In spite of a recognized importance as a zone for input from fine diameter primary afferent fibres, remarkably little is known about the functional attributes of different types of neurones comprising the spinal SDH. To gain information on relationships between morphology of SDH neurones and their physiological properties, we made tight-seal, whole-cell recordings using pipette microelectrodes filled with biocytin in spinal cord slices prepared from deeply anaesthetized (1.6 g kg^-1 urethane) weaned, golden hamsters. (The animals were killed by exsanguination.) Voltage- and current-clamp recordings were made from each neurone and subsequently histological processing was carried out to reveal intracellular biocytin. Criteria used to characterize neurones included: size and locus of the soma, the region and branching pattern of the dendrites and the axon, the frequency of spontaneous synaptic currents (sEPSCs), the latency and amplitude of synaptic currents evoked by dorsal root (DR) stimulation, the pattern of discharge to depolarizing current, and correlations between membrane voltage and current. Morphological and electrophysiological data were obtained for 170 neurones, 13 of which had somata in lamina I and 157 in lamina II. Almost all received excitatory input from the segmental DR. Over 75 % (133/170) of the sample fitted into one of nine categories, distinguished from one another by a combination of morphological and electrophysiological characteristics. Lamina I neurones were divided into two categories; one represented putative ventrolateral projection neurones (5/13) with prominent axons directed ventrolaterally and substantially lower input resistances than other (< 200 M¢) SDH neurones (> 400 M¢). Lamina II neurones included five morphological categories: islet, central, medial-lateral, radial, and vertical. The large central cell group (46) was divisible into three subsets based upon the responses to depolarization and the presence of an IA current. While a single physiological feature did not uniquely characterize any category, a combination of recording location plus certain electrophysiological characteristics make it reasonable to predict that a certain recording came from a neurone belonging to a particular category (e.g. a lamina I neurone with an input resistance < 200 M¢ was likely to be a V-L projection type; a lamina I neurone with a frequency of sEPSC > 5 s^-1 was a probable non-projection type; a lamina II cell with a transient response to depolarization and an IA current was likely to belong to a subset of central neurones). Thus, in the SDH there is considerable correlation between the diverse neuronal configurations and cellular functional attributes. This implies a diversity in the functional organization of the SDH in modulating and transmitting activity from fine dorsal root fibres

This work was supported by a research grant NS 10321 from the NINDS of the US NIH. We are grateful for the expert histological assistance of Kirk McNaughton.