Central connections of the nucleus mesencephalicus nervi trigemini in the mallard (Anas platyrhynchosL.).

Abstract

Background In the mallard duck, functionally distinct groups of jaw muscles are each innervated by a different subnucleus of the main trigeminal (mV) or facial (mVII) motor nucleus. The other subnuclei of mV and mVII innervate several head muscles, including lingual muscles. The reticular premotor cells of the trigeminal and facial jaw motor subnuclei occupy different areas in the parvocellular reticular formation (RPc). The cell bodies of jaw muscle spindle afferents are situated in the mesencephalic nucleus (MesV). In the present study, the central connections of MesV with jaw motor subnuclei and their premotor areas are investigated. Methods In a first series of experiments, horseradish peroxidase (HRP) injections were made in electrophysiologically identified trigeminal and facial subnuclei. In a second series of experiments, HRP was delivered iontophoretically at different parts of RPc. Anterograde tracing with tritiated leucine was used to confirm the central connections of MesV. Double labeling with fluorescent tracers was used to investigate whether MesV collaterals reach both the rostral and caudal parts of RPc. Results MesV projects to only two of the five different subnuclei of the trigeminal motor nucleus. The subnuclei that receive spindle afferents innervate jaw adductor muscles (mV2) or pro‐ and retractors of the mandible (pterygoid muscles; mV1). The three other subnuclei innervate jaw‐opener muscles or other head muscles. MesV fibers also project to the rostral part of the dorsolateral RPc (RPcdl), which serves as a premotor area for the motor subnuclei of adductor and pterygoid muscles. The intermediate part of RPcdl does not contain premotor cells of mV or mVII, and a clear projection of MesV to this area is absent. The caudal part of RPcdl projects to the mV and mVII subnuclei that innervate jaw‐opener muscles. This part of RPc receives a projection from the same MesV cells as the rostral RPcdl. The MesV projection to RPc does not include premotor cells of mV and mVII in the ventromedial part of RPc (RPcvm). Conclusions Spindle afferents from jaw‐closer muscles project only to mV subnuclei innervating jaw‐closer muscles (mV1, mV2) and to a population of premotor cells in the rostral RPcdl that innervates these subnuclei. The mixed population of premotor cells in RPcvm, which innervates both jaw‐opener and jaw‐closer subnuclei, does not receive a MesV projection. However, a premotor area for jaw‐opener subnuclei in the caudal part of RPcdl does receive MesV input and may serve as a relay through which proprioceptive information from jaw closer spindles can reach jaw opener muscles. Anat. Rec. 248:554‐565, 1997. © 1997 Wiley‐Liss, Inc.

Trigeminal and telencephalic projections to jaw and other upper vocal tract premotor neurons in songbirds: Sensorimotor circuitry for beak movements during singing

“…Thus, though examined in different cases, the terminal field resulting from injections in the lateral arcopallium and that resulting from injections in the trigeminal ganglion appeared to overlap in RPcvm. However, because ganglionic injections result in uptake of tracer by both cell bodies and fibres, the terminal field in RPcvm could in principle have resulted from the labeling MesV (proprioceptive) neurons that innervate jaw closer muscles (Arends and Dubbeldam, 1982; Bout et al, 1997). These neurons were invariably labeled by ganglionic injections, but in ducks they are known to project upon RPcdl and not upon RPcvm (Bout et al, 1997).…”

“…However, because ganglionic injections result in uptake of tracer by both cell bodies and fibres, the terminal field in RPcvm could in principle have resulted from the labeling MesV (proprioceptive) neurons that innervate jaw closer muscles (Arends and Dubbeldam, 1982; Bout et al, 1997). These neurons were invariably labeled by ganglionic injections, but in ducks they are known to project upon RPcdl and not upon RPcvm (Bout et al, 1997). In any case, because injections in RPcvm did not retrogradely label MesV neurons, but did retrogradely label ganglionic neurons (see above), it can be deduced that the source of the terminal field in RPcvm following ganglionic injections is, in fact, ganglionic neurons.…”