Pain is defined as an unpleasant sensory and emotional experience that is associated with actual or potential tissue damage, or described in terms of such damage.

The neuronal basis of pain:

An experiment was conducted that applied heat (stimulus) to a persons skin and used an electrode to measure the electrical activity in both nociceptors and thermoreceptors. This experiment demonstrated that pain perception requires specialized neurons and not simply a greater discharge of neurons that respond to harmless stimulus intensities (i.e. you need nociceptors to detect pain, thermoreceptors are not able to detect pain regardless of the intensity of the heat).

As the temperature increases slowly, the thermoreceptor responds, with action potentials, to low and moderate heat (below pain threshold, e.g., 40–43°C). After about 45°C, it stops responding, even if heat continues to rise. Nociceptors on the other hand are not active at lower temperatures (because there is no painful stimuli to respond to). These receptors only start firing once the pain threshold is reached (+- 43 degrees) and as the temperature increases beyond this point the number of action potentials being fired will increase indicating increasing pain.

Pain signaling follows the labelled line code, meaning that specific nerve fibers are “labelled “ for pain and thus the brain interprets any signal from them as pain no matter what.

<aside> 💡

If a signal comes from a nociceptor, the brain always interprets it as PAIN, even if it's a wrong or weak signal.

If a signal comes from a thermoreceptor, the brain interprets it as warmth or cold, never pain.

It’s about WHICH system the signal travels through, not necessarily how big or small the signal is. Each sensory neuron type has its own "labelled line" to the brain — and the brain trusts the label blindly.

</aside>

Classification of pain:

Pain can be classified based on duration and usefulness:

Duration:

• fast pain- sharp, rapid pain that you feel with impending tissue damage (when you get close to noxious stimuli) or just after tissue damage. This pain serves to inform you of the presence of the noxious stimulus, and it can act and disappear quite fast. This type of pain is very easy to localize and you can usually pinpoint exactly where the stimulus is. It is conducted by small myelinated fibers called A delta fibers.
• slow pain- dull and defuse pain that you feel a bit later after the actual tissue damage and it may last for hours/days. It serves to protect the damaged area to facilitate healing. It is conducted by C fibers which are thin unmyelinated fibers.

An experiment was conducted to decipher which fiber carried which type of pain (fast or slow). Blocking of the A delta fiber was associated with loss of the first/fast pain whereas blocking the C fiber was associated with loss of the slow/second pain.

Usefulness:

• physiologic pain -described as good pain because it is protective. It helps you to move away from potentially harmful stimuli and helps protect injured areas (area is sore so you are more careful with it) so that it can heal. This pain is acute and thus lasts only hours/sometimes days, and then it goes away during the healing process.
• pathologic pain- a chronic type of pain that stays well after tissue damage or can even occur without actual tissue damage. It is caused by nerve injury which may be due to toxins, ischemia or diabetes. It can last for months/years, and is very difficult to manage/modulate.

Look at bottom two only.

Pain transduction: