Spinal termination of nociceptive afferent fibres from deep tissues in the cat

In anaesthetized cats, the impulse activity in afferent fibres from the deep tissues of the lower hindlimb was recorded at the dorsal root entry zone with micropipettes filled with horseradish peroxidase solution. After identification of the fibres as nociceptive with the use of mechanical stimuli and ascertainment of the deep location of the receptive field (in fascia, muscle, tendon, ligament or joint capsule) the enzyme was injected iontophoretically into the axon. Histochemical processing of the tissue with diaminobenzidine was employed to visualize the spinal terminal arborizations of the units. The fibres conducted at 18.3-40.0 m/s; they showed two different patterns of termination: (1) units terminating exclusively in lamina I of the dorsal horn; (2) units possessing their main projection area in laminae IV/V with only a few collaterals and boutons in lamina I. Thus, the nociceptive fibres from deep tissues terminated in those laminae of the dorsal horn which are known to contain cells of origin of ascending nociceptive tracts.