Single unit activity in the rat's midbrain during nocifensive tail flick reaction.

Single units have been recorded extracellularly from the periaqueductal central grey and from adjacent structures in behaving rats during tail flick reflexes elicited by controlled radiant heat. Together with the spike recordings, the heart rate of the animal and a thoracic myogram as an indicator of overall motor activity, have been recorded. Twenty-seven medial midbrain units have been studied, which were all characterized by being spontaneously active and by receiving input from myelinated afferent fibres from the tail, as shown by weak transcutaneous electrical stimulation. These units showed a variety of changes in discharge pattern. The most frequently observed change in activity was an increase of firing in the pre-flick period (10 of 27 units) and a decrease of firing below the prestimulus level in the post-flick period (12 of 27 units). It is suggested that changes in the discharge patterns were more closely related to behavioral output than to the input side of nociceptive behaviour.