Abstract
Lamina I of the rat spinal cord contains neurons that project to various brain areas including thalamus, periaqueductal grey matter (PAG), lateral parabrachial area (LPb), caudal ventrolateral medulla and a region in dorsal medulla that includes the nucleus tractus solitarius and dorsal reticular nucleus. We have shown that spinothalamic lamina I neurons are infrequent in rat lumbar enlargement, where they constitute ∼5% of the estimated 400 projection neurons on each side of the L4 segment (Al-Khater and Todd, 2009). They are more numerous in cervical enlargement, but the total number of lamina I projection neurons in this region was not known. Here we have used paired injections of retrograde tracers into the brainstem to estimate the number of lamina I projection cells in the C7 segment. Our results suggest that there are ∼215 lamina I projection cells per side, and that spinothalamic cells therefore make up ∼42% of this population. The proportion of lamina I projection neurons labelled from PAG is higher in cervical than lumbar enlargement, while the proportion labelled from dorsal medulla is similar in the two regions. We also found that lamina I cells in L4 that project to the dorsal medulla are included in the population retrogradely labelled from LPb, thus confirming the estimate that there are around 400 lamina I projection cells in this segment.
Highlights
Lamina I of the dorsal horn (Rexed, 1952) is innervated by primary afferents that respond to noxious and/or thermal stimuli (Light and Perl, 1979; Sugiura et al, 1986), and contains many projection neurons that transmit this information to the brain (Todd, 2002; Willis and Coggeshall, 2004)
One injection was made into the left lateral parabrachial area (LPb), while the other was targetted on the periaqueductal grey matter (PAG), the caudal ventrolateral medulla (CVLM) or the dorsal medulla (NTS and DRt) on the left side
The main findings of this study were: (1) that in the C7 segment the great majority of lamina I neurons retrogradely labelled from PAG or CVLM were labelled from LPb, as we have previously reported for the mid-lumbar cord (Spike et al, 2003); and (2) that in both C7 and L4 segments most of the cells in this lamina that were labelled following injection of tracer into the dorsal medulla were labelled from LPb
Summary
Lamina I of the dorsal horn (Rexed, 1952) is innervated by primary afferents that respond to noxious and/or thermal stimuli (Light and Perl, 1979; Sugiura et al, 1986), and contains many projection neurons that transmit this information to the brain (Todd, 2002; Willis and Coggeshall, 2004). Based on the results of quantitative studies in which tracers were injected into LPb, PAG and CVLM, we estimated that there are ∼ 400 lamina I projection neurons on each side in the L4 segment of the rat, and that these make up approximately 6% of the total neuronal population in this lamina (Spike et al, 2003; Al-Khater et al, 2008). This estimate did not take account of lamina I neurons that were labelled from the dorsal medulla. The answer to the second question will enable us to provide a similar estimate for the cervical enlargement, and determine the proportion of projection cells that belong to the spinothalamic tract at this level
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