Biology session

The firing properties of a thalamic neuron are determined by large excitatory („driver”) terminals. According to recent data besides the well-known subcortical driver inputs the thalamus receives the same type of terminals from the cortical layer V. pyramidal cells. These inputs, just like the subcortical ones, target the proximal dendrite of the relay cell. According to the origin of the driver input thalamic nuclei can be divided into first order (driver input from the periphery) or higher order (driver input from the layer V. pyramidal cells) nuclei. However, co-distribution of the large cortical and subcortical terminals in the higher order nuclei has not been examined yet. In addition it is not known whether the two types of inputs have similar target selectivity and whether the two driver inputs from the cortex and from the periphery can converge on the same thalamic neuron.

To examine the co-distribution of the cortical and subcortical terminals in higher order thalamic nuclei, anterograde tracing from the S1 somatosensory cortex was combined with vesicular glutamate transporter type II (vGlut 2) immunostaining known to be a selective marker for subcortical glutamatergic terminals. The distribution of the subcortical terminals was inhomogeneous in the higher order posterior nucleus (Po) and homogeneous in the first order ventral posteromedial nucleus (VPM). The distribution of the large cortical terminals in the Po was also inhomogeneous (patches), but large cortical terminals were absent in the VPM. Three types of areas were distinguished in Po, vGlut 2 rich, vGlut 2 poor zones, and zones completely devoided of vGlut 2 positive terminals. Various numbers of large cortical terminals were found in all of the these areas. Cortical and subcortical terminals were often in close proximity to each other. This raise the possibility of the convergence of the two types of terminals on the same thalamic neuron. Correlated light and electron microscopic examination and parallel in vivo electrophysiological experiments unequivocally demonstrated the convergence of cotical and subcortical terminals on the same cell.

To reveal the morphological basis of the synaptic efficacy of the two pathways we examined the diameter of their postsynaptic elements. Both types of the terminals target comparable, thick proximal dendrites indicating that the they can have similar impact (drive) on the thalamic neurons, so their importance is similar in somatosensory functions. Our results furthermore highlight a new way of thalamic information processing, in which the thalamic neurons not only relay the activity of a single type of driver terminal to the cortex, but integrate two different types of driver information.