Expression of Fos-protein activated by tactile stimulation on the laryngeal vestibulum in the cat's lower brain stem

The blink reflex and its modulation - Part 1: Physiological mechanisms

The blink reflex (BR) is a protective eye-closure reflex mediated by brainstem circuits. The BR is usually evoked by electrical supraorbital nerve stimulation but can be elicited by a variety of sensory modalities. It has a long history in clinical neurophysiology practice. Less is known, however, about the many ways to modulate the BR. Various neurophysiological techniques can be applied to examine different aspects of afferent and efferent BR modulation. In this line, classical conditioning, prepulse and paired-pulse stimulation, and BR elicitation by self-stimulation may serve to investigate various aspects of brainstem connectivity. The BR may be used as a tool to quantify top-down modulation based on implicit assessment of the value of blinking in a given situation, e.g., depending on changes in stimulus location and probability of occurrence. Understanding the role of non-nociceptive and nociceptive fibers in eliciting a BR is important to get insight into the underlying neural circuitry. Finally, the use of BRs and other brainstem reflexes under general anesthesia may help to advance our knowledge of the brainstem in areas not amenable in awake intact humans. This review summarizes talks held by the Brainstem Special Interest Group of the International Federation of Clinical Neurophysiology at the International Congress of Clinical Neurophysiology 2022 in Geneva, Switzerland, and provides a state-of-the-art overview of the physiology of BR modulation. Understanding the principles of BR modulation is fundamental for a valid and thoughtful clinical application (reviewed in part 2) (Gunduz et al., submitted).

Medullary mediation of the laryngeal adductor reflex: A possible role in sudden infant death syndrome

The laryngeal adductor reflex (LAR) is a laryngeal protective reflex. Vagal afferent polymodal sensory fibres that have cell bodies in the nodose ganglion, originate in the sub-glottal area of the larynx and upper trachea. These polymodal sensory fibres respond to mechanical or chemical stimuli. The central axons of these sensory vagal neurons terminate in the dorsolateral subnuclei of the tractus solitarius in the medulla oblongata. The LAR is a critical, reflex in the pathways that play a protective role in the process of ventilation, and the sychronisation of ventilation with other activities that are undertaken by the oropharyngeal systems including: eating, speaking and singing. Failure of the LAR to operate properly at any time after birth can lead to SIDS, pneumonia or death. Despite the critical nature of this reflex, very little is known about the central pathways and neurotransmitters involved in the management of the LAR and any disorders associated with its failure to act properly. Here, we review current knowledge concerning the medullary nuclei and neurochemicals involved in the LAR and propose a potential neural pathway that may facilitate future SIDS research.

Distribution of Fos-Like Immunoreactivity, Catecholaminergic and Serotoninergic Neurons Activated by the Laryngeal Chemoreflex in the Medulla Oblongata of Rats

The laryngeal chemoreflex (LCR) induces apnea, glottis closure, bradycardia and hypertension in young and maturing mammals. We examined the distribution of medullary nuclei that are activated by the LCR and used immunofluorescent detection of Fos protein as a cellular marker for neuronal activation to establish that the medullary catecholaminergic and serotoninergic neurons participate in the modulation of the LCR. The LCR was elicited by the infusion of KCl-HCl solution into the laryngeal lumen of adult rats in the experimental group, whereas the control group received the same surgery but no infusion. In comparison, the number of regions of Fos-like immunoreactivity (FLI) that were activated by the LCR significantly increased in the nucleus of the solitary tract (NTS), the vestibular nuclear complex (VNC), the loose formation of the nucleus ambiguus (AmbL), the rostral ventral respiratory group (RVRG), the ventrolateral reticular complex (VLR), the pre-Bötzinger complex (PrBöt), the Bötzinger complex (Böt), the spinal trigeminal nucleus (SP5), and the raphe obscurus nucleus (ROb) bilaterally from the medulla oblongata. Furthermore, 12.71% of neurons with FLI in the dorsolateral part of the nucleus of the solitary tract (SolDL) showed tyrosine hydroxylase-immunoreactivity (TH-ir, catecholaminergic), and 70.87% of neurons with FLI in the ROb were serotoninergic. Our data demonstrated the distribution of medullary nuclei that were activated by the LCR, and further demonstrated that catecholaminergic neurons of the SolDL and serotoninergic neurons of the ROb were activated by the LCR, indicating the potential central pathway of the LCR.

Combined laryngeal inflammation and trauma mediate long-lasting immunoreactivity response in the brainstem sensory nuclei in the rat

Somatosensory feedback from the larynx plays a critical role in regulation of normal upper airway functions, such as breathing, deglutition, and voice production, while altered laryngeal sensory feedback is known to elicit a variety of pathological reflex responses, including persistent coughing, dysphonia, and laryngospasm. Despite its clinical impact, the central mechanisms underlying the development of pathological laryngeal responses remain poorly understood. We examined the effects of persistent vocal fold (VF) inflammation and trauma, as frequent causes of long-lasting modulation of laryngeal sensory feedback, on brainstem immunoreactivity in the rat. Combined VF inflammation and trauma were induced by injection of lipopolysaccharide (LPS) solution and compared to VF trauma alone from injection of vehicle solution and to controls without any VF manipulations. Using a c-fos marker, we found significantly increased Fos-like immunoreactivity (FLI) in the bilateral intermediate/parvicellular reticular formation (IRF/PCRF) with a trend in the left solitary tract nucleus (NTS) only in animals with combined LPS-induced VF inflammation and trauma. Further, FLI in the right NTS was significantly correlated with the severity of LPS-induced VF changes. However, increased brainstem FLI response was not associated with FLI changes in the first-order neurons of the laryngeal afferents located in the nodose and jugular ganglia in either group. Our data indicate that complex VF alterations (i.e., inflammation/trauma vs. trauma alone) may cause prolonged excitability of the brainstem nuclei receiving a direct sensory input from the larynx, which, in turn, may lead to (mal)plastic changes within the laryngeal central sensory control.

Brainstem regions involved in the expiration reflex. A c-fos study in anesthetized cats

Expression of the immediate-early gene c-fos, a marker of neuronal activation, was employed to localize brainstem neuronal populations functionally related to the expiration reflex (ER). Twelve spontaneously breathing, non-decerebrate, pentobarbital anesthetized cats were used. The level of Fos-like immunoreactivity (FLI) in 6 animals with repetitive ERs mechanically induced from the glottis (296+/-9 ERs) was compared to FLI in 6 control non-stimulated cats. Respiratory rate, arterial blood pressure, and end tidal CO(2) concentration remained stable during the experiment. In the medulla, increased FLI was found in the region of nucleus tractus solitarii (p<0.001), in the ventrolateral medulla along with the lateral tegmental field (p<0.01), and in the vestibular nuclei (p<0.01). In the pons, increased FLI was detected in the caudal extensions of the lateral parabrachial and Kölliker-Fuse nuclei (p<0.05). Within the rostral mesencephalon, FLI was enhanced in the midline area (p<0.05). A lower level of ER-related FLI compared to control animals was detected in the pontine raphe region (p<0.05) and the lateral division of mesencephalic periaqueductal gray (p<0.05). The results suggest that the ER is coordinated by a complex long loop of medullary-pontine-mesencephalic neuronal circuits, some of which may differ from those of other respiratory reflexes. The FLI related to the expulsive behavior ER differs from that induced by laryngeal stimulation and laryngeal adductor responses, particularly in ventrolateral medulla and mesencephalon.

Neuronal activation in the medulla oblongata during selective elicitation of the laryngeal adductor response

Swallow and cough are complex motor patterns elicited by rapid and intense electrical stimulation of the internal branch of the superior laryngeal nerve (ISLN). The laryngeal adductor response (LAR) includes only a laryngeal response, is elicited by single stimuli to the ISLN, and is thought to represent the brain stem pathway involved in laryngospasm. To identify which regions in the medulla are activated during elicitation of the LAR alone, single electrical stimuli were presented once every 2 s to the ISLN. Two groups of five cats each were studied; an experimental group with unilateral ISLN stimulation at 0.5 Hz and a surgical control group. Three additional cats were studied to evaluate whether other oral, pharyngeal, or respiratory muscles were activated during ISLN stimulation eliciting LAR. We quantified < or = 22 sections for each of 14 structures in the medulla to determine if regions had increased Fos-like immunoreactive neurons in the experimental group. Significant increases (P < 0.0033) occurred with unilateral ISLN stimulation in the interstitial subnucleus, the ventrolateral subnucleus, the commissural subnucleus of the nucleus tractus solitarius, the lateral tegmental field of the reticular formation, the area postrema, and the nucleus ambiguus. Neither the dorsal motor nucleus of the vagus, usually active for swallow, nor the nucleus retroambiguus, retrofacial nucleus, and the lateral reticular nucleus, usually active for cough, were active with elicitation of the laryngeal adductor response alone. The results demonstrate that the laryngeal adductor pathway is contained within the broader pathways for cough and swallow in the medulla.

Selective suppression of late laryngeal adductor responses by N-methyl-D-aspartate receptor blockade in the cat

Laryngeal adductor responses to afferent stimulation play a key role in airway protection. Although vital for protection during cough and swallow, these responses also must be centrally controlled to prevent airway obstruction by laryngospasm during prolonged stimulation. Our purpose was to determine the role of N-methyl-D-aspartate (NMDA) receptors in modulating early R1 responses (at 9 ms) and/or later more prolonged R2 responses (at 36 ms) during electrical stimulation of the laryngeal afferent fibers contained in the internal branch of the superior laryngeal nerve in the cat. The percent occurrence, amplitude, and conditioning of muscle responses to single superior laryngeal nerve (SLN) stimuli presented in pairs at interstimulus intervals of 250 ms were measured in three experiments: 1) animals that had ketamine as anesthetic premedication were compared with those who did not, when both were maintained under alpha-chloralose anesthesia. 2) The effects of administering ketamine in one group of animals were compared with increasing the depth of alpha-chloralose anesthesia without NMDA receptor blockade in another group of animals. 3) The effects of dextromethorphan (without anesthetic effects) were examined in another group of animals. In the first experiment, the occurrence of R2 responses were reduced from 95% in animals without ketamine premedication to 25% in animals with ketamine premedication (P = 0.015). No differences occurred in the occurrence, amplitude, latency, or conditioning effects on R1 responses between these groups. In the second experiment, the occurrence of R2 responses was reduced from 96 to 79% after an increase in the depth of anesthesia with alpha-chloralose in contrast with reductions in R2 occurrence from 98 to 19% following the administration of ketamine to induce NMDA receptor blockade along with increased anesthesia (P = 0.025). In the third experiment, R2 occurrence was reduced from 89 to 27% (P = 0.017) with administration of dextromethorphan while R1 response occurrence and amplitude did not change. In each of these experiments, NMDA receptor blockade did not have significant effects on cardiac or respiratory rates in any of the animals. The results demonstrate that NMDA receptors play an essential role in long latency R2 laryngeal responses to laryngeal afferent stimulation. On the other hand, early R1 laryngeal adductor responses are likely to involve non-NMDA receptor activation.

Mapping of brain stem neuronal circuitry active during swallowing

A poorly understood neural circuit in the brain stem controls swallowing. This experiment studied the swallowing circuit in the rat brain stem by means of fos immunocytochemistry. The fos protein is a marker of activated neurons, and under experimental conditions, repetition of a behavior causes the fos protein to be produced in the neurons involved in that behavior. The fos technique has been successfully used to delineate neural circuits involved in reflex glottic closure, cough, and vocalization; however, the technique has not been used to map the swallowing circuit. Nine rats were used in this study. Swallows were evoked in anesthetized rats for 1 hour, then, after a 4-hour delay to allow maximum fos production, the rats were painlessly sacrificed by perfusion. The brain stems were removed and sectioned in the frontal plane, and every fourth section was immunoreacted for fos protein. All sections were examined by light microscopy, and cells positive for fos were marked on drawings of brain stem structures for different levels throughout the brain stem. Control animals underwent sham experiments. After subtraction of the areas of fos labeling seen in controls, all experimental rats showed fos-labeled neurons in very discrete and localized areas, including practically all regions implicated by prior neurophysiology studies of swallowing. The distribution of labeled neurons was more dispersed through the brain stem than current theories of swallowing would suggest. Specifically, recent studies of swallowing control have focused on the nucleus of the solitary tract (NST) and the region surrounding the nucleus ambiguus (periambigual area) just rostral to the obex. These areas contained fos-labeled neurons, but unexpectedly, heavy labeling was found in the same areas caudal to the obex. Areas containing the heaviest labeling were specific subnuclei of the NST and surrounding reticular formation; the periambigual area; and the intermediate reticular zone in the pons and caudal medulla. Interestingly, none of these anatomic structures had uniform fos labeling; this finding suggests that the unlabeled areas are involved in other oromotor behaviors, or that the specific protocol did not activate the full population of swallowing-related neurons. A notable finding of this study is a candidate for the central pattern generator (CPG) of swallowing. Careful lesioning studies in cats strongly suggest that a region in the rostral-medial medulla contains the CPG for swallowing, although the exact location of the CPG was never pinpointed. In the homologous region of the rat brain stem, fos labeling was only found in a small group of neurons within the gigantocellular reticular formation that may be a candidate for the CPG. In summary, correlation with prior physiology experiments suggests that this experiment appears to have delineated many, if not all, of the components of the swallowing circuit for the first time in any mammal. In addition, other areas were found that might also be swallowing-related. One notable example is a small group of fos-labeled cells that may be the CPG for swallowing. Further studies are required to clarify the specific roles of the fos-labeled neurons seen in this study.

Central nervous pathways and control of the airways

Neural control of airway muscles and secretions is predominantly by excitatory parasympathetic and non-adrenergic, non-cholinergic innervations (excitatory and/or inhibitory depending on the species). Functionally distinct afferents effecting airway reflexes terminate in different but overlapping parts of the nucleus tractus solitarius, where integration of simultaneously evoked reflex responses occurs. Parasympathetic preganglionic neurones are located in the dorsal vagal nucleus and nucleus ambiguus, which also contains upper airway motoneurones. These output neurones receive inputs from the central respiratory network which modify the effectiveness of reflex activity. This is particularly important since many afferents evoking airway reflexes concurrently modify respiratory drive. Thus, their effect on the outflow is twofold, a direct reflex effect and an indirect respiratory action and these may facilitate or antagonise one another. Although there is reflex control of individual motor outflows, in some defined situations, e.g. swallowing and coughing a stereotypical pattern of motor outflow is evoked. The neural mechanisms underlying these aspects of airway control are discussed.

Modulation of laryngeal responses to superior laryngeal nerve stimulation by volitional swallowing in awake humans

Laryngeal sensori-motor closure reflexes are important for the protection of the airway and prevent the entry of foreign substances into the trachea and lungs. The purpose of this study was to determine how such reflexes might be modulated during volitional swallowing in awake humans, when persons are at risk of entry of food or liquids into the airway. The frequency and the amplitude of laryngeal adductor responses evoked by electrical stimulation of the internal branch of the superior laryngeal nerve (ISLN) were studied during different phases of volitional swallowing. Subjects swallowed water on command while electrical stimuli were presented to the ISLN at various intervals from 500 ms to 5 s following the command. Laryngeal adductor responses to unilateral ISLN stimulation were recorded bilaterally in the thyroarytenoid muscles using hooked wire electrodes. Early ipsilateral R1 responses occurred at 17 ms, and later bilateral R2 began around 65 ms. The muscle responses to stimuli occurring during expiration without swallowing were quantified as control trials. Responses to stimulation presented before swallowing, during the swallow, within 3 s after swallowing, and between 3 and 5 s after a swallow were measured. The frequency and amplitude of three responses (ipsilateral R1 and bilateral R2) relative to the control responses were compared across the different phases relative to the occurrence of swallowing. Results demonstrated that a reduction occurred in both the frequency and amplitude of the later bilateral R2 laryngeal responses to electrical stimulation for up to 3 s after swallowing (P = 0.005). The amplitude and frequency of ipsilateral R1 laryngeal responses, however, did not show a significant main effect following the swallow (P = 0.28), although there was a significant time by measure interaction (P = 0.006) related to reduced R1 response amplitude up to 3 s after swallowing (P = 0.021). Therefore, the more rapid and shorter unilateral R1 responses continued to provide some, albeit reduced, laryngeal protective functions after swallowing, whereas the later bilateral R2 responses were suppressed both in occurrence and amplitude for up to 3 s after swallowing. The results suggest that R2 laryngeal adductor responses are suppressed following swallowing when residues may remain in the laryngeal vestibule putting persons at increased risk for the entry of foreign substances into the airway.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Effects of Stimulus Intensity on Laryngeal Long Latency Responses in Awake Humans

Percutaneous electrical stimulation applied to the internal branch of the superior laryngeal nerve (ISLN) results in two long latency laryngeal adductor responses in awake humans: an ipsilateral thyroarytenoid (TA) R1 muscle response at 16 ms, and later bilateral TA R2 muscle responses at 60 ms. The purpose of this study was to determine whether a functional relationship existed between the R1 and R2 responses by gradually increasing the level of electrical stimulation from threshold to supramaximal levels. R1 amplitude increased linearly with stimulation intensity in 9 of the 11 subjects, whereas R2 only had a positive linear relationship in 3 subjects and a negative relationship with stimulation intensity in 1 subject. Significant negative relationships were found between response latency and stimulation intensify in 3 subjects for the R1 responses and 3 other subjects for the R2 responses. Overall, R1 amplitudes increased systematically, whereas R2 responses varied in latency and amplitude with increasing stimulus Intensity. Neither the latencies nor the amplitudes of the two responses were related after adjusting for stimulation intensity within subjects by using partial correlation coefficients. The R1 and R2 responses were functionally unrelated and most likely have different neural components.