Correlation of physiologically and morphologically identified neuronal types in human association cortex in vitro

1. We examined whether the three physiologically defined neuron types described for rodent neocortex were also evident in human association cortex studied in an in vitro brain slice preparation. We also examined the relationship between physiological and morphological cell type in human neocortical neurons. In particular, we tested whether burst-firing neurons were numerous in regions of human cortex that are susceptible to seizures. 2. Although we sampled regular-spiking and fast-spiking neurons, we observed no true burst-firing neurons, as defined for rodent cortex. We did find neurons that displayed a voltage-dependent shift in firing behavior. Because this behavior was due, in large part, to a low-threshold calcium conductance, we called these cells low-threshold spike (LTS) neurons. 3. Regular-spiking neurons and LTS neurons only differed in the voltage dependence of firing behavior and the first few interspike intervals (ISIs) of repetitive firing in response to small current injections (from hyperpolarized membrane potentials). Because of the general similarities between the two types, we consider the LTS cells to be a subgroup of regular-spiking cells. 4. All biocytin-filled regular-spiking neurons were spiny and pyramidal and found in layers II-VI. The lone filled fast-spiking cell was aspiny and nonpyramidal (layer V). The LTS neurons were morphologically heterogeneous. We found 80% of LTS neurons to be spiny and pyramidal, but 20% were aspiny nonpyramidal cells. LTS neurons were located in layers II-VI. 5. In conclusion, human association cortex contains two of three physiological cell types described in rodent cortex: regular spiking and fast spiking. These physiological types corresponded to spiny, pyramidal, and aspiny, nonpyramidal cells, respectively. We sampled no intrinsic burst-firing neurons in human association cortex. LTS neurons exhibited voltage-dependent changes in firing behavior and were morphologically heterogeneous: most LTS cells were spiny and pyramidal, but two cells were found to be aspiny and nonpyramidal. It is not clear whether the absence of burst-firing neurons or the morphological heterogeneity of LTS neurons are due to species differences or differences in cortical areas.