Download FYI information about sensory perception

CutaneousReceptors
http://biology.wsc.ma.edu/biology/courses/hoag/humbio/labs/senses/
Specialized sensory organs and free nerve endings in the skin provide four modalities of cutaneous
sensation. The modality and location of each sensation is determined by the specific sensory pathway in the
brain; the acuteness of sensation depends on the density of the cutaneous receptors.
Four independent modalities of cutaneous sensation have traditionally been recognized - warmth, cold,
touch, and pain. (Pressure is excluded because it is mediated by receptors deep in the dermis, and the
sensations of itch and tickle are usually excluded because of their mysterious origin.) Mapping these
sensations on the surface of the skin has revealed that the receptors are not generalized throughout the skin
but are clustered at different points.
Since the distribution is different for each of the four sensory modalities, earlier physiologists believed that
each sensation was mediated by a different sensory receptor, and this view was supported by histological
identification of different cutaneous receptors. Excision of areas of the skin from different sensory maps,
however, failed to reveal a different distribution of receptors, and more recent experiments have suggested
that the four sensations may arise from an analysis of complex patterns of sensory (afferent) impulses in the
brain.
http://www.hometrainingtools.com/article.asp?ai=1388&bhcd2=1272327480
Somatosensory System: The Ability To Sense Touch
Our sense of touch is controlled by a huge network of nerve endings and touch receptors in the skin known
as the somatosensory system. This system is responsible for all the sensations we feel - cold, hot, smooth,
rough, pressure, tickle, itch, pain, vibrations, and more. Within the somatosensory system, there are four
main types of receptors: mechanoreceptors, thermoreceptors, pain receptors, and proprioceptors.
Thermoreceptors: As their name suggests, these receptors perceive sensations related to the temperature
of objects the skin feels. They are found in the dermis layer of the skin. There are two basic categories of
thermoreceptors: hot and cold receptors.
Cold receptors start to perceive cold sensations when the surface of the skin drops below 95 º F. They are
most stimulated when the surface of the skin is at 77 º F and are no longer stimulated when the surface of
the skin drops below 41 º F. This is why your feet or hands start to go numb when they are submerged in
icy water for a long period of time.
Hot receptors start to perceive hot sensations when the surface of the skin rises above 86 º F and are most
stimulated at 113 º F. But beyond 113 º F, pain receptors take over to avoid damage being done to the skin
and underlying tissues.
Thermoreceptors are found all over the body, but cold receptors are found in greater density than heat
receptors. The highest concentration of thermoreceptors can be found in the face and ears (hence why your
nose and ears always get colder faster than the rest of your body on a chilly winter day).
3. Pain receptors: The scientific term is nocireceptor. "Noci-" in Latin means "injurious" or "hurt" which
is a good clue that these receptors detect pain or stimuli that can or does cause damage to the skin and other
tissues of the body. There are over three million pain receptors throughout the body, found in skin, muscles,
bones, blood vessels, and some organs. They can detect pain that is caused by mechanical stimuli (cut or
scrape), thermal stimuli (burn), or chemical stimuli (poison from an insect sting).
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These receptors cause a feeling of sharp pain to encourage you to quickly move away from a harmful
stimulus such as a broken piece of glass or a hot stove stop. They also have receptors that cause a dull pain
in an area that has been injured to encourage you not to use or touch that limb or body part until the
damaged area has healed. While it is never fun to activate these receptors that cause pain, they play an
important part in keeping the body safe from serious injury or damage by sending these early warning
signals to the brain.
Reference: Developed by Marjorie A. Murray, Ph.D.
http://faculty.washington.edu/chudler/twopt.html
Receptor density and the sizes of receptive fields of central neurons determine two-point discrimination
ability
What properties of the touch sensory system allow us to discriminate two points pushing on our skin even
when they are only 2 or 3 mm apart? In other words, the receptors must be packed closely enough so that a
probe stimulates one or more of them. High receptor density alone, however, cannot explain why the
fingertip can distinguish points so close together while the arm senses two points only when they are 35 to
40 mm apart. The second property necessary for fine two-point discrimination is that neighboring receptors
must connect to different CNS neurons, which in turn means that these CNS neurons must have small
receptive fields, as explained below.
Each sensory receptor connects through a series of relay neurons with a CNS neuron. A given central
neuron responds to all information from its input area (the skin area that is the gathering field for only that
CNS cell) as if it were coming from one point. This skin area is called the receptive field of the central
neuron. On the arm, each sensory receptor gathers information from a much larger skin area than a receptor
on the fingertip, and this receptor is also connected to a defined central neuron. This central neuron, like the
central "finger neuron", interprets all input as coming from one point, even though the skin area in this case
is much larger. In order for a person to feel two points, two separate central neuronal populations must be
activated by stimulation of their respective receptive fields. When this happens, two points are reported.
To summarize, two-point discrimination depends on activating two separate populations of neurons, and in
order to discriminate two closely placed points, the receptive fields of the neurons must be small. This in
turn means that the receptors must be densely packed in a sensitive area, so that two points very close
together activate different receptors.
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Sensory information from different receptors is combined at higher brain levels
Although individual receptors respond to only one type of stimulus, such as pressure or vibration, a
stimulus in the real world almost always activates several kinds of receptors simultaneously. To form a
representative picture of this in our minds, the different sensations must all "get together" somewhere in the
brain, and one place this happens is in cortical neurons called feature-detecting neurons. These neurons
each receive several different types of information from neurons in the primary somatosensory cortex
(which received their information from receptors). This integration of sensations allows us to experience an
ice cube as both smooth and cold, or to feel that sand at the beach contains different sized grains and may
be hot or cool. As this information is sent to higher brain centers, sensations also take on meaning because
of past experiences.
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Neurologists use two-point discrimination tests to check for nerve damage
Neurologists, doctors who specialize in diseases of the central (brain and spinal cord) and peripheral
(nerves to all the organs and muscles) nervous systems, sometimes test patients for two-point
discrimination. They may do this if they suspect a problem with sensory information entry to the skin, the
pathways to the brain, or the interpretation of sensory information. For example, if a patient has cut a finger
badly, a neurologist may test for two-point discrimination at the time of injury to see if the nerve was cut.
After the original injury has healed for a number of weeks, the neurologist will again test two-point
discrimination and compare it with the normal fingers to see if the nerve has regenerated.
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Reference: http://biology.wsc.ma.edu/biology/courses/hoag/humbio/labs/senses/
The Afterimage
The light that strikes the receptors of the eye stimulates a photochemical reaction in which the chemical
rhodopsin (within the rods) dissociates to form two chemical pigments. This chemical dissociation
produces electrical changes in the photoreceptors, which trigger a train of action potentials in the axons of
the optic nerve. These events cannot be repeated in a given receptor until the rhodopsin is regenerated, and
this requires a series of chemical reactions. In other words, after the visual pigment has been "bleached" by
the image of an object (i.e., after the rhodopsin has dissociated under the influence of light) a certain period
of time is required before that receptor can again be stimulated. When an eye that has adapted to a bright
light, such as a light bulb, is closed or quickly turned toward a wall, the bright image of the light will still
be seen. This is called a positive afterimage and is caused by the continued "firing" of the
photoreceptors. After a short period, the dark image of the light bulb (the negative afterimage) will
appear against a lighter background due to the "bleaching" of the visual pigment of the affected
receptors. According to the Young-Helmholtz theory of color vision, there are three systems of cones that
respond respectively to red, green, and blue (or violet) light, and all other colors are seen by the brain's
interpretation of mixtures of impulses from these three systems. Color discrimination will of course be
impaired if one system of cones is defective (color blindness), or if one system of cones has been
"bleached" by the continued viewing of an object. In the latter case, the positive afterimage will appear in
the complimentary color.
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