the senses
Each of the 5 senses consists of organs with specialized cellular structures that have receptors for specific stimuli.
These cells have links to the nervous system and thus to the brain.
Sensing is done at primitive levels in the cells and integrated into sensations in the nervous system
Sight
The eye is the organ of vision. It has a complex structure
consisting of a transparent lens that focuses light on the retina. The
retina is covered with two basic types of light-sensitive cells-rods and
cones. The cone cells are sensitive to color and are located in the
part of the retina called the fovea, where the light is focused by the
lens. The rod cells are not sensitive to color, but have greater
sensitivity to light than the cone cells. These cells are located
around the fovea and are responsible for peripheral vision and night
vision. The eye is connected to the brain through the optic nerve. The
point of this connection is called the "blind spot" because it is
insensitive to light. Experiments have shown that the back of the brain
maps the visual input from the eyes.
The brain combines the input of our two eyes into a single
three-dimensional image. In addition, even though the image on the
retina is upside-down because of the focusing action of the lens, the
brain compensates and provides the right-side-up perception.
Experiments have been done with subjects fitted with prisms that invert
the images. The subjects go through an initial period of great
confusion, but subsequently they perceive the images as right side up.
Color blindness or "Daltonism" is a common abnormality in human vision that makes it
impossible to differentiate colors accurately. One type of color blindness results in the inability to
distinguish red from green. This can be a real handicap for certain types of occupations.
To a colorblind person, a person with normal color vision would appear to have extrasensory perception.
However, we want to reserve the term "extrasensory perception" for perception that is beyond the range
of the normal.
Common Problems of Sight
Here are the four common refractive vision problems:
- Nearsightedness (myopia)
- Farsightedness (hyperopia)
- Astigmatism
- Presbyopia
In nearsightedness (myopia), the light from distant objects gets focused
in front of the retina rather than on it. Myopia happens usually when the
eyeball is too long; however, it is sometimes caused by too much focusing power
in the lens system. The result is that the person can see
close-up objects fine, but distant objects are blurry. Myopia can be corrected
by using a concave lens to diverge, or spread out, the light so that when it
passes through the lens system, it comes to focus on the retina.
Hearing
The ear is the
organ for hearing and balance. In hearing the ear detects vibrations,
their frequency (pitch) and amplitude (loudness). These become
nerve impulses carried to the brain. For balance the ear detects the direction
of motion, acceleration and head position related to
gravity.
There are 3 sections to
the ears: the outer ear, the middle ear, and the inner ear.
See the diagram below for the parts of the ear. It is very important
that you know these parts and their functions.
How We Hear?
When sound waves enter the ear they vibrate the eardrum. The eardrum
causes the ear ossicles (hammer,
anvil, stirrup) to move (vibrate). These vibrations arrive at the cochlea.
Within the cochlea is a liquid called lymph. The lymph is moved within the
cochlea. This movement of liquid stimulates sensory (sound) receptors in the
cochlea. There are about 24,000 sensory cells within our ears. These receptors
create an electrical impulse that travels up the auditory nerve and on to our
brain.
taste
Receptors for taste are located on the tongue at the taste buds.
The taste buds are at the top and the sides of the tongue. (See diagram below)
Some taste buds are also found in the lining of the throat.
Note that there are 4 different taste
receptors: sweet, sour, salt, and bitter. Note that the different types are
located at different areas of the tongue.
The
sense of taste is a combination of taste as well as smell, texture, and
temperature.
Cold
food tastes different than hot food. If you squeeze your nostrils closed when
you eat you will not taste the food as strongly. Also, if you have a cold your
sense of taste is diminished. We continue to taste some foods after we eat them
because bits of the food become lodged on out tongues and the taste receptors
are still sending impulses as a result.
Touch
The skin is the organ, which contains the receptors for
touch and temperature. The skin was discussed extensively in the excretion page. The receptors are found in
different concentrations at different areas of the body. An example is that
there are few touch receptors at the heal of the foot but many temperature
receptors at the elbow.
Smell.
The nose is the organ responsible for the sense of smell. The cavity
of the nose is lined with mucous membranes that have smell receptors
connected to the olfactory nerve. The smells themselves consist of
vapors of various substances. The smell receptors interact with the
molecules of these vapors and transmit the sensations to the brain. The
nose also has a structure called the vomeronasal organ whose function
has not been determined, but which is suspected of being sensitive to
pheromones that influence the reproductive cycle. The smell receptors
are sensitive to seven types of sensations that can be characterized as
camphor, musk, flower, mint, ether, acrid, or putrid. The sense of
smell is sometimes temporarily lost when a person has a cold. Dogs have
a sense of smell that is many times more sensitive than man's.
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