Membrane properties of morphologically identified X and Y cells in the lateral geniculate nucleus of the cat in vitro.

1. The membrane properties and the electrotonic features of cells in lamina A of the cat dorsal lateral geniculate nucleus (l.g.n.) were studied using an in vitro slice preparation. 2. Following intrasomatic injection of horseradish peroxidase (HRP) each neurone was classified as an X (n = 20) or a Y (n = 27) cell on the basis of its morphology. For both classes, the frequency distribution of soma area was similar to that reported in vivo where the identification of X and Y cells in lamina A of the cat l.g.n. was based on physiological criteria. 3. No difference was observed in the mean resting membrane potential between the two classes of cells. However, the input resistance (RN) of X cells was greater (82 M omega) and their membrane time constant (tau 0) longer (22 ms) than of Y cells (RN, 32 M omega; tau 0, 15 ms). 4. Using a simple neuronal model, the calculated electrotonic length (L) and the dendritic to somatic conductance ratio (rho) were similar for the two classes of cells. The mean value of L (0.7) and rho (1.9) suggests that both X and Y cells are electrically compact. 5. The specific membrane resistance (Rm, 28,000 omega cm2) of X cells, calculated using two different approaches, was found to be higher than that of Y cells (17,000 omega cm2). 6. The implication of these results for the integration of synaptic signals in the two classes of l.g.n. cells and the feasibility of differentiating between X and Y cells on the basis of their membrane properties are discussed.