Spinal Inhibitory Neurons that Modulate Cutaneous Sensory Pathways during Locomotion in a Simple Vertebrate

Abstract

During locomotion, reflex responses to sensory stimulation are usually modulated and may even be reversed. This is thought to be the result of phased inhibition, but the neurons responsible are usually not known. When the hatchling Xenopus tadpole swims, responses to cutaneous stimulation are modulated. This occurs because sensory pathway interneurons receive rhythmic glycinergic inhibition broadly in phase with the motor discharge on the same side of the trunk. We now describe a new whole-cell recording preparation of the Xenopus tadpole CNS. This has been used with neurobiotin injection to define the passive and firing properties of spinal ascending interneurons and their detailed anatomy. Paired recordings show that they make direct, glycinergic synapses onto spinal sensory pathway interneurons, and the site of contact can be seen anatomically. During swimming, ascending interneurons fire rhythmically. Analysis shows that their firing is more variable and not as reliable as other interneurons, but the temporal pattern of their impulse activity is suitable to produce the main peak of gating inhibition in sensory pathway interneurons. Ascending interneurons are not excited at short latency after skin stimulation but are strongly active after repetitive skin stimulation, which evokes vigorous and slower struggling movements. We conclude that ascending interneurons are a major class of modulatory neurons producing inhibitory gating of cutaneous sensory pathways during swimming and struggling.

Role of type-specific neuron properties in a spinal cord motor network

“…Activity in the dINs is clearly different to that in the other CPG neurons, with a stronger background (tonic) depolarisation and more prominent reciprocal, mid-cycle IPSPs, followed on each cycle by rebound firing (Li et al, 2006). In contrast: the background depolarisation in the cINs, aINs and MNs is much smaller, and the mid cycle inhibition is hardly visible as the membrane potential is close to the inhibitory reversal potential (Li et al, 2002(Li et al, , 2006. In these neurons the depolarisation is not sufficient for any to fire repetitively (as they are capable of doing to sustained current), or to fire on rebound.…”

“…Recordings from individual neurons were made under visual control using a ×40 water immersion lens on an upright compound microscope. The methods for fixation, neurobiotin staining and recording of neuron anatomy have been described previously (Li et al, 2002). All experiments comply with UK Home Office regulations and have been approved following local ethical review.…”

“…However, since membrane currents have not yet been characterized for the different neurons types, in most cases we used "emerging" parameters obtained by matching outputs of the model to physiological properties measured for each neuron type. Information was available from experiments on a wide range of these properties: neuronal input resistance, resting potential, firing threshold, current threshold for firing, action potential (AP) height and width, maximum rise and fall rates of the action potential, after-hyperpolarisation amplitude and delay, initial firing frequency to long, constant-current input, and the effect of increasing current on this frequency (see Table 1; Li et al, 2002Li et al, , 2004bLi et al, , d, 2006. The general equations are based on Hodgkin and Huxley (1952).…”

“…They fire repetitively to sustained injected current ( Fig. 1), but there are differences, for example in input resistance and firing frequency during current injection (Li et al, 2002).…”

“…These include: their passive and active responses to current injection, the transmitter pharmacology of synapses they make and receive (where some synaptic conductances have been studied in voltage-clamp recordings), and their activity during responses to sensory stimuli. Such detailed information has shown that 7 of the anatomical classes correspond to distinct functional classes: 1 type of sensory neuron innervating the skin (Clarke et al, 1984;Li et al, 2003); 2 types of sensory pathway neuron that have a role in the initiation of swimming (Li et al, 2003(Li et al, , 2004b; 3 types of premotor neuron that are active during swimming and may be components of the swimming central pattern generator (Dale, 1985;Li et al, 2002Li et al, , 2004dLi et al, , 2006; and motoneurons (Soffe and Roberts, 1982;Roberts et al, 1999). These detailed studies have recently provided evidence of functional sub-types within two anatomical classes (Li et al, unpublished).…”