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

Abstract

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.

Structure, innervation and response properties of integumentary sensory organs in crocodilians

“…As would be expected, responses to stimulation of the pre-maxilla, maxilla and quadratojugal of the upper jaw were recorded from the anterior regions of the ganglion, in proximity to the entrance of the maxillary nerve into the ganglion, and areas responsive to stimulation of the dentary were recorded from the posterior regions, near the mandibular nerve's division from the ganglion. The electrophysiologically derived topography of the crocodilian trigeminal ganglion was consistent with maxillary representations in the maxillo-mandibular lobe as documented in horseradish peroxidase tracer studies from hatchling chicks (Noden, 1980). In both the Nile crocodiles and alligators, receptive fields, some as small as the area of a single ISO, were sensitive to indentation thresholds produced by the finest von Frey filaments corresponding to a force of 0.078mN.…”

A novel spalt gene expressed in branchial arches affects the ability of cranial neural crest cells to populate sensory ganglia

“…Normally, neural crest cells contribute both neurons and glia of the trigeminal ganglia, with crest-derived neurons distributed throughout the proximal portion of the ganglion and non-neural/glial cells found both in the proximal and distal portions (Noden, 1980). Consistent with this, in control transfected embryos, GFPlabeled neural crest cells were distributed throughout the trigeminal ganglion in a relatively uniform pattern (Fig.…”

Do vertebrate neural crest and cranial placodes have a common evolutionary origin?

“…Differences in size, location, gene expression and preferred sensory modalities between placodal and neural crest derived cells in the profundal/trigeminal ganglia have been reported, but it remains to be resolved whether these two cell populations are functionally distinct or largely equivalent. (38,78,79) Differentiation of neurons in both neural crest and in placodes appears to be under the control of particular basic helix-loop-helix (bHLH) transcription factors, which also govern neurogenesis in many parts of the central nervous system. Neurogenins (Ngn1 or Ngn2) are expressed in proliferating precursors of sensory neurons and act as neuronal determination genes, which activate neuronal differentiation genes such as NeuroD.…”