Somatosensory properties of cuneocerebellar neurones in the main cuneate nucleus of the rat Nadia L Cerminara, Kalyanee Makarabhirom and John A Rawson Department of Physiology, Monash University, Clayton, Victoria, Australia Cells in the main cuneate nucleus (MCN) are known to provide a direct projection to the cerebellum, but the precise nature of the information these cells transmit to the cerebellum is unknown. The present study employed anatomical and electrophysiological procedures to determine the location of cuneocerebellar cells in the MCN, and their somatosensory properties in the rat. The location of neurones projecting to the cerebellum was determined with injections of the retrograde tracers, horseradish peroxidase or Fluoro-Gold in vermal and paravermal regions of the cerebellum. Topographically, the majority of retrogradely labelled cells in the MCN were found to lie primarily ventrolateral in the nucleus and rostral to the level of the obex. Single unit recordings from 69 well characterized MCN cells, identi®ed as projection cells by antidromic activation from stimulation of the inferior cerebellar peduncle, were classi®ed according to their responses to cutaneous stimulation and manipulation of joints and muscles. A slight majority of cells (37.7%) responded only to manipulation/stimulation of joints, and 30.4% of cells responded only to cutaneous stimulation. The remaining cells received convergent input from joint and cutaneous receptors. Cutaneous responsive cells all rapidly adapted to maintained stimuli, and had large receptive ®elds (RFs) that were generally located over the joints. These cells could be activated by passive movements of the forelimb that deformed the RF. They only discharged during movements and were silent during maintained limb positions. Cells responsive to punctate mechanical stimuli applied to the joint capsules, responded to passive movements of the forelimb, but typically only discharged towards the limits of joint movement, and adapted within a few seconds. Once adapted, small perturbations of joint position resulted in vigorous dynamic responses. The results indicate that the neurones in the MCN of the rat which project directly to the cerebellum are localized in the rostral half of the nucleus. They transmit predominantly dynamic information from joint and cutaneous receptors that are likely to be normally activated as a result of limb movements. These cells could signal information about evolving movements or disturbances of forelimb posture or stance. Keywords: cerebellum  spinocerebellar tracts  dorsal column nuclei  cuneate  brain mapping Cerminara NL, Makarabhirom K, Rawson JA. Somatosensory properties of cuneocerebellar neurones in the main cuneate nucleus of the rat. Cerebellum 2003; 2: 131–145 Introduction The cuneocerebellar tract (CCT) is generally thought to be the forelimb homologue of the dorsal spinocerebellar tract (DSCT), which transmits input from skin and muscle receptors in the hindlimb to the cerebellum. Unlike the DSCT, which originates from cells in the spinal cord, 1,2 the CCT has two quite distinct nuclei of origin in the medulla: the external cuneate nucleus (ECN)andtheinternalormaincuneatenucleus(MCN). The cerebellar projections from these nuclei are thought to form largely separate proprioceptive (P- CCT) and exteroceptive (E-CCT) subdivisions of the CCT, 3,4 with the external cuneate transmitting informa- tion from muscle receptors in the forelimb and the main nucleus providing cutaneous input. The subdivisions were initially made on the basis that cells in the ECN responded to stimulation of muscle nerves at Group I strength, whereas CCT cells in the MCN were activated by stimulation of cutaneous nerves and not by Group I muscle inputs. 3,4 However, it should be noted that these authors also reported that the majority of MCN cuneo- cerebellar tract cells were also activated by stimulation of muscle nerves at Group II strength, indicating a possible major contribution from muscle spindle secondary or other deep afferents. Subsequent electrophysiological studies have con- ®rmed that the ECN transmits Group I and II afferent input from the muscles in the neck, forelimb and upper trunk muscles to the cerebellum. 5–7 Studies using the method of transganglionic transport of HRP in the ECN to map the termination of individual muscles have also con®rmed these observations. 8–10 Apart from a study in the raccoon by Haring et al., 11 where it was suggested that the main input to MCN cuneocerebellar cells in this species is from muscle receptors, there is no data from receptive ®eld analyses to indicate precisely what information cells in the MCN transmit to the cerebellum. In the present study we have examined in detail both the origin of the cuneocerebellar Received 7 February 2001; Revised 14 May 2001; Accepted 15 May 2001 Correspondence: Nadia Cerminara, Department of Physiology, School of Medical Sciences, University of Bristol, University Walk, Bristol BS8 1TD, UK. Tel: 44 117 928 7809. Fax: 44 117 928 8923. E-mail: n.cerminara@bris.ac.uk j The Cerebellum 2003; 2: pp 131145 j 131  2003 Taylor & Francis DOI 10.1080/14734220310011371