Electrophysiological characterization of different types of neurons recorded in vivo in the motor cortex of the cat. I. Patterns of firing activity and synaptic responses.

1. Patterns of firing activity and characteristics of antidromic and synaptic responses to stimulation of the pyramidal tract at peduncular level [peduncular pyramidal tract (PP)] and the ventrolateral thalamic nucleus (VL) were studied in neurons of area 4 gamma of the motor cortex of awake, chronic cats using intracellular microelectrode techniques. The results offer a new functional classification of neocortical neurons based on electrophysiological properties of the 640 recorded cells. 2. Four classes of neurons were distinguished: (class i) inactivating bursting (ib) neurons (n = 60) including fast antidromic response PP (fPP) (n = 0), slow antidromic response PP (sPP) (n = 11), and no antidromic response PP cells (nPP) (n = 49); (class ii) noninactivating bursting (nib) neurons (n = 79), including fPP (n = 23), sPP (n = 0), and nPP cells (n = 56); (class iii) fast-spiking (fsp) neurons (n = 56), including fPP (n = 0), sPP (n = 0), and nPP cells (n = 56); and (class iv) regular-spiking (rsp) neurons (n = 445), including fPP (n = 96), sPP (n = 38), and nPP cells (n = 311). (Neurons in each classification were further separated by their antidromic responses to PP stimulation: fast PP (fPP) slow PP (sPP), or nPP cells, the latter not responding antidromically to electrical stimulation of the peduncle.) 3. Recurrent monosynaptic excitatory postsynaptic potentials (EPSPs) followed antidromic spikes elicited by PP stimulation in most (96%) fPP but much fewer (24%) sPP cells. In fPP cells, it was possible to separate the PP EPSPs into two monosynaptic EPSP components that were generated by other fPP and sPP cells, respectively. VL stimulation evoked monosynaptic EPSPs in 100% of fPP cells (vs. 63% of sPP cells) and antidromic action potentials in 16% of fPP cells (vs. 12% of sPP cells). 4. Firing activity consisted of single spike discharges in most PP cells; however, noninactivating bursting was observed in 19% of fPP cells, and inactivating bursting was observed in 23% of sPP cells (see below). In 18% of ib and 11% of nib/nPP neurons, VL stimulation elicited antidromic action potentials. Other bursting neurons proved to be PP cells with characteristic differences in axonal conduction velocity (see above). All PP cells among the nib cells were fPP, and all PP cells among the ib cells were sPP cells. All fsp neurons were found to be nPP cells, and none could be activated antidromically by VL stimulation. Thus the fsp pattern of discharge distinguished a unique class of nPP cells.(ABSTRACT TRUNCATED AT 400 WORDS)