FOUR ASPECTS OF TRIGEMINAL NUCLEUS AND A PARADOX

Abstract

The trigeminal nucleus and tract of cats was studied with microelectrode recording. Small-diameter peripheral fibers fail to penetrate long distances caudally in the descending tract so that cells in the rostral part of the nucleus respond as though all types of peripheral fiber ended on them, while caudal cells respond as though innervated mainly by large fibers. The nucleus is divided into four zones: principalis, oralis, interpolaris, and caudalis. Principalis contains cells with large receptive fields responding to a wide pressure range. Oralis is dominated by afferents from within the head rather than face skin. Interpolaris, lying lateral to n. cuneatus, contains cells with small receptive fields responding only to light pressure. Caudalis contains cells with wide receptive fields and responds to light pressure. One quarter of all cells responded to cooling, only cells innervated from the nares were found to be heat-sensitive. All temperature-responding cells also responded to pressure stimuli of their receptive fields. At no time were any cells encountered in the primary nuclei which responded only to intense pressure. No cells in the caudal part of the nucleus signalled a wide range of pressure stimuli. This failure to discover pain cells conflicts with the observation that facial analgesia follows section of the descending fibers. Solutions to the paradox are suggested, including the theory that pain reactions are set off by massive spatial and temporal summation of impulses from the primary nuclei.