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Chapter 29
The Senses
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.• temperature change• blood chemistry• pain• muscle contraction• blood pressure• touch Skeletal
muscle fiber
Specializedmuscle fibersConnectivetissue sheath
Nerve endings
Sensorynerve fiber (stretch receptor)
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.
How the inner ear senses gravity and
motion
1
3
Flow of fluid
Direction of body movement
Stimulation
Cilia ofhair cellsHair cells
Supportingcell
Sensorynervefibers
Saccule
Utricle
2
Otoliths
Gelatinousmatrix
Haircells
Supportingcells
Nerve
Fluid
Cupula
Semicircularcanals
Sensing Chemicals: Taste and Smell
• Embedded within the surface of the tongue are taste buds that contain many chemical 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.
Tastepapillae
Taste bud
Supportcell
Tastepore
Receptorcell withmicrovilli
Nerve fiber
Sensing Chemicals: Taste and Smell
• In the nose, chemically sensitive neurons are embedded within the epithelium of the nasal passage. Basal cell
Supportcell
Nasalpassage Axon
Receptorcell
Cilia
Olfactory nerve Olfactorymucosa
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.
Membranesupportinghair cellsHair cells
Branch ofauditorynerve
Membranecoveringhair cells
Fluid-filledcanals
Earcanal
Eustachiantube
Eardrum
Auditorynerve
Semicircularcanals
Ossicles
Cochlea
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
Opening
Canal
Cilia
Cupula
Sensorynerves
Afferentaxons
NerveLateral lineorgan
Lateral linescales
Haircell
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 is carried out by a special sensory apparatus called an eye.• Eyes contain
sensory receptors, called photoreceptors, that capture light energy.
• Many invertebrates have simple visual systems with photoreceptors clustered in an eyespot that can perceive the direction of light but cannot form an image.
Light
PhotoreceptorsEyespot
Pigment layer
Flatworm will turnaway from light
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 light-capturing molecule.
Eyes in three phyla of animals
Lenses
Retinularcell
Insect VertebrateMollusk
Opticnerve
Retina
Retina
Lens
LensOpticnerve
Eyemuscles
Opticnerve
(left): © David M. Dennis; (middle, right): © Corbis RF
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 eyeSclera
RetinaSuspensoryligament
Iris
Ciliary muscle
Lens
Cornea
Pupil
Artery
Fovea
Optic nerve
Choroid
Vein
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.
Pigmentdiscs
Outer segment
Connectingcilium
Inner segment
Mitochondria
Nucleus
Synapticterminal Rod Cone
Sensing Light: Vision
• 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.• There are three colors absorbed by these different
cone cells: blue, green, and red.
Lig
ht
abso
rpti
on
(p
erce
nt
of
max
imu
m)
400Wavelength (nm)
700600500
20
100
80
60
40
Bluecones420 nm
Redcones560 nm
Greencones530 nm
Rods500 nm
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.
Innerchamber
Pit
Outerchamber
Membrane
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.