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.

Impairment of the discrimination of the direction of single-whisker stimulation induced by the lemniscal pathway lesion.

In the rodent somatosensory system, stimulus information received by the whiskers is relayed to the barrel cortex via two parallel pathways, the lemniscal pathway and the paralemniscal pathway. The lemniscal pathway includes the principal trigeminal nucleus (Pr5) and the ventral posteromedial thalamic nucleus (VPm). The paralemniscal pathway includes the spinal trigeminal subnucleus interpolaris (Sp5i) and the medial division of posterior thalamic nucleus (POm). The purpose of this study was to investigate the roles of those pathways in perceptions of the direction of the single-whisker stimulation in the rat. Rats were trained to perform a go/no-go task that required the discrimination of forward or backward stimulation applied to their single whisker. When a selective lesion was made in VPm or Pr5, error rate for the task performance increased significantly. In contrast, when a selective lesion was made in POm or Sp5i, we found no significant change in performance. These results suggest that the lemniscal pathway plays more important roles in a discrimination of stimulus direction applied to the single whisker.