Previous | Next | To the beginning

Thalamocortical projection and electromagnetic activity in the cortex

Kazuo Sasaki

National Institute for Physiological Sciences, Okazaki 444-8585, Japan

The thalamus is histologically divided into two groups of neurons: The specific nuclei send their axons to the cortex and the non-specific nuclei to non-cortical structures. Low frequency (5 - 12 Hz) electrical stimulation of the former type of neurons elicits augmenting responses in the cortical area to which they project, whereas the latter induce recruiting responses in relatively wide cortical regions. However, the concept of the non-specific nuclei and their mode of projection (the so-called diffuse projection system) should be reinvestigated.

Laminar analysis of cortical potentials induced by thalamic stimulation by means of microelectrodes has revealed that there are two kinds of thalamo-cortical (T-C) projection cells, called deep and superficial T-C neurons [1]. Thus various T-C responses, including the augmenting and recruiting responses, can be explained by simple or combined forms of deep and superficial T-C signals. The superficial type, responsible for the recruiting response, was in fact identified electrophysiologically [2, 3]. Later, several histological investigations with new staining methods proved the existence of the two T-C projections sending their axons to cortical layers III - V and to layer I, respectively.

The deep and superficial T-C projections give EPSPs on the apical dendrites of pyramidal neurons, including extra- and intracellular currents. They are considered to be the main sources, respectively, of volume currents manifested on the surface of the scalp in EEG and ERP, and of intracellular currents through the apical dendrites in MEG [4].

It will also be shown experimentally how the extracellular volume currents are distorted by the shunting effect of the cerebrospinal fluid [5].

1. Sasaki, K., Staunton, H.P., and Dieckmann, G. Exp. Neurol. 26: 369-392, 1970
2. Sasaki, K., Matsuda, Y., Kawaguchi, S., and Mizuno, N. Exp. Brain. Res. 16: 89-103, 1972
3. Sasaki, K., Matsuda, Y., Oka, H., and Mizuno, N. Exp. Brain. Res. 22: 87-96, 1975
4. Sasaki, K., Nambu, A., Tsujimoto, T., Matsuzaki, R., Kyuhou, S., and Gemba, H. Brain Res. 5: 165-174, 1996
5. Sasaki, K., Shimono, T., Kawaguchi, S., and Yoneda, Y. Jap. J. Physiol. 19: 80-94, 1969

Previous | Next