The vestibular nuclei and vestibuloreticular connections in the mallard (Anas platyrhynchos L.). An anterograde and retrograde tracing study.

The vestibular apparatus provides information about the position and movements of the head. Craniocervical muscles position the head with respect to the upper part of the neck. Motoneurons innervating these muscles are located in the supraspinal nucleus and ventral horn of the rostral cervical cord. Premotor neurons of craniocervical muscles have been found in the medial two-thirds of the medullary reticular formation: the ventromedial part of the parvocellular reticular formation and the gigantocellular reticular formation. In the present study, projections from vestibular nuclei upon craniocervical premotor neurons were investigated using anterograde and retrograde tracers. Vestibulospinal fibers run bilaterally in the medial vestibulospinal tract and ipsilaterally in the lateral vestibulospinal tract. Vestibuloreticular projections are mainly ipsilateral, and originate from the n. vestibularis lateralis pars ventralis and pars dorsalis, and from the n. vestibularis descendens. Terminal labeling is found in the border zone between the parvocellular and gigantocellular reticular formation. These projections show that in addition to direct bilateral vestibulo-craniocervical projections an indirect vestibular pathway to craniocervical motor nuclei exists. The direct pathway probably is the neural substrate for the vestibulocollic reflex, whereas the vestibular projection upon the reticular formation might influence head orientation during various kinds of activities, such as pecking, preening and so on.

Location of reticular premotor areas of a motor center innervating craniocervical muscles in the mallard (Anas platyrhynchos L.).

The supraspinal nucleus (SSp) in the mallard, which lies in the rostral spinal cord and caudal brainstem, is a motor nucleus that forms the rostral continuation of the ventral horn. It contains part of the motoneurons innervating the craniocervical muscles. Injections with horseradish peroxidase (HRP) and wheat germ agglutinin conjugated to HRP (WGA) in the SSp were used to localize the craniocervical premotor neurons in the medullary reticular formation. A mixture of WGA and HRP (WGA/HRP) or biotinylated dextran amine (BDA) were injected in the different reticular areas to test the results. Small numbers of craniocervical premotor neurons were found bilaterally in the ventromedial part of the parvocellular reticular formation (RPcvm) and in the caudal extension of RPcvm, the nucleus centralis dorsalis of the medulla oblongata, and the gigantocellular reticular formation (RGc). In a second series of experiments, WGA/HRP and BDA injections in these reticular areas were used to visualize afferent fibers and terminals in the SSp. The combination of the two types of experiments shows that RPcvm and RGc contain modest numbers of craniocervical premotor neurons. Because the reticular formation also contains jaw and tongue premotor neurons and receives a variety of sensory projections, the present results suggest that the medullary reticular formation plays a role in the coordination of complex movements (e.g., feeding). The pattern of afferent and efferent connections of the reticular formation is used to redefine its subdivisions in the myelencephalon of the mallard.

A crossed projection from the optic tectum to craniocervical premotor areas in the brainstem reticular formation. An anterograde and retrograde tracing study in the mallard (Anas platyrhynchos L.).

The optic tectum in birds receives visual information from the contralateral retina. This information is passed through to other brain areas via the deep layers of the optic tectum. In the present study the crossed tectobulbar pathway is described in detail. This pathway forms the connection between the optic tectum and the premotor area of craniocervical muscles in the contralateral paramedian reticular formation. It originates predominantly from neurons in the ventromedial part of stratum griseum centrale and to a lesser extent from stratum album centrale. The fibers leave the tectum as a horizontal fiber bundle, and cross the midline through the caudal radix oculomotorius and rostral nucleus oculomotorius. On the contralateral side fibers turn to ventral and descend caudally in the contralateral paramedian reticular formation to the level of the obex. Labeled terminals are found in the ipsilateral medial mesencephalic reticular formation lateral to the radix and motor nucleus of the oculomotor nerve, and in the contralateral paramedian reticular formation, along the descending tract. Neurons in the medial mesencephalic reticular formation in turn project to the paramedian reticular formation. Through the crossed tectobulbar pathway visual information can influence the activity of craniocervical muscles via reticular premotor neurons.

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

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.

Do craniocervical and jaw motor nuclei receive input from the same population of reticular premotor neurons? A double labeling tracing study in the mallard (Anas platyrhynchos).

As part of a study concerning the organization of premotor areas in the medullary reticular formation in birds we used a fluorescent retrograde double labeling technique to localize the premotor neurons of the trigeminal (mV) and supraspinal motor nucleus (SSp). Diamidino Yellow injections in mV and Fast Blue injections in SSp demonstrated that mV and SSp do not share premotor neurons, but the premotor neurons form a mixed population in the ventromedial part of the parvocellular reticular formation.

Location of premotor neurons of the motor nuclei innervating craniocervical muscles in the mallard (Anas platyrhynchos L.).

Reticular premotor neurons of craniocervical muscles in the duck were localized with the retrograde tracer HRP and the anterograde tracer WGA-HRP. In the reticular formation neck premotor neurons were found in the gigantocellular reticular nucleus and in the ventromedial part of the parvocellular reticular nucleus rostral to the obex, and caudal to the obex in the nucleus centralis ventralis of the medulla. Results were compared with premotor areas of jaw muscles. The ventromedial part of the parvocellular reticular formation contains neck as well as jaw premotor neurons. This area may serve as the neural substrate for the coordination of neck and jaw movements.