Liquid to deaden pain?

[pivoxa15]

1. The problem statement, all variables and given/known data
To deaden pain in minor operations, a liquid that vaporises easily is often sprayed on the affected area. How does this prevent the pain being felt? Would water have the same effect?


2. Relevant equations
none


3. The attempt at a solution
Vaporises easily => absorbs heat quickly and leaves the skin. Does cooler skin at damaged area => less pain felt? How does one feel pain? Does one feel pain by sensory cells firing into the brain via the central nervous system? So a cooler temperature in the affected area => slower firing of these sensory cells? Hence brain registering pain slowly and decreased in magnitude as cool=>low energy on the cells behalf.

[Moonbear]

The question seems a bit misleading in the way it's asked. The vaporization of the pain killer isn't the basis of its function (the same pain killer works if injected or spread on the surface of the skin as an emulsion). The vaporization only provides the means of getting it onto the skin (i.e., by spraying it on places that are hard-to-reach in other ways).

Since you originally posted this question in Intro Physics, though, what class was the question asked in, and what was the general topic of the class the homework assignment was related to? That might give us some context of whether you're supposed to be understanding the pharmacology of this actual drug, or if you're supposed to be focusing on its physical properties (vaporization) in order to help you with this question.

[DaveC426913]

Didn't I read somewhere that you can't feel both heat and cold in the same place at the same time? Applying a cooling effect will confuse the mind's interpretation of signals from heat/pain receptors.

[pivoxa15]

It did appear in a physics textbook and that chapter was dealing with heats of vapourisation. Maybe to deaden pain, vapourisation of the pain killing liquid is unimportant but how would you compare between a liquid that vaporises easily and one that does not?

[pivoxa15]

Didn't I read somewhere that you can't feel both heat and cold in the same place at the same time? Applying a cooling effect will confuse the mind's interpretation of signals from heat/pain receptors.

Are you saying the feeling of pain and heat have the same receptors? It kind of make sense because I can associate pain with a hot feeling - sort of?

[DaveC426913]

Are you saying the feeling of pain and heat have the same receptors? It kind of make sense because I can associate pain with a hot feeling - sort of?No, I don't think so, but they're linked.

The four receptors are: heat, cold, pain and pressure.

[Revenged]

Their receptors are different but their neural pathways are the same... The spinothalamic tract communicates information about both pain (nociception) and temperature...



The classes of receptor are slightly different to what you are saying...

"Thermoreceptors" of which there are hot and cold receptors,

"Mechanoreceptors" that detect vibration and touch,

"Nociceptors" that detect pain

and "Proprioceptors" which detect where limbs are in space...

[Revenged]

Applying a cooling effect will confuse the mind's interpretation of signals from heat/pain receptors.

Perhaps... in the end of the day when you burn yourself you do run to put that burnt part under a tap... but i suspect it may be that we do this in order to reduce the activation of the pain and heat pathway (spinothalamic tract) and so reduce the activation of the sensory part of the brain (somatosensory cortex)... i doubt this would 'confuse the brain' as you put it...

[Revenged]

How does one feel pain? Does one feel pain by sensory cells firing into the brain via the central nervous system? So a cooler temperature in the affected area => slower firing of these sensory cells? Hence brain registering pain slowly and decreased in magnitude as cool=>low energy on the cells behalf.

No it doesn't work like that... Cold can also cause pain... If you try to place your hand in a bucket of ice it would hurt like hell...

The phamacological mechanisms behind pain control are complicated... I don't really know how to go into it since you haven't even told us what drug you are talking about...

[Math Is Hard]

A technician sprayed my arm with this Fluori-Methane Spray (http://www.walgreens.com/library/finddrug/druginfo1.jsp?particularDrug=Fluori-Methane&id=5734)before my IV was started for my wisdom teeth surgery. Worked great!

[Math Is Hard]

Perhaps... in the end of the day when you burn yourself you do run to put that burnt part under a tap... but i suspect it may be that we do this in order to reduce the activation of the pain and heat pathway (spinothalamic tract) and so reduce the activation of the sensory part of the brain (somatosensory cortex)... i doubt this would 'confuse the brain' as you put it...

His point about the confusion is valid. What he is talking about is this: say I have two flexible metal pipes and I run very hot water through one and very cold water through another. If I twist those pipes together and then grasp the twisted cord, simultaneously feeling hot and cold, I will feel an overall burning sensation and likely jerk my hand away. However, it is hard to say if this is related to how the vapocoolant spray works.

When that freezing cold blast hit my skin before my IV was inserted, it covered a large surface area - lots of neurons responding to lots of cold stimuli. Was it a big sensory distraction from the tiny area of the injection? In the old days, they used to give us a hard pinch near the site of the injection to distract us from the pain of the needle. Same idea.

The other thing that came to mind is that the freezing from vapocoolants temporarily constricts the skin capillaries, reducing blood flow. This could slow the metabolic processes in the nerve cells. I would assume that interferes with conduction, but I don't know for sure.

[DaveC426913]

Perhaps... in the end of the day when you burn yourself you do run to put that burnt part under a tap... ...
That is an independent phenomenon. Running cold water over the burn constricts blood flow and prevents swelling and further damage through rupturing of more cells.

[Revenged]

That is an independent phenomenon. Running cold water over the burn constricts blood flow and prevents swelling and further damage through rupturing of more cells.

Ok... I see...

[pivoxa15]

1. The problem statement, all variables and given/known data
To deaden pain in minor operations, a liquid that vaporises easily is often sprayed on the affected area. How does this prevent the pain being felt? Would water have the same effect?



So to answer the original question including the relevant biology, a liquid that vaporises easily is used so that it can get to hard to reach places. Water would only take heat away from the body which wouldn't do much to relieve pain?

[Sojourner01]

Agh, confusing terminology here. They (or maybe you) have used the term 'vapourisation' inaccurately. If it was vapourised, you wouldn't notice its contact with your skin. What they mean is that it's easily misted. Vapourisation is what occurs when the liquid evaporates from the skin- rapidly vapourising implies that it has a strong cooling effect.

This is probably a sidetrack, though.

[Fallen Seraph]

Perhaps a little too much is being read into this question? I'd reckon your original analysis is right? Certainly in the context of a phyics question. Is not the answer that the cooling from vaporisation leaves you numb, and so kills the pain?

[pivoxa15]

So cooling a spot makes you think that spot becomes numb? How is that explained biologically? Even though its a physics question, I think it would be good to learn a bit of the biology behind it.