Experiments were carried out in chloralose-anesthetized cats to study the responses of neurons in the lateral thalamus to excitation of afferent fibres from the knee joint. 2. Single- and multi-unit recordings were made with tungsten electrodes in dorsoventral penetrations through the ventral posterior lateral nucleus (VPL) during electrical stimulation of the medial articular nerve (MAN) of the cat's knee joint at an intensity sufficient to excite slowly conducting unmyelinated fibers. The locations of the recording sites were verified by recovering electrolytic lesion sites in histological sections (Nissl and cytochrome oxidase staining). 3. The average earliest latency for excitation of thalamic responses was 19.1 +/- 8.5 (SD) ms (n = 50). The threshold for excitation of most thalamic units was found to correspond to peripheral joint afferent fibers of the A-delta group. 4. The majority of neurons responding to MAN stimulation were found to be dorsal or ventral to the low-threshold cutaneous hindlimb region of the lateral division of the VPL (stereotaxic coordinates: AP 9.0-11.5; ML 7.0-9.5). In the ventral periphery of the VPL, most neurons responding to MAN stimulation (11/14) were wide dynamic range (WDR) with a discrete cutaneous receptive field on the hindpaw digits. Six WDR neurons were found dorsal to the hindlimb VPL with a convergent receptive field on the hindlimb (but not hindpaw digits). No nociceptive-specific knee joint units were found. 5. Other neurons were found dorsal to the hindlimb VPL with large receptive fields often encompassing the whole contralateral leg, including skin and deep hindlimb structures, possibly in a region described as the dorsal portion of the posterior complex (POd). Some neurons were found with no receptive field. 6. This study provides the first observations on the responses of lateral thalamic neurons to stimulation of the MAN of the cat knee joint. These results demonstrate a central pathway conveying impulses from specific deep joint afferents of the MAN to the peripheral region of the VPL and overlying region known as the POd, regions implicated in the transmission of nociceptive information.
