Re: How can a nerve distingiush between mild heat and severe heat?

Date: Mon Aug 28 10:00:03 2000
Posted By: Terry Hebert, Faculty, Universite de Montreal, Biochemistry, Montréal Heart Institute
Area of science: Neuroscience
ID: 964380219.Ns

Message:

Hello Vinod,

You ask a very interesting question. Somatic (i.e. body) sensory perception can be divided into 4 basic categories:

Tactile sensations elicited by mechanical stimulation applied to the body surface (touch etc.).
Proprioceptive sensations elicited by mechanical displacements of muscles and joints (i.e. how the body senses its position and the position of its limbs relative to one another).
Thermal sensations including separate cold and warm senses.
Pain sensation elicited by noxious stimuli.

Each of these modalities uses a particular type of receptor which can encode the features (quality and frequency) of a given set of stimuli. Pain sensation is mediated by "nociceptors" (from the Latin nocere, to injure). These are divided into separate receptors for mechanical pain, heat pain as well as nonspecialized "polymodal" receptors which respond to multiple types of noxious stimuli. To activate a receptor, a stimulus must be of a suitable intensity as well as of a suitable quality. Thus the answer to your question is that the intensity of a stimulus will determine the number of pain receptors which can be activated even though the repsonse by any given neuron will be identical. This is not always the case though as pain receptors are connected to neuronal axons of two particular fibre types- A(delta) and C fibres. A(delta) fibres carry fast pain sensations (abrupt, sharp pains) as their conduction velocity is higher than for C fibres which carry slow pain (the sickening burning sensation which sometimes follows fast pain) signals.

Remember, these receptors and the fibres that they are connected to are in the periphery. The signals that are generated travel up eventually to the central nervous system and are extensively processed along the way- as is the response whioh eventually results in movement or action aimed at ending the pain. What determines the specificity of the initial signal though is the receptor type (or types ) activated by the stimulus. For example, there are 5 different receptor types for tactile sensation as well as multiples types involved in sensation of pain, proprioception and temperature.

Hope this helps,

Terry

Reference:
Principles of Neural Science Kandell & Schwartz, 4th ed. McGraw-Hill Professional Publishing; ISBN: 0838577016

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