Nociceptors and Hot Foods

All spicy foods use one or more kind of spicy ingredients such as habanero, jalapeño, and red chile. Each country has a different method of cooking the spicy ingredient, but ultimately, the source of materials to produce spicy tastes is the key to maintain the taste. These spicy ingredients cause that taste because they contain a substance known as capsaicin.

What is capsaicin?

Capsaicin is a compound that has an effect of activating, or "turning on," certain receptors in the body. In particular, they respond to nociceptors, which are used to sense pain. "Capsaicin activates responses in a subset of nociceptive C fibers (polymodal nociceptors) by opening ligand-gated ion channels that permit the entry of Na⁺ and Ca²⁺."¹ In particular, the activation of capsaicin has a significant response to VR-1 channel, which interestingly can be activated by heating the compound with VR-1 channel to 43ºC.

Before going further, it is necessary to pause and define some of these definitions. First of all, with nociceptors, there are two types of fibers used in pain: Aδ fiber and C fiber. Aδ fiber is used for rapidly occurring pain whereas C fiber is for longer-lasting pain. For the purpose of this article regarding capsaicin, the only important thing is to realize that of the two types of fibers, capsaicin is particularly influenced by polymodal nociceptors, which are receptors that can be stimulated by means of mechanical or chemical causes. In other words, this indicates that if certain polymodal nociceptor is affected by capsaicin, then it is very possible that this receptor will always be responded by some other means of stimulus.

Why does Na⁺ and Ca²⁺ matter?

Aside from knowing that sodium is in salt (sodium chloride) and calcium is in milk, not everyone knows just exactly why sodium and calcium are crucial in our body. As the description for capsaicin activation indicated, channels that increase their conductance (also known as permeability) to sodium and calcium ions have tremendous effects in the body. Sodium, along with potassium and chloride, plays a pivotal role in determining the resting membrane potential of a given cell. In electrophysiology, the resting membrane potential of a certain cell can be calculated by using the Goldman-Hodgkin-Katz equation.

Resting membrane potential is important because when it increases, or depolarizes, from its usual negative number, an action potential can be generated. Here is a very good animation of an action potential that discusses sodium and potassium from Purves's Neuroscience on-line accompanying website.

Now, as far as calcium goes, this ion is important in various ways, ranging from muscle contraction to release of neurotransmitters like acetylcholine. Controlling the level of calcium is one of the main tasks of body's attempts to maintain homeostasis.

Going back to capsaicin...

In a simple, vernacular English, eating a spicy food with capsaicin in it feels "hot" because in the molecular level, the reaction of body is identical to say, putting your tongue in a hot material (not exactly same since it will probably hurt more to put your tongue on a hot material). VR-1 channel can be activated by both heat and capsaicin, so even though your brain realizes that capsaicin isn't a "hot" substance (temperature-wise), the molecular response in the body is same.

If you are bored after reading this, here is my "assignment" to you - why would our body have developed similar mechanisms to hot and spicy substances?

Works Cited

¹ Dale Purves, et al., Neuroscience, 4^th ed. (Sunderland: MA: Sinauer Associates, Inc., 2008) 234.