MORPHOLOGY AND MEMBRANE-PROPERTIES OF NEURONS IN THE CAT VENTROBASAL THALAMUS IN-VITRO

1. The morphological (n=66) and electrophysiological (n=41) properties of eighty-six thalamocortical (TC) neurones and those of one interneu rone in the cat ventrobasal (VB) thalamus were examined using an in vi tro slice preparation. The resting membrane potential for thirty-seven TC neurones was -61.9 +/- 0.7 mV, with thirteen neurones exhibiting d elta oscillation with and without DC injection. 2. The voltage-current relationships of TC neurones were highly non-linear, with a mean peak input resistance of 254.4 M Omega and a mean steady-state input resis tance of 80.6 M Omega between -60 and -75 mV. At potentials more posit ive than -60 mV, outward rectification led to a mean steady-state inpu t resistance of 13.3 M Omega. At potentials more negative than -75 mV, there was inward rectification, consisting of a fast component leadin g to a mean peak input resistance of 14.5 M Omega, and a slow time-dep endent component leading to a mean steady-state input resistance of 10 .6 M Omega. 3. Above -60 mV, three types of firing were exhibited by T C neurones. The first was an accelerating pattern associated with litt le spike broadening and a late component in the spike after-hyperpolar ization. The second was an accommodating or intermittent pattern assoc iated with spike broadening, while the third was a burst-suppressed pa ttern of firing also associated with spike broadening, but with broade r spikes of a smaller amplitude. All TC neurones envoked high frequenc y (310-520 Hz) burst firing mediated by a low threshold Ca2+ potential . 4. Morphologically TC neurones were divided into two groups: Type I (n = 31 neurones) which had larger soma, dendritic arbors that occupie d more space, thicker primary dendrites and daughter dendrites that fo llowed a more direct course than Type II (n = 35). The only electrophy siological differences were that Type I neurones (n = 16) had smaller peak input and outward rectification resistance and spike after-hyperp olarization, but greater peak inward rectification resistance, and exh ibited delta oscillation less often than Type II (n = 13).5. The morph ologically identified interneurone exhibited no outward rectification, only moderate inward rectification, and no high frequency firing asso ciated with the offset of negative current steps below -55 mV. This in terneurone had a regular accommodating firing pattern, but the spike a fter-hyperpolarization had a late component, unlike the accommodating firing in TC neurones. 6. Therefore, the differentiation of TC neurona l types in the cat VB thalamus based on their morphology was reflected by differences in peak input resistance, outward rectification and sp ike after-hyperpolarization, which could be accounted. for by their di fference in soma size. More importantly, the firing pattern of the maj ority of TC neurones in the cat VB thalamus were different from those of TC neurones in other sensory thalamic nuclei. 7. Thalamocortical ne urones in the cat VB thalamus were also clearly distinguishable from t he interneurone based on the presence of their prominent outward recti fication, peak inward rectification and robust low threshold Ca2+ pote ntials.