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Chapter 27Sense Organs

Sensory Receptors Respond to Stimuli

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27.1 Sensory receptors can be divided into five categories

Sensory receptors - allow organisms to respond to stimuli Chemoreceptors respond to chemical substances in the

immediate vicinity Taste and smell depend on this

Pain receptors (nociceptors) sense excessive temperature and mechanical pressure in addition to chemicals (some of which are released by damaged tissues)

Electromagnetic receptors are stimulated by changes in electromagnetic waves

Photoreceptors, present in the eyes, are sensitive to visible light Thermoreceptors are stimulated by changes in temperature

Humans have thermoreceptors located in the hypothalamus and skin Mechanoreceptors, including touch receptors, are stimulated by

mechanical forces, which most often result in pressure of some sort

Airborne sound waves are converted to fluid-borne pressure waves that can be detected by mechanoreceptors in the inner ear

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Figure 27.1A Electromagnetic receptors in gray whales help them migrate

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Figure 27.1B Thermoreceptors that are sensitive to infrared energy help pythons find their prey

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Figure 27.1C The ears of some bats use echolocation to find their prey

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27.2 Sensory receptors communicate with the CNS

Sensory receptors are the first element in a reflex arc We are only aware of a reflex action when sensory information

reaches the brain Some sensory receptors are free nerve endings or

encapsulated nerve endings, while others are specialized cells closely associated with neurons The stronger the stimulus, the greater the frequency of nerve

impulses Before sensory receptors initiate nerve impulses, they

carry out some integration, the summing up of signals Example: Sensory adaptation is a decrease in response to a

stimulus The functioning of sensory receptors makes a significant

contribution to homeostasis Without sensory input, we would not receive information about

our internal and external environments27-7

Figure 27.2 Nerve impulses from sensory receptors result in sensation and perception in the brain

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Chemoreceptors Are Sensitive to Chemicals

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27.3 Chemoreceptors are widespread in the animal kingdom

Sensory receptors responsible for taste and smell are sensitive to certain chemical substances in food, liquids, and air In the housefly, chemoreceptors are on feet Antennae of insects detect airborne pheromones,

chemical signals passed between members of the same species

In vertebrates, chemoreceptors are located in the nose, mouth, and skin Snakes and other vertebrates possess vomeronasal

organs (VNO), a pair of pitlike organs located in the roof of the mouth

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Figure 27.3 A male moth responds to a species-specific sex attractant

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27.4 Mammalian taste receptors are located in the mouth

In mammals, taste receptors are a type of chemoreceptor located in taste buds

There are at least four primary types of taste Sweet, sour, salty, and bitter A fifth taste, called unami, may exist for certain flavors of

cheese, beef broth, and some seafood Taste buds for each of these tastes are located

throughout the tongue Certain regions may be most sensitive to particular tastes

Tip of the tongue is most sensitive to sweet tastes Margins to salty and sour tastes Rear of the tongue to bitter tastes

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Figure 27.4 Taste buds in humans

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27.5 Mammalian olfactory receptors are located in the nose

In humans the sense of smell (olfaction) depends on 10 to 20 million olfactory cells Structures are located within olfactory epithelium in

the roof of the nasal cavity Olfactory cells are modified neurons

Each cell ends in a tuft of about five olfactory cilia that bear receptor proteins for odor molecules

Each olfactory cell has only 1 out of 1,000 different types of receptor proteins

Olfactory bulbs have direct connections with limbic system and centers for emotions and memory Why certain smells trigger certain memories 27-14

Figure 27.5 Olfactory cells in humans

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Photoreceptors Are Sensitive to Light

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27.6 The vertebrate eye is a camera-type eye

The human eye is an elongated sphere and has three layers or coats Sclera - opaque, white, fibrous layer that covers most of the eye

Conjunctiva covers exposed surface of the sclera and lines the inside of the eye lids

In front of the eye, the sclera becomes the cornea through which light passes

Choroid - middle, thin, dark-brown layer contains many blood vessels and a brown pigment that absorbs stray light rays

Front of the eye, the choroid becomes the iris, which regulates the size of the pupil

The pupil regulates the amount of light entering the eye The lens lies directly behind the iris and the pupil

Retina - located in posterior compartment and contains photoreceptors

Rods are very sensitive to light, but they do not see color Cones give us the ability to distinguish colors Fovea centralis, where cone cells are densely packed and where light is

focused No rods and cones where the optic nerve exits the retina

No vision is possible in this area, and is therefore called a blind spot27-17

Figure 27.6 Anatomy of the human eye

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES 27.7 Protect your eyes

from the sun Evidence suggests that macular degeneration and

cataracts are caused by long-term exposure to sun’s ultraviolet rays

It is recommended that people in sunny climates or who work outdoors wear sunglasses that absorb ultraviolet light

Sunglass Association of America devised a system for categorizing sunglasses Cosmetic lenses absorb 20% of UV-A and 60% of visible light General-purpose lenses absorb at least 60% of UV-A, and 60–92%

of visible light Special-purpose lenses block at least 60% of UV-A and 20–97% of

visible light

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27.8 The lens helps bring an object into focus

Focusing mostly occurs at the cornea as light passes from an air to a fluid medium Lens provides additional focusing as visual accommodation

occurs for close vision Shape of the lens is controlled by the ciliary muscle within the

ciliary body Because close work requires contraction of the ciliary

muscle, it very often causes muscle fatigue known as eyestrain With normal aging, the lens loses its ability to accommodate for

near objects People frequently need reading glasses by middle age

Aging, or exposure to the sun, also makes the lens subject to cataracts Lens becomes opaque, and incapable of transmitting light rays

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Figure 27.8A Focusing on a distant object

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Figure 27.8B Focusing on a near object

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES 27.9 The inability to form a

clear image can be corrected LASIK Surgery

Many nearsighted people are now choosing to undergo eye surgery

LASIK stands for laser in-situ keratomileusis, which means carving of the cornea to reshape it

Adults affected by common vision problems (nearsightedness, farsightedness, or astigmatism) respond well A small flap of conjunctiva is lifted to expose the cornea Then the laser is used to remove tissue from the cornea

After the procedure, the flap of conjunctiva is put back in place and allowed to heal on its own Most patients achieve vision close to 20/20, but chances for

improved vision are based on how good eyes were before surgery27-23

Figure 27.9 Common abnormalities of the eye with possible corrective lenses

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27.10 The retina sends information to the visual cortex

Retina has three layers of neurons: Rod cell and cone cell layer Bipolar cell layer Ganglion cell layer

Vision begins once light has been absorbed by pigment molecules embedded in the membrane of the rods and cones Rhodopsin is a molecule made up of the protein opsin and a light-

absorbing molecule called retinal When rod absorbs light, rhodopsin splits into opsin and retinal

Leads to cascade of reactions and closure of ion channels in rod’s plasma membrane

Cones are located primarily in the fovea centralis and are activated by bright light Detect the fine detail and the color of an object Color vision depends on three different kinds of cones which

contain pigments B (blue), G (green), and R (red) pigments 27-25

Mechanoreceptors Are Involved in Hearing and Balance

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27.11 The mammalian ear has three main regions

Outer ear - gathers sound waves Consists of the pinna (external flap) and the auditory canal Opening of auditory canal is lined with fine hairs and glands

Middle ear - amplifies sound waves Begins at the tympanic membrane (eardrum) Ends at a bony wall containing two small openings covered by

membranes called the oval window and the round window Three small bones lie between the tympanic membrane and

the oval window Called the ossicles, individually they are the malleus (hammer),

the incus (anvil), and the stapes (stirrup) Inner ear - filled with fluid

Semicircular canals and also the vestibule are both concerned with balance

The cochlea is concerned with hearing27-27

Figure 27.11 Anatomy of the human ear

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27.12 Hair cells in the inner ear detect sound vibrations

Sound waves travel by the successive vibrations of molecules When many waves strike the tympanic membrane, it vibrates

Malleus takes the pressure from the tympanic membrane and passes it, by means of the incus, to the stapes that strikes the oval window

Stapes vibrates the membrane of the oval window with a force that has been multiplied about 20 times by the movement of the ossicles

force allows sound waves to become fluid pressure waves in inner ear Eventually pressure waves disappear at round window

Sensory Coding Each part of the organ of Corti responds to different wave frequencies, or

pitch Pitch sensation we experience depends on which region of the basilar

membrane vibrates and which area of the brain is stimulated Volume is a function of the size (amplitude) of sound waves Loud noises cause the fluid within the vestibular canal to exert more

pressure and the basilar membrane to vibrate to a greater extent 27-29

Figure 27.12 Mechanoreceptors for hearing

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES 27.13 Protect your ears

from loud noises Age-associated nerve deafness, results from stereocilia

damage due to exposure to loud noises and is preventable Any noise above a level of 80 decibels could damage the hair

cells of the organ of Corti Eventually, the stereocilia and then the hair cells disappear

completely The first hint of danger could be temporary hearing loss,

a “full” feeling in the ears, muffled hearing, or tinnitus (e.g., ringing in the ears) Aside from loud music, noisy indoor or outdoor equipment, such

as a rug-cleaning machine or a chain saw, is also damaging to hearing

Some medicines are ototoxic Anticancer drugs, most notably cisplatin, and certain antibiotics

(e.g., streptomycin, kanamycin, gentamicin) make the ears especially susceptible to hearing loss 27-31

Figure 27.13A Normal hair cells in the organ of Corti

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Figure 27.13B Damaged hair cells in the organ of Corti

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27.14 The sense of balance occurs in the inner ear

Rotational balance - mechanoreceptors for balance detect rotational and/or angular movement of the head Involves the semicircular canals arranged so that

there is one in each dimension of space Because there are three semicircular canals, each

ampulla responds to head movement in a different plane of space

Gravitational balance - straight-line movement of the head in any direction Depends on the utricle and saccule - two

membranous sacs located in the vestibule Utricle is especially sensitive to horizontal movements

of the head, while the saccule responds best to vertical (up-down) movements 27-34

Figure 27.14A The receptors for rotational balance are in the ampullae of the semicircular canals

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Figure 27.14B The receptors for gravitational balance are in the utricle and saccule of the vestibule

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES 27.15 Motion sickness

can be disturbing Certain types of movement are likely to cause

motion sickness Seasickness may result from the rocking or swaying

of a boat Airsickness can occur due to abrupt changes in

altitude during a plane trip or the jerky motions of turbulence

Motion sickness arises when the brain is bombarded with conflicting sensory input One way to avoid motion sickness is to avoid reading

while you are in motion If you are on a train, try to avoid facing backward If you are on a plane, reserve a seat over the wings,

where the plane is most stable 27-37

27.16 Other animals respond to motion

Lateral line system of fishes guides them in their movements and in locating other fish System detects movements of nearby objects much like the

sensory receptors in the human inner ear detect motion In bony fishes, the lateral line receptors are located within a

canal that has openings to the outside Receptor is a hair cell with cilia embedded in a gelatinous

cupula When the many cupulae bend due to pressure waves, the hair

cells initiate nerve impulses Statocysts are found in cnidarians, snails (molluscs),

and lobsters and crabs (arthropods) These organs give information only about the position of the

head; they are not involved in the sensation of movement When the head stops moving, a small particle called a

statolith stimulates the cilia of the closest hair cells and these cilia generate impulses that indicate the position of the head

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Figure 27.16A Lateral line system in fishes

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Figure 27.16B A statocyst of a lobster

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Connecting the Concepts:Chapter 27

Animal’s information exchange with internal and external environments depends on a few types of sensory receptors Chemoreceptors in taste and olfactory cells Photoreceptors such as eyes Mechanoreceptors such as the hair cells for hearing and balance

Senses are not equally developed in all animals Birds that live in forested areas signal that a territory is occupied by

singing, because it is difficult to see a bird in a tree, as most birders know Insectivorous bats find prey in the dark by sending out a series of sound

pulses and listen for the echoes that come back Electric fishes of Africa and Australia have electroreceptors that can detect

disturbances in an electrical current they emit into the water to detect prey and obstacles

Through the evolutionary process, animals tend to rely on those stimuli and senses that are adaptive to their particular environment and way of life In all cases, sensory receptors generate nerve impulses that travel to the

brain, where sensation occurs27-41