Figure 3. Projection neurons.

a | This image shows a transverse section through the contralateral side of the L4 segment of a rat that received injections of two retrograde tracers: Fluorogold into the lateral parabrachial area (LPb) and cholera toxin B subunit (CTb) into the caudal ventrolateral medulla (CVLM). The section was immunostained for CTb (red), Fluorogold (green) and NeuN (blue). Injection of tracers into both of these regions has been previously shown to label virtually all lamina I projection neurons⁵⁹. Cells retrogradely labelled from the LPb appear green, those labelled from the CVLM are red, and those that received label from both sites are yellow. Note the high density of double-labelled cells in lamina I, but that even here they are outnumbered by other neurons (blue). Lamina II contains virtually no projection cells at lumbar levels, and there are scattered retrogradely labelled cells in deeper laminae and in the lateral spinal nucleus (LSN). b | A summary of quantitative data from a series of studies of projection neurons in the L4 segment of the rat spinal cord^58,59,65-67. These data suggest that there are approximately 400 projection neurons in lamina I (~5% of the total neuronal population in this lamina¹¹⁶) (blue numbers). Most of these can be retrogradely labelled following injection of tracer into the LPb or CVLM, with smaller numbers projecting to the other sites. Studies involving paired injections indicate that virtually all lamina I cells that are retrogradely labelled following tracer injection into thalamus, periaqueductal grey matter (PAG) or nucleus of the solitary tract (NTS) also project to LPb. Note that as spinal projections to the CVLM terminate near the main bundle of ascending axons, retrograde labelling from this region may include cells labelled through uptake of tracer into fibres of passage, rather than from axon terminals. The numbers of neurokinin 1 receptor (NK1R)-expressing projection neurons in laminae III-IV that can be labelled from each site are also shown (red numbers). There are around 24 of these cells per side in the L4 segment (corresponding to ~0.1% of the neurons in this lamina¹¹⁶). c | A horizontal section through lamina I scanned to reveal NK1R (green) and the glycine receptor-associated protein gephyrin (magenta). Two NK1R-positive neurons and a giant lamina I cell (outlined by gephyrin puncta) are visible. Although retrograde labelling was not performed, all lamina I neurons of this size are known to be projection cells^46,83. d | An example of a NK1R-expressing lamina III projection neuron in a sagittal section. The cell is retrogradely labelled with CTb (magenta) that had been injected into the LPb. Note the extensive dorsal dendrites that are labelled with the NK1R antibody (green), and can be followed into lamina I. Several retrogradely labelled lamina I cells are also visible in this field. Scale bars = 100 μm (a,d), 50 μm (c).

Part a reproduced, with permission, from REF. 158. Part c reproduced, with permission, from REF^82,83 © 2008 Society for Neuroscience. Part d reproduced, with permission, from REF ⁵⁸ © 2000 Wiley.