79 Chapter 6 Less than 15% of the spinothalamic fibers originate from neurons in lamina I in cat Esther-Marije Klop, Leonora J. Mouton and Gert Holstege Neurosci. Lett.; 360(3): 125-128 (2004) ABSTRACT Lamina I neurons sending their axons into the spinothalamic tract are thought to play a crucial role in nociception, but many spinothalamic fibers do not originate from lamina I neurons. In cat, no consensus exist about what percentage of the spinothalamic tract cells is located in lamina I. After wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) injections that covered large parts of the thalamus, retrogradely labeled cells were plotted and counted in all segments of the spinal cord. Results show that, averaged over all spinal segments, the percentage of labeled lamina I neurons was 4.9-14.2%. These results demonstrate that, in contrast to what is concluded in several previous studies, lamina I in the cat provides only a limited part of the total spinal input to the thalamus. INTRODUCTION The spinothalamic tract is best known for its role in the transmission of information concerning nociception, temperature and crude touch from spinal cord to thalamus. The pathway is usually associated with projections from Rexed’s lamina I, but no consensus exists about what portion of all spinothalamic fibers originates in lamina I neurons. Studies in rat, cat and monkey (rat: Kemplay and Webster, 1986; Burstein et al., 1990b; cat: Trevino and Carstens, 1975; Carstens and Trevino, 1978a; Carstens and Trevino, 1978b; Comans and Snow, 1981; Wiberg and Blomqvist, 1984b; monkey: Trevino and Carstens, 1975; Willis et al., 1979; Apkarian and Hodge, 1989a) have reported that the vast majority of the spinothalamic cells is not located in lamina I. On the other hand, according to Craig et al. (1989b), in cat about half of the spinothalamic tract originates in lamina I. This view has been adopted in subsequent papers (Craig, 1989a; Zhang et al., 1996) as well as in recent reviews (Craig, 2003c). These discrepancies might be caused by differences in thalamic injection sites and/ or the selection of the studied spinal segments, especially since the contribution of lamina I cells to the spinothalamic tract differs between spinal segments. Only in rat all segments of the spinal cord were studied after large injections that covered almost all parts of the thalamus (Kemplay and Webster, 1986; Burstein et al., 1990b). Until now, in cat only one or a few segments of the spinal cord were studied (Trevino and Carstens, 1975; Carstens and Trevino, 1978a; Carstens and Trevino, 1978b; Comans and Snow,