Electrophysiological characterization of different types of neurons recorded in vivo in the motor cortex of the cat. II. Membrane parameters, action potentials, current-induced voltage responses and electrotonic structures.

1. Electrical properties of four functional classes [inactivating bursting (ib), noninactivating bursting (nib), fast spiking (fsp), and regular spiking (rsp)] of neurons in the motor cortex of conscious cats were studied with the use of intracellular voltage recording and single-electrode voltage-clamp (SEVC) techniques. Evaluations were made of action potentials and afterpotentials, current-voltage (I-V) relationships, and passive cable properties. Values of membrane potential (Vm), input resistance (RN), membrane time constant (T0), and firing threshold (T50) were also measured. The data were used to extend the electrophysiological classifications of neurons described in the companion paper. 2. Average values of Vm (from -63 to -66 mV), action-potential amplitudes (from 72 to 77 mV), and firing threshold (-54 mV) were not statistically different in different types of neurons. However, the magnitude of intracellularly injected depolarizing current required to induce spike discharge at 50% probability varied significantly (from 0.6 to 1.1 nA) among cell types. The mean RN and T0 measured at Vm varied between 8.3 and 19.8 M omega, and 7.2 and 15.1 ms, respectively, in the cell classes. 3. Action potentials were overshooting. Their mean duration at half amplitude varied from 0.25 to 0.73 ms among different cell types. Three types of action-potential configurations were distinguished. Type I action potentials found in nib and rsp neurons were relatively fast and had a depolarizing afterpotential (DAP) as well as fast and slow after hyperpolarizations (fAHPs, sAHPs). Type II action potentials found in ib and rsp cells had relatively slow rise and decay phases, DAPs, and sAHPs. Their fAHPs were small or absent. Type III action potentials were found exclusively in fsp cells, had very short durations, prominent fAHPs, but no sAHPs. 4. Steady-state I-V relationships were determined by measuring voltage responses to 0.2- to 1.0-nA hyperpolarizing, rectangular current pulses at different membrane potentials. Both RN and T0 exhibited nonlinear behavior over wide ranges of membrane potential; however, between -65 and -75 mV, the I-V relationships varied little, and they appeared constant in most cells. The steady-state values of RN increased with decreasing, and decreased with increasing the membrane potential in all but fsp cells. The I-V relationships were virtually linear in fsp neurons. 5. Transient I-V relationships were studied by measuring voltage responses to depolarizing and hyperpolarizing, rectangular current pulses of increasing amplitude from a preset membrane potential of -70 mV.(ABSTRACT TRUNCATED AT 400 WORDS)