2. THERMAL SENSATIONS
Dr Ghulam Mustafa

3. Learning Objectives Thermal Receptors Temp range
Warm receptors
Cold receptor
Temp range
Mechanism of Stimulation of Thermal Receptors
Adaptation of thermal Receptors
Spatial Summation of Thermal Sensations.
Thermal sensations pathways

4. Thermal Receptors
The human being can perceive different gradations of cold and heat
From freezing cold
To cold
To cool
To indifferent
To warm to hot to burning hot.
Three types of sensory receptors:
Cold receptors
Warmth receptors
Pain receptors.

5. The cold and warmth receptors
Located immediately under the skin
At discrete separated spot
3 to 10 times as many cold spots as warmth spots

6. Warm receptors The existence of distinctive warmth nerve endings
Free nerve endings
Warmth signals are transmitted mainly over Type C nerve fibers
Transmission velocities of only 0.4 to 2 m/sec.

7. Cold receptor
Small type A delta Myelinated nerve ending that branches a number of times
Tips - protrude into the bottom surfaces of basal epidermal cells.
Signals - transmitted from these receptors
Via type A delta nerve fibers
At velocities of about 20 m/sec
Some cold sensations -transmitted in type C nerve fibers

8. Temp range 1) A pain fiber stimulated by cold, (2) A cold fiber,
(3) A warmth fiber, and
(4) A pain fiber stimulated by heat.

9. Very Cold Region +10° to 15°C Above about 30°C At 45°C
cold-pain fibers are stimulated
+10° to 15°C
cold receptors begin to be stimulated,
peak stimulation at about 24°C
Above about 30°C
warmth receptors begin to be stimulated,
fade out at about 49°C.
At 45°C
heat-pain fibers begin to be stimulated
damage to the cold endings caused by the excessive heat

12. Freezing cold and burning hot sensations feel almost alike.

13. Mechanism of Stimulation of Thermal Receptors
Cold and warmth receptors
Stimulated by changes in their metabolic rates
Alters the rate of intracellular chemical reactions
More than twofold for each 10°C change.
Not from direct physical effects of heat or cold on the nerve endings
Chemical stimulation of the endings

14. Adaptation of Thermal Receptors
Cold receptor is suddenly subjected to an abrupt fall in temperature
Strongly stimulated at first
Stimulation fades
Rapidly during the first few seconds
Progressively more slowly during the next 30 minutes
Thermal senses respond markedly to changes in temperature

15. Spatial Summation of Thermal Sensations.
Number of cold or warm endings in any one surface area of the body - Less
Difficult to judge gradations of temperature
When small skin areas are stimulated.
When a large skin area - stimulated - at once
The thermal signals from the entire area summate.

16. Thermal sensations pathways
On entering the spinal cord
Signals travel for a few segments upward or downward in the tract of Lissauer
Terminate mainly in Laminae I, II, and III of the dorsal horn
Small amount of processing
Cross to the opposite anterolateral sensory tract
Terminate in
(1) Reticular areas of the brain stem and
(2) Ventrobasal complex of the thalamus.

17. Also relayed
to the cerebral somatic sensory cortex from the ventrobasal complex
Neuron in cortical somatic sensory area I has been found by microelectrode studies to be directly responsive to
either cold or
warm stimuli
on a specific area of the skin.
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18. Thank you