Reorganization of the projection from the sensory cortex to the motor cortex following elimination of the thalamic projection to the motor cortex in cats; Golgi, electron microscope and degeneration study.

Changes of terminal connections of projection fibers from area 2 of the sensory cortex to the motor cortex following chronic lesion in the thalamus were examined using the electron microscope. The lesioned areas included nucleus ventralis anterior, n. ventralis lateralis and rostral part of n. ventralis posterolateralis. The synaptic sites were identified using the Golgi impregnation method to identify postsynaptic neurons in the motor cortex and the axonal degeneration method to identify presynaptic terminals of fibers originating from area 2. The following results were obtained. (1) The number of degenerating terminals per unit area in the motor cortex was increased to nearly twice that in normal animals. (2) The number of degenerating terminals synapsing with stellate cells was not increased but stayed more or less the same as in normal animals. (3) The number of degenerating terminals contacting pyramidal cells increased substantially, to more than twice that in normal animals. (4) These newly formed synapses were found on proximal dendritic shafts of the pyramidal cells in both layers III and V, suggesting that these synapses occupied the spaces where the thalamocortical terminals were located. (5) The functional significance of these newly formed synapses was discussed in relation to the recovery of motor function following thalamic lesion.