Chapter 18: The Senses
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Types of Sensory Receptors
• Chemoreceptors respond to chemical
substances, such as changes in pH, or the
senses of taste and smell.
• Pain receptors are chemoreceptors that
respond to chemicals from damaged
tissues.
• Mechanoreceptors respond to mechanical
forces.
• The senses of hearing and balance both
involve mechanoreceptors.
• Proprioceptors (mechanoreceptors) in
tendons around joints make us aware of
position; pressoreceptors in arteries detect
blood pressure changes, and stretch
receptors in lungs detect degree of
inflation.
• Thermoreceptors respond to temperature
changes; there are both warm receptors
and cold receptors.
• Photoreceptors respond to light energy.
• Special photoreceptors called rods result
in black-and-white vision, while cones
detect color.
How Sensation Occurs
• Sensation occurs when nerve impulses
reach the cerebral cortex.
• Perception is an interpretation of the
meaning of sensations.
• The sensation that results depends on the
part of the brain receiving the impulses.
• Receptors may integrate signals before
sending nerve impulses.
• Sensory adaptation occurs when a stimulus
continues but the receptor decreases its
response.
Sensation
• These proprioceptors allow the muscles to
maintain the proper length and tension, or
muscle tone.
• The knee-jerk reflex involves muscle
spindles.
• Signals to the CNS from muscle spindles
help maintain balance and posture.
• Golgi tendon organs are proprioceptors
with the opposite effect.
Muscle spindle
Cutaneous Receptors
• The dermis of the skin contains sensory
receptors for touch, pressure, pain, and
temperature (warmth and cold).
• Three types of cutaneous receptors are
sensitive to fine touch:
1) Meissner corpuscles are concentrated in
finger tips, lips, tongue, nipples, and
genital areas;
2) Merkel discs are found where the
epidermis meets the dermis; and
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3) free nerve endings (root hair plexus)
around hair follicles all detect touch.
Three different types of pressure
receptors are Pacinian corpuscles,
Ruffini endings, and Krause end bulbs.
Temperature receptors are simply free
nerve endings in the epidermis; some
are responsive to cold and others are
responsive to warmth, although there are
no structural differences between them.
Sensory receptors in human skin
• Pain Receptors
• Nociceptors are pain receptors on internal
organs and may be sensitive to
temperature, pressure, or chemicals.
• Referred pain occurs when stimulation of
internal pain receptors is felt as pain from
the skin.
• Referred pain most likely happens
because of shared nerve pathways
between the skin and internal organs.
Sense of Taste
• The taste buds located in papillae on the
tongue contain taste cells that
communicate with sensory nerve fibers.
• Microvilli on taste cells contain receptor
proteins that match chemicals in food.
• The brain determines the taste according
to a “weighted average” of incoming
impulses from taste buds sensitive to
either sweet, sour, salty, or bitter tastes.
Taste buds
Sense of Smell
• Olfactory cells (modified neurons) are
located in epithelium in the roof of the nasal
cavity.
• After molecules bind to receptor proteins on
the varied cilia of olfactory cells, nerve
impulses lead to olfactory areas of the
cerebral cortex.
• The perceived odor is determined by the
combination of olfactory cells stimulated.
• The effects of smell and taste combine.
Olfactory cell location and anatomy
Anatomy of the Eye
• The eye has three layers.
• The sclera is the outer layer seen as the
white of the eye and includes the
transparent bulge in the front of the eye
called the cornea.
• The choroid is the middle, darkly
pigmented layer that absorbs stray light
rays; it also becomes the iris that regulates
the size of the pupil.
• Behind the iris, the choroid thickens and
forms the ciliary body.
• The ciliary body contains the ciliary
muscle, which controls the shape of the
lens for near and far vision.
• The lens divides the eye into two
compartments: the anterior compartment
(containing aqueous humor) and the
posterior compartment (containing vitreous
humor).
• Rod cells and cone cells are located in the
retina that forms the inner layer.
• The retina lines the back half of the eye
and has cone cells densely packed in one
area called the fovea centralis.
• Sensory fibers from the retina form the
optic nerve leading to the brain.
Anatomy of the human eye
Focusing
• The cornea and the lens focus light rays
on the retina.
• To see a close object, the ciliary muscles
change the lens shape to provide visual
accommodation.
• After age 40, the lens is less able to
accommodate and near vision is less
acute.
• Cataracts occur when the lens becomes
opaque; sun exposure might be a factor in
developing cataracts.
Focusing
Photoreceptors
• Both rod cells and cone cells have an
outer segment with membranous disks
containing embedded pigments.
• Rods contain a deep purple pigment called
rhodopsin that is composed of retinal
(made from vitamin A) and the protein
opsin.
• Rods are numerous and provide
peripheral vision, perception of motion,
and vision in dim light at night.
• When a rod absorbs light, rhodosin splits
into opsin and retinal, leading to a cascade
of reactions and the closing of rod
membrane ion channels.
• Inhibitory neurotransmitters are no longer
released from the rod.
• Breakdown of rhodopsin in rods thus
initiates nerve impulses.
• Cones have three different pigments (red,
green and blue) made from retinal and
opsin; opsin varies between the three.
Photoreceptors in the eye
Integration of Visual Signals in the
Retina
• The retina has three layers of neurons:
rods and cones are near the retina, bipolar
cells are in the middle, and the innermost
layer contains ganglion cells that carry
impulses to the optic nerve.
• The rod and cones synapse with the
bipolar cells, which in turn synapse with
ganglion cells that initiate nerve impulses.
• As signals pass from one layer to the next,
integration occurs because each layer
contains fewer cells than the previous layer.
• However each cone connects directly to one
ganglion cell, while a hundred rods may
synapse with only one ganglion cell.
• It is likely that much processing occurs in the
retina before impulses are sent to the brain.
• There are no rods and cones where the optic
nerve exits the retina; this is the blind spot.
Structure and function of the retina
Integration of Visual Signals in the
Brain
• The visual pathway begins with the retina
and passes through the thalamus before
reaching the cerebral cortex.
• The visual pathway and the visual cortex
split the visual field apart, but the visual
association areas rebuild it so we correctly
perceive the entire visual field.
Optic chiasma
Abnormalities of the Eye
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• Color Blindness
The most common abnormality is a lack of red
and/or green cones.
• Distance Vision
Nearsighted individuals (elongated eyeball)
cannot see distant objects; this is corrected by
a concave lens.
Farsighted individuals (shortened eyeball) see
distant objects well but not up close; this is
corrected by a convex lens.
Astigmatism occurs with an uneven cornea or
lens.
Common abnormalities of the eye
Anatomy of the Ear
• The ear is divided into three parts.
• The outer ear consists of the pinna and
the auditory canal, which direct sound
waves to the middle ear.
• The middle ear begins at the tympanic
membrane (eardrum) and contains the
ossicles: the malleus, incus, and stapes
that amplify sound waves.
• The malleus is attached to the tympanic
membrane, and the stapes is attached to
the oval window, which is covered by
membrane.
• The inner ear contains semicircular canals
and vestibule involved in equilibrium, and
the cochlea for hearing.
Anatomy of the human ear
Process of Hearing
• Sound waves enter the auditory canal and
vibrate the tympanic membrane.
• If the vibrations are strong enough, the
outer and middle portions (ossicles) of the
ear convey and amplify the sound waves
about 20 times and vibrate against the oval
window.
• These vibrations set up pressure waves
within the fluid of the cochlea.
• The cochlea contains the spiral organ
consisting of hair cells on the basilar
membrane whose stereocilia are
embedded within the tectorial membrane.
• Vibrations within the cochlea cause the
sterocilia to vibrate against the tectorial
membrane, thus generating nerve
impulses.
• Different regions are sensitive to different
frequencies or pitch.
• When the stereocilia of the hair cells bend,
nerve impulses are generated in the
cochlear nerve and are carried to the
brain.
Mechanoreceptors for hearing
• Gravitational Equilibrium
• Stimulation of hair cells within the utricle
and the saccule, two sacs located in the
vestibule, by the slippage of calcium
carbonate granules or otoliths, provide
impulses that tell the brain the direction of
movement of the head.
• The movement of the otoliths provides a
sense of gravitational equilibrium.
Mechanoreceptors for equilibrium
• Proprioceptors in muscles and joints help
the body maintain balance and posture.
• Cutaneous receptors in the skin respond
to touch, pressure, pain, and temperature
(both warmth and cold).
• In the mouth, the microvilli of taste cells
have membrane protein receptors that
respond to certain molecules.
• Olfactory cells within the olfactory
epithelium respond to molecules and
result in a sense of smell.
• Photoreceptors for sight contain visual
pigments, which respond to light rays.
• Some integration occurs in the retina of
the eye before nerve impulses are sent to
the brain.
• Sensory receptors for hearing are hair
cells in the cochlea of the inner ear that
respond to pressure waves.
• Sensory receptors for balance are hair
cells in the vestibule and semicircular
canals of the inner ear that respond to the
tilt of the head and to the movement of the
body, respectively.