Identification of gamma-aminobutyric acid-immunoreactive axon endings associated with mesencephalic periodontal afferent terminals and morphometry of the two types of terminals in the cat supratrigeminal nucleus

Projection and synaptic connectivity of trigeminal mesencephalic nucleus neurons controlling jaw reflexes

Neurons in the trigeminal mesencephalic nucleus (Vmes) receive deep sensation (proprioception) from jaw-closing muscle spindles and periodontal ligaments and project primarily to the jaw-closing motoneuron pool (jaw-closing nucleus) of the trigeminal motor nucleus and to the supratrigeminal nucleus. Numerous articles have described the morphology and physiology of the central projections of Vmes afferents originating from the muscle spindles and periodontal ligaments. However, no report has provided a detailed description of projection and synaptic connectivity, especially of single afferents, and their functional implications. In this review, we reanalyze data obtained by single intra-axonal recording and labeling of functionally identified Vmes muscle spindle afferents and periodontal ligament afferents and by electron microscopic observation of their projection features and synaptic organization of boutons, to compare the data for the jaw-closing nucleus and supratrigeminal nucleus. Our analysis shows that each Vmes afferent type has characteristic projection pattern and synaptic feature that may be important in jaw-reflex control.

Prenatal development of NMDA receptor composition and function in trigeminal neurons

The prenatal development of neural circuits for rhythmical oral-motor behaviors used for feeding is essential for the survival of the newborn mammal. The N-methyl-D-aspartate (NMDA) receptor plays a critical role in brainstem circuits underlying postnatal oral-motor behaviors. To understand a role for the NMDA receptor in the emergence of sucking behavior we conducted physiological and immunohistochemical experiments using fetal rats. Physiology experiments examined the development of the NMDA dose response of the brainstem circuit responsible for generating rhythmical trigeminal activity by recording trigeminal motor outputs using an in vitro preparation. The high dose of NMDA agonist bath application affected the mean cycle duration of rhythmical trigeminal activity (RTA) at both embryonic day (E) 18-19 and E20-21 in comparison with standard concentration of NMDA agonist. NMDA receptor immunohistochemistry studies, using antibodies directed against subunits NR1, NR2A, NR2B, NR3A and NR3B were performed to determine the prenatal regulation of NMDA subunits in trigeminal motoneurons (Mo5), and mesencephalic trigeminal neurons (Me5) between E17 to E20. In Mo5, NR1, NR2A, NR2B and NR3A immunoreactivity was observed throughout the time frame sampled. NR3B immunoreactivity was not observed in Mo5 or Me5. In Mo5, there was a significant decrease in the percentage of NR2B immunoreactive neurons between E17 and E20, and a concurrent increase in the NR2A/NR2B ratio between E17 and E20. In Me5, NR1, NR2A and NR3A immunoreactivity was observed throughout the time frame sampled; a significant decrease in the percentage of NR2A immunoreactive neurons between E17 and E20, and NR3A immunoreactive neurons between E17 and E18 occurred. The timing of subunit changes between E17 and E18 is coincident with the prenatal emergence of rhythmical jaw movements, and in vitro rhythmical trigeminal activity, shown in earlier studies. Our data suggest that NMDA receptor plays an important role in the development and function of prenatal oral-motor circuits.

Neurobiology of orofacial proprioception

Primary sensory fibers innervating the head region derive from neurons of both the trigeminal ganglion (TG) and mesencephalic trigeminal nucleus (MTN). The trigeminal primary proprioceptors have their cell bodies in the MTN. Unlike the TG cells, MTN neuronal somata are centrally located within the brainstem and receive synaptic inputs that potentially modify their output. They are a crucial component of the neural circuitry responsible for the generation and control of oromotor activities. Gaining an insight into the chemical neuroanatomy of the MTN is, therefore, of fundamental importance for the understanding of neurobiology of the head proprioceptive system. This paper summarizes the recent advances in our knowledge of pre- and postsynaptic mechanisms related to orofacial proprioceptive signaling in mammals. It first briefly describes the neuroanatomy of the MTN, which is involved in the processing of proprioceptive information from the face and oral cavity, and then focuses on its neurochemistry. In order to solve the puzzle of the chemical coding of the mammalian MTN, we review the expression of classical neurotransmitters and their receptors in mesencephalic trigeminal neurons. Furthermore, we discuss the relationship of neuropeptides and their corresponding receptors in relaying of masticatory proprioception and also refer to the interactions with other atypical neuromessengers and neurotrophic factors. In extension of previous inferences, we provide conclusive evidence that the levels of transmitters vary according to the environmental conditions thus implying the neuroplasticity of mesencephalic trigeminal neurons. Finally, we have also tried to give an integrated functional account of the MTN neurochemical profiles.

The synaptic microcircuitry associated with primary afferent terminals in the interpolaris and caudalis of trigeminal sensory nuclear complex

Previous ultrastructural studies indicating a higher number of axoaxonic contacts on individual low-threshold mechanoreceptive afferents in the principalis (Vp) than in the oralis (Vo) of cat trigeminal sensory nuclear complex (TSNC) suggest that the synaptic microcircuitry associated with primary afferents manifests unique differences across the sensory nuclei of TSNC. To address this issue, we analyzed synaptic microcircuits associated with fast adapting vibrissa afferent terminals in the interpolaris (Vi) and caudalis (Vc, laminae III/IV) by using intraaxonal injections of horseradish peroxidase (HRP) in cats. Forty-two and 65 HRP-labeled boutons were analyzed in the Vi and Vc, respectively. The labeled boutons contained clear, spherical vesicles. They most frequently formed asymmetric axodendritic synapses and were commonly postsynaptic to unlabeled axon terminals containing pleomorphic vesicles (p-endings) with symmetric junctions. The examination of synaptic contacts over the entire surface of individual boutons indicated that the afferent boutons made contacts with an average of two postsynaptic targets in the Vi and Vc. In contrast, axoaxonic contacts, and labeled boutons participating in synaptic triads, where p-endings contacted both the boutons and their postsynaptic targets, were, on average, higher in the Vi than in the Vc. These results suggest that the output of sensory information conveyed through low-threshold mechanoreceptive afferents is more strongly controlled at the level of the first synapse by presynaptic and postsynaptic mechanisms in the Vi responsible for sensory discriminative functions than in the Vc for sensorimotor reflexive functions.

Ultrastructure of jaw muscle spindle afferents within the rat trigeminal mesencephalic nucleus

This study examined the ultrastructures of neuronal elements within trigeminal mesencephalic nucleus by labeling masseteric mesencephalic neurons and masseter motoneurons with injection of horseradish peroxidase into masseteric muscle. Of eight horseradish peroxidase-labeled muscle spindle afferents examined, four terminals showed synaptic contact with labeled dendrites of masseteric motoneurons, two with labeled somata, and the remaining two with unlabeled dendrites. A few of the labeled dendrites showed intimate contact with the somata of the trigeminal mesencephalic nucleus neurons. These results provide morphological evidence of synaptic contact of recurring masseteric muscle spindle afferents with the trigeminal mesencephalic nucleus somata and also suggest the presence of electrical synapses between the somata of the trigeminal mesencephalic nucleus neurons and dendrites of jaw-closing motoneurons.

Neural mechanisms controlling jaw-jerk reflex in the cat

Signal substances of axon terminals presynaptic to jaw spindle Ia afferents and their ultrastructural features were examined using a combination of intra-axonal horseradish peroxidase injection and postembedding immunogold-labeling techniques in cats. A total of 35 axon terminals presynaptic to 22 horseradish peroxidase-labeled Ia boutons were examined. Of the 35 presynaptic axon terminals, 14 (40%) were immunoreactive for both gamma-aminobutyric acid and glycine, 9 (26%) for gamma-aminobutyric acid alone and 9 (26%) for glycine alone. The bouton volume, mitochondrial volume, active zone area, and apposed surface area were larger for Ia boutons than for presynaptic axon terminals, while each of the values is similar among the three types of presynaptic axon terminals. These results suggest that gamma-aminobutyric acid and glycine play an important role for modulating the jaw-jerk reflex presynaptically and that the smaller size of presynaptic axon terminals is important to prevent action potential generation from Ia afferents.

Synaptic Inputs to Trigeminal Primary Afferent Neurons Cause Firing and Modulate Intrinsic Oscillatory Activity

In this paper, we investigated the influence of synapses on the cell bodies of trigeminal muscle spindle afferents that lie in the trigeminal mesencephalic nucleus (NVmes), using intracellular recordings in brain stem slices of young rats. Three types of synaptic responses could be evoked by electrical stimulation of the adjacent supratrigeminal, motor, and main sensory nuclei and the intertrigeminal area: monophasic depolarizing postsynaptic potentials (PSPs), biphasic PSPs, and all or none action potentials without underlying excitatory PSPs (EPSPs). Many PSPs and spikes were abolished by bath-application of 6,7-dinitroquinoxaline (DNQX) alone or combined with d,l-2-amino-5-phosphonovaleric acid (APV), suggesting that they are mediated by non– N-methyl-d-aspartate (NMDA) and NMDA glutamatergic receptors, while some action potentials were sensitive to bicuculline, indicating involvement of GABAA receptors. A number of cells showed spontaneous membrane potential oscillations, and stimulation of synaptic inputs increased the amplitude of the oscillations for several cycles, which often triggered repetitive firing. Furthermore, the oscillatory rhythm was reset by the stimulation. Our results show that synaptic inputs to muscle primary afferent neurons in NVmes from neighboring areas are mainly excitatory and that they cause firing. In addition, the inputs synchronize intrinsic oscillations, which may lead to sustained, synchronous firing in a subpopulation of afferents. This may be of importance during rapid biting and during the mastication of very hard or tough foods.

Synaptic organization of monosynaptic connections from mesencephalic trigeminal nucleus neurons to hypoglossal motoneurons in the rat

Synaptological characteristics of synapses between axonal boutons of the trigeminal mesencephalic nucleus (Vme) neurons and the hypoglossal nucleus (XII) motoneurons (MNs) were studied using biotinylated dextran amine (BDA) anterograde labeling combined with horseradish peroxidase (HRP) retrograde transport in the rat. BDA was initially iontophoresed into Vme unilaterally and 7 days later HRP was injected into the anterior two-thirds of the ipsilateral tongue. After histochemical reactions, BDA anterogradely labeled boutons were seen to appose closely to somata and dendrites of HRP retrogradely labeled MNs in XII by light microscopy. A total of 212 BDA-labeled Vme boutons were examined ultrastructurally, which had an average diameter of 1.3 +/- 0.4 microm and contain small clear spherical vesicles. Eighty-eight percent of Vme boutons (187/212) synapsed on dendrites of HRP-labeled XII MNs. Twenty-five Vme boutons (25/212, 12%) made synapses with somata of HRP-labeled XII MNs. Thirty-five percent (74/212) of BDA-labeled Vme boutons were also contacted by unlabeled P-type terminals. Presynaptic P-type terminals contained spherical (47%, 35/74), pleomorphic (43%, 32/74), and flattened (10%, 7/74) synaptic vesicles. Thus, P-type terminals (as a presynaptic element), BDA-labeled Vme boutons, and XII MNs constitute axoaxodendritic and axoaxosomatic synaptic triads. There are four types of synaptic microcircuits in XII neuropil: synaptic convergence, synaptic divergence, presynaptic inhibition synaptic circuits, and feedforward regulation circuits. This detailed ultrastructure examination of the synaptic organization between Vme neurons and XII MNs provides insights into the synaptic mechanisms of the trigeminal proprioceptive afferents involved in the jaw-tongue reflex and coordination during oral motor behaviors.

Synaptic organization of tooth pulp afferent terminals in the rat trigeminal sensory nuclei

Previous studies provide evidence that a structure/function correlation exists in the distinct zones of the trigeminal sensory nuclei. To evaluate this relationship, we examined the ultrastructure of afferent terminals from the tooth pulp in the rat trigeminal sensory nuclei: the principalis (Vp), the dorsomedial part of oral nucleus (Vdm), and the superficial layers of caudalis (Vc), by using transganglionic transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). A total of 93 labeled boutons were serially sectioned, in which some sections were incubated with gamma-aminobutyric acid (GABA) antiserum. Almost all labeled boutons formed asymmetric contact with nonprimary dendrites, in which more than half of labeled boutons in the Vc made synapses with their spines. The labeled boutons could be divided into two types on the basis of numbers of dense-cored vesicles (DCVs) in a boutons: S-type and DCV-type. Almost all labeled boutons in the Vp and Vdm were S-type, whereas two types were distributed evenly in the Vc. In contrast to DCV-type boutons, the S-type was frequently postsynaptic to unlabeled axon terminals containing a mixture of round, oval, and flattened vesicles (p-endings) and forming symmetrical synapses. Most p-endings examined were immunoreactive to GABA. The frequency of axoaxonic contacts was higher for labeled boutons in the Vp than in the Vdm and Vc. These results suggest that the three structures of trigeminal sensory nuclei serve distinct functions in nociceptive processing.

Synaptic relationships between hair follicle afferents and neurones expressing GABA and glycine-like immunoreactivity in the spinal cord of the rat

gamma-Aminobutyric acid (GABA) and glycine have been implicated in the inhibition of sensory pathways in the dorsal horn of the spinal cord. The object of this study is to investigate the interactions between neurones immunoreactive for GABA and/or glycine and hair follicle afferent terminals labelled by intracellular injection with neurobiotin. GABA and glycine-like immunoreactivity in axons and dendrites in synaptic contact with the afferent terminals was demonstrated by using a postembedding immunogold method, and serial section reconstruction was used to show the distribution and nature of these interactions in lamina III of the dorsal horn. Most afferent boutons (94%) were postsynaptic at axo-axonic synapses: 67% of presynaptic boutons presynaptic to the afferent terminals were immunoreactive for GABA and glycine, 24% for GABA alone, and 7% for glycine alone. Only a small percentage of dendrites postsynaptic to afferent boutons appeared to belong to inhibitory interneurones: 3% were immunoreactive for GABA and glycine, 10% for glycine alone, but 87% were immunoreactive for neither antibody. Many afferent boutons were the central terminals of what appeared to be type IIb glomeruli and were involved triadic synaptic arrangements at which boutons presynaptic to an afferent terminal also made axodendritic contacts with dendrites postsynaptic to the afferent. Many of the presynaptic boutons involved in the triads were immunoreactive for GABA and glycine. Because afferent terminals do not themselves express glycine receptors (Mitchell et al. [1993] J. Neurosci. 13:2371-2381), glycine may therefore act on dendrites postsynaptic to hair follicle afferent terminals at these triads.

Quantitative ultrastructural analysis of glycine- and gamma-aminobutyric acid-immunoreactive terminals on trigeminal alpha- and gamma-motoneuron somata in the rat

Detailed knowledge of the inhibitory input to trigeminal motoneurons is needed to understand better the central mechanisms of jaw movements. Here a quantitative analysis of terminals contacting somata of jaw-closing (JC) and jaw-opening (JO) alpha-motoneurons, and of JC gamma-motoneurons, was performed by use of serial sectioning and postembedding immunogold cytochemistry. For each type of motoneuron, the synaptic boutons were classified into four groups, i.e., immunonegative boutons or boutons immunoreactive to glycine only, to gamma-aminobutyric acid (GABA) only, or to both glycine and GABA. The density of immunolabeled boutons was much higher for the alpha- than for the gamma-motoneurons. In the alpha-motoneuron populations, the immunolabeled boutons were subdivided into one large group of boutons containing glycine-like immunoreactivity only, one group of intermediate size harboring both glycine- and GABA-like immunoreactivity, and a small group of boutons containing GABA-like immunoreactivity only. The percentage of immunolabeled boutons was higher for JC than JO alpha-motoneurons, the most pronounced difference being observed for glycine-like immunoreactivity. In contrast, on the somatic membrane of gamma-motoneurons, the three types of immunoreactive bouton occurred at similar frequencies. These results indicate that trigeminal motoneurons are strongly and differentially controlled by premotoneurons containing glycine and/or GABA and suggest that these neurons play an important role for the generation of masticatory patterns.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Quantitative ultrastructure of slowly adapting lingual afferent terminals in the principal and oral nuclei in the cat

Previous studies provide evidence that a structure/function correlation exists in the cytoarchitectonically different zones of the trigeminal sensory nuclei. To extend this relationship, we examined the ultrastructural features of trigeminal primary afferent neurons in the cat dorsal principal nucleus (Vpd) and the rostrodorsomedial oral nucleus (Vo.r) using intra-axonal labeling with horseradish peroxidase and morphometric analyses. All labeled boutons contained round synaptic vesicles. Eighty-two percent of the boutons in the Vo.r and 99% of the boutons in the Vpd were presynaptic to nonprimary dendrites. The remaining boutons in the Vo.r were presynaptic to somata (8%) or primary dendrites (10%). The average number of postsynaptic profiles per labeled bouton did not differ in the Vpd and Vo.r. Most labeled boutons in the two nuclei were postsynaptic to unlabeled axon terminals with pleomorphic vesicles (p-ending). The number of p-endings per labeled bouton was higher in the Vpd than Vo.r A morphometric analysis indicated that labeled bouton volume and apposed surface area were larger in the Vpd than Vo.r while active zone area and vesicle number did not differ. All these parameters were larger than those of p-endings in each nucleus. In both labeled boutons and p-endings, the parameters were positively correlated with bouton size. These results suggest that sensory information conveyed through trigeminal afferents is more strongly controlled at the level of the first synapse by presynaptic mechanisms in the Vpd than in the Vo.r, while the efficacy of transmission at primary afferent synapses does not differ.

GABA and glycine-like immunoreactivity at axoaxonic synapses on 1a muscle afferent terminals in the spinal cord of the rat

The object of this study was to analyze the synaptic interactions of identified muscle spindle afferent axon terminals in the spinal cord of the rat. Group 1a muscle afferents supplying the gastrocnemius muscle were impaled with microelectrodes in the dorsal white matter of the spinal cord and stained by intracellular injection with Neurobiotin. Postembedding immunogold techniques were used to reveal GABA- and glycine-like immunoreactivity in boutons presynaptic to afferent terminals in the ventral horn and the deep layers of the dorsal horn. Serial-section reconstruction was used to reveal the distribution of synaptic contacts of different types on the afferent terminals. The majority of afferent boutons received axoaxonic and made axodendritic or axosomatic synaptic contacts. In the ventral horn, 91% of boutons presynaptic to the afferent terminals were immunoreactive for GABA alone and 9% were immunoreactive for both GABA and glycine. The mean number of axo-axonic contacts received per terminal was 2.7, and the mean number of synaptic contacts at which the terminal was the presynaptic element was 1.4. In the deep layers of the dorsal horn, 58% of boutons presynaptic to afferent terminals were immunoreactive for GABA alone, 31% were immunoreactive for GABA and glycine, and 11% for glycine alone. The mean number of axoaxonic contacts received per afferent terminal in this region was 1.6 and the mean number of synaptic contacts at which the terminal was the presynaptic element was 0.86. This clearly establishes the principle that activity in 1a afferents is modulated by several neurochemically distinct populations of presynaptic neuron.

Role of parabrachial nucleus in submandibular salivary secretion induced by bitter taste stimulation in rats

When rats lick a bitter taste solution such as quinine-hydrochloride, they secrete profuse amounts of saliva. The salivation has a higher flow rate than that induced by other qualities of taste stimulation: sweet, salty, and sour. The present study is aimed to clarify the neural mechanism of the quinine-evoked salivation by means of behavioral, neuroanatomical, and electrophysiological experiments. Behaviorally, submandibular salivary secretion and rejection behavior (gaping) were observed in normal rats, as well as in rats chronically decerebrated at the precollicular level. In chronically decerebrate rats, these quinine-evoked reactions were strongly suppressed by destruction of the medial part of the parabrachial nucleus, including the so-called taste area, and ventral part of the parabrachial nucleus, including the pontine reticular formation. Neuroanatomical study using a retrograde tracer, Fluoro-gold, revealed that the neurons sending their axons to the superior salivatory nucleus, parasympathetic secretory center, were located mainly in the pontine reticular formation ventral to the parabrachial nucleus, not in the parabrachial taste area. Extracellular neural activity was recorded from the parabrachial region in decerebrate rats, and responsiveness to taste stimulation, jaw movements, and electrical stimulation of the superior salivatory nucleus was examined. Neurons responsive to both taste stimulation and antidromic stimulation of the superior salivatory nucleus were found in the pontine reticular formation ventral to the parabrachial nucleus, which responded well to quinine and HCl taste stimuli. Neurons in the parabrachial taste area could respond to four qualities of taste stimulation, but not to antidromic stimulation of the salivary center. These results suggest that aversive taste information from the parabrachial taste area reaches the salivary secretory center via the reticular formation ventral to the parabrachial nucleus.

Distribution pattern of inhibitory and excitatory synapses in the dendritic tree of single masseter alpha-motoneurons in the cat

Little is known about the differences in the distributions of inhibitory and excitatory synapses in the dendritic tree of single motoneurons in the brainstem and spinal cord. In this study, the distribution of gamma-aminobutyric acid (GABA)-, glycine-, and glutamate-like immunoreactivity in axon terminals on dendrites of cat masseter alpha-motoneurons, stained intracellularly with horseradish peroxidase, was examined by using postembedding immunogold histochemistry in serial ultrathin sections. The dendritic tree was divided into three segments: primary (Pd) and distal (Dd) dendrites and intermediate (Id) dendrites between the two segments. Quantitative analysis of 175, 279, and 105 boutons synapsing on 13 Pd, 54 Id, and 81 Dd, respectively, was performed. Fifty percent of the total number of studied boutons were immunopositive for GABA and/or glycine and 48% for glutamate. Among the former, 27% showed glycine immunoreactivity only and 14% were immunoreactive to both glycine and GABA. The remainder (9%) showed immunoreactivity for GABA only. As few as 3% of the boutons were immunonegative for the three amino acids. Most boutons immunoreactive to inhibitory amino acid(s) contained a mixture of spherical, oval, and flattened synaptic vesicles. Most boutons immunoreactive to excitatory amino acid contained clear, spherical, synaptic vesicles with a few dense-cored vesicles. When comparisons of the inhibitory and excitatory boutons were made between the three dendritic segments, the proportion of the inhibitory to the excitatory boutons was high in the Pd (60% vs. 37%) but somewhat low in the Id (46% vs. 52%) and Dd (44% vs. 53%). The percentage of synaptic covering and packing density of the inhibitory synaptic boutons decreased in the order Pd, Id, and Dd, but this trend was not applicable to the excitatory boutons. The present study provides possible evidence that the spatial distribution patterns of inhibitory and excitatory synapses are different in the dendritic tree of jaw-closing alpha-motoneurons.

Brainstem mechanisms underlying feeding behaviors

The essential elements controlling trigeminal motoneurons during feeding lie between the trigeminal and facial motor nuclei. These include populations of neurons in the medial reticular formation and pre-motoneurons in the lateral brainstem that reorganize to generate various patterns. Orofacial sensory feedback, antidromic firing in spindle afferents and intrinsic properties of motoneurons also contribute to the final masticatory motor output.