THERMAL SENSATIONS Dr Ghulam Mustafa

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

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.

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

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.

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

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.

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

Freezing cold and burning hot sensations feel almost alike.

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

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

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.

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.

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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