The responses of relay cells in the human thalamic ventral caudal nucleus (Vc) was explored in a set of experiments designed to study neuronal firing patterns evoked in humans by pain and thermal cutaneous stimuli. Cells responsive to cold or painful stimuli have been found to produce low threshold spike (LTS) bursting preferentially in response to these stimuli. This appears to be an intrinsic thalamic phenomenon and not a firing pattern produced by input from the periphery. Microstimulation within the Vc during awake surgical procedures demonstrates that cold and pain sensations can be evoked by short bursts of electrical stimuli in a pattern similar to that observed during neuronal LTS bursts. These results strongly suggest that that cold and pain sensations may be mediated in part by human thalamic LTS burst firing patterns.

Light microscopic observations employing intravital methylene blue staining and impregnation by the zinc iodide-osmium tetroxide technique are presented for intraepidermal nerve fibre endings of the intervibrissal fur in the mystacial pad of the rat snout. Both procedures revealed anatomical details of the intraepidermal nerve fibre plexus in epidermal hillocks often located very close to the mouths of hairs. These nerve fibres appeared to resemble those described in previous immunohistochemical studies as cluster or bush endings. The methylene blue preparations demonstrated the existence of an intensely stained enlargement at the site of the branching point of the nerve fibres which seemed to be functionally related to the development of such nerve fibre plexuses. Due to their close association with hairs, these nerve fibre plexuses are most likely to be mechanoreceptive. Additionally, solitary varicose nerve fibres were found loosely distributed within the epidermis. The visualisation of 2 different morphological types of nerve fibre endings extends the validity of the concept of punctate sensibility into the epidermis. Methylene blue staining appeared to be somewhat superior to the zinc iodide-osmium tetroxide technique. Due to their selectivity for intraepidermal nerve fibres, the methods applied here supplement immunohistochemical procedures in a helpful manner.