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Anatomy & Physiology (Open + Free)
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Unit 13:: Nervous System
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Sensory Modalities and Adaptation
449
Describe the ty pes of
inform ation (m odality ) detected
by the receptors associated with
the som esthetic senses and the
phenom enon of adaptation.
The receptors that provide information for somesthetic senses come in a variety of anatomical and functional
types. Each specific receptor will respond to only one type of functional signal (such as mechanical or
chemical, but not both). If the information is transmitted all the way to the brain’s cortex, we perceive a
sensation. The type of perception that this information leads to is called a modality. There are four main
modalities typically recognized as part of the somesthetic senses. These include temperature, touch, pain
(nociception), and position and movement (proprioception). Yet each of these can be further subdivided
into sub-modalities (sometimes called stimulus modalities). For example, the modality of pain can be
subdivided into sharp, dull and aching.
There is not general agreement over how sensory modalities should be categorized and subcategorized. In
some classification schemes the general sense of touch is replaced with pressure. In others, there may be
differences in the subcategories of touch. One classification scheme is presented in the table below.
One Scheme for Categorizing Sensory Modalities
Main Modality
Subdivision 1
Touch
Crude touch
Discriminative touch
Subdivision 2
Touch
Pressure
Vibration
Flutter
Pain (Nociception)
Sharp
Dull
Aching
Temperature
Hot
Cold
Proprioception
Position
Muscle length
Muscle tension
Joint pressure
Movement
Muscle length
Muscle tension
Joint pressure
Joint angle
In subdiv ision 2 , touch is a sensation produced under conditions of v ery little skin displacem ent, while pressure requires
displacem ent of the skin and underly ing tissues. Flutter is a sensation produced by a stim uli acting at a lower frequency than
v ibration.
The stimulus modalities shown in the subdivision 1 and subdivision 2 columns in the table might contribute to
more complex sensations when combined under certain conditions. For example, there is evidence that a
tickle occurs with simultaneous activation of certain touch and pain receptors – as long as other conditions are
appropriate. As you are probably aware, others can tickle you, but you generally cannot tickle yourself, even
if you can activate the same sensory receptors. Thus input from other systems seems to be able to affect our
sensations. There is still a lot for scientists to learn about sensory perception.
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Sensory Adaptation
When first jumping into cool water, you may endure a wave of sensory information “reminding you” that the
water is cool. Yet minutes later, you may be “used to” the water temperature, or have adapted to it. This
change in perception did not occur because the water warmed up, but because the sensory receptors that
originally responded to the change in temperature are no longer sending signals to the CNS, or are sending
them at a decreased rate. This example indicates how a rapidly adapting receptor might function; it provides
significant signals to the CNS about the original change in temperature that the body experiences, but then
adapts such that it sends fewer signals thereafter.
Not all sensory receptors are rapidly adapting. Certain pain receptors seem to have little adaptation, or are
very slowly adapting, so that as long as the pain stimulus is applied, the person continues to “get the
message”! The general rates of adaptation for specific sensory receptors are indicated in the above table.
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