Farmasevtika instituti farmakologiya va klinik farmatsiya kafedrasi

Pain Is a Protective Mechanis . 
Pain occurs whenever tissues are being damaged, and it 
causes the individualto react to remove the pain stimulus. Even such simpleactivities as sitting for a 
long time on the ischia can cause tissue destruction because of lack of blood flow to the skinwhere it is 
compressed by the weight of the body.
Types of Pain and Their Qualities
—Fast Pain and Slow Pain 
Pain has been classified 
into two major types: 
fast pain 
and 
slow pain. 
Fast pain is felt within about 0.1 second after a pain 
stimulus is applied, whereas slow pain begins only after 1 second or more and then increases 
slowlyover many seconds and sometimes even minutes. During the course of this chapter, we shall see 
that the conductionpathways for these two types of pain are different and that each of them has specific 
qualities.
Pain Receptors and Their Stimulation 
Pain Receptors Are Free Nerve Endings. 
The pain receptors in the skin and other tissues 
are all free nerve endings. They are widespread in the superficial layers of the 
skin, 
as well as in certain 
internal tissues, such as the 
periosteum, 
the 
arterial walls, 
the 
joint surfaces, 
and the
falx 
and 
tentorium 
in the cranial vault. Most other deeptissues are only sparsely supplied with pain endings; 
nevertheless,any widespread tissue damage can summate to cause the slow-chronic-aching type of pain 
in most of these areas. 
Rate of Tissue Damage as a Stimulus for Pain 
The average person begins to perceive pain when the skin is heated above 45 °C, as shown in 
Figure 48-1. This is alsothe temperature at which the tissues begin to be damagedby heat; indeed, the 
tissues are eventually destroyed if the temperature remains above this level indefinitely. Therefore, it is 
immediately apparent that pain resultingfrom heat is closely correlated with the 
rate at whichdamage 
to the tissues is occurring 
and not with the total damagethat has already occurred.