The Senses

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The Senses We have 5 senses touch (including
pressure) smell taste hearing vision Each
sense has specialized receptor cells associated
with it. Receptor cells are depolarized (channels
open to allow sodium to rush in) by the stimulus.
The depolarization (a generator potential) leads
to synaptic activity where the receptor cell
connects to a sensory neuron.
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Like the post-synaptic potentials that sum to
determine whether a nerve impulse will arise, the
generator potentials from receptors sum to
determine whether (and at what frequency) nerve
impulses will arise in sensory neurons. Generator
potentials thus determine one aspect of neural
coding frequency of impulses (frequency coding)
as an indication of intensity of stimulation. The
other aspect of coding is called line labeling.
Your brain knows what kind of stimulus it is by
which neurons are delivering the information.
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Errors can arise in line labeling close your
eyes press gently a finger gently against your
eyelid. When you press and when you release the
pressure most of us will see what we interpret
as colour. The pressure caused some generator
potentials in receptor cells of the retina. We
misinterpret the stimulus as if it were
coloured light. Now lets consider each of the
senses individually
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Touch (and other skin senses) What is classed
within this sense are touch, pain, temperature
(both hot and cold) and pressure. There are
different sensory cells for these senses. Touch
and pain are both sensed by free nerve endings
in the skin and some touch by enclosed endings
(Meissners corpuscles). What differs is their
sensitivity. Touch sensors can be exquisitely
sensitive, e.g. on the face, lips and fingertips.
Free nerve endings are frequently wrapped around
the base of hair follicles.
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All touch sensors are distorted and become leaky
by the stimulus. For free endings around hair
follicles, amount of bending determines the size
of the generator potential, and, thus, the
frequency of nerve impulses. Pain sensors,
looking essentially identical, are insensitive.
Most of their response is the result of physical
damage to the nerve ending. Temperature sensors
are generally enclosed nerve endings (bulbs of
Krause). The enclosure is simple.
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The last type is the pressure sensors. They are
enclosed in an onion-like leyered structure
called a Pacinian corpuscle). The layering has
the effect of averaging out the pressure over a
small region.
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Chemoreceptors (taste and smell) Taste and smell
are closely related. Most tastes and many smells
are compound responses involving both senses.
There is a greater diversity of types (what they
respond to) among smell receptors than among
taste ones. Four different taste receptors are
generally distinguished sweet near the tip
of the tongue salty along the sides, but
nearer the front sour along the sides but
toward the rear bitter the rear surface of the
tongue
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A taste bud (shown below) looks somewhat like an
orange, with a number of cells clustered together
like segments of the orange. At the top of the
cells are microvilli at an opening, a pit in the
tongue. Chemicals dissolved in saliva bind to
receptor proteins in the cell membranes of
microvilli and stimulate the receptor cells.
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Taste receptors are not neuronal, and are
replaced by other bud cells (segments of the
orange). There is continuing cell division in
each bud, and segments mature to become active
sensory receptors. Smell Sensory cells for
olfaction are located in the olfactory bulb in
the uppermost part of the nasal cavity. Here,
each receptor is a neuron, whose axon passes
through a spongy bit of bone (the ethmoid bone)
and to the olfactory region of the cerebrum. The
receptor cells have long cilia that extend into
the nasal cavity. The cilia act as the receptive
surface.
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Molecules dissolved or suspended in mucus
stimulate the receptors by molecular
shape-dependent binding. In many animals it may
take only one or very few molecules of an odorant
to produce action potentials in receptor axons.
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How many odors are there? There are probably only
a few categories maybe 7or less, but multiple
receptors respond to single, complex odors, so
that a trained perfume sniffer can distinguish
gt10,000 different odors. Here is one
categorization etherial small molecules like
simple anaesthetics and solvents camphorous
like tiger balm musky large, complex
molecules like the secretions from mustelid
(e.g. skunk) anal glands floral also large
molecules, but based on carbon chains and ring
structures minty different ring structures
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Hearing (and balance) We hear sound over a range
of frequencies from 20 Hz to 20,000 Hz (when
were young, then parts of the cochlea stiffen,
and we lose some response to the high
frequencies). Velocity of sound propagation
(1100km/hr, or Mach 1) is distinct from
frequency, measured in Hertz (Hz). The sensory
receptors for hearing are hair cells located in
the cochlea of the inner ear. For sound to
stimulate those hair cells, there is a relatively
complex apparatus that transmits vibration to the
cochlea
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The pinna (external ear) acts like an
old-fashioned ear trumpet, collecting sound
waves from the environment and passing them
down the auditory canal. At the inner end of the
canal is the eardrum, a thin membrane. Against it
on the inside is the first of a set of 3 tiny
bones (the hammer, the anvil, and the stirrup, in
order) that transmit the vibration (and amplify
the amplitude of the waves) to the oval window.
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The oval window is the entry point into the
cochlea for vibration. You can think of the
cochlea as a little like a French horn played
backwards. The oval window brings vibration to
the cochlea at the cider end of the coil.
However, the cochlea is not a tube with only a
single channel
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The cochlea actually has 3 channels, the
vestibular canal (or scala vestibuli), the
cochlear duct (or scala media) and the tympanic
canal (scala timpani). The tympanic and
vestibular canals are open to each other at the
far (small) end of the cochlea. The oval window
is at the large end of the vestibular canal.
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Vibrations pass down the vestibular canal and
back up the tympanic canal. The vibration energy
is dissipated at the round window at the head of
the tympanic canal. So how does this vibration
stimulate hearing? Lying on the bottom of the
cochlear canal, which is also the top of the
tympanic canal, is the receptor organ, the organ
of Corti.
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The receptor cells are the hair cells. The hairs
projecting from their upper surface lie against
the tectorial membrane. Its gelatinous and
pretty much stationary. When sound waves pass
through the tympanic canal, the organ of Corti
vibrates up and down. The hairs of the receptors
are bent, and produce generator potentials when
they bend. O.K., so we can sense sound. How do we
distinguish frequencies? The cochlea changes in
diameter along its length. Only a specific region
of the membrane at the top of the tympanic canal
resonates to any given frequency.
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You can begin to see how if we look at the
cochlea as if it were unrolled
High frequencies excite the part of the organ of
Corti nearest the oval window, and low
frequencies excite the region near the apex.
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Sound intensity is determined by the amplitude of
the sound wave, the amplitude of movement of the
floor of the tympanic canal, and finally the
amount the hairs are bent. Sound can get too
loud. We have a protective response muscles
attached to the middle ear bones dampen
amplitude. However, the hairs can take only so
much. Repeated exposure to very loud sound
eventually begins to break off hairs hearing
loss occurs. Where, in an electron micrograph of
a healthy ear, the hair cells look like a forest,
some rock musicians (and others similarly
exposed) look like the forest has been clearcut.
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Balance and orientation Above the oval window are
the semicircular canals (3 of them) and the
saccule and utricle. The saccule and utricle are
open saces lined with hair cells, and with
calcium carbonate (limestone) granules inside.
See, you do have rocks in your head! The granules
fall to the bottom of the sacs due to gravity,
bending hairs of cells beneath. Orientation
(right side up, upside down, lying on your
side,) is indicated by which cells are producing
generator potentials.
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The 3 semicircular canals are oriented in the
three possible planes. They are fluid filled,
with hair cells lining them. The hairs move in
the fluid breeze. Stand still, and there is no
breeze. Start moving, and the fluid in the
canals has inertia, it falls behind and hairs
are bent, causing generator potentials. Stop and
the same inertia keeps the fluid moving more
generator potentials.
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Note that the big signals are sent when you begin
or stop moving. The semicircular canals are
sending signals indicating acceleration. If you
spin for a while, the fluid catches up and
signals stop. Then you stop spinning. Your eyes
tell you youve stopped, but the semicircular
canals are sending a conflicting signal. Thats
believed to be what causes dizziness and motion
sickness.
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Vision Most animals are sensitive to light. The
simplest have photoreceptors that can cause an
animal to move (a kinesis) until it reaches a
dark area. This sort of response persists into
flatworms and others that have eyecups. Higher
invertebrates and vertebrates have camera-like
eyes with a single lens. The only real difference
between our eyes and those of a squid is the
orientation of the light receptors in the retina
in the squid they are pointed toward the light
source in ours they are facing backwards. That
makes the squid eye more sensitive in its
low-light environment.
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The analogies to a camera The sclera (the white
of your eye) is the body of the camera The
choroid is a black layer like the black paint
inside the camera it prevents reflections The
lens focuses images onto your retina. It can get
thicker or thinner due to contraction or
relaxation of the ciliary body (muscle) The
cornea is the clear covering in the front. Since
it is a curved surface, it really does most of
the focusing (at least for objects gt 6m away),
but is not adjustable. The iris controls how
much light enters the eye
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Our ability to accommodate, to change the shape
of the lens by making it thicker, rounder to
focus on nearer objects, and thinner to focus on
more distant objects, decreases with age. The
lens loses the elasticity that allows it to
thicken and round to focus on near objects. The
problem is called presbyopia, which means old
eye in Greek.
A stiffer lens doesnt thicken this much in old
age.
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The final analogy is the retina. It functions as
the film in your camera-eye.
At the front are ganglion cells they are neurons
whose axons form the optic nerve. They integrate
information from a number of bipolar cells. In
the middle are bipolar cells. They collect input
from one or more receptor cells, and pass it on
to one or more ganglion cells.
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At the back are the receptor cells. There are two
types Cones provide colour information in the
central focus area of the retina, called the
fovea. These cells are packed very tightly
together, and contain one of three pigments,
called photopsins, that respond to different
colours of light. Rods are present in the
remainder of the retina. They contain the pigment
rhodopsin provide only black-and- white
information. They are, however, far more
sensitive to low light.
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The discs on the outer segments of the receptors
contain the pigments. Light causes the pigments
to change conformation, opening channels in the
membrane and causing a generator potential.
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How does this become a visual perception? The
ganglion cells are the first level of
integration. They are excited by impulses from
some bipolar cells, but inhibited by others. What
results is what is called a receptive field,
typically on-center and off-surround.
On if this area is lit
Off (inhibited) if this area is illuminated
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This is the information passed to the visual
cortex. There, individual cells (the first
level of integration in the brain) sum the
information coming from particular groups of
ganglion cells. In the cat, the groups added
formed bars (lines) of an image. Heres what
happens when a bar of light shines onto the
retina at different angles
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There are two more (and more complex) levels of
visual integration in the cat. You can see how
some simple perceptions might occur a V is two
simple cells whose bars are at angles to each
other summed at the next level More complex
objects and perceptions are harder to explain
mechanically. They work, and we recognize objects
as complex as the human face of someone familiar.
This, as well as other aspects of biology,
sometimes seem almost magical.