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Chapter 8
The Special Senses
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Senses
Somatic senses
Involve receptors from more than one place in the body
Help coordinate muscle movement
Maintain body temperature
Special senses
Extremely sensitive receptors
Supply us with detailed information about the world around us
Sight
Sound
Smell
Taste
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Special Senses
Special senses include
Photoreceptors for vision
Mechanoreceptors for hearing and balance
Chemoreceptors for smell and taste
Sensory adaptation
The perception that sense simply decreases to the point where we are not aware of it any longer
Special senses evolved to protect organisms from danger as they move through their environment so they can reach reproductive age
Anything that affects reproduction can have species-wide effects
For example, color vision evolved to help animals find ripe fruit
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Olfaction
Smelling
Occurs in the upper chambers of the nasal passages
Sensory receptors respond to chemicals dissolved in the mucus lying over them
When a receptor binds its specific odor molecule
A sensory impulse is sent to the olfactory bulb and on to the brain
Messages (signals) travel along nerve fibers
Neural connections between the olfactory bulb and the limbic system explain why smells trigger memories and emotions
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Olfaction
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Gustation
Taste
The sense of taste is linked with the sense of smell
Receptors for taste are in roughly 10,000 taste buds
Most of which are on the tongue
In small bumps called papillae
Taste buds can distinguish five categories of taste
Sweet, sour, salty, bitter, and umami (savory)
When stimulated, taste bud receptor cells send information on to the brain
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Gustation
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Hearing
The ear has three functional parts
The outer ear
Composed of the pinna and external auditory canal
Both capture sound waves and funnel them to the middle ear
The middle ear
Ear drum (tympanic membrane) marks the beginning of the middle ear
Compression waves in the air (sound) cause the membrane to vibrate
Converting sound into mechanical motion
Attached to the inside of the tympanic membrane is the malleus
The vibrating tympanic membrane moves the malleus, which in turn moves the incus through a synovial joint
The stapes (ossicle) is the final small bone of the middle ear
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Hearing
The inner ear
Beyond the stapes lies the oval window
A membrane that functions like the tympanic membrane
The oval window bounces in response to movement of the stapes
Creating “fluid waves” in the inner ear
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10. The Middle and Inner Ear
The entire middle and inner ear are within a hollow portion of the temporal bone
This area is filled with air and communicates with the external environment through the eustachian tube (auditory tube)
The cochlea of the fluid-filled inner ear has three compartments
Vestibular canal, organ of Corti, and tympanic canal
The tectorial membrane lies on top of the organ of Corti and rests on hair cells, which are the sensory receptors of the auditory system
Hair cells do not have hairs - rather, they have organelles called stereocilia which look like hairs - stereocilia are sound-sensing organelles
As the stapes pushes air against the membrane, the stereocilia move and a nerve impulse is created in the neuron of that particular hair cell
Each part of the tectorial membrane is sensitive to a different pitch
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Hearing
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Hearing
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13. Equilibrium is Housed in the Inner Ear
Equilibrium is the sense of balance
The vestibule and semicircular canals of the inner ear are responsible for the two types of equilibrium
Static equilibrium (also called gravitational equilibrium)
The physical response to gravity that tells us which direction is down
Dynamic equilibrium (also called rotational equilibrium)
Detects acceleration or deceleration of your head
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Static Equilibrium
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Dynamic Equilibrium
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Vision
The eye has three layers
Sclera (fibrous layer)
Choroid (vascular layer)
Retina (nervous layer)
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17. Eye Structures and Functions
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18. Visual Acuity and the Lens
Visual acuity requires the eye to focus entering light onto the retina at the back of the eyeball
The changing of lens shape to view nearby objects is called accommodation
Accommodation gets more difficult with age
Because the lens continues to add layers over time
Making the lens thicker and stiffer
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Visual Impairments
Nearsightedness and farsightedness are both caused by the lens' inability to accommodate light properly
Nearsightedness
The eye is too long for the lens to focus the light rays on the retina
Results in forming an image that is spread out and fuzzy when it hits the retina
Farsightedness is the opposite of nearsightedness
Astigmatism
The cornea is imperfectly shaped, resulting in an uneven pattern of light hitting the retina
Some areas of the image are in focus, but not others
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Visual Impairments
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The Retina
Behind the lens lies a large chamber filled with vitreous humor, a gel-like fluid that holds the retina in place
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22. The Retina is Sensitive to Light
Composed entirely of layers of neurons
Rods and cones are neurons that detect light - the photoreceptors
Photoreceptors detect light in the retina
The cones respond to bright light, providing color vision and resolution
Cones are concentrated near the center of the retina, where incoming light is strongest
The rods function in low levels of light, providing only vague images
Rods are spread across the periphery of the retina
The bipolar cells and ganglionic cells are interneurons that carry the action potential generated by the photoreceptors to the brain
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Anatomy of the Retina
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24. Rods and Cones Use Different Chemical Mechanisms
When a photon of light hits a rod, a neural response is initiated via the chemical rhodopsin
Rhodopsin is a visual pigment that responds to low levels of white light
Light splits it into retinal and opsin, which causes a series of events that generate an action potential
Cones
Use the visual pigments retinal and opsin
There are three types of cones, each of which is sensitive to different wavelengths of light
Red
Green
Blue
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Rods and Cones
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The Path of Vision
Visual nerve impulses first pass toward the front of the eye
From the photoreceptors to the bipolar neurons
The bipolar neurons transmit the impulse to the ganglionic cells
In the anterior of the retina
Ganglionic cells collect impulses from a small cluster of bipolar cells
And pass them to the brain via the optic nerve
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The Path of Vision
Vision occupies more space in the brain than any other special sense
Visual impulses travel along the optic nerve, through the thalamus to the occipital lobe of the brain
Some impulses cross to the opposite side of the brain at the optic chiasma
The view from the right eye is partially projected on the left side of the visual cortex of the cerebrum, and that from the left is partially projected on the right side
The image reaching the occipital lobe is upside down and inverted
The brain must flip and invert the image before it makes sense to us
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Sensory Loss
Vision and hearing can diminish with age
Some hearing loss is due to mechanical malfunctions
Conduction deafness
Sound is poorly conducted from the outer ear to the inner ear
Hearing aids can help those suffering from conduction deafness by increasing the amplitude of sound that enters the ear
Nerve deafness
Cannot be resolved by a hearing aid
Sound is either not detected by the cochlear nerves or the nerve impulse is not transmitted to the brain
Cochlear implants convert sound vibrations into electrical impulses and have shown some promise in treating nerve deafness
© 2013 by John Wiley & Sons, Inc. All rights reserved.