Modulation of synaptic transmission from segmental afferents by spontaneous activity of dorsal horn spinal neurones in the cat

1. We examined, in the anaesthetised cat, the influence of the neuronal ens embles producing spontaneous negative cord dorsum potentials (nCDPs) on seg mental pathways mediating primary afferent depolarisation (PAD) of cutaneou s and group I muscle afferents and on Ia monosynaptic activation of spinal motoneurones. 2. The intraspinal distribution of the field potentials associated with the spontaneous nCDPs indicated that the neuronal ensembles involved in the ge neration of these potentials were located in the dorsal horn of lumbar segm ents, in the same region of termination of low-threshold cutaneous afferent s. 3. During the occurrence of spontaneous nCDPs, transmission from low-thresh old cutaneous afferents to second order neurones in laminae III-VI, as well as transmission along pathways mediating PAD of cutaneous and Ib afferents , was facilitated. PD of Ia afferents was instead inhibited. 4. Monosynaptic reflexes of flexors and extensors were facilitated during t he spontaneous nCDPs. The magnitude of the facilitation was proportional to the amplitude of the 'conditioning' spontaneous nCDPs. This led to a high positive correlation between amplitude fluctuations of spontaneous nCDPs an d fluctuations of monosynaptic reflexes. 5. Stimulation of low-threshold cutaneous afferents transiently reduced the probability of occurrence of spontaneous nCDPs as well as the fluctuations of monosynaptic reflexes. 6. It is concluded that the spontaneous nCDPs were produced by the activati on of a population of dorsal horn neurones that shared the same functional pathways and involved the same set of neurones as those responding monosyna ptically to stimulation of large cutaneous afferents. The spontaneous activ ity of these neurones was probably the main cause of the fluctuations of th e monosynaptic reflexes observed under anaesthesia and could provide a dyna mic linkage between segmental sensory and motor pathway.