Cranial nerves represent a notoriously complex province of the neuroanatomical landscape of the vertebrates. Here, we offer a selection of the anatomic, genetic, and developmental features of their efferent component that are often misrepresented, ignored or controversial, as a complement to more exhaustive treatments of the subject. Our description reveals that efferent (or "motor") neurons in vertebrates represent a vague anatomic category (such as that of interneurons) rather than a true neuron type; That motor neurons fall into three bona fide types, segregated on the rostro-caudal axis of the central nervous system; That each of the three types is highly related to a type of preganglionic autonomic neuron; and that this genetic and topographical arrangement of three motor/preganglionic types correlates, not perfectly yet remarkably, with three broad physiological functions.

The first phase of feeding consists in the procurement of solid foods from the environment by biting, and their preparation for swallowing by chewing. These actions require the precise coordination of tens of orofacial muscles for the jaw and tongue. The seat for this motor patterning is known to reside in the reticular formation, a complex and poorly mapped region of the hindbrain, but the neuron groups involved are still elusive. Here, we characterize a group of excitatory reticular interneurons located in the supratrigeminal area that express the homeodomain transcription factor Phox2b. This nucleus-Sup5Phox2b-is premotor to both jaw-closing and jaw-opening motoneurons and receives direct input from cranial sensory afferents, motor cortex, and satiation related nuclei. Its activity differentially tracks lapping, biting, and chewing movements, suggesting its involvement in the elaboration of distinct orofacial motor patterns in vivo. Acute global activation or inhibition of Sup5Phox2b by optogenetics interrupt volitional feeding sequences. Thus, Sup5Phox2b is an obligatory subcortical node, topologically and genetically defined, in the neural circuits that control the oral phase of feeding in mice.

The first phase of feeding consists in acquiring solid foods from the environment by biting, and their preparation for swallowing by chewing. These actions require the precise coordination of tens of orofacial muscles for the jaw and tongue. The siege for this motor patterning is known to be in the reticular formation, a complex and poorly mapped region of the hindbrain, but the neuron groups involved are still elusive. Here, we characterize a group of reticular interneurons located in the supratrigeminal area that express the homeodomain transcription factor Phox2b . This nucleus — Sup5 Phox2b — is premotor to both jaw-closing and jaw-opener motoneurons and receives direct input from cranial sensory afferents, motor cortex and satiation related nuclei. Its activity differentially tracks lapping, biting and chewing movements, suggesting its involvement in the elaboration of distinct orofacial motor patterns in vivo. Acute global activation or inhibition of Sup5 Phox2b by optogenetics both interrupt volitional feeding sequences. Thus, Sup5 Phox2b is an obligatory subcortical node, topologically and genetically defined, in the neural circuits that control the oral phase of feeding. Teaser A genetically defined cluster of neurons in the hindbrain is an essential relay for biting and chewing food. ### Competing Interest Statement The authors have declared no competing interest.

In the rat, the activity of laryngeal adductor muscles, the crural diaphragm, and sympathetic vasomotor neurons is entrained to the postinspiratory (post-I) phase of the respiratory cycle, a mechanism thought to enhance cardiorespiratory efficiency. The identity of the central neurons responsible for transmitting respiratory activity to these outputs remains unresolved. Here we explore the contribution of the Kölliker-Fuse/parabrachial nuclei (KF-PBN) in the generation of post-I activity in vagal and sympathetic outputs under steady-state conditions and during acute hypoxemia, a condition that potently recruits post-I activity. In artificially ventilated, vagotomized, and urethane-anesthetized rats, bilateral KF-PBN inhibition by microinjection of the GABAA receptor agonist isoguvacine evoked stereotypical responses on respiratory pattern, characterized by a reduction in phrenic nerve burst amplitude, a modest lengthening of inspiratory time, and an increase in breath-to-breath variability, while post-I vagal nerve activity was abolished and post-I sympathetic nerve activity diminished. During acute hypoxemia, KF-PBN inhibition attenuated tachypneic responses and completely abolished post-I vagal activity while preserving respiratory-sympathetic coupling. Furthermore, KF-PBN inhibition disrupted the decline in respiratory frequency that normally follows resumption of oxygenation. These findings suggest that the KF-PBN is a critical hub for the distribution of post-I activities to vagal and sympathetic outputs and is an important contributor to the dynamic adjustments to respiratory patterns that occur in response to acute hypoxia. Although KF-PBN appears essential for post-I vagal activity, it only partially contributes to post-I sympathetic nerve activity, suggesting the contribution of multiple neural pathways to respiratory-sympathetic coupling.NEW & NOTEWORTHY Inhibition of neurons in the pontine Kölliker-Fuse/parabrachial complex (KF-PBN) differentially inhibited postinspiratory (post-I) activity in vagal and sympathetic outputs. The strong recruitment of post-I vagal activity that occurs in response to hypoxemia is selectively abolished by KF-PBN inhibition. This suggests that 1) post-I activity in vagal and sympathetic outputs may be generated by partially independent mechanisms and 2) neurons in the KF-PBN are a preeminent source of drive for the generation of eupneic post-I activity.

The pelvic organs (bladder, rectum and sex organs) have been represented for a century as receiving autonomic innervation from two pathways — lumbar sympathetic and sacral parasympathetic — by way of a shared relay, the pelvic ganglion, conceived as an assemblage of sympathetic and parasympathetic neurons. Using single cell RNA sequencing, we find that the mouse pelvic ganglion is made of four classes of neurons, distinct from both sympathetic and parasympathetic ones, albeit with a kinship to the former, but not the latter, through a complex genetic signature. We also show that spinal lumbar preganglionic neurons synapse in the pelvic ganglion onto equal numbers of noradrenergic and cholinergic cells, both of which therefore serve as sympathetic relays. Thus, the pelvic viscera receive no innervation from parasympathetic or typical sympathetic neurons, but instead from a divergent tail end of the sympathetic chains, in charge of its idiosyncratic functions.

Identification d’un mécanisme moléculaire et cellulaire contrôlant les apnées obstructives chez la souris. Nous avons caractérisé les neurones inhibiteurs du tronc cérébral exprimant le facteur de transcription Mafa (1) anatomiquement à l’aide de tracages viraux monosynaptiques (2) fonctionnellement en induisant par génetique intersectionnelle l’expression de récepteurs DREADD dans ces neurones. Le rôle de ce facteur de transcription dans ces neurones sur la fonction respiratoire des souris été étudiée par plethysmographie chez des mutants knock-out et knock-in d’une mutation 4A qui empêche la phosphorylation de ce facteur de transcription par GSK3. Nous avons montré que l’activation des neurones inhibiteurs exprimant le facteur de transcription Mafa suffit à induire de nombreuses apnées. Certains de ces neurones inhibiteurs, situés dans le tronc cérébral postérieur sont des neurones pré-moteurs du muscle géniohyoide qui contribue à ouvrir les voies aériennes supérieures. Nous avons identifié Gad2 comme étant un gène cible direct de Mafa, dont l’activation et la stabilité dépendent de sa phosphorylation par GSK3. In vivo, lorsque cette phosphorylation est abolie par la mutation 4A, les souriceaux nouveaux nés mutants présentent des apnées obstructives de plus en plus nombreuses au cours de la première journée de vie. Un antagoniste du GABA peut compenser transitoirement ce phénotype apnéique. La phosphorylation de Mafa adapte l’intensité inhibitrice sur les motoneurones des voies aériennes supérieures qui, lorsqu’elle est augmentée, induit des apnées obstructives.

Human speech can be divided into short, rhythmically timed elements, similar to syllables within words. Even our cries and laughs, as well as the vocalizations of other species, are periodic. However, the cellular and molecular mechanisms underlying the tempo of mammalian vocalizations remain unknown. Furthermore, even the core cells that produce vocalizations remain ill-defined. Here, we describe rhythmically timed neonatal mouse vocalizations that occur within single breaths and identify a brainstem node that is necessary for and sufficient to structure these cries, which we name the intermediate reticular oscillator (iRO). We show that the iRO acts autonomously and sends direct inputs to key muscles and the respiratory rhythm generator in order to coordinate neonatal vocalizations with breathing, as well as paces and patterns these cries. These results reveal that a novel mammalian brainstem oscillator embedded within the conserved breathing circuitry plays a central role in the production of neonatal vocalizations.

The ability to discriminate competing external stimuli and initiate contextually appropriate behaviours is a key brain function. Neurons in the deep superior colliculus (dSC) integrate multisensory inputs and activate descending projections to premotor pathways responsible for orienting, attention and defence, behaviours which involve adjustments to respiratory and cardiovascular parameters. However, the neural pathways that subserve the physiological components of orienting are poorly understood. We report that orienting responses to optogenetic dSC stimulation are accompanied by short-latency autonomic, respiratory and electroencephalographic effects in awake rats, closely mimicking those evoked by naturalistic alerting stimuli. Physiological responses were not accompanied by detectable aversion or fear, and persisted under urethane anaesthesia, indicating independence from emotional stress. Anterograde and trans-synaptic viral tracing identified a monosynaptic pathway that links the dSC to spinally projecting neurons in the medullary gigantocellular reticular nucleus (GiA), a key hub for the coordination of orienting and locomotor behaviours. In urethane-anaesthetized animals, sympathoexcitatory and cardiovascular, but not respiratory, responses to dSC stimulation were replicated by optogenetic stimulation of the dSC-GiA terminals, suggesting a likely role for this pathway in mediating the autonomic components of dSC-mediated responses. Similarly, extracellular recordings from putative GiA sympathetic premotor neurons confirmed short-latency excitatory inputs from the dSC. This pathway represents a likely substrate for autonomic components of orienting responses that are mediated by dSC neurons and suggests a mechanism through which physiological and motor components of orienting behaviours may be integrated without the involvement of higher centres that mediate affective components of defensive responses.

While apneas are associated with multiple pathological and fatal conditions, the underlying molecular mechanisms remain elusive. We report that a mutated form of the transcription factor Mafa (Mafa4A) that prevents phosphorylation of the Mafa protein leads to an abnormally high incidence of breath holding apneas and death in newborn Mafa4A/4A mutant mice. This apneic breathing is phenocopied by restricting the mutation to central GABAergic inhibitory neurons and by activation of inhibitory Mafa neurons while reversed by inhibiting GABAergic transmission centrally. We find that Mafa activates the Gad2 promoter in vitro and that this activation is enhanced by the mutation that likely results in increased inhibitory drives onto target neurons. We also find that Mafa inhibitory neurons are absent from respiratory, sensory (primary and secondary) and pontine structures but are present in the vicinity of the hypoglossal motor nucleus including premotor neurons that innervate the geniohyoid muscle, to control upper airway patency. Altogether, our data reveal a role for Mafa phosphorylation in regulation of GABAergic drives and suggest a mechanism whereby reduced premotor drives to upper airway muscles may cause apneic breathing at birth.

It has long been known that orofacial movements for feeding can be triggered, coordinated, and often rhythmically organized at the level of the brainstem, without input from higher centers. We uncover two nuclei that can organize the movements for ingesting fluids in mammals. These neuronal groups, defined by unique transcriptional codes and developmental origins, IRt Phox2b and Peri5 Atoh1 , are located, respectively, in the intermediate reticular formation of the medulla and around the motor nucleus of the trigeminal nerve. They are premotor to all jaw-opening and tongue muscles. Stimulation of either, in awake animals, opens the jaw, while IRt Phox2b alone also protracts the tongue. Moreover, stationary stimulation of IRt Phox2b entrains a rhythmic alternation of tongue protraction and retraction, synchronized with jaw opening and closing, that mimics lapping. Finally, fiber photometric recordings show that IRt Phox2b is active during volitional lapping. Our study identifies one of the long hypothesized subcortical nuclei underpinning a stereotyped feeding behavior.

The ability to discriminate competing, external stimuli, and initiate contextually appropriate behaviors, is a key brain function. Neurons in the deep superior colliculus (dSC) integrate multisensory inputs and activate descending projections to premotor pathways responsible for orienting and attention, behaviors which involve adjustments to respiratory and cardiovascular parameters. However, the neural pathways that subserve the physiological components of orienting are poorly understood. We report that orienting responses to optogenetic dSC stimulation are accompanied by short-latency autonomic, respiratory and electroencephalographic effects in awake rats, closely mimicking those evoked by naturalistic alerting stimuli. Physiological responses were not accompanied by detectable aversion or fear and persisted under urethane anesthesia, indicating independence from emotional stress. Moreover, autonomic responses were replicated by selective stimulation of dSC inputs to a subregion in the ventromedial medulla containing spinally-projecting premotor neurons. This putative disynaptic pathway from the dSC represents a likely substrate for autonomic components of orienting. ### Competing Interest Statement The authors have declared no competing interest.

It has long been known that orofacial movements for feeding can be triggered, coordinated, and often rhythmically organized at the level of the brainstem, without input from higher centers. We uncover two nuclei that can organize the movements for ingesting fluids in mice. These neuronal groups, IRtPhox2b and Peri5Atoh1, are marked by expression of the pan-autonomic homeobox gene Phox2b and are located, respectively, in the intermediate reticular formation of the medulla and around the motor nucleus of the trigeminal nerve. They are premotor to all jaw-opening and tongue muscles. Stimulation of either, in awake animals, opens the jaw, while IRtPhox2b alone also protracts the tongue. Moreover, stationary stimulation of IRtPhox2b entrains a rhythmic alternation of tongue protraction and retraction, synchronized with jaw opening and closing, that mimics lapping. Finally, fiber photometric recordings show that IRtPhox2b is active during volitional lapping. Our study identifies one of the subcortical nuclei underpinning a stereotyped feeding behavior.

The superior colliculus (SC) is a sensory integration hub in the dorsal brainstem where multimodal information is combined and, depending on the saliency of the competing sensory inputs, appropriate motor commands and supportive autonomic changes initiated. In rodents, the SC is indispensable for initiating behavioral responses to stereotypical visual stimuli that resemble approaching objects, such as looming (an expanding overhead black circle). Here we report that presentation of overhead looming or naturalistic stimuli drove acute surges in blood pressure in telemetered conscious rats, an effect that was replicated by optogenetic stimulation of the deep SC (dSC). dSC stimulation also evoked increases in respiratory rate and tail vasoconstriction in the absence of detectable anxiety‐like behaviors and continued to exert excitatory effects on heart rate, respiratory rate, and sympathetic nerve activity under urethane anesthesia. The objective of the current study was to identify the central pathways responsible for mediating these physiological effects. Anterograde labeling of dSC neurons revealed a previously uncharacterized axonal projections to brainstem cell groups associated with arousal and autonomic control, including the locus coeruleus, A5 group and, most extensively, neurons within a region that spanned the medullary gigantocellular and raphe cell groups, collectively called the rostral ventromedial medulla (RVMM). Optogenetic stimulation of dSC terminals within the RVMM recapitulated some of the sympathetic and respiratory effects evoked by dSC stimulation, and optogenetic dSC activation evoked powerful excitatory effects on extracellular recordings of putative RVMM sympathetic premotor neurons, suggesting that elements of the physiological response dSC stimulation are mediated by direct activation of medullary autonomic neurons. To investigate the contribution of environmental stimuli to the excitability of this pathway we conducted single‐unit recordings of SC neuronal responses to visual and acoustic stimuli using high‐density silicon probes. In addition to responding to stereotypical audio‐visual looming stimuli, we report the presence of SC neurons with higher‐order visual capabilities relating to object detection that differ by subregion and are several orders of magnitude more complex than previously recognized. These tuning properties were also found in subpopulations of opto‐tagged SC neurons that project to the RVMM. Our data suggest that the SC is not only capable of nuanced object recognition, but can translate naturalistic visual cues into fast‐acting autonomic changes via direct medullary projections Support or Funding Information Research was supported by the NHMRC and Hillcrest Foundation

[This corrects the article DOI: 10.3389/fnins.2019.00897.].

Repetitive hypoxia is a key feature of obstructive sleep apnoea (OSA), a condition characterized by intermittent airways obstruction. Patients with OSA present with persistent increases in sympathetic activity and commonly develop hypertension. The objectives of this study were to determine if the persistent increases in sympathetic nerve activity, known to be induced by acute intermittent hypoxia (AIH), are mediated through activation of the pituitary adenylate cyclase activating polypeptide (PACAP) signaling system. Here, we show that the excitatory neuropeptide PACAP, acting in the spinal cord, is important for generating the sympathetic response seen following AIH. Using PACAP receptor knockout mice, and pharmacological agents in Sprague Dawley rats, we measured blood pressure, heart rate, pH, PaCO2, and splanchnic sympathetic nerve activity, under anaesthesia, to demonstrate that the sympathetic response to AIH is mediated via the PAC1 receptor, in a cAMP-dependent manner. We also report that both intermittent microinjection of glutamate into the rostroventrolateral medulla (RVLM) and intermittent infusion of a sub-threshold dose of PACAP into the subarachnoid space can mimic the sympathetic response to AIH. All the sympathetic responses are independent of blood pressure, pH or PaCO2 changes. Our results show that in AIH, PACAP signaling in the spinal cord helps drive persistent increases in sympathetic nerve activity. This mechanism may be a precursor to the development of hypertension in conditions of chronic intermittent hypoxia, such as OSA.

Introduction The superior colliculus plays key roles in the immediate processing of threatening sensory stimuli and generates rapid behavioural responses that are critical for survival. We have previously found that disinhibition of the deep superior colliculus (dSC) unmasks coordinated respiratory, sympathetic and somatomotor outputs that are independent of processing in higher centres. We postulate that these effects are a result of direct innervation of autonomic and respiratory medullary neurons based on the results of neuroanatomical tracing studies that identified dSC synaptic contacts on spinally projecting neurons within the rostral ventromedial medulla (RVMM). In the present study we investigate the physiological significance of this relay and compare effects of dSC stimulation to activation of dSC‐RVMM terminals. Methods We used an AAV vector to express Channelrhodopsin2 (ChR2) in the dSC and chronically implanted a fibre optic cannula into the dSC. After transgene expression electrophysiology experiments were conducted under urethane anesthesia with rats instrumented to record blood pressure, diaphragmatic EMG and splanchnic sympathetic nerve activity (SNA). One subset of experiments then positioned the optrode in the RVMM to deliver light to the terminal projections from the dSC. For the other subset of experiments, extracellular recordings were made of spinally projecting RVMM neurons. Results dSC photoactivation evoked an increase in respiratory frequency (35±8.7%, P=<0.01, N=7) and heart rate (11±2.6 bpm, P=<0.01, N=7), and a modest change in blood pressure when compared to controls. Stimulus‐triggered averaging of SNA revealed short‐latency excitatory potentials in most cases. Photoactivation of ChR2‐expressing dSC‐RVMM terminals evoked similar effects on cardiorespiratory outputs as dSC stimulation, with an increase in respiratory frequency (14 ±9.2%, P<0.001, N=6) and blood pressure (13 ±2 mmHg P<0.01, N=7), as well as qualitatively similar sympathoexcitation. In preliminary experiments that recorded extracellular action potentials in bulbospinal RVMM neurons we recorded excitatory effects of dSC stimulation in 6/8 spontaneously active neurons. Conclusion We conclude that optogenetic activation of dSC neurons that project to the RVMM can drive cardiorespiratory effects that are consistent with our previous findings and that dSC stimulation evokes excitatory effects in some RVMM bulbospinal neurons. This confirms our hypothesis of the existence of a previously uncharacterized excitatory pathway between the dSC and RVMM. Ongoing experiments will functionally characterize RVMM neurons that receive dSC input. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

The mammalian nervous system is comprised of a seemingly infinitely complex network of specialized synaptic connections that coordinate the flow of information through it. The field of connectomics seeks to map the structure that underlies brain function at resolutions that range from the ultrastructural, which examines the organization of individual synapses that impinge upon a neuron, to the macroscopic, which examines gross connectivity between large brain regions. At the mesoscopic level, distant and local connections between neuronal populations are identified, providing insights into circuit-level architecture. Although neural tract tracing techniques have been available to experimental neuroscientists for many decades, considerable methodological advances have been made in the last 20 years due to synergies between the fields of molecular biology, virology, microscopy, computer science and genetics. As a consequence, investigators now enjoy an unprecedented toolbox of reagents that can be directed against selected subpopulations of neurons to identify their efferent and afferent connectomes. Unfortunately, the intersectional nature of this progress presents newcomers to the field with a daunting array of technologies that have emerged from disciplines they may not be familiar with. This review outlines the current state of mesoscale connectomic approaches, from data collection to analysis, written for the novice to this field. A brief history of neuroanatomy is followed by an assessment of the techniques used by contemporary neuroscientists to resolve mesoscale organization, such as conventional and viral tracers, and methods of selecting for sub-populations of neurons. We consider some weaknesses and bottlenecks of the most widely used approaches for the analysis and dissemination of tracing data and explore the trajectories that rapidly developing neuroanatomy technologies are likely to take.

The etiology of hypertension, the world's biggest killer, remains poorly understood, with treatments targeting the established symptom, not the cause. The development of hypertension involves increased sympathetic nerve activity that, in experimental hypertension, may be driven by excessive respiratory modulation. Using selective viral and cell lesion techniques, we identify adrenergic C1 neurons in the medulla oblongata as critical for respiratory-sympathetic entrainment and the development of experimental hypertension. We also show that a cohort of young, normotensive humans, selected for an exaggerated blood pressure response to exercise and thus increased hypertension risk, has enhanced respiratory-related blood pressure fluctuations. These studies pinpoint a specific neuronal target for ameliorating excessive sympathetic activity during the developmental phase of hypertension and identify a group of pre-hypertensive subjects that would benefit from targeting these cells.