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Electrophysiological properties of neonatal rat motoneurons studies in vitro


, : Electrophysiological properties of neonatal rat motoneurons studies in vitro. Journal of Physiology (Cambridge) 370: 651-680

The electroresponsive properties of neonatal lumbar spinal motoneurones were studied using isolated, hemisected spinal cords from neonatal rats aged 3-12 days. The extracellular and intracellular responses to electrical stimulation of the ventral and dorsal root were studied as well as the intracellular response to current injection. Field potentials recorded in the lateral motor area following electrical stimulation of lumbar ventral roots had a triphasic positive-negative-positive wave form. The negative component did not return to the base line smoothly but exhibited a 'shoulder' where the negativity increased in duration. Following electrical stimulation of the dorsal root, presynaptic field potentials were recorded upon activation of the afferent axons as well as following synaptic activation of interneurones and motoneurones. The input resistances of neonatal motoneurones determined from the slope of current-voltage plots were high compared wih the adult. The resistance decreased with age with a mean of 18.1 M.OMEGA. for animals 3-5 days old 8.8 M.OMEGA. for animals 6-8 days old and 5.4 M.OMEGA. for animals 9-11 days old. Values for the membrane time constant were similar to those in the adult with a mean of 4.5 ms. Action potentials elicited by ventral or dorsal root stimulation or by intracellular current injection were marked by a pronounced after-depolarization (a.d.p.) and an after-hyperpolarization (a.h.p.). The amplitude of the a.h.p. varied with that of the a.d.p. The amplitude of excitatory post-synaptic potentials (e.p.s.p.s) elicited by electrical stimulation of the dorsal root was affected by intracellular current injection. Two types of e.p.s.p.s were distinguished: those with a biphasic reversal (early phase first) and those in which the early phase was unaffected by inward current injection while the later phase was reversed. Unlike in the adult, the reversals could be achieved with low current levels and the amplitude of both types of e.p.s.p. was increased by inward current injection. Inhibitory post-synaptic potentials (i.p.s.p.s) were elicited by dorsal or ventral root stimulation. The amplitude of these i.p.s.p.s was diminished and reversed in sign with inward current injection and their amplitude was enhanced with outward current injection. Activation of neonatal motoneurones with long current pulses revealed that there is one steady-state firing range. The firing properties of neonatal motoneurones differ from those of adult rat in that (a) there is no clear threshold for repetitive firing, (b)there is a low maximum firing frequency (near 30 Hz) and (c)the slope of the firing frequency-injected current (f-I) plot has a high gain. These results are discussed with respect to previous findings in adult rat and eat and with reference to the properties of the developing nervous and muscular systems.

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