I. The Eyes Have It
A. A bumblebee gathering nectar at a flower has an entirely different type of eye from
that of a vertebrate.
1. The eye of an insect is called compound because it has many visual units, each
sending its own data to the brain, where a mosaic image is produced.
2. The compound eye produces a crude image, but it is a taskmaster at detecting
3. Not only can the insect see color, but its eyes can also respond to ultraviolet
B. In contrast to the mosaic image of the compound eye, the vertebrate camera-type
eye has one lens for its many photoreceptors, and the brain forms a single image
after receiving data from the eyes.
1. Resolving power is good, but the eye is relatively large and heavy.
C. The vertebrate camera-type eye contains the sensory receptors for light, called the
rods and the cones.
1. The rods function well in dim light, but produce an image that is indistinct and
2. Cones show color and have terrific resolving power.
D. Humans have cones of only three different colors (blue, green, and red), but see
1. Birds, with cones of four to five different colors, have superior color vision to
that of humans.
E. Previously, scientists thought that the compound eye and the camera-type eye
evolved separately, and perhaps many times over, in the animal kingdom.
1. But now evo-devo geneticists tell us that the same genes are active whether an
animal has a compound eye or a camera-type eye.
F. Predatory mammals tend to have stereoscopic vision.
1. Animals with eyes facing sideways, such as rabbits and zebras, do not have
stereoscopic vision, but they do have panoramic vision.
II. Sensory Receptors______________ _______
Critical concepts include: types of sensory receptors, and communication from perception to sensation via CNS and PNS.
28.1 Sensory receptors differ but function similarly
A. All animals have sensory receptors that allow them to respond to stimuli.
B. There are only five common categories of sensory receptors:
1. Chemoreceptors respond to chemical substances in the immediate vicinity.
a) Ex: taste, smell
2. Pain receptors (also called nociceptors) are sometimes classified as a type
a) However, pain receptors respond to excessive temperature and
mechanical pressure as well.
3. Electromagnetic receptors are stimulated by changes in electromagnetic
a) Ex: vision, electromagnetic waves
4. Thermoreceptors are stimulated by changes in temperature.
5. Mechanoreceptors are stimulated by mechanical forces, which most often
result in pressure of some sort.
a) Ex: hearing, balance, touch
C. Function of sensory receptors
1. In complex animals, sensory receptors in the peripheral nervous system
(PNS) send information to the brain and spinal cord of the central nervous
system (CNS), which integrates sensory input before directing a motor
2. When nerve impulses arrive at the cerebral cortex, sensation, which is
the conscious perception of stimuli, occurs.
D. Sensory receptors are the first element in a flex arc.
1. However, we are only aware of a reflex action once input has reached the
a) At that time, the brain integrates information received from various
sensory receptors an sensation occurs.
E. Some sensory receptors are associated with neurons. They may be free nerve
endings or encapsulated nerve endings, while others are specialized cells
closely associated with neurons.
1. If the stimulus is sufficient, nerve impulses begin and are carried by a
sensory nerve fiber to the CNS.
a) The stronger the stimulus, the greater the frequency of nerve impulses.
F. All sensory receptors initiate nerve impulses; the resulting sensation depends
on the part of the brain receiving the impulses.
G. Before sensory receptors initiate nerve impulses, they carry out some
integration, the summing up of signals.
1. One type of integration is called sensory adaptation, a decrease in the
response to a stimulus.
H. The functioning of sensory receptors makes a significant contribution to
III. Chemical Senses_________________________ _
Critical concepts include: examples of chemoreceptors, mammalian taste receptors, and mammalian olfactory receptors.
28.2 Chemoreceptors are exemplified by taste buds and olfactory cells
A. The sensory receptors responsible for taste and smell are termed
chemoreceptors because they are sensitive to certain chemical substances in
food, including liquids, and in air.
1. Chemoreceptors are believed to be the most primitive sense.
2. The antennae of insects detect airborne pheromones, which are chemical
signals passed between members of the same species.
3. In vertebrates, chemoreceptors are located in the nose, mouth, and skin.
B. Taste buds
1. In mammals, including humans, taste receptors are a type of chemoreceptor
located in taste buds.
2. Adult humans have approximately 3,000 taste buds, located primarily on
3. Taste buds have supporting cells, and a number of elongated taste cells end
in microvilli at a taste pore.
a) The microvilli, which project from the taste cells into the taste pore,
bear receptor proteins for certain molecules.
4. There are at least four primary types of taste: sweet, sour, salty, and bitter.
a) A fifth taste, called umami, may exist for certain flavors of cheese, beef
broth, and some seafood.
b) Taste buds for each of these tastes are located throughout the tongue,
although certain regions may be most sensitive to particular tastes.
5. The brain appears to survey the overall pattern of incoming sensory
impulses and to take a “weighted average” of their taste messages as the
C. Olfactory cells
1. The chemical sense of smell is well developed in mammals, especially in
carnivores, which use it to track down their prey.
2. In humans and other mammals, the sense of smell, or olfaction, is
dependent on between 10 and 20 million olfactory cells.
a) These structures are located within an olfactory epithelium high in the
roof of the nasal cavity.
b) Olfactory cells are modified neurons.
c) Nerve fibers from similar olfactory cells lead to the same neuron in the
olfactory bulb, an extension of the brain.
3. An odor contains many odor molecules that activate a characteristic
combination of receptor proteins.
4. The olfactory bulbs have direct connections with the limbic system and its centers for emotions and memory.
IV. Sense of Vision___________________ _______
Critical concepts include: anatomy of the human eye, vision diseases, role of the lens, defects in the lens, and the role of the retina in vision.
28.3 The vertebrate eye is a camera-type eye
A. The human eye is an elongated sphere about 2.5 cm in diameter that has three
layers: the sclera, the choroid, and the retina.
1. The outer layer, the sclera, is an opaque, white, fibrous layer that covers
most of the eye.
2. A mucous membrane called the conjunctiva covers the exposed surface of
the sclera and lines the inside of the eyelids.
3. In front of the eye, the sclera becomes the cornea, which is transparent.
4. The middle, thin, dark-brown layer, the choroid, contains many blood
vessels and a brown pigment that absorbs stray light rays.
a) Toward the front of the eye, the choroid becomes the donut-shaped iris,
which regulates the size of an opening called the pupil.
b) The pupil, like the aperture on a camera lens, regulates the amount of
light entering the eye.
c) The ciliary body contains the ciliary muscles, which control the shape
of the lens for near and far vision.
5. The lens, with a membranous capsule, lies directly behind the iris and the
a) The lens divides the cavity of the eye into two compartments.
b) A basic, watery solution called aqueous humor fills the anterior
6. The third layer of the eye, the retina, is located in the posterior
compartment, which is filled with a clear, gelatinous material called the
a) The retina contains the photoreceptors, called the rod cells and cone
b) The retina has a very special region called the fovea centralis, where
cone cells are densely packed.
c) Sensory fibers in the optic nerve take nerve impulses to the visual
cortex of the brain.
d) The place where the optic nerve exits is called the blind spot.
B. The lens
1. When we look at an object, light rays pass through the pupil and focus on the
2. The lens, however, provides additional focusing power as visual
accommodation occurs for close vision.
3. The shape of the lens is controlled by the ciliary muscle within the ciliary
a) When we view a distant object, the ciliary muscle is relaxed.
b) When we view a near object, the ciliary muscle contracts.
C. Aging, or possibly exposure to the sun, also makes the lens subject to cataracts.
1. Currently, surgery is the only viable treatment for cataracts.
How Biology Impacts Our Lives
28A Protect Your Eyes from the Sun
A. The three most frequent causes of blindness are retinal disorders, glaucoma,
1. Retinal disorders include diabetic retinopathy and macular degeneration.
2. People who have experienced acute glaucoma report that the eyeball feels
as heavy as a stone.
3. In cataracts, cloudy spots on the lens of the eye eventually pervade the
B. Accumulating evidence suggests that both macular degeneration and cataracts,
which tend to occur in the elderly, are caused by long-term exposure to the
ultraviolet rays of the sun.
1. It is recommended, therefore, that everyone, especially people who live in
a sunny climate or work outdoors, wear sunglasses that absorb ultraviolet
C. The Sunglass Association of America has devised a system for categorizing
sunglasses based on the absorption of UV-A and visible light.
D. Healthcare providers have noted an increased incidence of cataracts in heavy
How Biology Impacts Our Lives
28B The Inability to Form a Clear Image Can Be Corrected
A. If we can see what is designated as size 20 letters from 20 feet away, you are
said to have 20/20 vision.
1. People who can easily see a near object but have trouble seeing an
optometrist’s chart 20 feet away are said to be nearsighted, a condition
a) In order to see distant objects, nearsighted people may wear concave
lenses, which diverge the light rays so that the image can be focused on
2. People who can easily see the optometrist’s chart 20 feet away but cannot
easily see near objects are farsighted, a condition called hyperopia.
a) In order to see near objects, these individuals may wear a convex lens.
3. When the cornea or lens is uneven, the image is fuzzy.
a) This condition, called astigmatism, can be corrected by wearing an
unevenly ground lens to compensate for the uneven cornea.
B. LASIK surgery
1. Rather than wearing glasses or contact lenses, many people are now choosing
to undergo LASIK eye surgery.
2. LASIK stands for laser in-situ keratomileusis, which results in reshaping of
3. Typically, adults affected by common vision problems respond well to
28.4 The retina sends information to the visual cortex
A. Rod cells contain numerous visual pigment molecules that absorb light.
1. The visual pigment in rods is a deep-purple pigment called rhodopsin.
2. When a rod absorbs light, rhodopsin splits into opsin and retinal,
leading to a cascade of reactions and the closure of ion channels in the rod
cell’s plasma membrane.
3. These nerve impulses go to the visual areas of the cerebral cortex.
4. Rod cells are very sensitive to light, and therefore, are suited to night
B. Cone cells are located primarily in the fovea centralis and are activated by
1. They allow us to detect the fine detail and color of an object.
2. Color vision depends on three different kinds of cones, which contain
pigments called the B (blue), G (green), and R (red) pigments.
3. Each pigment is made up of opsin and retinal.
4. In color blindness, usually one type of cone is defective or deficient in
C. The retina has three layers of neurons: the rod and cone cells, the bipolar cell
layer, and the ganglion cell layer.
1. The rods and cones synapse with the bipolar cells, which in turn synapse with
ganglion cells, whose axons are optic nerve fibers.
2. There are many more rods and cones than ganglion cells.
a) The sensitivity of cones versus rods is mirrored by how directly they
connect to ganglion cells.
D. As signals pass to bipolar cells and ganglion cells, integration occurs.
1. Considerable processing takes place in the retina before ganglion cells
generate nerve impulses, which are carried in the optic nerve to the visual
2. Because the image is inverted and reversed, it must be righted in the brain
for us to correctly perceive the visual field.
V. Sense of Hearing and Balance _________________________
Critical concepts include: regions of the mammalian ear, mechanism of hearing, sensory coding, hearing loss, sense of balance relating to the ear, motion sickness, and other sensory receptors regarding motion.
28.5 The mammalian ear has three well-developed regions
A. The ear has three distinct divisions: the outer, middle, and inner ear.
1. The function of the outer ear is to gather sound waves.
a) It consists of the pinna and the auditory canal.
2. The middle ear begins at the tympanic membrane and ends at a bony wall
containing two small openings covered by membranes.
a) These openings are called the oval window and the round window.
b) Three small bones lie between the tympanic membrane and the oval
window. Collectively called the ossicles, individually they are the
malleus, the incus, and the stapes.
3. The auditory tube extends from the middle ear to the nasopharynx and
permits equalization of air pressure.
4. The inner ear is filled with fluid.
a) It has three areas: the semicircular canals and the vestibule are both
concerned with balance; the cochlea is concerned with hearing.
1. Path of sound waves through the outer and middle ear
2. Sound waves travel by the successive vibrations of molecules.
3. When a large number of waves strike the tympanic membrane, it moves
back and forth ever so slightly.
4. The malleus takes the pressure from the tympanic membrane and passes it,
by means of the incus, to the stapes, which strikes the oval window.
a) The stapes vibrates the membrane of the oval window with a force that
has been multiplied.
b) Eventually the pressure waves disappear at the round window.
C. Mechanoreceptors for hearing
1. These receptors are located in the snail-shaped cochlea, a major part of the
2. The organ of Corti, by which we hear, consists of little hair cells that occur
along the length of the basilar membrane.
a) The hair cells have extensions called stereocilia, which are embedded in
the gelatinous tectorial membrane.
b) The hair cells now generate nerve impulses that travel in the cochlear
nerve to the brain stem.
c) When these impulses reach the auditory areas of the cerebral cortex,
they are interpreted as sound.
D. Sensory coding
1. Each part of the organ of Corti is sensitive to different wave frequencies, or
a) The pitch sensation we experience depends on which region of the
basilar membrane vibrates and which area of the brain is stimulated.
2. Volume is a function of the size (amplitude) of sound waves.
How Biology Impacts Our Lives
28C Protect Your Ears from Loud Noises
A. When we are children, the middle ear is subject to infections that can lead to
hearing impairment if not treated promptly.
B. With age, the mobility of the ossicles decreases, and in the condition called
otosclerosis, new filamentous bone grows over the stirrup, impeding its
movement and causing hearing loss.
1. Surgical treatment is the only remedy for this type of deafness, which is
called conduction deafness.
C. Another type of hearing loss, called age-associated nerve deafness, results from
stereocilia damage due to exposure to loud noises.
1. This type of deafness is preventable, if care is taken.
D. Noise is measured in decibels, and any noise above a level of 80 decibels could
result in damage to the hair cells of the organ of Corti.
1. Eventually, the stereocilia and then the hair cells disappear completely.
2. The first hint of danger could be temporary hearing loss, a “full” feeling in
the ears, muffled hearing, or tinnitus (ringing in the ears).
3. If exposure to noise is unavoidable, specially designed noise reduction earmuffs or earplugs are available.
4. Loud music, noisy indoor or outdoor equipment, motorcycles,
snowmobiles, motorcross bikes, gun shots, etc., can all damage hearing.
E. Some medicines are ototoxic.
1. They can make the ears susceptible to hearing loss.
a) Ex: anticancer drugs and antibiotics
How Life Changes
28D Evolution of the Mammalian Ear
A. Invertebrates have body parts that resemble those in the mammalian ear.
1. For example, insects have tympanic membranes in various places, including
their front legs.
2. Stereociliated hair cells located beneath the membrane generate nerve
impulses when sound waves cause the membrane to vibrate.
3. Gravitational balance organs called statocysts are found in cnidarians,
molluscs, and arthropods.
B. The outer ear of birds and mammals has a recessed tympanic membrane, and
the inner ear contains a utricle and a saccule, which are gravitational balance
organs that respond to the movement of particles called otoliths.
C. Fishes have semicircular canals and a utricle and saccule.
1. Fishes also have a lateral line system that admits water and contains
hair cells enclosed by a gelatinous cap called a cupula.
2. This unique system has no equivalent in terrestrial vertebrates.
D. Mammals have a unique middle ear due to the presence of the three ossicles.
1. The other terrestrial vertebrates have only one ossicle, a stapes.
2. Paleontologists believe that the malleus and incus are derived from the
jawbone of a reptile.
28.6 Mammalian balance receptors are in the inner ear
A. The mechanoreceptors for balance detect rotational and/or angular
movement of the head (rotational balance) and also straight-line
movement of the head in any direction (gravitational balance).
B. Rotational balance
1. Mechanoreceptors in the semicircular canals detect rotational balance.
a) The three semicircular canals are arranged so that there is one in each
dimension of space.
b) The base of each of the three canals, called the ampulla, is slightly
c) Little hair cells are located within the ampulla.
d) As fluid within a semicircular canal flows over and displaces a cupula,
the stereocilia of the hair cells bends.
e) This changes the pattern of signals carried by the vestibular nerve to the
2. When we spin, the cupula slowly moves in the same direction we are
spinning, and bending of the stereocilia causes hair cells to send messages
to the brain.
a) When we stop spinning, the slow-moving cupula continues to move in
the direction of the spin, and the stereocilia bend again indicating that we
3. Certain types of movement are likely to cause motion sickness.
a) Ex: seasickness, airsickness
b) Motion sickness is the most common cause of vertigo.
c) Fortunately, motion sickness subsides soon after the triggering motion
d) One way to avoid motion sickness is to avoid reading while you are in
C. Gravitational balance
1. Mechanoreceptors in the utricle and saccules detect movement of the head
in the vertical or horizontal planes.
a) The utricle and the saccule are two membranous sacs located in the
inner ear near the semicircular canals.
b) Both of these sacs contain little hair cells, whose stereocilia are
embedded within a gelatinous material called an otolithic membrane.
c) Calcium carbonate granules, or otoliths, rest on this membrane.
2. When the body is still, the otoliths in the utricle and the saccule rest on the
otolithic membrane above the hair cells.
a) When the head bends or the body moves in the horizontal and vertical
planes, the otoliths are displaced.
b) The otolithic membrane sags, bending the stereocilia of the hair cells
c) The frequency of nerve impulses in the vestibular nerve indicates
whether you are moving up or down.
3. These data reach the cerebellum, which uses them to determine the
direction of the movement of the head at that moment.
a) The cerebellum coordinates skeletal muscle contraction to correct our
position in space if necessary.
VI. Somatic Senses__________________________________________ _______
Critical concepts include: proprioceptors and touch receptors.
28.7 Mammalian proprioceptors are located in skeletal muscles_______________
1. The receptors that maintain muscle tone and also help maintain the
body’s balance and posture are called proprioceptors.
2. Muscle spindles act to increase muscle contraction and tension.
3. Golgi tendon organs decrease muscle tension.
4. The result is a muscle that has the proper length and tension, or muscle tone.
5. When a muscle relaxes and its length increases, the muscle spindle is stretched,
and nerve signals to the spinal cord are generated.
a) A reflex action then occurs.
1. The skin is composed of two layers: the epidermis and the dermis.
a) The dermis contains receptors, which make the skin sensitive to touch,
pressure, pain, and temperature (warmth and cold).
b) The touch receptors respond to mechanical stimuli that result in our sense of
touch, its location and intensity.
1. When damage occurs due to mechanical, thermal, or electrical stimulation, or a
toxic substance, cells release chemicals that stimulate pain receptors.
a) Stimulation of internal pain receptors is felt as pain from the skin, as well as
the internal organs. This is called referred pain.