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Chapter 14
The Senses
Points to Ponder
• What are sensory receptors?
• How do we detect the sense of taste and
smell?
• What is the anatomy of the eye?
• How do we focus images?
• What are some eye abnormalities?
• What is the anatomy of the ear?
• Which parts function in balance and which
parts function in hearing?
14.1 Sensory receptors and sensations
Sensory receptors
• Sensory receptors – dendrites specialized to
detect certain types of stimuli
– Exteroceptors: detect stimuli from outside the
body (e.g. taste, hearing, vision).
• Continually send messages to the CNS regarding
environmental conditions
– Interoceptors: receive stimuli from inside the
body (e.g. change in blood pressure)
• Receptors are activated only when stimuli is present
1. Pressoreceptores: change in blood pressure
2. Osmoreceptors: change in water-salt balance
3. Chemoreceptors: monitor pH of the blood
Types of sensory receptors
• Chemoreceptors – respond to nearby chemicals
– Taste, smell, monitor pH in the blood
• pH low  chemoreceptors in carotid arteries and aorta activated 
breathing rate increases  more CO2 is expired  blood ph raises
– Pain receptors – a type of chemoreceptors
• respond to chemicals released by damaged tissue
• Photoreceptors – respond to light energy
– Sensitive to light rays and provide us with vision
– Rod cells: black and white vision
– Cone cells: color vision
• Thermoreceptors – stimulated by temperature
changes
– Located in the hypothalamus and the skin
Types of sensory receptors
• Mechanoreceptors – respond to mechanical forces
such as pressure
– Hearing:
• sound waves are converted to fluid-borne pressure waves that are
detected y mechanoreceptors in the inner ear
– Balance and equilibrium:
• mechanoreceptors response to fluid-borne pressure waves in the
vestibule and semicircular canals of the inner ear
– Touch receptors: skin
– Pressoreceptors: in the certain arteries
– Proprioceptors: respond to stretching of muscle fibers,
tendons, joints, and ligaments
• Allow body to determine position of our limbs
14.1 Sensory receptors and sensations
Senses and the receptors involved
How does sensation occur?
1. Sensory receptors respond
to environmental stimuli
2. Nerve impulses travel to
cerebral cortex
3. Sensation (conscious
perception) of stimuli occurs
4. Sensory adaptation,
decrease in stimulus
response, can occur with
repetitive stimuli (i.e. odor)
– Two Possible Pathways to
adaptation
1. Sensory receptors stop
sending impulses to brain
2. RAS filters out ongoing stimuli
- Sensory info  brain stem 
thalamus  to cerebral
cortex via RAS
Sensation
- Sensory receptors
-
Free nerve endings or specialized cells associated
with neurons
Plasma membrane of the receptors contains
receptor proteins that react to the stimulus
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Ex. Chemoreceptors bind to particular molecule
Once receptor is stimulated
-
Ion channels open, and ions flow across the plasma
membrane
Impulse carried to the PNS and CNS
Travel ascending tract to the brain  cerebral cortex  to
particular area depending on impulse (i.e. visual, auditory)
-
Results in sensation and perception
Proprioceptors and Cutaneous Receptors
• Sensory receptors in the muscles, joints,
tendons, internal organs and skin
• Sensory receptor stimulation  impulse
travels to spinal cord  travels ascending
tracts to the brain  somatosensory areas
of the cerebral cortex
Proprioceptors
• Mechanoreceptors involved in reflex actions that maintain
muscle tone
• Determines position of limbs by detecting degree of muscle
relaxation, stretch of tendons, and movement of ligaments
• Muscle spindles
– Sensory nerve endings wrapped around muscle cells
– Nerve impulses generated when muscle relaxes and stretching is low
Cutaneous receptors
•
•
Receptors in the dermis make the skin sensitive to touch,
pressure, pain and temperature
Temperature receptors in epidermis
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–
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Free nerve endings
Hot and cold receptors
Receptors sensitive
to touch in dermis
1. Meissner corpuscles
2. Merkel disks
3. Root hair plexus
•
Receptors sensitive
to pressure in dermis
1. Pacinian corpuscles
2. Ruffini endings
Pain Receptors
• Sensitive to chemicals released by damaged
issues
• Drugs inhibit the synthesis of certain chemicals
• Referred Pain
– Pain from an internal organ is referred to the skin
– Nerve impulse from pain receptors of internal organs
travel to the spinal cord and synapse with neurons
also receiving impulses from the skin
• i.e. pain from the heart is felt in the left shoulder and arm
Senses of Taste and Smell
• Chemical senses
– Taste cells and olfactory cells have
chemoreceptors sensitive to particular
molecules in food and air
Taste receptors
• Taste receptors = taste buds
~ 3,000 taste buds mostly on the tongue
– Also present on hard palate, pharynx, and epiglottis
• Sensitive to sweet, sour, salty and bitter tastes in food
• 80-90% of what we perceive as taste is actually due to the
sense of smell
• From Taste buds  Brain
–
–
–
–
–
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Taste bud opens at taste pore
Taste bud has supporting cells and taste cells with microvilli
Tastant binds to chemoreceptors of taste cells
Nerve impulse is generated in sensory nerve fibers
Impulse travels to thalamus
Relayed to the gustatory (taste) cortex
Cortex integrates incoming info and gives us our sensation
Taste Receptors
14.3 Senses of taste and smell
Smell receptors
• 10-20 million olfactory cells (modified neurons) in the roof of
the nasal cavity
• Each cell end has olfactory cilia with receptors for odorants
(odor molecules)
• From Olfactory receptors  Brains
– Each olfactory cells has receptors for one odorant, but nerve fibers
from different cells lead to the same neuron in the olfactory bulb
– Odorant bind to chemoreceptors on olfactory cilia
– Stimulation of olfactory cell
– Impulse travels via that sensory nerve fiber
– Synapse with neuron in olfactory bulb
– Impulse travels via the olfactory tract to the
• Olfactory areas of the cerebral cortex
• Limbic system: center for emotion and memory
• Number of olfactory cells decline with age, remaining ones
become less sensitive
Olfactory Receptors
14.4 Sense of vision
Anatomy of the eye
• 2 compartments:
– Anterior chamber:
• between cornea and lens filled with a clear fluid called aqueous
humor
• Typically drains via ducts
• Glaucoma occurs when drainage ducts are blocked
– Resulting pressure compresses arteries that serve nerve fibers of
retina resulting is partial or complete blindness
– Posterior chamber:
• most of the eye, behind the lens contains gelatinous material
called vitreous humor
• Made of 3 layers/coats –
A. Sclera: mostly white and fibrous except the cornea
B. Choroid: darkly, pigmented vascular layer
C. Retina: inner layer containing photoreceptors
14.4 Sense of vision
Anatomy of the eye
14.4 Sense of vision
A. The eye: Sclera
• Sclera – the white of the eye that maintains
eye shape
– Cornea: transparent portion of the sclera that
is important in refracting light
– Pupil: a hole that allows light into the eyeball
14.4 Sense of vision
B. The eye: Choroid
• Choroid – middle layer that absorbs light
rays that are not absorbed by the retina
– Iris: donut-shaped, colored structure that
regulates the size of the pupil
• Color of irus correlates with its pigmentation
– Ciliary body: a structure behind the iris that
contains a muscle that control the shape of the
lens
• Lens – attached to the ciliary body and
functions to refract and focus light rays
14.4 Sense of vision
Anatomy of the eye
• The lens is flexible,
transparent and concave
• The lens accommodates,
changes shape, to focus light
on retina to form an image
– Shape controlled by ciliary
muscles
– Distant objects
• Ciliary muscles relax
• Suspensory ligament stretch
• Lens remains flat
– Near objects
• Ciliary muscle contract
• Releasing tension on
ligaments
• Lens rounds up
• As we age the lens loses
elasticity and we use glasses
to correct for this
The eye: The lens
14.4 Sense of vision
C. The eye: Retina
• Contains photoreceptors
– rods and cones
• Rods are sensitive to light
• Cones require bright light and see wavelengths of
light (color)
• The fovea centralis is an area of the retina
– densely packed with cones where images are focused
• Sensory receptors from the retina form the optic
nerve
– Optic nerve takes impulses to the brain
• The blindspot is where the optic nerve attaches
and lacks vision
14.4 Sense of vision
Anatomy of the retina
C. The eye: Photoreceptors of the retina
• Embedded in the membrane of the retina
• Rods:
– Contain a visual pigment called rhodopsin
• Made of opsin and retinal (light absorbing molecule)
Stimulation of Photoreceptors
1. Rod absorbs light
2. Rhodopsin splits into opsin and retinals
3. This split leads to reactions that cause the closure of ion
channels in the rod cell’s plasma membrane
4. Release of inhibitory transmitter molecules from rod’s synaptic
vesicles ceases
5. Signal goes to other neurons in the retina
– Important for peripheral and night vision
– Vitamin A is important for proper functioning
C. The eye: Photoreceptors of the retina
• Cones:
– Located mostly in the fovea
– Allow us to detect fine detail and color
– 3 different kinds of cones containing red,
green and blue pigments
• Each pigment is made up of retinal and
opsin but with differences in the opsin
structure which accounts for individual
absorption patterns
14.4 Sense of vision
Rods and cones in the retina
Visual Pathway to the Brain
1. Stimulation of Photoreceptors (previously explained)
2. Photoreceptors synapse with bipolar cells and stimulate cells
3. Bipolar cells synapse ganglion cells and stimulate these cells
– Integration of signals coming from photoreceptors are processed by the
bipolar and ganglion cells before ganglion cells generate nerve
impulses
4. Nerve impulse is generated and travels via the sensory fibers
(axons) of ganglion cells which become optic nerve
5. Optic nerve carry impulses from the eyes to the optic chiasma
(X shape, crossing of optic nerve fibers) and optic tract
6. Optic tract synapse with nuclei within thalamus
7. Thalamic nuclei axons form optic radiations that take nerve
impulses to the visual cortex in the occipital lobe
14.4 Sense of vision
Summary of eye structures
14.4 Sense of vision
Abnormalities of the eye
• Color blindness – genetic disease most common in males
– usually cannot see red or green
• Cataracts – lens of the eye is cloudy
• Glaucoma – fluid pressure builds up in the eye
• Astigmatism – cornea or lens is uneven leading to a fuzzy
image
• Nearsightedness – eyeball is too long making it hard to see
far away objects
– Rays focus in front of the retina
– Concave corrective lens improve vision
• Farsightedness – eyeball is too short making it hard to see
near objects
– Rays focus behind the retina
– Convex corrective lens improve vision
14.4 Sense of vision
Abnormalities of the eye that are corrected
with lenses
14.5 Sense of hearing
Anatomy of the ear
• The ear functions in hearing and balance
• 3 divisions:
A. Outer ear: functions in hearing; filled with air
B. Middle ear: functions in hearing; filled with air
C. Inner ear: functions in hearing and balance;
filled with fluid
14.5 Sense of hearing
A. The ear: Outer ear
• Includes:
– Pinna: the external ear flap that catches sound
waves
– Auditory canal: directs sound waves to the
tympanic membrane
• lined with fine hairs and modified sweat glands that
secrete earwax
– Was helps guard against entrance of foreign materials
14.5 Sense of hearing
B. The ear: Middle ear
• Includes:
– Tympanic membrane (eardrum): membrane
that vibrates to carry the wave to the bones
– 3 small bones called ossicles: amplify sound
waves
• Malleus (hammer), incus (anvil), stapes (stirrup)
• Malleus adheres to tympanic membrane
• Stapes touches the oval window
– Auditory tube: a tube that connects from the
throat to the middle ear and is used to equalize
pressure so the eardrum does not burst
14.5 Sense of hearing
Following the sound wave
14.5 Sense of hearing
C. The ear: Inner ear
•
Important for both hearing and balance
•
3 areas: cochlea, semicircular canals, vestibule
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Stapes (middle ear bone) vibrates and strikes the
membrane of the oval window causing fluid waves in
the cochlea
•
Cochlea – hearing
•
Vestibule – gravitational equilibrium
•
Semicircular canals – rotational equilibrium
14.5 Sense of hearing
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Begin at the auditory canal
Travel by vibrations of molecules
Waves strike the tympanic membrane
Malleus takes the pressure from the inner surface of the
tympanic membrane and passes it to incus, and to the stapes
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Auditory Pathway
This method multiplies the pressure about 20 times
Stapes strikes the membrane of the oval window causing it to
vibrate
Pressure is passed to the fluid within the cochlea
Cochlae contains the organ of Corti (spiral organ) sense
organ containing hairs for hearing
1. Pressure waves cause the basilar membrane to move up and down
2. Bending of embedded hairs (stereocilia) occurs when they are pushed
against the tectorial membrane
3. Bending of stereocilia causes the generation of a nerve impulses that
travels to cochlear nerve and then to brain
Pitch and Volume
• Pitch
– Determined by varying wave frequencies that are
detected by different parts of the organ of Corti
– Sensation depends on the region of the basilar
membrane that vibrates and the area of the auditory
cortex that is stimulated
• Volume
– Determined by the amplitude of sound waves
– More pressure in canal causes basilar membrane to
vibrate to a greater extent
• Result is increased stimulation interpreted as increased
volume
14.5 Sense of hearing
14.Sense of equilibrium
The inner ear: Semicircular canals and
vestibule
• Detects movement of the head in the
vertical and horizontal planes
(gravitational equilibrium)
– Depends on hair cells in the utricle and
saccule
• Detects angular movement (rotational
equilibrium)
– Depends on hair cells at the base of each
semicircular canal (ampulla)
14.5 Sense of hearing
The inner ear: Balance
Rotational Equilibrium Pathway
• Mechanoreceptors in semicircular canals
– Detect rotation/angular movement of head
• Three semicircular canals
– One is each dimension of space
• Base of each canal = ampulla
– Each ampulla responds to head rotation in a different plane
of space
• Little hair cells found in ampulla are associated with
a cupula
• As fluid within a semicircular canal flows over at
displaces the cupula
• Stereocilia bend, and pattern of impulses carried by
the vestibular nerve to the brain changes
• Brain uses info from hair cells to maintain equilibrium
Gravitational Equilibrium Pathway
• Mechanoreceptors in the utricle and saccule
– Detect movement of head in the vertical and horizontal place
• Utricle and saccule contain little hair cells
• Stereocilia are embedded within otolithic membrane
– Otolithic consists of otoliths (calcium carbonate granules)
• When Body is Still
– otoliths in the utricle and the saccule rest on the otolithic membrane
above the hair cells
• When Head Bends
– Otoliths are displace and the otolithic membrane sags
– Stereocilia of the hair cells bend
– If stereocilia move toward the kinocilium
• nerve impulses increase in the vestibular nerve
– If stereocilia move away from the kinocilium
• nerve impulses decrease in the vestibular nerve
– Frequency of nerve impulses indicates if movement is up or down