrients and oxygen to the lens, iris, and cornea.
3. The conjunctiva
a. lines the inner surface of the eyelids and folds back over the white of the eye (but not the cornea).
b. Conjunctivitis is an inflammation of the conjuctiva by bacteria or a virus.
4. A gland above the eye secretes tears that
a. clean and
b. moisten the eye.
5. The lens focuses light onto the retina by bending light rays. Focusing can occur in two ways.
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a. In squids and fishes, the lens focuses by moving back and forth.
b. In mammals, the lens focuses by changing shape using
i. muscles attached to the choroid and
ii. ligaments that suspend the lens.
C. 29.9 CONNECTION: Artificial lenses or surgery can correct focusing problems
1. Visual acuity is the ability of the eyes to distinguish fine detail.
a. Visual acuity is measured by reading standardized eye charts from a distance of 20 feet.
b. The ability to see normally at 20 feet is 20/20 vision.
2. Three vision problems are common.
a. Nearsightedness is the inability to focus on distant objects, usually caused by an
eyeball that is too long.
b. Farsightedness is the inability to focus on close objects, usually caused by an eyeball that is too
short.
c. Astigmatism is blurred vision caused by a misshapen lens or cornea.
d. Corrective lenses can bend light rays to compensate for each of these problems.
D. 29.10 The human retina contains two types of photoreceptors: rods and cones
1. The human retina contains two types of photoreceptors.
a. Rods
i. contain the visual pigment rhodopsin, which can absorb dim light, and
ii. can detect shades of gray in dim light.
b. Cones
i. contain the visual pigment photopsin, which absorbs bright colored light, and
ii. allow us to see color in bright light.
2. When rhodopsin and photopsin absorb light,
a. they change chemically, and
b. the change alters the permeability of the cell’s membrane.
c. The resulting receptor potential triggers a change in the release of neurotransmitter from the
synaptic terminals.
d. This release initiates a complex integration process in the retina.
V. Taste and Smell
A. 29.11 Taste and odor receptors detect chemicals present in solution or air
1. Taste and smell depend on chemoreceptors that detect specific chemicals in the
environment.
2. Chemoreceptors
a. in taste buds detect molecules in solution and
b. lining the nasal cavity detect airborne molecules.
B. Taste and smell interact. Much of what we taste is really smell.
C. Taste receptors
1. are located in taste buds on the tongue and
2. produce five taste sensations:
a. sweet,
b. salty,
c. sour,
d. bitter, and
e. umami (the savory flavor of meats and cheeses).
D. 29.12 CONNECTION: “Supertasters” have a heightened sense of taste
1. About 25% of humans are “supertasters” with up to three times the sensitivity to bitter.
2. Supertasters are more likely to
a. avoid spinach, broccoli, cabbage, coffee, and alcoholic beverages and
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b. be obese.
E. 29.13 Review: The central nervous system couples stimulus with response
1. The nervous system
a. receives sensory information,
b. integrates it, and
c. commands appropriate responses, either an action or no action.
Word Roots
aqua- = water (aqueous humor: the clear, watery solution that fills the space between the lens and the cornea
in the vertebrate eye)
audit- = hearing (auditory canal: part of the vertebrate outer ear that channels sound waves from the pinna or
outer body surface to the eardrum)
chemo- = chemical (chemoreceptor: a sensory receptor that detects chemical changes within the body or a
specific kind of molecule in the external environment)
coch- = a snail (cochlea: a coiled tube in the inner ear of birds and mammals that contains the hearing organ,
the organ of Corti)
electro- = electricity (electromagnetic receptor: a sensory receptor that detects energy of different wavelengths, such as electricity, magnetism, and light)
fovea- = a pit (fovea: the eye’s center of focus, a location of high photoreceptor concentration on the retina)
mechano- = an instrument (mechanoreceptor: a sensory receptor that detects physical deformations in the
body’s environment associated with pressure, touch, stretch, motion, or sound)
photo- = light (photopsin: one of a family of visual pigments in the cones of the vertebrate eye that absorb
bright, colored light; photoreceptor: a type of electromagnetic receptor that detects light)
rhodo- = red (rhodopsin: a visual pigment located in the rods of the vertebrate eye that absorbs dim light)
sclero- = hard (sclera: a tough layer of connective tissue forming the outer surface of the vertebrate eye)
semi- = half (semicircular canals: fluid-filled channels in the inner ear that detect changes in the head’s rate of
rotation or angular movement)
thermo- = heat (thermoreceptor: a sensory receptor that detects heat or cold)
vitre- = glass (vitreous humor: the jellylike substance that fills the space behind the lens in the
vertebrate eye)
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