Effects of dorsal root section on spinocervical tract neurones in the cat

1. In nine cats, dorsal roots L(6) and L(7) were sectioned under pentobarbitone anaesthesia using strict aseptic precautions. In two cats all lumbosacral roots except L(6) were sectioned under similar conditions. 27-75 days after the dorsal rhizotomies, under chloralose anaesthesia, neurones of the spinocervical tract (s.c.t.) were recorded electrophysiologically both extracellularly and intracellularly, their response properties and receptive fields were analysed, and they were injected with horseradish peroxidase (HRP). Twenty-five cells were recorded extracellularly and twenty were recorded intracellularly and stained with the enzyme.2. Of the twenty neurones stained with HRP, one was located in caudal L(5), eleven in L(6) and eight in L(7). Five cells had no receptive field, either before or after current injection, two cells had no field before injection but after current injection responded to either electrical stimulation of peripheral nerve or to brusque tapping on the limb; six neurones responded only to tapping before current injection and two of these showed a more clearly defined receptive field after current injection; six neurones had a receptive field and responded with impulses to hair movement and/or pressure within a well-defined area but this area was surrounded by skin from which excitatory post-synaptic potentials could be elicited; only one neurone had an apparently normal receptive field.3. Of the twenty-five neurones recorded extracellularly, eleven had no receptive field, four responded only to tapping on the limb, three had receptive fields consisting of an area of low threshold plus an area responding to brusque or noxious mechanical stimulation, and seven neurones had apparently normal receptive fields.4. Neurones with no observable receptive fields were either silent with no resting activity, or had very low rates of background activity consisting of isolated single impulses or short bursts of impulses. No neurones in de-afferented segments had high rates of background activity.5. There were no alterations in the normal somatotopy of the dorsal horn. All fifteen s.c.t. neurones injected with HRP and which had a localizable receptive field had fields appropriately located in the somatotopic map. There were no electro-physiological signs of inappropriate connexions.6. The dendritic trees of fifteen neurones injected with HRP were reconstructed. With the exception of one cell which appeared grossly disorganized (but which was the one cell with an apparently normal receptive field), there were no obvious differences between the dendritic trees of de-afferented cells and normal cells from the same and other cats. Their dendritic trees had similar rostro-caudal, medio-lateral and dorso-ventral extents to normal cells, and appeared to exhibit similar degrees of branching. There was a suggestion that de-afferented neurones had rather more of their dorsally-directed distal dendrites invading lamina II than normal cells, but the sample size was too small to make this more than a possibility.7. It is concluded that after dorsal rhizotomy, s.c.t. neurones lose part or all of their afferent input and that within 75 days do not show any signs of new or inappropriate connexions.