Download The Senses

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.
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.
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 let’s consider each of the senses individually:
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 (Meissner’s 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.
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.
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.
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
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.
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.
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.
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 >10,000
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
Hearing (and balance)
We hear sound over a range of frequencies from ~20
Hz to 20,000 Hz (when we’re 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…
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.
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…
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.
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.
The receptor cells are the hair cells. The hairs
projecting from their upper surface lie against the
tectorial membrane. It’s 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.
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.
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.
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.
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.
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
you’ve stopped, but the semicircular canals are sending
a conflicting signal. That’s believed to be what causes
dizziness and motion sickness.
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.
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 > 6m away), but is
not adjustable.
The iris controls how much light enters the eye
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 doesn’t thicken this much in old age.
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.
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-andwhite information. They are,
however, far more sensitive
to low light.
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.
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 “offsurround”.
On if this area
is lit
Off (inhibited)
if this area is
illuminated
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’.
Here’s what happens when
a ‘bar’ of light shines onto
the retina at different angles:
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.