Somatosensory Systems, Pain NeuroReport 0959-4965 # Lippincott Williams & Wilkins On the relation of rat's external cuneate activity to global parameters of forelimb posture Genoveffa Giaquinta, Antonino Casabona, Maria Stella Valle, Gianfranco Bosco 1 and Vincenzo Perciavalle CA Department of Physiological Sciences, University of Catania, viale Andrea Doria 6, 95125 Catania, Italy; 1 Department of Physiology, University of Minnesota, Minneapolis, MN, USA CA Corresponding Author USING anesthetized adult rats, we studied the relation- ships between the activity of cells belonging to the external cuneate nucleus (ECN) and passive forelimb positions. In essence, we sought to distinguish between a representation of limb position based on local limb parameters (individual joint angles, for example) or a representation based on more global parameters such as the length and the orientation of the limb axis. Using multivariate regression analyses we found that most neurons showed strong linear relationships with the length and the orientation of the limb axis. Relation- ships to individual joint angles were, instead, rather weak and in most cases not signi®cant. This result implies an extensive integration of sensory information at the level of second order sensory neurons. Neuro- Report 10:3075±3080 # 1999 Lippincott Williams & Wilkins. Key words: Cerebellum; Forelimb position; Propriocep- tion; Sensorimotor integration Introduction A controversial issue in sensorimotor research con- cerns the organization of sensory information within the central nervous system. At the level of the primary sensory cortex, the classical view of a somatotopical organization has been challenged by experiments in behaving monkeys, which showed that the activity of individual cortical neurons may relate to global parameters of arm movement and posture [1,2]. With regard to the role of low order neurons in the organization and processing of sen- sory information, it was generally thought that they relayed relatively unprocessed information to higher centers in the CNS. However, there is increasing evidence that a more complex processing of sensory information may occur even at the level of spinal neurons. For example, a large degree of sensory convergence has been shown on single spinocerebel- lar neurons [3±6], indicating the possibility of a quite extensive processing of sensory information. Recently, Bosco et al. [7,8] investigated the func- tional relevance of this complex pattern of sensory convergence by recording the activity of dorsal spinocerebellar tract (DSCT) neurons during passive hindlimb positioning in the anesthetized cat. These studies pointed out that DSCT neurons might represent hindlimb position in terms of limb end- point coordinates (limb axis length and orientation) rather than with respect to more local parameters, such as individual joint angles. Interestingly, beha- vioral studies suggested that similar limb endpoint coordinates might be the controlled variables in maintaining stance and during locomotion [9,10]. In addition, for the control of reaching and pointing it has been suggested that intermediate stages in sensorimotor processing may involve a similar type of reference frame. However, except for the cortical data mentioned above, there are not many studies that address this issue at a lower level of sensory processing. Earlier anatomical [11] and neurophysiological studies [12] based on rather focal sensory stimulation favored the idea of a somatoto- pic arrangement of sensory information within the main and external cuneate nuclei. However, recent ultrastructural [13] and functional studies [14] have suggested a more integrative role for the external cuneate nucleus circuitry. This could have a relevant functional impact for both cerebellum and somato- sensory system. In fact, the ECN not only repre- sents one of the major sensory input sources to the spinocerebellum [15,16] but also projects to the ventrobasal complex of the thalamus [17,18], provid- ing some of the proprioceptive input to the somato- sensory cortex. These considerations provided us with the ratio- nale for investigating more systematically the type of forelimb sensory representation by ECN neu- NeuroReport 10, 3075±3080 (1999) Vol 10 No 14 29 September 1999 3075