When you first jump into a swimming pool on a really warm day, the water feels rather chilly at first, but then you get used to the cold temperature. Why?

Answered by Tamara Cox, RN, Eastern Idaho Public Health

Human skin contains different kinds of sensory receptors (cells) that can identify several distinct types of sensations, such as tapping, vibration, pressure, pain, heat, and cold. Each receptor is triggered by a specific stimulus. Thermoreceptors detect temperature changes and send electrical pulses through sensory nerves to the brain where the signals are processed.

thermo1We are equipped with some thermoreceptors that are activated by cold conditions and others that are activated by heat. Warm receptors will turn up their signal rate when they feel warmth—or heat transfer into the body. Cooling—or heat transfer out of the body—results in a decreased signal rate. Cold receptors, on the other hand, increase their firing rate during cooling and decrease it during warming.

Something interesting happens when you expose receptors to a specific sensation such as heat for a long time: they start to tire out and decrease their activity, thereby you will no longer notice the sensation as much.

Desensitize Your Thermoregulators

Could desensitization also alter our sensitivity to what we feel next? Try this activity and found out.
Materials: 3 pots or bowls large enough to submerge both hands, warm water, room-temperature water, ice water, towel, clock

  1. Fill one pot with very cold water or ice water.
  2. Fill the second pot with room-temperature water.
  3. Fill the third pot with warm water. Make sure the water isn’t too hot; you’ll need to comfortably leave your hands in the water.
  4. Submerge your right hand in the pot with cold water.
  5. Put your left hand in the pot with warm water.
  6. Leave your hands in the pots for two minutes. Check your observation of the temperature of the water in each pot again. Does the cold water still feel as cold as it did at first? What about the warm water? Did the temperature of the water in the pots change or has your perception of the temperature changed?
  7. Remove your hands from the pots and immediately place both hands in the pot with room-temperature water. How would you describe the temperature of the water? Do both hands feel the same temperature?

thermo2

You will probably be experiencing a difference in temperature sensation between the two hands. Even though both hands are now in the same container, and experiencing the same temperature, the left hand should feel hot, while the right hand should find the water pretty chilly.

You are experiencing something called a sensory adaptation. In this experiment when the right hand is placed in cold water, the cold sensitive thermoreceptors are activated causing an electrical pulse which passes down the sensory nerve in the fingertips and hands to the brain.

On the other side, when the left hand is placed in the warm water, the warm thermoreceptors are activated.

If your hand is exposed to heat for a long time then the hot sensitive receptors, will, much like muscles after a long workout, start to get tired. They become less sensitive to the stimulus. The same things happen to the cold receptors; if your hand is exposed to the cold for a long time then the thermoreceptors become less sensitive to cold.

When you then moved your right hand to a warmer environment,the cold sensitive receptors had adapted, but the warm receptors had not and your left hand perceived the lukewarm container to be warmer than it really was.

On the left side, you effectively wore out your hot sensitive nerve endings and when you moved your hand to a colder environment the hot sensitive receptors had adapted, but the cold receptors had not, so the right hand perceived the middle container to be colder than it really was.

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