Download The Senses

The Senses
Lecture Outline
I. What are sensory receptors?
A. Sensory receptors change environmental stimuli into nerve impulses.
 Refer to Table 33.1 for a list of the different types of environmental stimuli
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II.
In order to be detected, an environmental stimulus must be strong enough to open
ion channels in the membranes of receptor cells, thus depolarizing them and
initiating a generator potential
A generator potential may initiate an action potential in the sensory neurons that
synapse with the sensory receptor, and the impulse is transmitted to the brain
Impulses conducted to specific sensory areas of the cerebral cortex produce
conscious sensations
The special senses include sight, hearing, taste and smell
General somatic senses include receptors for heat, cold, pain, and pressure
Receptors that sense the internal environment are located within the walls of blood
vessels
Receptors that provide information about body position and movement are located
in the muscles, tendons, joints, and inner ear
How do animals sense their internal and external environments and their position in
space?
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Receptors within blood vessels and organs sense the internal environment.
Temperature-sensitive neurons in the hypothalamus, act as the body’s thermostat,
constantly monitoring the temperature of the blood
Homeostatic mechanisms include shivering or sweating, contracting or dilating the
blood vessels in the skin, and an increase or decrease in cellular respiration (which
produces heat)
Other receptors located in the walls of arteries are sensitive to the levels of carbon
dioxide and oxygen in the blood, as well as its pH
o Signals are sent to respiratory centers in the medulla and pons that regulate
the rate of respiration
 Pain receptors are widely distributed throughout the body
B.
Receptors within skeletal muscles, tendons, and the inner ear sense position in
space.
 Proprioceptors sense the position of the body in space
o Muscle spindles are specialized muscle cells that keep track of the degree to
which the muscles are contracted
o A stretch receptor is wrapped around the muscle spindle that detects muscle
stretching
 Other stretch receptors are found in vertebrate tendons, and in the tissue
surrounding joints
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III.
Vertebrates have nerve endings in the skin that are sensitive to temperature.
Thermoreceptors are sensitive to coldness and warmth
Can some animals detect stimuli humans cannot?
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IV.
Mechanoreceptors detect stimuli such as touch, movement, and pressure.
Fish and aquatic amphibians have a complex system of mechanoreceptors called a
lateral line system that detects sound vibrations, bodily movements, and other
pressure changes in the environment produced by water currents
Invertebrates have a variety of mechanoreceptors that are sensitive to touch
Human skin has free nerve endings that detect pain, temperature, itch, and
movement
Merkel’s disks and Meissner’s corpuscles are two types of touch receptors in the
human skin
Pacinian corpuscles are pressure receptors located deeper in the skin, particularly in
the nipples and external genitals of both sexes
Some animals can detect infrared radiation, and others can detect UV light.
Pit vipers, such as rattlesnakes, have pit organs in the head that can detect the heat
given off by another animal
Some insects are sensitive to the UV light emitted from flowers
Some animals can detect electrical fields.
Some fish, salamanders, and the duck-billed platypus have electroreceptors
Some electric eels and electric catfish generate an electric field strong enough to
stun a potential prey
Some electric fish generate a weak electric field that is used as a type of radar to
detect objects in the environment
Some migratory animals and those with homing instincts detect magnetic fields.
Magnetic fields affect a wide variety of animals, including salmon, salamanders,
turtles, hornets, honeybees, and homing pigeons
Animals affected by magnetic fields have magnetic granules in their bodies
How do the senses of taste, smell, sight, hearing, and balance work?
A.
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Taste buds and olfactory receptors sense chemicals.
Chemoreception is the ability to sense chemicals
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Chemoreceptors may be found in the skin of animals, in their legs, their mouths, or
in bristles on the head
 Chemoreception is used to find food or mates, locate enemies, or to warn other
members of the species of danger
 A directed movement toward or away from a chemical stimulus is a positive or
negative chemotaxis
 Taste
o Taste is the detection of chemicals in solution
o In humans, the four primary tastes detected by taste buds are sweet, salty, bitter,
and sour
o Taste buds are microscopic structures shaped like onions, each made up of from
30 to 80 receptor cells
 Smell
o Olfactory receptors sense chemicals in the air
o In humans, olfactory receptors, equipped with tiny sensitive cilia,
are located in the nasal epithelium
B.
Rods and cones within the retina of the eye sense light.
 Photoreception is the sensing of visible light, which is the
manifestation of certain wavelengths of the electromagnetic
spectrum
 Photoreception in organisms other than humans
o Plants and certain protists can detect light
o Euglena is an example of a protist with a stigma, or
eyespot, that is sensitive to light
o Various species of worms and arthropods have
light-sensitive eyes that do not form images
o Many insects have compound eyes, consisting of
many lenses
o Certain mollusks (squid and octopus) have imageforming eyes similar to the eyes of vertebrates
o Positive phototropism is movement toward light
(example = housefly) , while negative phototropism
if movement away from light (example =
cockroach)
 Sight
o Refer to Figure 33.16 for the detailed
anatomy of the human eye
o The eyeball is protected by a tough
outer covering called the sclera
o The transparent cornea admits light,
which passes through the aqueous
humor, is focused by the lens, and
then passes through the vitreous
humor before striking the retina
o Failure to focus an image on the
retina results in blurry vision, usually
caused either by an elongated eyeball
(producing nearsightedness) or a
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C.
shortened eyeball (producing
farsightedness)
The iris controls the amount of light
entering the eye by varying the size
of the pupil
Rod-shaped cells in the retina are
sensitive to the presence of light, and
not its color
There are three types of cone-shaped
cells in the retina, each sensitive to a
different range of wavelengths of
light (red, blue, and green)
The area of sharpest vision on the
retina is the fovea, which contains
the greatest concentration of rods
Impulses generated in the rods and
cones of the retina are transmitted via
the optic nerve to the visual cortex of
the cerebrum
Depth perception is due to the
different positions of the two eyes,
each viewing an object from a
slightly different angle
Receptor cells of the inner ear sense pressure waves created by sound waves.
 Sound waves are mechanical
energy resulting from the
vibration of an object, which
causes compression and
decompression waves in air,
water, or solid materials
 Sound reception in organisms
other than humans
o Many animal, such as elephants, homing pigeons, and probably hippos,
perceive low-frequency sounds, called infrasound
o Elephants use infrasound as mating calls and warnings
o Dogs hear high-pitched sounds
o Whales, dolphins, shrews, some birds, and bats can hear extremely highpitched sounds
o Bats use extremely high-pitched sounds for echolocation, or animal sonar
 Hearing
o The pinna of the external ear funnels sound waves into the
external auditory canal, which leads to the tympanic
membrane
o The eustachian tube connects the middle ear to the pharynx,
and permits an equalization of pressure on both sides of the
tympanic membrane
o Vibrations of the tympanic membrane are transferred
through three tiny bones, the malleus, incus, and stapes, to
the fluid-filled cochlea of the internal ear for interpretation
o The cochlea contains an elongate organ of Corti, that is
covered with hair cells embedded in a gelatinous material
o Vibrations directed into the cochlea cause these hairs to
vibrate in concert with specific wavelengths of vibration
o The vibrations of the cells, and basilar membrane to which
they are attached, generate nerve impulses in neurons that
are transmitted through the auditory nerve to the brain,
where they are interpreted as sound
D.
The vestibule and semicircular canals of the inner ear detect position and direction
of movement.
 Most animals have gravity receptors (statocysts) that
are sensitive to the movement of small grains
(statoliths) located within a specialized organ, where
they rest on sensory hairs
 Displacement of the stones relative to the force of
gravity causes the statoliths to stimulate sensory
cells, which then send information to the CNS
 In humans, and other vertebrates, a bulge in the
midsection of the inner ear, called the vestibule,
contains the utricle and saccule, which contain
otoliths that stimulate cilia when the head is moved
away from right side up and this information is fed
to the CNS
 Positioned above the saccule and utricle are three
fluid-filled semicircular canals positioned at right
angles to one another
 The semicircular canals contain ciliated sensory
cells that detect movements of the head and send
this information to the CNS