Chapter 29 The Senses
All animal senses originate in sensory receptors,specialized cells or neurons that are tuned to the
–conditions of the external world and
–the internal organs.
All sensory receptors
–trigger an electrical signal (action potential) from neuronsand
–send information to the central nervous system.
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29.3 Specialized sensory receptors detect fivecategories of stimuli
There are five categories of sensory receptors.
1. Pain receptors detect dangerous stimuli including highheat and pressure.
2. Thermoreceptors detect heat or cold.
3. Mechanoreceptors respond to
–mechanical energy,
–touch,
–pressure, and
–sound.
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4. Chemoreceptors
–include sensory receptors in our nose and taste buds and
–respond to chemicals.
5. Electromagnetic receptors respond to
–electricity,
–magnetism, and
–light (sensed by photoreceptors) in humans.
29.3 Specialized sensory receptors detect fivecategories of stimuli
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Figure 29.3A
Heat
Light touch
Epidermis
Pain
Cold
Hair
Dermis
Nerve
to brain
Connective
tissue
Hair
movement
Strong
pressure
29.4 The ear converts air pressure waves to actionpotentials that are perceived as sound
The human ear channels sound waves
–from the outer ear with a flap-like pinna,
–down the auditory canal,
–to the eardrum, which separates the outer ear from themiddle ear,
–to a chain of bones in the middle ear (malleus, incus, andstapes), and
–to the fluid in the coiled cochlea in the inner ear that islined with hair-like nerve cells.
–The Eustachian tube connects the pharynx to the middleear, permitting pressure equalization.
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Figure 29.4A
Outer ear
Inner ear
Pinna
Auditory
canal
Eardrum
Middle ear
Eustachian
tube
Pressure waves transmitted to the fluid of thecochlea
–stimulate hair cells in parts of the cochlea (organ of Cortiand basilar membrane) and
–trigger nerve signals to the brain.
29.4 The ear converts air pressure waves to actionpotentials that are perceived as sound
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Deafness is the loss of hearing.
Deafness can be caused by the inability to detectsounds resulting from
–middle-ear infections,
–a ruptured eardrum, or
–stiffening of the middle-ear bones.
Deafness
–can also result from damage to sensory receptors orneurons and
–is often progressive and permanent.
29.4 The ear converts air pressure waves to actionpotentials that are perceived as sound
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29.5 The inner ear houses our organs of balance
Three organs in the inner ear detect body positionand movement. These include
–three semicircular canals and
–two chambers, the utricle and the saccule.
–All three of these structures operate by bending of hairson hair cells (clusters of hair cells are called cupula).
The three semicircular canals detect changes inthe head’s rotation or angular movement.
The utricle and saccule detect the position of thehead with respect to gravity.
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Figure 29.5
Semicircular
canals
Nerve
Cochlea
Saccule
Utricle
Flow of fluid
Cupula
Flow
of fluid
Cupula
Hairs
Hair
cell
Nerve fibers
Direction of body movement
29.7 Several types of eyes have evolvedindependently among animals
The ability to detect light plays a central role in thelives of nearly all animals.
All animal light detectors are based on cells calledphotoreceptors that contain pigment molecules thatabsorb light.
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29.7 Several types of eyes have evolvedindependently among animals
Most invertebrate eyes include some kind of light-detecting organ.
One of the simplest organs is the eye cup,
–used by platyhelminths called planarians,
–which senses light intensity and direction.
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Figure 29.7a
Dark
pigment
Eyecups
29.7 Several types of eyes have evolvedindependently among animals
Two major types of image-forming eyes haveevolved in the invertebrates.
1. Compound eyes of insects
–consist of up to several thousand lenses
–function as acute motion detectors, and
–usually provide excellent color vision.
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Figure 29.7B
29.7 Several types of eyes have evolvedindependently among animals
2.In single-lens eyes
–light enters the front center of the eye through a small opening,the pupil, controlled by an iris,
–passes through a single disklike lens, and
–is focused onto the retina, which consists of many photoreceptorcells and connects to the optic nerve that goes to the brain.
–The center of focus is the fovea, where photoreceptor cells arehighly concentrated.
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Figure 29.7C
Sclera
Ligament
Cornea
Iris
Pupil
Aqueous
humor
Lens
Vitreous
humor
Blind spot
Artery
and vein
Optic
nerve
Fovea
(center of
visual field)
Retina
Choroid
29.7 Several types of eyes have evolvedindependently
The outer surface of the human eyeball is a tough,whitish layer of connective tissue called the sclera.
–At the front of the eye, the sclera becomes thetransparent cornea,which
–lets light into the eye and
–also helps focus light along with ligaments that change theshape of the lens.
–The sclera surrounds a pigmented layer called thechoroid (posterior iris). The anterior choroid forms theiris, which gives the eye its color.
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29.7 Several types of eyes have evolvedindependently
The lens and ciliary body divide the eye into twofluid-filled chambers.
1.The large chamber behind the lens is filled with a jellylikevitreous humor.
2.The smaller chamber in front of the lens contains thethinner aqueous humor.
–These humors
–help maintain the shape of the eyeball and
–circulate nutrients and oxygen to the lens, iris, and cornea.
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