ELECTROPHYSIOLOGY OF CAT ASSOCIATION CORTICAL-CELLS IN-VIVO - INTRINSIC-PROPERTIES AND SYNAPTIC RESPONSES

1. The intrinsic properties and synaptic responses of association cort ical neurons (n = 179) recorded from cat's areas 5 and 7 were studied in vivo. Intracellular recordings were performed under urethane anesth esia. Resting membrane potential (V(m)) was -71.7 +/- 1.2 (SE) mV, amp litude of action potential was 83.7 +/- 2.3 mV, and input resistance w as 18.4 +/- 1.8 MOEMGA. Cells were identified ortho- and antidromicall y from lateroposterior and centrolateral thalamic nuclei and from homo topic foci in the contralateral cortex. Physiologically identified neu rons were intracellularly stained with Lucifer yellow (LY) and found t o be pyramidal-shaped elements (n = 21). 2. We classified the neurons as regular-spiking and intrinsically bursting cells. Regular-spiking c ells were further classified as slow-and fast-adapting according to th e adaptation of spike frequency during long-lasting depolarizing curre nt pulses. 3. Regular-spiking, slow-adapting neurons had a monophasic afterhyperpolarization (AHP) or a biphasic AHP with fast and medium co mponents (fAHP, mAHP). Slow-adapting behavior was observed in 84% (n = 119) of the regular-spiking cells.4. Regular-spiking, fast-adapting c ells only fired a train of spikes at the beginning of the pulse. There after, the V(m) remained as a depolarizing plateau, occasionally trigg ering some spikes. These neurons had a monophasic AHP and represented 16% (n = 23) of the regular-spiking neurons. 5. Intrinsically bursting neurons (n = 37) were observed in 20% of neocortical cells at depolar ized V(m). Their action potential was followed by a marked depolarizin g afterpotential (DAP). Rhythmic (4-10 Hz) bursts occurred during long -lasting depolarizing current pulses. 6. Small (3-10 mV), fast (1.5-4 ms), all-or-none depolarizing potentials were triggered by depolarizin g current pulses. They are tentatively regarded as dendritic spikes re corded from the soma because their rate of occurrence changed as a fun ction of the V(m) and they were eventually blocked by hyperpolarizatio n. 7. Synaptic stimulation of either thalamic or homotopic contralater al cortical areas elicited a sequence of excitatory postsynaptic poten tials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). Two comp onents of the EPSP were revealed. At a hyperpolarized V(m), the initia l component of the EPSP increased in amplitude, whereas the secondary component was blocked. Repetitive (10 Hz) stimulation of the thalamus or contralateral cortex elicited incremental responses. The augmentati on phenomenon was due to an increase in the secondary component of the EPSP. The cortically elicited augmenting responses survived extensive thalamic lesions. A short IPSP and a long-lasting IPSP were evoked by thalamic or cortical stimulation. The short IPSP had a reversal poten tial at about -70 mV and was reversed by increasing intracellular chlo ride concentration.