Other Sensory Receptors
Modules 7.1-7.3
Figure
portraying the sound waves of compressed and decompressed air molecules
radiating outward from the vibrating source
Figure
showing the major features of sound waves – the 2 we will talk about are
1) the
amplitude of the waves (how big are the waves of air molecules, which determine
the loudness that we perceive
2) the
frequency of the waves per second (how fast is the original vibration and the
waves it produces) which determines the pitch that we perceive. Frequency may
be expressed as cps (cycles per second) or “Hertz”
Diagram
of the Structure of the Ear (Outer, Middle & Inner Ear)
Sound waves are focused into the external auditory canal by
the somewhat funnel shaped pinna (outer ear) and cause vibration of the
tympanic membrane (or eardrum) at the end of the canal. The backside of the
eardrum is attached to a series of 3 tiny bones (the “ossicles” or,
individually, the hammer, anvil and stirrup shaped bones) which reach across
(kind of like the bones of your arm) the air-filled middle ear, so that the
last bone presses against the fluid filled inner ear (or cochlea) at a point
called the oval window.
With each movement of the eardrum the stirrup presses
against the cochlea causing the fluid inside to move (kind of like pressing on
a water balloon), and when the eardrum vibrates back in the opposite direction,
so does the stirrup, and the fluid inside the cochles sloshes back to its
original location. The cochlea is shaped like a snail shell – a long spiral –
and the fluid movements are from the wide beginning part of the spiral towards
the inner most part of the spiral (and then sloshing back again). Each
different frequency of sound produces a
slightly different pattern of fluid movement inside the cochlea, with the
“peak” of the fluid wave occurring at a different point within the length of
the cochlea spiral.
Figure of
interior of cochlea : If we could “unroll” the spiral shape of the cochlea and
then cut a cross section through it to look at the interior, we’d see that inside there are membranes that
divide the interior into 3 lengthwise “canals”, with the auditory receptors
(hair cells) lined up on the floor (basilar membrane) of the middlemost of these canals. The fluid
movements down the length of the cochlea cause this basilar membrane to move up
and down on the surface of those “waves”.
Detail of
the arrangement of the Hair Cells on the Basilar Membrane: The hair cells are
lined up in a structure called the Organ of Corti, which sits on the basilar
membrane and runs the entire length of the cochlea spiral. Hair cells are
called that because they have hair-like structures coming out of the top of
each cell. The tips of those hairs just barely touch an overhanging membrane
(tectorial membrane) when no sound is entering the ear.
(Imagine that someone standing still on a trampoline
(basilar membrane) in a roofed carport (roof= tectorial membrane) can just
barely touch the roof with their arms (the hairs) completely extended. But if
they bounce on the trampoline (i.e. when the basilar membrane moves up and down
because of fluid waves) they would be able to touch the roof more firmly and in
fact would have to bend their arms to prevent them from smashing against the
roof . Similarly the hair of the hair cells are pressed and bent against the tectorial membrane by
the fluid movements triggered by sounds. That pressure is what actually
generates electrical changes in these receptors cells which can then be
transmitted to the brain, alerting it
about the presence of a sound.
Auditory
Hair Cell Closeup Photograph
Photo of many hair cells lined up on Basilar Membrane
Figure
portraying
Fluid
Waves Traveling Down Length of Cochlea Causing Basilar Membrane Movement Up and
Down
Different
Pitches MaximallyActivate Different hair cells down length of cochlea (because
of fluid movement differences). High pitches activate hair cells at the
beginning of cochlea (close to oval window), middle pitches activate hair cells
about midway down the length and with low pitches the peak of the “wave” and
hair cell activation reaches the inner most tip of the cochlea spiral.
Photo: Normal
& “Trampled” Hair Cells Exposed to Loud Sounds (hair cells get squashed
down by excessive exposure to loud sound (like a rock concert) like grass gets
trampled at a crowded picnic. (If this is an occasional event they perk back
up, but repeated abuse can permanently damage hair cells, just like the grass
in a heavily used path eventually dies.) If the noise is always a specific
frequency (e.g. the frequencies of a jet engine), only the hair cells activated
by that particular frequency will be damaged.
Sound Localization
•
Relies on brain
detecting differences in what 2 ears hear to locate sounds on left or right
–
Intensity differences
(high frequencies especially)
–
Time of onset
differences (any)
–
Phase differences (low
frequencies especially)
If you only had one good ear you could move your head to see
which direction is louder.
•
Tell front sounds from
back sounds because of pinna-related differences (sounds from behind us are
more muffled)
Air Pressure Must be between middle ear and
outside world balanced for Normal Hearing
(e.g. it may get unbalanced when you go up or down in an
airplane, and your hearing will get more and more muffled until you “pop” your
ears to equalize the pressure on both sides of the eardrum.
Types of Deafness
•
Conductive Deafness –
auditory stimulus does not pass normally through middle ear to cochlea
•
Nerve Deafness –
deafness due to damage to inner ear hair cells or auditory nerve
Knowing
the Arrangement and Functioning of the Hair Cells Allowed the development of Cochlear
Implants (electrodes fed into cochlea to directly stimulate auditory nerves in
those without functioning hair cells.
Auditory nerves enter brainstem (auditory input processed in
several brainstem areas (superior olive of medulla, inferior colliculus of
midbrain, medial geniculate nucleus of thalamus), before reaching the auditory
cortex in the top gyrus of the temporal lobe (where our conscious awareness and
analysis of the sound takes place). The strip of primary auditory cortex is
organized in a “tonotopic” fashion (from cells that process deep bass sounds to
cells that process high pitches)
Vestibular System – Another Set of Fluid
Filled Structures of the Inner Ear, Just Above Cochlea
•
Sense movement of head
(and body)
•
Use that input to :
–
Help maintain balance
–
Control eye movements
when head moves
n
Contributes to our motor control
Vestibular System Figure showing 3 semi-circular fluid
filled canals, each oriented in a different plane, but all joined,at their base
to 3 sphere- shaped structures. (Imagine you had a mini-boom box with sphere
shaped speakers on either side. There are 3 large semi-circular handles on the
boom box – one vertically oriented handle on the top which attaches at the
front and the back of the top panel, one vertical handle sticks out from the
side of the box, attaching to the top and bottom of the back panel, and a third
handle is a horizontal handle on the back of the box, attaching parallel to the
bottom)
There are hair cells located within the end of each of the semicircular canal, close to the
junction with the spherical structures. With each movement of your head fluid
sloshes within your semicircular canals, more so in particular canals depending
on the movement. When you spin on your chair one canal is activated, when you
bend forward another is activated, when you tilt sideways another is maximally
activated. (so canals are sensitive to spinning, tilting, bending). Fluid
movement causes friction against hairs cells, initiating electrical messages
(about position and movement), similar to transduction in the cochlea.
Rocks in Your Head
The sphere shaped structures also contain a row of hair
cells inside. In one sphere (the utricle) that row of hair cells is
horizontally positioned on the floor of the sphere and in the other (the
saccule) the row of hair cells is vertically oriented, with the hair cells lined
up on the side of the sphere. In both spheres the tips of the hair cells are
embedded in a jelly like layer with rock-like crystals stuck to the surface of
the jelly. Each time you move forwards or backwards (in your car for example)
those rock crystals drag in the opposite direction, causing pull or friction
against the hair cells beneath them. (Analagous to how, whatever I have setting
on the seat of my car tends to move back on the seat when I accelerate and
forward on the seat when I brake – the rock crystals in the utricle are doing
the same. The crystals in the saccule drag across their hair cells when you go
up or down in an elevator, allowing you to “feel” which way you are going.
Vestibular overstimulation triggers motion
sickness and/or dizziness.
•
Vestibular malfunction
can cause Meniere’s Disease- extreme dizziness, nausea, vomiting, dysequilibrium,
difficulty moving because of faulty balance and postion perception.
We have a variety of cutaneous receptors that respond to
touch, temperature pain, etc.
Sensations from the body feed into the dorsal roots of our
31 pairs of spinal nerves (and 1 pair of cranial nerves) as we described
earlier this semester. Each pair of spinal nerves collects sensations from its
specific “slice” of the body surface. This “territory” is known as its
dermatome (“skin region”). We are going to concentrate on pain.
The Experience of Pain
•
Tissue injury
leads to release of irritating chemicals (histamine, prostaglandins &
others) which activate pain receptors and also make receptors more sensitive;
in addition some receptors specifically activated by high heat or acid
•
Receptors release
glutamate (when pain is mild) & Substance P (when pain is more intense)
•
Experience
influenced by other sensory inputs: Melzack
& Wall’s Gate theory or gate-control theory of pain: the message sending by
the neurons receiving those glutamate and Substance P transmissions mentioned
above can be modified by nearby tiny inter-neurons. If those inter-neurons are
activated by either 1) other incoming non-pain sensory messages OR 2) pain
suppression commands coming down from the brain OR 3) the presence of opiate
drugs or natural endorphins, the inter-neurons can inhibit or decrease the
number of pain messages relayed on to the brain (“closing the gate” to some of
the pain messages). The neurotransmitter released by those inter-neurons is
enkephalin.
Figure showing destination of incoming body
sensations: Somatosensory Cortex
Taste
Taste
buds containing the taste receptors are located along the sides of the
individual “bumps” or Fungiform Papillae that we see on the surface of our
tongue.
Figure of
Taste Buds Along Sides of Papilla
Individual taste receptors have hair like cilia which stick
out of the taste bud into the crevice between adjacent papillae, where those
cilia are bathed by saliva or liquids in the mouth.
•
Many receptors in
a bud
•
Replaced every
10-14 days like skin cells
•
Have excitable
membranes and release transmitter like neurons
•
Have receptors
sensitive to salty, sour, sweet, bitter and “umami” (first 2 open ion channels,
last 3 activate G protein associated metabotropic receptors)
Taste
•
Humans vary in #
of taste buds and taste sensitivity
–
E.g. genetically
based difference in ability to taste the bitter compound PTC (see also p.10)
•
Low sensitivity (tt), medium (Tt), and high
sensitivity (TT)
–
We also vary in
the number of fungiform papillae & taste buds.
–
About 25% have
less than 1000 (“nontasters”), 25% have 10,000+ (“supertasters”), and about 50%
of us are in the middle (“tasters”)
–
Supertasters are
oversensitive to sweet, bitter, and capsaicin (hot pepper) and fatty feel of
food.
Taste Cortex?
•
Some taste messages go
to the tongue region of somatosensory cortex, but fMRI studies show that the
greatest activation from taste occurs in a cortical region hidden deep in the
lateral fissure: the insula
• Experiencing flavor requires both taste and
smell.
Figure
showing location of Olfactory Receptors in patch of nasal membrane at the very
top of nasal passages.
Olfactory
Receptors also have cilia – these stick out from the nasal membrane so that
molecules of chemical in the air entering our nasal passages can bind to
specifically shaped receptors on the cilia. The chemicals that produce
different smells have different shapes.
Cluster of axons from olfactory receptors pass through tiny
pinpoint holes in the bone beneath the frontal lobes, and synapse in the long
skinny “olfactory bulbs” on the bottom of the frontal lobe (not part of frontal
cortex – separate ancient structures you can lift up from the surface of the
frontal lobe – about ¼ inch wide and 3 inches long, attaching to limbic system
structures.
Olfactory
Nerves in a precarious position in head injuries because the brain moves within
skull and can shear off those axon connections in car accident, for example.
The VNO
•
A 2nd
(unconscious) olfactory system for sensing pheromones (pheromones are low-level
chemical signals released by 1 member of a species that influence the
physiology and/or behavior of another member of species)
•
Prominent in
other species; almost gone in humans
Human Pheromones
•
Pheromone inducing
menstrual synchrony in women living together
•
Male pheromone
stimulating menstrual regularity in females