Spatio-temporal information coding in the cuneate nucleus

Abstract

The dorsal column nuclei, cuneatus and gracilis, receive somesthetic information impinging on projection cells and local inhibitory interneurons. The presence of these interneurons allows spatio-temporal progressive coding of information that can be modeled [E. Sánchez, J. Aguilar, C. Rivadulla, A. Canedo, The role of Glycinergic Interneurons in the Dorsal Column Nuclei, Neurocomputing 58–60 (2004) 1049–1055 [14]] using their known synaptic connections with projection cells [J. Aguilar, C. Rivadulla, C. Soto, A. Canedo, New corticocuneate cellular mechanisms underlying the modulation of cutaneous ascending transmission in anesthetized cats, J. Neurophysiol. 89 (2003) 3328–3339 [1], J. Aguilar, C. Soto, C. Rivadulla, A. Canedo, The lemniscal-cuneate recurrent excitation is suppressed by strychnine and enhancedd by GABAA antagonists in the anesthetized cat, Eur. J. Neurosci. 16 (2002) 1697–1704 [2], J. Mariño, L. Martinez, A. Canedo, Sensorimotor integration at the dorsal column nuclei, NIPS 14 (1999) 231–237 [9]. Here we explore the dependency of the processing time required to complete the progressive coding with regard to cutaneous stimuli varying in size and regularity of the intensity profile.

Introduction

The dorsal middle region of the dorsal column nuclei (DCN) is constituted for two classes of neurons, glutamatergic cells projecting into the contralateral medial lemniscus and local interneurons releasing GABA, glycine or both neurotransmitters [11]. The cat's DCN receive cortical input from the primary somatosensory cortex [6], [10], [12], [18] and primary glutamatergic afferents topographically aligned [4], [7], [8], [13].

Recent studies in the cuneate nucleus (CN) using intracellular as well as extracellular recording combined with microiontophoresis have revealed that: (i) the cuneate neurons projecting to the medial lemniscus present a center-surround antagonism [5], (ii) the internal circuitry of the cutaneous sector of the cat's CN is such that the projecting cells with matched receptive fields monosynaptically activate each other through recurrent collaterals re-entering the nucleus, while inhibiting other projection neurons with different RFs [2], and (iii) the cortico-cuneate cells [1] and primary afferents [16] with matched RFs activate and disinhibit aligned cuneo-lemniscal neurons and inhibit other neighbouring projection neurons with unmatched RFs. The activation at the centre of the RF is produced through NMDA and non-NMDA glutamate receptors, the lateral inhibition is produced through GABAergic interneurons and the disinhibition is mediated by serial glycinergic-GABAergic-projection cells interactions [1], [2], [16].

The above results are the basis to determine the influences over each projecting neuron and were used to develop a computational model for the CN [14]. Both projection neurons and interneurons are represented as MacCulloch-Pits processing units. Concretely, the activity of the processing units representing the projection neurons is under the modulating influence of primary afferent, collateral recurrent and corticocuneate inputs affecting these cells as described above. The different weight values w_ji model the synaptic interactions among the distinct classes of neurons and are grouped into matrixes whose values allow for adjusting the contribution of each neuronal class to the network representing the CN.