Somatotopic and functional organization of the avian trigeminal ganglion: an HRP analysis in the hatchling chick.

While the somatotopic organization of many central systems is well characterized, that of peripheral sensory neurons has not been adequately defined. This is especially true for the trigeminal ganglion. By applying HRP subcutaneously at each of 14 sites and also intramuscularly, it is possible to determine whether the location of sensory neurons within the ganglion reflects their peripheral projections. There is no discernible somatotopic organization of neurons in the ophthalmic lobe. However, the location of maxillary neurons in the maxillo-mandibular lobe is organized with the most posterior cells projecting to sites closest to the ganglion and with neurons located more anteriorly projecting to progressively more distant sites. There is a less well defined organization in the superior-inferior axis of the ganglion, and none along its proximal (root) to distal axis. Mandibular exteroceptive neurons are found primarily in the anterior region of the maxillo-mandibular lobe, while mandibular proprioceptive cells are located in the proximo-central part of this lobe. In most cases there is a considerable scattering of horseradish peroxidase (HRP)-filled neurons. Projections to contralateral ganglia, the trigeminal motor nucleus, and the trigeminal mesencephalic nucleus were also examined. Cytologically, the hatchling trigeminal consists of two interspersed types of neurons: large, lightly staining and smaller, darkly staining cells. Previous experiments have proved that these two cell types do not correspond to each of the embryonic precursors of trigeminal neurons, the neural crest and placodal cells. In this study HRP was found localized in both classes of neurons following injection at all sites, including jaw-closing muscles. This indicates that the dual cytology is not correlated with either distribution of peripheral fibers or exteroceptive vs. proprioceptive functions. The possibilities that the two types of neurons may have different central projections and/or may be related to visceral vs. somatic afferent functions are discussed.