Download Sensory Receptors and the Special Senses

16 - Sensory Receptors and the
Special Senses
Taft College Human Physiology
The Function of Sensory
Receptors
• Sensory receptors are of great survival value to the
body.
• They allow us to monitor changes in our internal and
external environment and in so doing, maintain
homeostasis.
• The stimuli detected by the sensory receptors act to
depolarize a membrane, then the impulses will travel to
the CNS via afferent (sensory) neurons.
• The nerve impulses for different senses are basically the
same.
• It is the connection to various parts of the brain that
causes you to interpret them differently.
• Theoretically, if you were able to surgically swap the
attachment of cranial nerves for hearing and vision at the
brain, you would hear lightning and see thunder.
• We will focus on our attention on 2 main receptors: our
eyes and ears.
The Eye
• Eye- The eye is a complex:
• exteroreceptor (extero = outside)
it is designed to pickup stimuli outside the
body
• photoreceptor (photo = light)
it is designed to detect light stimuli.
Innervation of the Eye
• The eye is innervated by 4 cranial nerves.
• II Optic nerve = nerve of vision. It has the
sensory function of detecting light stimuli.
• 3 cranial nerves provide motor impulses for eye
movement. Eye movement is accomplished by
several muscles that insert into the tough outer
coat of the eye (sclera).
• III Oculomotor
• IV Trochlear
• VI Abducens
The Anatomy of the Eye Involved
in the Physiology of Vision
• The eyeball is made of 3
layers or coats (tunics):
The outer fibrous coat,
the middle vascular coat,
and the inner nervous
coat.
• We will only discuss the
inner nervous coat in
detail and selected parts
of the other 2 as needed
to understand the basic
physiology of sight.
Optic Nerve
1. Fibrous coat
2. Vascular coat
3. Nervous coat = retina
Nervous coat = retina
•
•
•
•
•
The retina receives light and
converts it into nerve impulses.
The retina contains 2 types of
photoreceptors: rods (120
million) and cones (6 million)and
is continuous with the optic
nerve.
Rods- high sensitivity. Respond
to even low light levels. Night
vision. Detect black, white, and
shades of gray, no color. Detect
shapes and movement.
Cones- low sensitivity. Only
functional in bright light. Color
discrimination. 3 kinds of cones:
one for each color- red, green,
and blue.
Colors of the spectrum other
than red, blue ,and green are
interpreted from interaction of all
3 kinds of cones.
II Optic Nerve
Retina with rods and cones
Rods show summation, Cones do not.
•
•
•
•
•
•
Rods
Rods and cones are ‘wired’ differently.
Cones
Cones are ‘wired’ on a 1:1 ratio with
bipolar cells. This allows for great
resolution for detailed viewing, in color!
But it does take a lot of light to allow
this type of vision.
Rods
Rods are ‘wired’ on a ~100: 1 ratio with
bipolar cells. Therefore many subthreshold stimuli may act together in an
additive way to depolarize the neuron
(bipolar cells) = summation. This allows
for great sensitivity for night vision, but
only in gray tones. Movement can be
detected but resolution is poor.
Nocturnal animals have many rods and
a mirror like reflective layer that gathers Light Path
light (tapetum lucidum), so have greater
night vision sensitivity than our own.
Cones
Rods
Bipolar
cells
Ganglion
cells
Optic (II) Nerve
Fovea Centralis and Optic Disc
•
•
•
•
•
•
•
In the exact center of the posterior
part of the retina is an area called the
macula lutea.
In this structure is a depression
containing an area of highly packed
cone cells, called the fovea centralis.
Only cones are found here, no rods.
The fovea centralis is the point of
greatest visual acuity = ability to see
detail.
The optic disc is where the optic
nerve exits the eye. There are so many
axons (neurons) that converge to form
the optic nerve here that there is no
room for rods or cones.
The optic disc is located medial to the
fovea.
The optic disc is commonly called is
the ‘blind spot’ of the eye as no photo
receptors (rods or cones) exist. It can
be easily demonstrated.
Other Important Structures of the Eye
•
•
•
•
•
Iris/Pupil - The iris is the colored
portion of the eye shaped like a
flattened donut.
The iris is a muscular diaphragm
with a central hole (pupil) that
regulates the amount of light
entering the eye.
Cornea - Found in the outer fibrous
coat and is continuous with the
sclera (white of the eye) but
transparent.
Anterior 1/6 of eyeball that bulges
forward to participate in focusing
light (80%) on the retina.
Corneal transplants are the most
common and successful organ
transplants. Due to avascular
anatomy, therefore no rejection.
Now, plastic corneas can be used
as well.
Cornea
Other Important Structures of the Eye
•
•
•
•
•
•
•
Lens
The lens is attached by suspensory
ligaments that encircle the lens.
These attach it to the ciliary body
which surrounds the lens.
The ciliary muscles in the ciliary
body control the shape of the lens.
The ciliary muscles are relaxed
(tight ligaments and flattened lens)
for distant viewing (far vision).
The ciliary muscles are contracted
(loose ligaments and rounded lens)
for close inspection (near vision) of
objects.
Eye Fatigue - Since the ciliary
muscles are contracted for close
vision, it makes sense that your
eyes may fatigue during reading or
close inspection of objects.
Changing the shape of the lens to
focus the image on the retina is
called accommodation.
Ciliary muscles relaxed,
suspensory ligaments tight
flattened
Ciliary muscles contracted can lead to fatigue
suspensory ligaments relaxed
rounded
Physiology of Vision
•
•
•
•
•
•
•
•
•
•
•
•
•
•
The rods contain a visual pigment called rhodopsin.
In the presence of light rhodopsin breaks down into 2 parts: retinal and opsin .
light
Rhodopsin (dark color)
retinal + opsin (colorless products)
dark
The reaction is the reverse in the dark.
In light the rhodopsin splits, and it becomes colorless or ‘bleached’.
The splitting of rhodopsin causes depolarization of the neuron and impulses
travel to the brain and are perceived as light.
The intensity of light will determine the number of rhodopsin molecules that
split.
Dark adaptation – In sunlight, rhodopsin molecules are continually broken
down. When you go into dark theatre, you must wait for enough rhodopsin to
build up so you can see (about 5 minutes for 50%).
Light adaptation - An opposite effect happens when you go from the dark to a
sunny place. Initially you may see nothing but bright light and no images, as so
much rhodopsin is being broken down.
Your eye has to decrease its sensitivity by breaking down all the rhodopsin.
This why you squint and initially protect your eyes in bright light.
Rhodopsin is bleached as fast as it is regenerated in the daylight. This is called
‘light adaptation’.
The cones for color vision work with visual pigments as well but we will not
cover.
Eye Disorders
•
•
•
•
•
•
•
•
•
•
•
•
Color Blindness: an inherited condition.
The inability to distinguish certain color differences results from absence or
deficiency of one or more of the 3 cone photopigments.
Most common is red-green color blindness where the red or green sensitive
pigment is missing and person cannot distinguish between red and green.
It is a sex-linked trait, most commonly in males, as an inherited trait from their
mother.
Glaucoma: increased pressure of the aqueous humor due to blockage (canal of
Schlemm).
The pressure may cause irreversible damage to the retina.
This is a painless condition affecting 2% over age 40. This why during an eye
exam the clinician directs a puff of air or other method to detect your
intraocular pressure.
What drug has been used in treatment?
Cataract: the lens becomes opaque. The lens proteins change shape.
Usually caused by aging. Other causes: trauma, infection, diabetes, UV
exposure).
Corrected by surgical removal and implantation of an artificial lens.
The earlier artificial lenses were fixed focus, now they may accommodate.
Eye Disorders
•
•
•
•
•
•
Refraction (light bending)
abnormalities
Nearsighted (myopic) (long eye)
Light rays focus in front of retina.
Need a concave corrective lens.
Farsighted (hypermetropic)
(short eye)
Light rays focus behind the retina.
Need a convex corrective lens.
Astigmatism- blurred vision due to
imperfections of the curvature of the
cornea (or lens). Can be corrected by
corrective lens.
Eye Disorders
•
•
•
•
Age-related (Senile) Macular Degeneration (AMD)
Begins as hardening of the arteries in the fovea that deprives the
retina of nourishment.
May progress as new blood vessels grow over the macula lutea.
Person loses ability to see straight ahead while peripheral vision is
maintained.
Early symptoms cause vision problems of blurring or distortion in
central field progressing to blindness. Causes are not well understood
but occurs over age 50.
•
Presbyopia - By age 40, the lens has lost much of its elasticity. So, the
minimum distance that an object can be focused (near point of vision)
moves out and away from the eyes. This is because the light rays
cannot be bent enough by lens to focus on the retina.
•
Normal near point = 4 inches
•
•
So, this is why bifocals are needed for reading after 40 years if age!
This is a loss of accommodation and is called presbyopia (older
persons vision).
40 years = 8 inches
60 years = 31 inches.
Correcting Eye Disorders (FYI)
•
•
1.
2.
3.
4.
5.
•
LASIK = (Laser assisted in-situ
keratomileusis.)
= surgery to correct the curvature of the
cornea for farsightedness,
nearsightedness, and astigmatism.
Anesthetic drops are placed in the eye.
A circular flap of tissue is cut from the
center of the cornea.
The flap is folded out of the way and the
cornea is precisely reshaped using a
laser and computer.
The circular flap is placed back in
position over the treated area.
A patch is placed over the eye
overnight. The flap rapidly reattaches to
the cornea.
Corneal transplant -pic
The Ear and Hearing
• Introduction
• The ear is a mechanoreceptor responsible
for sensing hearing and equilibrium
(balance).
• The ear is a specialized receptor for sound
waves.
• VIII cranial nerve = vestibulocochlear
(auditory) nerve is the nerve of hearing
and balance.
Anatomy and Physiology of the Ear
•
•
•
•
•
The ear consists of 3 major parts: the outer (external), middle, and
inner(internal) ear.
1. External (outer) ear
Consists of the auricle (pinna), a funnel like projection that collects sound waves
and directs them to the external auditory canal (meatus) and on to the tympanic
membrane (eardrum).
The canal is lined by large hairs and ceruminous glands that secrete cerumen
(earwax). The wax lining and hairs help to prevent dust and insects from
damaging the tympanic membrane.
Sound waves vibrate the tympanic membrane which vibrates the malleus = 1st of
3 ear ossicles.
Anatomy and Physiology of the Ear
•
•
•
•
•
•
•
•
•
•
2. Middle Ear
Consists of an air filled chamber in the temporal bone that houses the tiny bones of the ear =
ear ossicles.
The ear ossicles transmit and amplify vibrations of the tympanum to the inner ear,
specifically to the oval window.
The ear ossicles, in order from tympanum to inner ear:
1. Malleus = Hammer
2. Incus =
Anvil
3. Stapes = Stirrup This ossicle transfers vibration to the oval window in the inner ear.
Entering the middle ear chamber is the auditory tube (pharyngotympanic, or Eustachian)
responsible for equalizing the pressure on both sides of the tympanic membrane. The
equalization with atmospheric pressure can be accomplished by yawning or swallowing.
What happens when you ears “pop”. Why does the sound become louder?
Pathogens may travel through the eustachian tube from nose and throat to the middle ear.
Anatomy and Physiology of the Ear
•
•
•
•
3. Inner ear (labyrinth)
The inner ear is filled with liquid and
houses the organs essential for hearing (cochlea) and
equilibrium (semicircular canals and vestibule).
The last of the ear ossicles, the stapes, is connected to a small
membrane called the oval window. Located on the other side of
this membrane is the fluid filled chamber, the cochlea.
The cochlea contains fluid, channels, and membranes that
transmit vibrations to the spiral organ (organ of Corti), the organ
of hearing.
The vestibulocochlear VIII nerve carries information to the brain.
=Equilibrium
=Hearing
Physiological Steps in Sound Perception
1.
2.
3.
4.
5.
6.
7.
8.
The auricle directs sound waves to the
into the external auditory canal.
The eardrum vibrates slowly in response
to low frequency (low pitched sounds)
and rapidly in response to high frequency
sounds. Higher volume causes greater
displacement of eardrum. Lower volume
causes less displacement of the eardrum.
The vibration of the eardrum is
transmitted and amplified by the ear
ossicles.
The stapes pushes the oval window in
and out.
The movement of the oval window sets
fluid (perilymph) pressure waves in
motion in the cochlea.
Pressure waves travel through the
cochlea and cause the round window to
bulge (9).
The pressure waves push the basilar
membrane back and forth.
The hair cells on the basilar membrane
hit the tectorial membrane. The bending
of the hair cells causes impulse to travel
along the vestibulocochlear nerve to the
brain and is interpreted as sound.
2 Components of Sound
1. Volume = Loudness - determined by
degree of movement the basilar
membrane and number of hair cells
stimulated.
3
1
2
4
5
9
7
8
6
8
2. Frequency = Pitch – determined by
area of the basilar membrane that is
stimulated, proximal = high frequency,
distal = low frequency. Note- low freq
sounds travel further than high freq.
Physiological Steps in Sound Perception
(continued)
8.
•
The hair cells on the basilar membrane hit the tectorial
membrane. The bending of the hair cells causes
impulse to travel along the vestibulocochlear nerve to
the brain and is interpreted as sound.
The spiral organ (Organ of Corti), the actual organ of
hearing consists of 3 components, the basilar
membrane, hairs cells, and tectorial membrane.
Pitch
Pitch
• Pitch is perceived as a result of the region of basilar membrane that is
‘tuned’ to vibrate at specific pressure frequencies set up in the cochlear
fluid by movement of the oval window.
• The fibers in the basilar membrane that span its width (like harp strings)
along the length of the cochlea, vibrate in resonance with specific wave
frequencies.
– The fibers are shortest near the oval window and get progressively
longer toward the cochlear apex.
– The shorter fibers vibrate with high frequency waves (20k Hz) and the
longer fibers vibrate with lower frequencies (20 Hz).
HIGH
FREQUENCY
MEDIUM
FREQUENCY
LOW
FREQUENCY
Volume and Pitch =
2 Components of Sound
• Volume = Loudness
is based on degree of
displacement of the
basilar membrane
– bending of hairs is
in direct proportion
to volume.
• Pitch = Frequency
is based on the region
of the cochlea the
vibrations stimulate.
Hearing Loss
•
•
•
•
If your ears ever ring following a loud noise, you have done
permanent damage to your hair cells.
Loud noise causes the hair cells to crash into the tectorial
membrane and bend the hair cells.
It then takes greater volume for them to respond.
The purpose of a hearing aid is to raise the volume of the
sound so it may move the basilar membrane enough for the
damaged hair cells to reach the tectorial membrane and send
impulses to the brain.
Tectorial Membrane
Hair Cells
Basilar Membrane
Healthy
Damaged
Physiology of Equilibrium
• Equilibrium of the body is reached through
the interpretation of responses of 1) head
to movement, 2) visual input, 3) stretch
receptor input from muscles and tendons.
• We will discuss the input from the inner
ear which is effected by movement of the
head.
• 2 types of balance (equilibrium) are
assisted by the inner ear:
– Static equilibrium
– dynamic equilibrium.
•
•
•
•
•
•
•
•
1. Static Equilibrium is the maintenance of body
position (mainly the head) relative to the force of
gravity.
Mainly due to sensing of movement in the vestibule
by hairs cells similar to those used in hearing.
These receptors sense linear acceleration
movement of the head only, not rotational
movement.
The hair cells project into a membrane called the
otolithic membrane that is gelatinous in nature.
Otoliths (calcium carbonate crystals) sit on the
surface of this membrane.
As tilt your head forward or side to side, or move up
and down in an elevator, the heavy otoliths and
otolithic membrane bend the hairs of the hair cells.
The same thing happens when you stop moving, the
inertia in the membrane bends the hairs.
Depending on which direction the hair is bent, it
either causes depolarization, so nervous impulses
are triggered in the vestibular nerve.
These impulses are transmitted to the brain stem
(vestibular nuclei) and cerebellum for interpretation
with other input and then a skeletal motor response
is sent.
Static
Equilibrium
Otoliths
respond to
gravity
Hair
cells
Dynamic Equilibrium
•
•
•
•
•
2. Dynamic Equilibrium is the maintenance of body position (mainly
the head) in response to sudden movements such as rotation,
acceleration, and deceleration (balance while moving).
3 semicircular canals that lie at right angle to each other in 3 planes (2
vertical, 1 horizontal), are the structures for sensing dynamic
equilibrium.
These structures are lined with hair cells that are imbedded in
gelatinous membrane. As the head moves, the endolymph in the
canals flows over the hairs and bends them. Since the 3 canals are in
3, 90 degree planes, rotational movement in any plane and direction
will be sensed.
If you keep spinning around, the endolymph will eventually spin at a
similar speed. When you stop, it keeps going and bends the hairs
again, only in the opposite direction. This tells the brain you have
slowed or stopped.
Key Point for equilibrium sensing: the rigid bony inner ear with
attached hair cells moves with the body, while the fluids and gels with
otoliths are free to move at various rates depending on the forces
acting on them.
Motion Sickness
•
•
•
•
Motion sickness- a common equilibrium
disorder. Probably due to sensory mismatch
of visual input sensing a fixed position (your
cabin on a ship) and your vestibule sensing
movement (the rough seas).
The brain receives conflicting info and is
confused which somehow leads to motion
sickness.
Warning signs that precede nausea and
vomiting include: increased salivation, pallor,
rapid deep breathing, sweating.
Prevent before the fact by using antimotion
drugs (ex: Dramamine- dimenhydrinate,
scopolamine- skin patch) that depress
vestibular input.