This system controls aspects of body function mainly by means of two hormones, both catecholamines. The two hormones, adrenaline and nor-adrenaline are produced by the adrenal medulla and by the nerve endings of sympathetic nerve fibres (nor-adrenaline only). They are involved in controlling autonomic functions (e.g. heart rate). The proportion of each hormone released by the adrenal medulla can vary from species to species as well as with age.
The activity of this system can be measured by taking serial blood samples and measuring concentrations of the catecholamines, adrenaline and nor-adrenaline (e.g. Mellor et al 2002). However, there can be difficulties in obtaining samples of adequate quality over the correct time period and results are usually available only after considerable delay. The assays are also expensive.
An alternatively approach could be to monitor heart rate, blood pressure (Peers et al 2002), diameter of the pupils, skin resistance and / or peripheral blood flow. Several of these changes occur in parallel. However, changes in heart rate may also occur in response to handling, eating, exercise, extraneous noises etc. Consequently, it is difficult to use changes in heart rate without using (remote) monitoring devices attached to the animal for long periods, preferably to include before, during and after the episode of pain. Holton et al (1998) suggested that these parameters were of little use clinically in determining the presence of pain in dogs after surgical procedures.