Cervicothalamic tract termination and its cortical connections: anatomical studies in cats and ferrets

Sammanfattning: The termination and cortical connections of the cervicothalamic tract (CTT) were studied by neuroanatomical tract tracing techniques in cats and ferrets. After injections of wheat germ agglutinin-horseradish peroxidase conjugate or biotinylated dextran amine (BDA) into the cat lateral cervical nucleus (LCN), the most abundant CTT termination is present in the ventral posterior lateral nucleus (VPL). The CTT terminates in the peripheral rim of the VPL and in between its lateral and medial subdivisions, regions defined in cats by sparse Cat-301 immunolabeling. Scattered CTT terminations are also seen in the vental periphery of the ventrobasal complex [VBvp; ventral posterior inferior nucleus (VPI) in ferrets] and in the medial nucleus of the posterior complex (POm). Dense and focused CTT termination is also present in the medial extension of the magnocellular nucleus of the medial geniculate nucleus (MGmc). Most of the CTT terminals in VPL are clustered. In contrast, in the VBvp and the POm widely spaced terminals dominate. In the MGmc, clustered terminals are found in close relation to Cat-301-positive neurons. Although the LCN is smaller in ferrets, its organization and termination in the thalamus are closely samilar to those in cats. Following BDA injections into the LCN and choleragenoid injections into the first somatosensory cortex (SI) or a region including the second somatosensory cortex (SIIr) in ferrets, numerous juxtapositions are evident between CTT terminals and proximal parts of labeled VPL neurons. In the POm, both proximal and distal juxtapositions were evident, whereas in the VPI only distal juxtapositions are detected. Quantitative analysis demonstrated that a higher percentage of SIIr- than SI-projecting VPL neurons was overlapped by CTT terminals. These findings suggest that the spinocervicothalamic pathway foremost is involved in functions assigned to the second somatosensory cortex, e.g., tactile learning and sensorimotor integration.