The Senses
Anatomy & Physiology I
Chapter 11
The Senses
• Protect a person by detecting changes in the
environment
• Stimulus – an environmental change that initiates or
stimulate a nerve impulse
• Sensation – when a specialized area of the cerebral
cortex interprets the nerve impulse and the stimulus
then becomes something we experience
• Environmental change >> stimulus >> threshold
stimulus >> sensory receptors >> sensory neuron>>
CNS >> interpretation>> sensation (what we
experience or sense)
• Threshold stimulus - minimum amount of stimulus
required to generate a nerve impulse
Sensory Receptors –
Types based on Structure
• Free dendrite of a sensory neuron; Example, pain,
temperature
• End-organ - the modified dendrite ending of a
sensory neuron enclosed in a capsule -Pacinian &
Meissner corpuscles for pressure & touch
receptors
• Specialized cell associated with an afferent
neuron (afferent/ascending/to the CNS); rods &
cones in the eye
Sensory Receptors
Sensory Receptors –
Types based on Stimulus
• Chemoreceptors – detect chemicals in solution;
taste & smell
• Photoreceptors – respond to light; eye retina
• Thermoreceptors – detect temperature change; skin
• Mechanoreceptors – respond to movement,
vibration, pressure, stretch; located in skin, joints
(monitor body position), hearing & equilibrium in
ear
Types of senses based on the
distribution of their receptors
• Special senses; sensory
receptors localized in a special
sense organ
–
–
–
–
–
Vision
Hearing
Equilibrium
Taste
Smell
• General senses; sensory
receptors widely distributed
throughout the body
–
–
–
–
–
Pressure
Temperature
Pain
Touch
Position
The Eye - Protective Structures
• Skull bones form walls of eye
orbit; over half of posterior eye
• Upper & lower eyelids; anterior
eye
• Eyelashes & eyebrows; anterior
eye
• Conjunctiva; lines inner surface
of eyelids & covers the visible
portion of the white of the eye
(sclera); produces mucus & is
highly vascular
• Lacrimal glands; produce tears
which lubricate & produce
enzyme that protects against
infection
– Flow from superior lateral eye
into inferior medial nasolacrimal
duct
Lacrimal Gland & Conjunctiva
Coats of the Eyeball
• 3 tunics (coats)
• Sclera – outermost tunic
made of tough connective
tissue; white of the eye
because made of collagen &
no blood vessels
• Choroid – coat made of
delicate connective tissue;
extensive blood supply
which are visualized during
eye exam; prevents light
from scattering throughout
eye
• Retina – actual receptor
layer; contains rods & cones
which generate visual nerve
impulses
What are some structures that
protect the eye?
What are the names of the tunics
of the eyeball?
Pathway of Light Rays & Refraction
• Refraction – bending of light
rays as they pass from one
substance to another
substance of a different
density
• Allows light from a very large
area to be focused on a very
small area of the retina
• Cornea
• Aqueous humor
• Lens
• Vitreous body
Light Refraction in the Eye
• Cornea – transparent,
colorless continuation of
sclera that covers the pupil;
the window of the eye
• Aqueous humor – watery
fluid that fills the eye anterior
to the lens; aids in refraction
& maintains eye shape
• Lens – clear circular structure
with biconcave surface made
of firm, elastic material; can
change in thickness & focus
near or far
• Vitreous body – soft gel that
fills entire space posterior to
the lens; aids in refraction &
maintains eye shape
Layers of the Retina
• Pigmented layer –
deepest layer just
anterior to choroid
• Rods & cones –
receptors of the eye
• Connecting neurons
that carry impulses
toward the optic
nerve
Rods
• Rods – highly sensitive to
light
– function in dim light but do
not provide sharp image
• Dark adaptation; the time
it takes for rods to begin
working in a darkened area
– 120 million each retina
– Distributed towards the
periphery of the retina
– See shades of gray; no
colors
– Rhodopsin – visual purple
pigment that is sensitive to
light; requires vitamin A;
lack of this pigment leads
to night blindness
Cones
• Cones – sensitive to color
– Function only in bright light
– 6 million per retina
– Localized in center of retina
• fovea centralis, pit near the optic nerve;
area of greatest visual acuity
• surrounded by the macula lutea
• Optic disk; point where the optic nerve
arises in the retina; no rods or cones in
this area; blind spot on the retina
– Sees sharp images
– Pigments sensitive to red, green, blue
• Hereditary lack of pigment can lead to
colorblindness in males
Fovea & Macula lutea
Fovea (dark pink) &
Macula lutea (yellowish)
Visual Impulses
• Light stimulates rods &
cones which stimulate
neurons that eventually
merge to form the optic
nerve (CN II)
• Some optic nerve fibers
crosses at optic chiasma
• Visual center in the
occipital cortex of the
cerebrum interprets
Eye Muscles – Extrinsic & Intrinsic Groups
• Extrinsic
– 6 voluntary on outer
surface of eye
– Controlled by CN III, IV,
VI (oculomotor,
trochlear, abucens)
– Convergence – pulling
the eyeballs in a
coordinated fashion so
there is one visual field
Eye Muscles – Extrinsic & Intrinsic Groups
• Intrinsic
– Involuntary muscles within the
eyeball
– Controlled by CN III
(oculomotor)
– Iris; pigmented part of the eye
composed of 2 sets of muscles
that control pupil size
• Circular muscle constricts in
bright light
• Radial muscle constricts in dim
light
– Ciliary muscle; holds the lens
of the eye in place by
suspensory ligaments
– Accomodation – ciliary muscle
constriction changes the shape
of the lens to allow for near &
far vision
Intrinsic Eye Muscle Actions
• The light rays from a close
object diverge (separate) more
than do the light rays from a
distant object >>the lens must
become more rounded to
bend the light rays more
• When the ciliary muscle is
relaxed, tension on the
suspensory ligaments keeps
the lens in a more
flattened shape. For close
vision, the ciliary muscle
contracts
• For close vision, the ciliary
muscle contracts, which draws
the ciliary ring forward and
relaxes tension on the
suspensory ligaments. The
elastic lens then recoils and
becomes thicker
Nerve Supply to Eye
Two sensory nerves supply the eye:
• CNII – optic nerve; carries visual impulses from retinal
rods & cones to the thalamus (diencephalon) to the
visual center in occipital lobe of cerebrum
• CNV – trigeminal nerve, opthalmic branch; carries pain,
touch & temperature impulses from the eye to the brain
• There are no retinal rods and cones in the area of the
optic nerve. Consequently, no image can form on the
retina at this point, which is known as the blind spot or
optic disk
Nerve Supply to the Eye
Three nerves carry motor impulses to the eyeball muscles:
• CNIII – oculomotor; largest motor nerve to the eyeball; supplies
voluntary & involuntary muscles to all but two eye muscles
• CNIV – trochlear; supplies superior extrinsic eye muscle
• CNVI – abducens; supplies lateral extrinsic eye muscles
Steps in Vision
•
•
•
•
•
•
•
•
Light refracts (bends)
Muscles of the iris adjust the pupil
Ciliary muscle adjusts the lens (accomodation)
Extrinsic eye muscles produce convergence
(coordinate to allow one visual field)
Light stimulates rods & cones
Optic nerve transmits impulses to thalamus
Thalamus transmits impulses to occipital lobe
Occipital lobe cortex interprets impulses
Errors of Refraction
• Hyperopia – farsightedness
– Usually due to abnormally short eyeball (flat cornea)
– Light focuses behind retina
– The lens can thicken only to a given limit to accommodate for
near vision
– move an object away from the eye to see it clearly
– Glasses with convex lenses that increase light refraction
• Myopia – nearsightedness
– Usually due to abnormally long eyeball
– Light focuses in front of retina
– Distant objects appear blurred and may appear clear only if
brought near the eye
– A concave lens corrects for myopia
• Astigmatism – blurred vision
– Cornea or lens curves irregularly, bending light incorrectly
A. Normal
C. Hyperopia
B. Myopia
D. Astigmatism
Eye Disorders
• Strabismus – deviation of the eye
due to lack of coordination of eye
muscles
• In convergent strabismus, the eye
deviates toward the nasal side
• In divergent strabismus, the
affected eye deviates laterally.
• if not corrected brain will not
develop to see properly
• Amblyopia – loss of vision in a
healthy eye because it cannot
work properly with the other eye
Eye Disorders
• Conjuntivitis – inflammation of conjunctiva
– Pinkeye – conjunctivitis caused by infection; is usually
caused by cocci or bacilli
– Inclusion conjunctivitis is an acute eye infection caused by
Chlamydia (AKA Trachoma)
• Corneal laceration – most common eye injury & if
untreated can result in blindness
– Cornea is avascular so it is possible to receive transplant
without rejection
– Enucleation – removes the eye due to traumatic injury
• Cataract – opacity of the lens which can lead to
blindness
• Glaucoma – excess pressure in the eyeball due to
aqueous humor not being reabsorbed into blood
Conjunctivitis & Cataract
Glaucoma
Disorders related to retina
• Diabetic retinopathy – retina damaged by
vascular hemorrhages & overgrowth
• Retinal detachment – retina separates from
underlying layer as the result of trauma or
fluid accumulation between tunics of eye
• Macular degeneration – macula lutea
deteriorates & distorts visual field
Diabetic Retinopathy
Detached Retina
Macular Degeneration
The Ear - Structure
• Outer ear – external ear to the tympanic membrane
• Middle ear – contains 3 bones (ossicles) of ear &
eustachian tube
• Inner ear – contains sensory receptors for hearing &
equilibrium
Outer Ear
• Pinna – aka auricle; the
external ear
• External auditory canal
– aka external auditory
meatus
– Contains ceruminous
glands
• Tympanic membrane –
aka eardrum
– Vibrates as sound
waves enter the ear
Middle Ear - Ossicles
• Small cavity containing 3
ossicles (bones) that
amplify sound waves
received by tympanic
membrane
• Malleus (hammer) –
attached to tympanic
membrane by handle &
head attaches to incus
• Incus (anvil) – connects to
malleus & stapes
• Stapes (stirrup) –
connects to oval window,
the membrane of inner
ear
The middle ear- ossicles
Middle Ear – Eustachian Tube
• Eustachian tube –
connects middle ear
to pharynx
• Allows pressure to
equalize on the 2 sides
of tympanic
membrane
Inner Ear aka Bony Labyrinth
• 3 separate divisions of sensory receptors
– Vestibule, semicircular canals & cochlea
– Perilymph – fluid of inner ear
– Membranous labyrinth – within bony labyrinth & filled with
endolymph
Inner Ear aka Bony Labyrinth
• Vestibule – 2
bony chambers
that contain
equilibrium
receptors
• Semicircular
canals – 2 bony
tubes that
contain
equilibrium
receptors
Inner Ear aka Bony Labyrinth
• Cochlea – bony coil that
contains hearing
receptors
• Round window –
membrane through
which sound waves
leave the inner ear
Hearing
• Organ of Corti – sensory organ
of hearing
– Consists of ciliated receptor
cells
• Located inside membranous
cochlea aka cochlear duct
• Wave against the roof of the
cochlear duct (tectorial
membrane)
• Stimulates cochlear nerve
(auditory branch of
vestibulocochlear nerve CN
VIII)
• Sound waves leave inner ear
through round window
Steps of Hearing
• Sound waves vibrate
tympanic membrane
• Amplified by ossicles in
middle ear
• Reach the oval window &
create waves in the inner
ear fluids
• Vibrating the chochlear duct
• Causing cilia or organ of
Corti to wave against the
tectorial membrane
• Cochlear nerve stimulated
(branch of CNVIII)
• The temporal lobe of
cerebrum interprets stimuli
•
•
•
•
Hearing
Organ of Corti differentiates both pitch (tone) & intensity (loudness)
Higher pitched tones near the base
Lower pitched tones near the top
Loud sounds stimulate more cells & produce more vibrations,
sending more impulses to the brain
Equilibrium - Balance
• Sensory receptors are
ciliated cells located in
vestibule & semicircular
canals
• Shifting in position of
cilia within the thick
fluid that surrounds
them generates a nerve
impulse
• Send impulses to
vestibular branch of CN
VIII (vestibulocochlear)
Equilibrium - Balance
• Static equilibriumSensing the position
of head or body
when moving in a
straight line
– Due to vestibule
receptors known as
macula
– Fluid surrounding
maculae contains
crystals called
otoliths which drag
the fluid & increase
pull of gravity
Detail of Macula
Equilibrium
• Dynamic equilibrium
– function during
spinning or moving
in different
directions
– Semicircular canal
receptors known as
cristae
Ear Disorders
• Otitis media –
inflammation of the
middle ear
– Children susceptible
because short
eustachian tube
• Otitis externia –
inflammation of
external auditory canal aka swimmer’s ear
Hearing Loss
• Conductive hearing loss – interference with sound waves
passing through ear
– Foreign body obstruction
– Tympanic membrane damage
– Otosclerosis – hereditary malfunction of stapes
• Sensorineural hearing loss – cochlea, CNVIII or temporal
lobe of brain improperly transmit or interpret neural stimuli
– Prolonged exposure to loud noises
– Certain drugs or toxins
– Cochlear implant can stimulate cochlear nerve directly,
bypassing receptor cells
– Presbycusis – age related hearing loss due to atrophy of sensory
receptors an CNVIII fibers
Loud Noise Damage to Ear
Taste - Gustation
• Tongue receptors aka taste buds
• Taste buds are stimulated only if
the substance to be tasted is in
solution or dissolves in the fluids
of the mouth
–
–
–
–
Sweet – tip of the tongue
Salty – middle of tongue
Sour – middle sides of tongue
Bitter – posterior tongue
• Transmitted along CN VII & IX
(facial & glossopharyngeal) to
frontal cortex; no special center
in cortex
• Other tastes include water,
alkaline, metallic
• Tastes can be combined with
smells
Smell - Olfaction
• Important for
protection, taste,
triggering memory,
sexual behavior
• Receptors located in
superior nasal cavity
• Conduct impulses
along olfactory nerve
(CNI) to olfactory
center in temporal
lobe of cerebral
cortex
General Senses
• Located throughout
body
• Touch
• Pressure
• Temperature
• Position
• Pain
General Senses
Touch –
The touch receptors, tactile (TAKtil) corpuscles, are found mostly
in the dermis of the skin and
around hair follicles
• Meissner corpuscles in skin,
also near hair follicles; # of
corpuscles determines
sensitivity to touch
– Lips, tip of tongue, fingertips
very sensitive
– Back, back of hand less sensitive
• Pressure – Pacinian corpuscles
in skin, also near joints
– sensory end-organs
– Can respond even when a person
is under anesthesia
General Senses
• Temperature – free nerve endings; dendrite branches
– Widely distributed in skin
– Separate branches for heat & cold
– Stimulus travels to hypothalamus (diencephalon)
• Position – proprioceptors - receptors located in joints,
muscles, tendons
– Kinesthesia – sense of body movement – where the body
is in time & space
– Inform brain about muscle contraction/tendon tension
– Muscle tone, posture, coordination, complicated skills
– Stimulus travels to cerebellum
Temperature & Proprioceptors
General Senses
• Pain – free nerve endings;
widely distributed in skin,
muscles, joints, and
scattered sparsely in
viscera & blood vessels
• Nocioceptors – nerve
endings that transmit
pain signals
– Sharp – acute, fast Adelta
fibers
– Dull – slow, chronic C fibers
Pain
• The 5th vital sign
• An unpleasant sensory & emotional
experience associated with actual or potential
tissue damage or described in terms of such
damage
• Pain is always subjective
• Pain is whatever the experiencing person says
it is, existing whenever he or she says it does
Neuroanatomy of Pain
• Nocioception – how noxious stimuli are
perceived as pain
• 3 phases of nocioception
– Transduction – a noxious stimulus occurs
peripherally and travels to the spinal cord
– Transmission – pain impulse moves from the
level of the spinal cord to the brain
– Perception – conscious awareness of a
painful sensation
• Modulation – pain is inhibited
– The brain sends signals to slow down or
impede the pain impulse
Sources of Pain
• Visceral pain – from organs
• Deep somatic pain – from structures like vessels,
bones, muscles, joints
• Cutaneous pain – from skin
• Referred pain – pain felt at a particular site but
originates at a different location innervated by the
same spinal nerve
• Neuropathic pain – an abnormal neurological
processing of pain signals
– the most difficult to assess & treat
– Pain often perceived long after the site of injury heals
Referred Pain
Different Pain Scales
Neonatal Pain Scoring
Pain Relief
• Analgesics – drugs that relieve pain
– Nonnarcotic analgesics – act locally to reduce
inflammation i.e. NSAIDs
– Narcotics – act on CNS to alter the perception of pain
• Anesthetics – drugs that block pain
• Endorphins – chemicals released from the brain to
modulate pain; massage can activate endorphins
• Heat – reduces chronic pain as long as there is no
inflammation
• Cold – acts as an anesthetic; reduces inflammation;
when in doubt, ice
• Relaxation or distraction techniques
Sensory Adaptation
• Sensory receptors exposed to continuous
stimulus adjust so sensation becomes less
acute; warmth, cold & light pressure adapt
rapidly
• Pain receptors do not adapt; C fiber (slow
pain) receptors become more sensitive if
exposed to continuous stimulation
Chronic Pain
Chronic Pain