Neurons with distinctive firing patterns, morphology and distribution in laminae V–VII of the neonatal rat lumbar spinal cord

Abstract

It is generally accepted that neurons in the ventral spinal grey matter, a substantial proportion of which can be regarded as constituents of the spinal motor apparatus, receive and integrate synaptic inputs arising from various peripheral, spinal and supraspinal sources. Thus, a profound knowledge concerning the integrative properties of interneurons in the spinal ventral grey matter appears to be essential for a fair understanding of operational principles of spinal motor neural assemblies. Using the whole cell patch clamp configuration in a correlative physiological and morphological experimental approach, here we demonstrate that the intrinsic membrane properties of neurons vary widely in laminae V-VII of the ventral grey matter of the neonatal rat lumbar spinal cord. Based on their firing patterns in response to depolarizing current steps, we have classified the recorded neurons into four categories: 'phasic', 'repetitive', 'single' and 'slow'. Neurons with firing properties characteristic of the 'phasic', 'repetitive' and 'single' cells have previously been reported also in the superficial and deep spinal dorsal horn, but this is the first account in the literature in which 'slow' neurons have been recovered and described in the spinal cord. The physiological heterogeneity in conjunction with the morphological correlation and distribution of neurons argues that different components of motor neural assemblies in the spinal ventral grey matter possess different signal processing characteristics.

ERG Conductance Expression Modulates the Excitability of Ventral Horn GABAergic Interneurons That Control Rhythmic Oscillations in the Developing Mouse Spinal Cord

“…It is noteworthy that a similar classification of firing patterns of ventral interneurons (with a strong preponderance of tonic cells) (Szucs et al, 2003;Theiss and Heckman, 2005) has been reported for newborn spinal cord preparations, indicating that the observed differences were not caused by culturing conditions.…”

“…Such changes occurred in coincidence with the critical transformation of spontaneous activity from bursting to sporadic discharges (Whelan, 2003;Rosato-Siri et al, 2004). Previous studies reported similar neuronal firing classes in the ventral horn of the postnatal rodent spinal cord, with phasic versus tonic (or repetitive) cells distributed in a comparable proportion (Szucs et al, 2003;Theiss and Heckman, 2005).…”

“…All neurons were held at Ϫ60 mV membrane potential, a value close to their resting potential (see Materials and Methods). Cells were kept at Ϫ60 mV (because their spike threshold was approximately Ϫ50 mV) to prevent spontaneous firing (Szucs et al, 2003), which obscures stimulusevoked responses.…”

“…1 A, B) (Prescott and de Koninck, 2002;Szucs et al, 2003;Theiss and Heckman, 2005;Lu et al, 2006): (1) "tonic" cells, which continuously fired APs without apparent accommodation; (2) "adapting" cells, which discharged an early burst of APs followed by adaptation; (3) "delay" cells, which generated APs after a lag; (4) "irregular" cells without a discernible pattern of AP discharge; and (5) "transient" cells, which generated a single AP only. Interestingly, the distribution of the five neuronal classes was maturation dependent.…”

Electrophysiological and Pharmacological Properties of Locomotor Activity-Related Neurons in cfos-EGFP Mice

“…The similar firing pattern has been reported in rat spinal neurons in ventral horn (Szucs et al 2003;Theiss and Heckman 2005;Theiss et al 2007), superficial horn (Prescott and De Koninck 2002), and deep dorsal horn (Hochman et al 1997). In response to a depolarizing step current, neurons of type 1 elicited only one to two spikes (Fig.…”