Difference in the functional significance between the lemniscal and paralemniscal pathways in the perception of direction of single-whisker stimulation examined by muscimol microinjection

Tactile information received by the whiskers of rodents is processed along several parallel pathways. A pathway that particularly includes the principal trigeminal nucleus (Pr5) and the thalamic ventral posterior medial nucleus (VPm) is called “lemniscal”, and a pathway that includes the spinal trigeminal subnucleus interpolaris (Sp5i) and the thalamic posterior medial nucleus (POm) is called “paralemniscal”. We trained rats to discriminate between two directions of stimulation applied to their single whiskers (forward or backward) to investigate how these pathways contributed to their perception of the direction of the single-whisker stimulation, and injected muscimol into either the lemniscal or paralemniscal nucleus while rats performed this task. The correct rate dropped significantly after muscimol injections into Pr5 or VPm, whereas we found no significant effects on discrimination after muscimol injections into Sp5i or POm. These results suggest that the lemniscal system is involved in enabling the direction of the single-whisker stimulus to be discriminated than the paralemniscal.