THE SENSES
CHAPTER 29
PROCESSING SENSORY
PERCEPTION
• The sensory nervous system tells the CNS
what is happening.
• Sensory neurons carry impulses to the CNS from
special cells, called sensory receptors, that
detect changes outside and inside the body.
• Particularly complex sensory receptors, made up
of many cells and tissue types, are called sensory
organs.
PROCESSING SENSORY
PERCEPTION
•
The path of sensory information to the CNS
is a simple one composed of three stages:
1. Stimulation is when a stimulus impinges on a
sensory receptor.
2. Transduction is the conversion of the stimulus to
an electrical potential, or nerve impulse.
3. Transmission by a sensory neuron conducts the
impulse along an afferent pathway to the CNS.
PROCESSING SENSORY
PERCEPTION
• All sensory receptors are able to initiate
nerve impulses by opening or closing
stimulus-gated channels.
• Except for visual photoreceptors, these channels
are sodium ion channels.
• Exteroreceptors sense stimuli that arise in the
external environment.
• Interoreceptors sense stimuli that arise from within
the body.
PROCESSING SENSORY
PERCEPTION
• The body uses a variety of interoreceptors to
respond to different aspects of its internal
environment.
Sensory
•
•
•
•
•
•
temperature change
blood chemistry
pain
muscle contraction
blood pressure
touch
nerve fiber (stretch receptor)
Nerve endings
Skeletal
muscle fiber
Specialized
muscle fibers
Connective
tissue sheath
SENSING GRAVITY AND MOTION
• Sensory receptors that detect gravity are
hair cells in the inner ear
• The tips of the hair cells project into a gelatinous
matrix with embedded particles called otoliths.
• The otoliths shift in the matrix in response to the
pull of gravity, stimulating hair cells.
• Sensory receptors in the semicircular canals
of the inner ear detect changes in motion.
Otoliths
Gelatinous
matrix
Hair
cells
Supporting
cells
Utricle
1
2
Semicircular
canals
Saccule
Fluid
Flow of fluid
Nerve
Cupula
Cilia of
hair cells
Hair cells
HOW THE INNER
EAR SENSES
GRAVITY AND
MOTION
Supporting
cell
Stimulation
Sensory
nerve
fibers
3
Direction of body movement
SENSING CHEMICALS: TASTE
AND SMELL
• Embedded within the surface
of the tongue are taste buds
that contain many chemical Taste
papillae
receptors.
• Chemicals from food dissolve in
saliva and contact the taste cells
• The tastes perceived are salty, sour,
sweet, bitter, and umami (a
“meaty taste”).
• The “hot” sensation of foods, such
as chili peppers, is detected by
pain receptors, not chemical
receptors.
Taste bud
Support
cell
Taste
pore
Receptor
cell with
microvilli
SENSING CHEMICALS: TASTE AND
SMELL
• In the nose,
chemically sensitive
neurons are
embedded within the
epithelium of the
nasal passage.
Olfactory nerve
Nasal
passage
Olfactory
mucosa
Axon
Basal cell
Support
cell
Receptor
cell
Cilia
SENSING SOUNDS: HEARING
• When you hear a sound, you
are detecting the air vibrating,
as waves of pressure push the
eardrum membrane in and
out.
• On the other side of the eardrum
are three small bones, called
ossicles, that transfer the vibration
to the inner ear fluid.
• Sound receptors within the
cochlea of the inner ear interpret
sound as changes in fluid that
move a sensitive membrane.
• The membrane vibrates
differently according to sounds
of different frequencies.
Ossicles
Semicircular
canals
Auditory
nerve
Cochlea
Ear
canal
Eardrum
Eustachian
tube
Fluid-filled
canals
Branch of Membrane
covering
auditory
hair cells
nerve
Hair cells
Membrane
supporting
hair cells
SENSING SOUNDS: HEARING
• A lateral line system provides
a sense of “distant touch”.
• Vibrations carried through the
fish’s environment travel
down a longitudinal canal
and other canals in the fish’s
skin.
• These vibrations produce
movements of cupula that
contain hair cells, causing the
hair cells to bend.
• This stimulates sensory
neurons.
Lateral line
Lateral line
scales
Opening
Canal Lateral line
organ
Nerve
Cupula
Hair
cell
Afferent
axons
Sensory
nerves
Cilia
SENSING SOUNDS: HEARING
• Some mammals
perceive distance by
means of sonar.
• Using echolocation, they
emit sounds and then
determine the time it
takes these sounds to
reach an object and
return .
• Examples of mammals
that use sonar are bats,
shrews, whales, and
dolphins.
SENSING LIGHT: VISION
• The perception of light Photoreceptors
Eyespot
is carried out by a
Light
special sensory
apparatus called an
eye.
Pigment layer
• Eyes contain sensory
receptors, called
photoreceptors, that
Flatworm will turn
capture light energy.
away from light
• Many invertebrates have simple visual systems with
photoreceptors clustered in an eyespot that can
perceive the direction of light but cannot form an
image.
SENSING LIGHT: VISION
• Well-developed, image-forming eyes have
evolved in four animal phyla:
•
•
•
•
annelids
mollusks
arthropods
vertebrates
• These are examples of convergent evolution
although all use the same type of lightcapturing molecule.
EYES IN THREE PHYLA OF ANIMALS
Lenses
Optic
nerve
Retina
Optic
nerve
Lens
Optic
nerve
Eye
muscles
Retinular
cell
Retina
Insect
Mollusk
(left): © David M. Dennis; (middle, right): © Corbis RF
Vertebrate
Lens
SENSING LIGHT: VISION
• The vertebrate eye works like a lens-focused
camera.
• Light first passes through a transparent covering
called the cornea.
• a lens helps to focus the light from the cornea to
the rear of the eye.
• The shape of the lens can be adjusted by ciliary muscles.
• The iris, located between the cornea and the
lens, acts as a shutter to control the amount of
light that enters the eye.
• The pupil is the transparent zone in the middle of the iris.
• The retina is an array of photoreceptors in the
back of the eye.
THE STRUCTURE OF THE HUMAN EYE
Suspensory
ligament
Sclera
Retina
Choroid
Lens
Optic nerve
Cornea
Vein
Pupil
Artery
Iris
Ciliary muscle
Fovea
SENSING LIGHT: VISION
• The retina is the light-sensing
portion of the eye and
contains two kinds of
photoreceptors:
• Rods are very sensitive to light
intensity but do not detect
color or produce sharp images.
• Cones can detect color and
produce sharp images.
• The center of the vertebrate
retina contains a tiny pit,
called the fovea, that is
densely packed with cones
and produces the sharpest
image.
Pigment
discs
Outer segment
Connecting
cilium
Inner segment
Mitochondria
Nucleus
Synaptic
terminal
Rod
Cone
• Three kinds of cone
cells provide us
with color vision.
• Each possesses a
different version of
the opsin protein,
which affects the
wavelength of light
absorbed by retinal.
Light absorption (percent of maximum)
SENSING LIGHT: VISION
Blue
cones
420 nm
Green Red
Rods
cones cones
500 nm 530 nm 560 nm
100
80
60
40
20
400
600
500
Wavelength (nm)
• There are three colors absorbed by these different
cone cells: blue, green, and red.
700
SENSING LIGHT: VISION
• Color blindness occurs
when individuals are
not able to perceive all
three colors.
• It typically occurs due to
an inherited lack of one
or more types of cones.
• It is a sex-linked trait, so
men are more likely to
be colorblind than
women.
SENSING LIGHT: VISION
• In primates and most predators the image each
eye sees is slightly different because each eye
views the object from a different angle.
• This slight displacement permits binocular vision, the ability
to perceive 3-D images and to sense depth or the distance
to an object.
• Other animals, such as prey animals, have eyes located on
the sides of the head, enlarging the overall visual field but
preventing binocular vision.
OTHER VERTEBRATE SENSES
• Vertebrates can sense their environment via parts of
the electromagnetic spectrum other than visible
light.
• Heat
• Pit vipers possess a pair of heat-detecting pit organs
located on either side of the head between the eye and
the nostril.
Pit
Outer
Membrane
chamber
Inner
chamber
OTHER VERTEBRATE SENSES
• Electricity
• Elasmobranches (sharks, rays, and skates) have
electroreceptors.
• Magnetism
• Eels, sharks, bees, and many birds can navigate
along the magnetic field lines of the earth.