CHAPTER 29 THE SENSES. PROCESSING SENSORY PERCEPTION The sensory nervous system tells the CNS what is happening. Sensory neurons carry impulses to the.

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.