The mode of cerebello-thalamic relay transmission investigated with intracellular recording from cells of the ventrolateral nucleus of cat's thalamus.

Abstract

1. Cerebello-thalamo-cortical relay transmission was studied by intracellular recording from cells of the ventrolateral (VL) nucleus of cat's thalamus. The VL relay cells were identified by their antidromic invasion from the pericruciate cortex as well as by their monosynaptic activation from the cerebellar nuclei. 2. Impulses conducting along the cerebello-thalamic pathway at a mean velocity of 18 m/sec produced in VL relay cells excitatory postsynaptic potentials (EPSPs) monosynaptically. Cerebello-thalamic transmission was characterized by relatively large sizes of unitary EPSPs that amounted to 0.8–3.2 mV in peak amplitude. Spontaneously arising EPSP-noise had a similarly large unitary size. 3. The spot from which the EPSPs were produced with the lowest threshold was determined for each of 50 VL relay neurones. The EPSP origin thus determined lay within both lateral and interpositus nuclei. 4. The cerebellar-evoked EPSPs were often followed by inhibitory postsynaptic potentials (IPSPs) with an additional delay of 0.9–2.5 msec. The possibility is suggested that this inhibitory pathway contains at least one interneurone, presumably located within VL. For 10 VL relay cells, the low threshold points for inducing the polysynaptic IPSP were found to be distributed within the interpositus nucleus. 5. Stimulation to the pericruciate cortex produced EPSPs monosynaptically in VL relay cells, the pathway having a conduction velocity of 4–14 m/sec. The time course of the cortical-evoked EPSPs was characteristically slow, in contrast to that of the cerebellar induced EPSPs. This suggests that cortico-thalamic fibers make contact with the peripheral dendrites of the VL neurones, whereas the cerebello-thalamic ones impinge onto their somas or proximal dendrites.