The medulla oblongata caudal to the obex was explored with glass capillary microelectrodes filled with fast green dye in urethan-chloralose-anesthetized cats. Concurrently, the trigeminal integument was mechanically stimulated. Two classes of units, which were activated by innocuous mechanical stimulation of the ipsilateral trigeminal integument, were identified. Both of them showed a somatotopic organization. 2. Units maximally activated by tactile stimulation of a small receptive field in the ipsilateral trigeminal integument were located in pars magnocellularis of the trigeminal subnucleus caudalis. Receptive field falling within, the mandibular distribution were found in the dorsomedial part of pars magnocellularis, maxillary fields occurred next, and ophthalmic fields were represented ventrolaterally. The rostrum (the mouth and nose) was represented just behind the obex, and the receptive field represented gradually shifted to the periphery of the face along the rostrocaudal axis, finally proceeding to dermatomes of cervical segments. At each transverse plane, tactile sensation of the most rostral segment of the represented integument projected onto the marginal zone of pars magnocellularis immediately adjacent to substantia gelatinosa, while the most peripheral segment projected onto the core of quasi-semicircular pars magnocellularis. The result is in contrast with the scheme proposed by previous investigators. 3. Within the lateral part of subnucleus reticularis dorsalis medullae oblongatae, which is ventromedially contiguous with pars magnocellularis, trigeminal units homologous with lamina V units in the spinal dorsal horn were identified. Typically, these units had a graded response in the center of the receptive field, but responded only to strong mechanical stimuli applied to the periphery of the receptive field. In the center of the receptive field, tactile stimulation of a small restricted area was effective, and the tactile receptive field showed a somatotopic organization. The mandibular distribution was represented dorsomedially, the maxillary distribution next to that, and the ophthalmic distribution laterally. Furthermore, a rostrocaudal differentiation similar to that observed with tactile units in pars magnocellularis was found. Hence, a double somatotopic representation of tactile sensation within the caudal medulla oblongata was indicated.
