The sensory trigeminal system in birds: Input, organization and effects ofperipheral damage. A review

The primary sensory trigeminal system in birds comprises the mesencephalic trigeminal nucleus and the brigeminal ganglion with projections to the prin cipal sensory nucleus (PrV) and the descending tract with its subnuclei. Ot her cranial nerves can contribute to PrV and the descending system that tog ether form the somatosensory system of the head. There is also a propriocep tive component. The somatosensory system comprises a component serving tact ile sense and a nociceptive component. The former processes information fro m many mechanoreceptors in beak and tongue; both PrV and subnuclei of the d escending system are involved. The nociceptive component consists of small ganglion cells projecting presumably to layers I and TI of the caudal subnu cleus of the descending trigeminal system and cervical dorsal horn; this is the only trigeminal region showing immunoreactivity for substance P. The e ffects of amputation of the tips of the beak of chickens (debeaking) are es timated by fiber counts in electron microscopic preparations of the trigemi nal branches innervating that area, and by cell counts in Nissl stained sec tions of the trigeminal ganglion. Our data indicate that debeaking causes a loss of exteroceptive units, but not of nociceptive units. Comparison of s ections stained for the presence of substance P (immunohistochemistry) did not reveal a long-term effect on the nociceptive system suggestive of the o ccurrence of chronic pain.