Sensory cells detect stimuli: chemical flavorings in your food; light emitted by your TV screen; tension in a muscle as you grasp a computer mouse; sound waves produced by your MP3 player; touch sensations as you hold someone’s hand; or other sensations such as electricity, cold, or heat.
Figure 29.2A on the facing page shows sensory transduction occurring when sensory receptor cells in a taste bud detect sugar molecules, as when you lick an ice cream cone.
In Figure 29.2B, the taste bud on the left has sensory receptors that respond to sugar, and the taste bud on the right responds to salt. The graphs in Figure 29.2B also indicate how action potentials communicate information about the intensity of stimuli. In each case, the left part of the graph represents the rate at which the sensory neurons in the taste bud transmit action potentials when the taste receptors are not stimulated. The right side of each graph shows that the rate of transmission depends on the intensity of the stimulus. The stronger the stimulus, the more neurotransmitter released by the receptor cell and the more frequently the sensory neuron transmits action potentials to the brain.
Specialized sensory receptors detect five categories of stimuli.
One of the most sensitive chemoreceptors in the animal kingdom is on the antennae of the male silkworm moth Bombyx mori (Figure 29.3C). The antennae are covered with thousands of sensory hairs (visible in the micrograph). The hairs have chemoreceptors that detect a sex pheromone released by the female.
The ear converts air pressure waves to action potentials that are perceived as sound. The human ear is really two separate organs, one for hearing and the other for maintaining balance.
(Figure 29.4B). Both the outer ear and middle ear are common sites of childhood infections (called swimmer’s ear and otitis media, respectively).
These pressure waves make your eardrum vibrate with the same frequency as the sound (Figure 29.4E). The frequency, measured in hertz (Hz), is the number of vibrations per second.
High-pitched sounds, such as high notes sung by a soprano, generate high-frequency waves. Low-pitched sounds, like the low notes made by a bass, generate low-frequency waves.
Several organs in the inner ear detect body position and movement.
Boating, flying, or even riding in a car can make us dizzy and nauseated, a condition called motion sickness. Because motion sickness can be a severe problem for astronauts, the National Aeronautics and Space Administration (NASA) conducts research on the problem. NASA has discovered that some people can learn to consciously control body functions, including the vomiting reflex. Astronauts receive intensive training in how to exert “mind over body” when zero gravity starts to induce motion sickness.
Most invertebrates have some kind of light-detecting organ. an important advantage for flying insects and other small animals often threatened by predators. also provide excellent color vision. Some species, such as honeybees, can see ultraviolet light (invisible to humans), which helps them locate certain nectar-bearing flowers. single-lens eye (squids)- (The single-lens eye found in squids and other invertebrates differs in some of the details.)
Humans have single-lens eyes that focus by changing position or shape
A gland above the eye secretes tears, a dilute salt solution that is spread across the eyeball by blinking and that drains into ducts that lead into the nasal cavities. This fluid cleanses and moistens the eye surface. Excess secretion in response to eye irritation or strong emotions causes tears to spill over the eyelid and fill the nasal cavities, producing sniffles. Some scientists speculate that emotional tears play a role in reducing stress.
If you can do this accurately, you have normal (20/20) acuity in each eye. This means that from a distance of 20 feet, each of your eyes can read the chart’s line of letters designated for 20 feet. Nearsighted– Nearsightedness A nearsighted eyeball is longer than normal (also known as myopia) is corrected by glasses or contact lenses that are thinner in the middle than at the outside edge. The lenses make the light rays from distant objects diverge as they enter the eye.
Farsighted people see distant objects normally but cannot focus on close objects.
The human retina contains two types of photoreceptors named for their shapes. The relative numbers of rods and cones an animal has correlates with whether an animal is most active during the day or night. Each human eye contains about 125 million rod cells and 6 million cone cells. (Rhodopsin is derived from vitamin A, which is why vitamin A deficiency can cause “night blindness.”)
RODS DO NOT DETECT COLOR.
Your senses of smell and taste depend on receptor cells that detect chemicals in the environment. Chemoreceptors in your taste buds detect molecules in solution; chemoreceptors in your nose detect airborne molecules.
Umami describes the savory flavor common in meats, cheeses, and other protein-rich foods, as well as the flavor-enhancing chemical monosodium glutamate (MSG).
- specialized neurons or other cells that are
tuned to the conditions of the external world or
internal organs. - It sweeps its wide head
back and forth, like a
the shore with a metal
- Electroreception, the
ability to sense electric
Sensory receptors convert stimulus energy
to action potentials
In stimulus detection, the receptor cell
converts one type of signal (the stimulus)
to another type, an electrical signal.
This conversion, called sensory
transduction, produces a change in the
cell’s membrane potential (the potential
energy stored by the plasma membrane
of the receptor cell.
• Changes in the flow of
ions create a graded
change in membrane
potential called a
Sensory adaptation, the
tendency of some
sensory receptors to
become less sensitive
when they are stimulated
Five Categories of Stimuli
1. Pain Receptors
respond to excess
heat or pressure or to
from damaged or
detect either heat
are stimulated by
various forms of
mechanical energy, such
as touch and pressure,
stretching, motion, and
Five Categories of Stimuli
• light touch- transduce
very slight inputs of
into action potentials.
• pressure sensor-
lying deeper in the
skin, is stimulated by
• touch receptor
around the base of
the hair, detects hair
Another type of mechanoreceptor
(not shown) is found in our skeletal
muscles. Sensitive to changes in
muscle length, stretch receptors
monitor the position of
include the sensory
receptors in our nose and
taste buds, which are attuned
to chemicals in the external
environment, as well as some
receptors that detect
chemicals in the body’s
include sensors in some of
our arteries that can detect
Five Categories of Stimuli
detected as Energy
occurring as electricity,
magnetism, or various
wavelengths of light.
eyes, are probably the most
common type of
Photoreceptors detect the
electromagnetic energy of light,
Five Categories of Stimuli
For each of the following
senses in humans, identify
the type of receptor: seeing,
tasting, hearing, smelling.
Hearing and Balance
Basic principle: the stimulation of long
projections on hair cells (mechanoreceptors) in
1. Outer ear
2. Middle ear
3. Inner ear
1. outer ear consists of
the flap-like pinna— the
fleshy structure we
commonly refer to as
our “ear”—and the
2. The pinna and the
auditory canal collect
sound waves and
channel them to the
eardrum, a sheet of
tissue that separates the
outer ear from the middle
When sound pressure waves strike the eardrum,
the eardrum vibrates and passes the vibrations to
three small bones:
the hammer (more formally, the malleus), anvil
(incus), stirrup (stapes)
The middle ear
also opens into a
passage called the
with the pharynx
(back of the throat),
pressure to stay
equal on either
The inner ear consists of fluid-filled channels in
the bones of the skull. Sound vibrations or
movements of the head set the fluid in motion.
One of the channels, the cochlea (Latin for
“snail”), is a long, coiled tube.• Our actual hearing
organ, the organ of
Corti, is located within
the middle canal.
• The organ of Corti
consists of an array
of hair cells
embedded in a
the floor of the middle
Sound waves, which move as pressure
waves in the air, are collected by the pinna
and auditory canal of the outer ear.
Volume and Pitch
The brain senses a sound as an increase in the
frequency of action potentials from the auditory
How is the quality of the sound
“The higher the volume (loudness) of sound, the
higher the amplitude (height) of the pressure wave
The pitch of a sound depends on the frequency
of the sound waves.
Deafness can be caused by the inability of the
ear to conduct sounds, resulting from middle-ear
infections, a ruptured eardrum, or stiffening of
the middle-ear bones (a common age-related
Can also result from damage to sensory
receptors or neurons.
In recent years, however, many deaf people
have received cochlear implants, electronic
devices that convert sounds to electrical
impulses that stimulate the auditory nerve
structures lie next to the
cochlea (Figure 29.5)
and include three
semicircular canals and
two chambers, the
utricle and the saccule.
The ear contains three
that detect changes in
the head’s rate of
The inner ear houses our organs of balanc
What causes motion sickness?
Motion sickness is believed to be
caused when the brain receives
signals from equilibrium receptors in
the inner ear that conflict with visual
signals from the eyes.
Symptoms maybe relieved by
closing the eyes, limiting head
movements, or focusing on a stable
2 types of image-forming
1. compound eye consists
of up to several
thousand light- etectors
2. Single-lens eye
evolved independently in
The human eye has a
small opening at the
center of the eye, the
pupil, through which
doughnut shaped iris
changes the diameter
of the pupil to let in
more or less light.
After going through
the pupil, light passes
through a single
The lens focuses
light onto the retina,
which consists of
Photoreceptor cells are highly concentrated at
the retina’s center of focus, called the fovea.
The outer surface
of the human
eyeball is a tough,
whitish layer of
called the sclera.
At the front of the
eye, the sclera
cornea, which lets
light into the eye
and also helps
The sclera surrounds a
pigmented layer called the
choroid. The anterior choroid
forms the iris, which gives the
eye its color.
The lens and ciliary
body divide the eye
into two fluid-filled
The large chamber
behind the lens is
filled with jellylike
The much smaller
chamber in front of
the lens contains the
• A thin mucous
membrane helps keep
the outside of the eye
moist. This membrane,
called the conjunctiva
Arrange the following eye parts into
the correct sequence encountered
by photons of light traveling into
the eye: pupil, retina, cornea, lens,
vitreous humor, aqueous humor.
Cornea aqueous humor pupil
vitreous humor lens
Artificial lenses or surgery can
correct focusing problems
Reading from an eye chart measures your visual
acuity, the ability of your eyes to distinguish fine
People with nearsightedness cannot focus well
on distant objects, although they can see well at
short distances (the condition is named for the
type of vision that is unimpaired).
Farsightedness (also known as hyperopia) is
the opposite of nearsightedness. It occurs when
the eyeball is shorter than normal, causing the
lens to focus images behind the retina.
2 types of photoreceptors:
1. Cones- are stimulated by bright light and can
distinguish color, but they contribute little to
• Cones contain visual pigments called
photopsins, which absorb bright, colored
2. Rods are extremely sensitive to light and
enable us to see in dim light, though only in
shades of gray.
Rods contain a visual pigment called
rhodopsin, which can absorb dim light.
Taste and Smell
Taste and odor receptors detect chemicals present
in solution or air
Olfactory (smell) receptors are sensory neurons
that line the upper portion of the nasal cavity
and send impulses along their axons directly to
the olfactory bulb of the brain
Many animals rely heavily on their sense of
smell for survival. Most other mammals have a
much more discriminating sense of smell than
Odors often provide more information than
visual images about food, the presence of
mates, or danger.four familiar taste perceptions:
sweet, sour, salty, and bitter and
UMAMI (Japanese for “delicious”)