Graphic-digitizer analysis of axon spectra in ethmoidal and lingual branches of the trigeminal nerve

The Mammalian Diving Response: Inroads to Its Neural Control

The mammalian diving response (DR) is a remarkable behavior that was first formally studied by Laurence Irving and Per Scholander in the late 1930s. The DR is called such because it is most prominent in marine mammals such as seals, whales, and dolphins, but nevertheless is found in all mammals studied. It consists generally of breathing cessation (apnea), a dramatic slowing of heart rate (bradycardia), and an increase in peripheral vasoconstriction. The DR is thought to conserve vital oxygen stores and thus maintain life by directing perfusion to the two organs most essential for life-the heart and the brain. The DR is important, not only for its dramatic power over autonomic function, but also because it alters normal homeostatic reflexes such as the baroreceptor reflex and respiratory chemoreceptor reflex. The neurons driving the reflex circuits for the DR are contained within the medulla and spinal cord since the response remains after the brainstem transection at the pontomedullary junction. Neuroanatomical and physiological data suggesting brainstem areas important for the apnea, bradycardia, and peripheral vasoconstriction induced by underwater submersion are reviewed. Defining the brainstem circuit for the DR may open broad avenues for understanding the mechanisms of suprabulbar control of autonomic function in general, as well as implicate its role in some clinical states. Knowledge of the proposed diving circuit should facilitate studies on elite human divers performing breath-holding dives as well as investigations on sudden infant death syndrome (SIDS), stroke, migraine headache, and arrhythmias. We have speculated that the DR is the most powerful autonomic reflex known.

The mammalian diving response: an enigmatic reflex to preserve life?

The mammalian diving response is a remarkable behavior that overrides basic homeostatic reflexes. It is most studied in large aquatic mammals but is seen in all vertebrates. Pelagic mammals have developed several physiological adaptations to conserve intrinsic oxygen stores, but the apnea, bradycardia, and vasoconstriction is shared with those terrestrial and is neurally mediated. The adaptations of aquatic mammals are reviewed here as well as the neural control of cardiorespiratory physiology during diving in rodents.

The rat: a laboratory model for studies of the diving response

Underwater submersion in mammals induces apnea, parasympathetically mediated bradycardia, and sympathetically mediated peripheral vasoconstriction. These effects are collectively termed the diving response, potentially the most powerful autonomic reflex known. Although these physiological responses are directed by neurons in the brain, study of neural control of the diving response has been hampered since 1) it is difficult to study the brains of animals while they are underwater, 2) feral marine mammals are usually large and have brains of variable size, and 3) there are but few references on the brains of naturally diving species. Similar responses are elicited in anesthetized rodents after stimulation of their nasal mucosa, but this nasopharyngeal reflex has not been compared directly with natural diving behavior in the rat. In the present study, we compared hemodynamic responses elicited in awake rats during volitional underwater submersion with those of rats swimming on the water's surface, rats involuntarily submerged, and rats either anesthetized or decerebrate and stimulated nasally with ammonia vapors. We show that the hemodynamic changes to voluntary diving in the rat are similar to those of naturally diving marine mammals. We also show that the responses of voluntary diving rats are 1) significantly different from those seen during swimming, 2) generally similar to those elicited in trained rats involuntarily "dunked" underwater, and 3) generally different from those seen from dunking naive rats underwater. Nasal stimulation of anesthetized rats differed most from the hemodynamic variables of rats trained to dive voluntarily. We propose that the rat trained to dive underwater is an excellent laboratory model to study neural control of the mammalian diving response, and also suggest that some investigations may be done with nasal stimulation of decerebrate preparations to decipher such control.

Assessment of Upper Respiratory Tract and Ocular Irritative Effects of Volatile Chemicals in Humans

Accurate assessment of upper respiratory tract and ocular irritation is critical for identifying and remedying problems related to overexposure to volatile chemicals, as well as for establishing parameters of irritation useful for regulatory purposes. This article (a) describes the basic anatomy and physiology of the human upper respiratory tract and ocular mucosae, (b) discusses how airborne chemicals induce irritative sensations, and (c) reviews practical means employed for assessing such phenomena, including psychophysical (e.g., threshold and suprathreshold perceptual measures), physiological (e.g., cardiovascular responses), electrophysiological (e.g., event-related potentials), and imaging (e.g., magnetic resonance imaging) techniques. Although traditionally animal models have been used as the first step in assessing such irritation, they are not addressed here since (a) there are numerous reviews available on this topic and (b) many rodents and rabbits are obligate nose breathers whose nasal passages differ considerably from those of humans, potentially limiting generalization of animal-based data to humans. A major goal of this compendium is to inform the reader of procedures for assessing irritation in humans and to provide information of value in the continued interpretation and development of empirical databases upon which future reasoned regulatory health decisions can be made.

Somatostatin inhibits the excitability of rat small-diameter trigeminal ganglion neurons that innervate nasal mucosa and project to the upper cervical dorsal horn via activation of somatostatin 2a receptor

This study investigated whether somatostatin (SST) modulates the excitability of nociceptive trigeminal ganglion (TRG) neurons that innervate the nasal mucosa and project to the upper cervical (C(1)) dorsal horn by using perforated-patch clamping, retrograde-labeling, and immunohistochemistry. Fluorogold (FG) retrograde labeling was used to identify the rat TRG neurons innervating the nasal mucosa, while microbeads (MB) were used to label neurons projected onto the superficial layer of the C(1) dorsal horn. FG-labeled small-diameter TRG neurons exhibited SST(2A) receptor immunoreactivity (19%) and half of these neurons were also labeled with MB. In whole-cell current-clamp mode, most (72%) of the dissociated FG-/MB-labeled TRG neurons were hyperpolarized by application of SST. The hyperpolarization was evoked by SST in a concentration-dependent manner (0.1-10 microM) and the responses were associated with a decrease in the cell input resistance. The minimum concentration to elicit a significant hyperpolarization was 1 microM. The repetitive firings during a depolarizing pulse were significantly reduced by SST (1 microM) application. The hyperpolarization and decreased firing evoked by SST were both blocked by the SST(2) receptor antagonist, CYN154806 (1 microM). Under voltage-clamp conditions, SST (1 microM) significantly increased the voltage-gated K(+) transient (I(A)) and sustained (I(K)) currents and these increases were abolished by coapplication of CYN154806 (1 microM). In the presence of both 4-aminopyridine (6 mM) and tetraethylammonium (10 mM), no significant changes in the membrane potential in response to SST application were found. These results suggest that modulation of trigeminal nociceptive transmission in the C(1) dorsal horn by activation of SST(2A) receptors occurs at the level of small-diameter TRG cell bodies and/or their afferent terminals, and that this may be related to regulation of protective upper-airway reflexes.

Objective evaluation of iatrogenic lingual nerve injuries using the jaw-opening reflex

The extent of reflex inhibition of masseteric electromyographic activity, after an electrical stimulus applied to lingual mucosa, was used as a test of the ability of the lingual nerve to conduct nerve impulses and this was compared with the results of standard clinical tests. Two groups of subjects were assessed: healthy subjects (n=10) and patients with lingual nerve injuries (n=17). The patients were tested 8-9 weeks after their injury and retested 6 months later when they were retrospectively allocated to either a temporary injury or a permanent injury group. The group measure of reflex inhibition after stimulation of the tongue on the opposite side to the injury was no different from the same measure in controls, whereas two-point discrimination did differ. Group measures of inhibition and of subjective function after stimulation on the side of the injury were significantly different from controls whereas light touch and two-point discrimination were not. There was good agreement between quantified masseteric inhibition and subjective function, but it was not possible at 8-9 weeks after the injury to differentiate between those that would recover and those that would be permanent.

A light microscopical study on the structure of traumatic neuromas of the human lingual nerve

OBJECTIVE

To determine the morphologic characteristics of traumatic neuromas resulting from damage to the lingual nerve during the surgical removal of lower third molar teeth.

STUDY DESIGN

Using light microscopy, we examined hematoxylin and eosin-stained sections of neuromas removed at the time of microsurgical nerve repair in 31 patients. Changes in fascicular pattern were quantified and evidence of inflammation was recorded. Statistical comparisons were made between the sections from patients with and without symptoms of dysesthesia, and with sections of normal lingual nerve obtained from organ donor retrieval patients.

RESULTS

The neuromas were found to contain large numbers of small and haphazardly arranged regenerating nerve fascicles within a densely collagenous and fibroblastic stroma. The mean number of fascicles was 31 (+/- SD 28) in normal lingual nerve, but 462 (+/- 366) within traumatic neuromas. Mean fascicle diameter was 44 (+/- 10) microm in neuromas, but 273 (+/- 101) microm in normal nerve. A chronic mononuclear cell inflammatory infiltrate was observed in 42% of neuroma specimens, and histologic signs of inflammation were frequently seen in patients with symptoms of dysesthesia.

CONCLUSIONS

Damage to the lingual nerve during third molar removal results in marked changes to the fascicular pattern and sometimes the presence of chronic inflammation in the injured nerve. These changes could contribute to the altered electrophysiological properties of axons trapped within traumatic neuromas, but we found no significant differences between the specimens studied from patients with or without symptoms of dysesthesia.

Electrical stimulation of the anterior ethmoidal nerve produces the diving response

Stimulation of the upper respiratory tract usually produces apnea, but it can also produce a vagally mediated bradycardia and a sympathetically mediated increase in peripheral vascular resistance. This cardiorespiratory response, often called the diving response, is usually initiated by nasal stimulation. The purpose of this research was to investigate the anterior ethmoidal nerve (AEN) that innervates the nasal mucosa of muskrats (Ondatra zibethicus). Electrical stimulation of the AEN (typically 50 Hz, 100 micros and 500 microA) produced immediate and sustained bradycardia and cessation of respiration similar to that of the diving response. Heart rate (HR) significantly decreased from 264+/-18 to 121+/-8 bpm, with a concurrent 4.2+/-0.9 s apnea, during the 5 s stimulation period. BP decreased from 97.9+/-4.8 to 91.2+/-6.4 mmHg. Using estimations from (1) cross-sectional areas of AEN trigeminal ganglion cells labeled with WGA-HRP, and (2) electron microscopic analysis of the AEN, we found that approximately 65% of the AEN is composed of unmyelinated C-fibers. In addition, 72.4% of myelinated fibers from the nerves that innervate the nasal passages were of small diameter (<6 microm, presumably Adelta fibers). Thus, the AEN of the muskrat contains a high concentration of small diameter fibers (89.8%). We conclude that electrical stimulation of small diameter fibers within the AEN of muskrats can produce the cardiovascular and respiratory responses similar to that of the diving response.

Responses of single lingual nerve fibers to thermal and chemical stimulation

The goals of this study were to characterize the responses of: (1) thermally-sensitive fibers of the lingual branch of the trigeminal nerve to cooling from 35 degrees to 10 degrees C at a rate of 1 degrees C/s; and (2) these neurons to a mid-range concentration of NaCl (150 mM), glucose (150 mM), citric acid (0.3 mM), and quinine-HCl (3 mM) at 35 degrees and 25 degrees C. A cluster analysis of 47 neurons' responses to cooling revealed two major groups and one minor group. Group 1 neurons (n=19) had a shorter latency, exhibited faster time-to-peak activity, and responded over a smaller range of temperature compared to Group 2 neurons (n=22). Group 3 neurons (n=6) exhibited the longest response latency and responded over a wider cooler range of temperature. Twenty-five out of thirty-one thermally-sensitive, non-tactile lingual neurons responded weakly to at least one chemical stimulus, with some neurons responding to 2, 3, or all 4 chemical stimuli. Group 1 neurons responded to more chemical stimuli at 35 degrees C, while Group 2 neurons responded more at 25 degrees C. Under their optimal temperature conditions, Group 1 and Group 2 neurons responded most often to citric acid and least often to glucose, with NaCl and Q-HCl eliciting an intermediate number of responses. As a whole, the responses of thermally-sensitive fibers to chemical stimulation were modest at best with an absence of chemical specificity. There was no evidence of a 'best' stimulus, although there was a suggestion of temporal coding.

Central projections of nerves innervating the rabbit maxillary sinus localized using wheat germ agglutinin-horseradish peroxidase or choleragenoid-horseradish peroxidase

Central projections of nerves innervating the rabbit maxillary sinus were localized by using wheat germ agglutinin-horseradish peroxidase (WGA-HRP) or choleragenoid-horseradish peroxidase (B-HRP). Tracer was placed into the left maxillary sinus; rabbits were killed 3 or 5 days later, and histochemical localization of transported WGA-HRP or B-HRP was performed. Labeled cell bodies (437-545/animal) were seen in the ipsilateral trigeminal ganglion. Very few labeled cell bodies (zero to three/animal) were observed in the contralateral ganglion. The area of cell bodies labeled by WGA-HRP appeared similar to the area of cell bodies labeled by B-HRP. Transganglionic projections from either tracer were localized to lamina II of the ipsilateral subnucleus caudalis. In addition, WGA-HRP labeling was occasionally observed in lamina I. No labeling was present in other areas of the brainstem. In contrast to the above results, other studies have demonstrated that B-HRP produces terminal-like labeling in deeper layers of the gray matter. We injected B-HRP into the infraorbital nerve and sciatic nerve, which are known to contain projections to deep layers of the gray matter. Labeling was observed in the deep layers of the medullary or spinal dorsal horn 5 days later, suggesting that nerves innervating the sinus only project to superficial laminae. These results suggest that neurons in superficial laminae of the subnucleus caudalis may be important for the reflex initiation of the increased glandular secretions in the maxillary sinus during sinusitis.

Trigeminally-mediated alteration of cardiorespiratory rhythms during nasal application of carbon dioxide in the rat

Stimulation of the upper respiratory tract with air-borne irritants can result in dramatic alterations of cardiorespiratory rhythms that include apnea, bradycardia and selective peripheral vasoconstriction. Since carbon dioxide can stimulate receptors in the nasal passages, we wanted to determine if this odorless gas can induce the same autonomic changes as air-borne irritants. Passing 100% carbon dioxide through the nasal passages of rats anesthetized with chloralose-urethane produced apnea, a vagally-mediated bradycardia and a sympathetically-mediated increase in mean arterial blood pressure. Application of atropine blocked the bradycardia without affecting respiratory or blood pressure changes, while injection of prazosin eliminated blood pressure responses but did not affect heart rate or apnea. There were no significant autonomic responses to nasal application of 10, 25 or 50% carbon dioxide. The responses were mediated through the trigeminal innervation of the nasal mucosa since they could be blocked when the anesthetic procaine was applied to the nasal cavity. We conclude that these cardiorespiratory responses are due to stimulation of trigeminal nociceptors located within the nasal mucosa.

Nasal receptors responding to cold and l-menthol airflow in the guinea pig

The aim of this study was to demonstrate the presence of nasal 'cold' receptors, through recordings of action potentials from the ethmoidal nerve (EN), in guinea pigs and to characterize their responsiveness to l-menthol and capsaicin. Constant flows (400 ml/min) of room air (20 degrees C), warm air (45 degrees C), room air containing l-menthol, and cold air (-5 degrees C) were directed into the nasal cavity in the inspiratory direction via a nasopharyngeal catheter in the anesthetized guinea pigs breathing spontaneously through a tracheostomy. The ethmoidal afferent activity was increased by cold air, and to a greater extent by l-menthol but hardly by warm air. After topical anesthesia of the nasal cavity with 2% lidocaine, cold air and l-menthol no longer stimulated the EN. L-menthol noticeably stimulated the EN even after repeated capsaicin instillation into the nose, but these values were lower than those following the l-menthol stimulus before the 1st capsaicin treatment. These results suggest that the ethmoidal nerve in guinea pigs has cold-sensitive receptors which consist of both small myelinated fibers and C-fiber endings.

Nasal receptors responding to noxious chemical irritants

This study was performed to investigate the chemoreception of trigeminal afferents in the nose. Single unit activity was recorded from the anterior ethmoidal nerve in the anesthetized guinea pig breathing through a tracheostomy during nasal instillation of capsaicin (0.3 mM), nicotine (6 mM) and ammonia (1.5 M) solutions or with distilled water. Out of 36 fibers recorded, nineteen were stimulated by capsaicin, six by nicotine and seventeen by ammonia. Among those fibers, two were stimulated by both capsaicin and nicotine, six by both capsaicin and ammonia and one nicotine-responsive fiber was also stimulated by ammonia. A large proportion of capsaicin- and nicotine-responsive fibers exhibited long lasting discharges (170.4 +/- 17.7 sec and 120.7 +/- 29.3 sec, respectively), and were not stimulated by the second application of the same substance. However, fibers responding to ammonia discharged for a shorter time (31.5 +/- 6.5 sec), indicating a rapid adaptation. These results indicate that the ethmoidal nerve possesses a well-developed responsiveness to noxious stimuli. The nociceptive component of this nerve may be related to the various cardiorespiratory responses that can be elicited from the nasal cavity and also to local axonal reflexes (neurogenic inflammation) due to the release of chemical mediators from C-fiber endings.

Postnatal development of the anterior ethmoidal nerve in cats: unmyelinated and myelinated nerve fiber analysis

This is the first quantitative electron microscopic study of anterior ethmoidal nerve in adult and newborn cats. The adult nerve comprises about 1,000 myelinated fibers including A delta (65%) and A beta (35%) fibers and 6,000 unmyelinated fibers. At birth, only 27% of the adult myelinated fibers complement is already present. The immaturity of the nerve is discussed in relation to that of the sneeze reflex.

Recovery of the jaw-opening reflex after lesions of the lingual nerve in the rat

In three groups of rats, lesions were produced in the right lingual nerves near the base of the tongue; the three types of injury inflicted (cryogenic, crush, and stretch) are reputed to spare the epineurium but produce different degrees of intraneural damage. In regular assessments of recovery, an electrical stimulus (sufficient to elicit the jaw-opening reflex) was applied to either side of the tongue in turn; the amplitude of the reflex was measured as the isometric force of jaw opening. The size of the reflex response to stimulation of the injured side was followed up to 4 months post-lesion, with the response elicited from the control side used as the reference level. The reflex was absent when the experimental side was stimulated immediately after creation of a lesion; the first sign of reflex recovery was found at about 15 days post-operative. Subsequently, in 84% of the animals, the reflex activity elicited from the experimental side increased until it exceeded that elicited from the reference side; this relative hyperreflexia started 1-4 months post-lesion and had a highly variable duration. There was no difference in the incidence, latency, or duration of the hyperreflexia following any of the three types of lesion. The hyperreflexia found in this study is not readily explained by existing hypotheses of the mechanisms underlying post-lesion hyperesthesia or central neuronal hyperexcitability.

Selectivity of lingual nerve fibers to chemical stimuli

The cell bodies of the lingual branch of the trigeminal nerve were localized in the trigeminal ganglion using extracellular recordings together with horseradish peroxidase labeling from the tongue. Individual lingual nerve fibers were characterized with regard to their conduction velocities, receptive fields, and response to thermal, mechanical, and chemical stimuli. Fibers were classified as C, A delta, A beta, cold, and warm. The chemical stimuli included NaCl, KCl, NH4Cl, CaCl2, menthol, nicotine, hexanol, and capsaicin. With increasing salt concentration the latency of the response decreased and the activity increased. The responses elicited by salts (to 2.5 M), but not nonpolar stimuli such as menthol, were reversibly inhibited by 3.5 mM of the tight junction blocker, LaCl3. These data suggest that salts diffuse into stratified squamous epithelia through tight junctions in the stratum corneum and stratum granulosum, whereupon they enter the extracellular space. 11 C fibers were identified and 5 were characterized as polymodal nociceptors. All of the C fibers were activated by one or more of the salts NaCl, KCl, or NH4Cl. Three C fibers were activated by nicotine (1 mM), but none were affected by CaCl2 (1 M), menthol (1 mM), or hexanol (50 mM). However, not all C fibers or even the subpopulation of polymodals were activated by the same salts or by nicotine. Thus, it appears that C fibers display differential responsiveness to chemical stimuli. A delta fibers also showed differential sensitivity to chemicals. Of the 35 characterized A delta mechanoreceptors, 8 responded to NaCl, 9 to KCl, 9 to NH4Cl, 0 to CaCl2, menthol, or hexanol, and 2 to nicotine. 8 of 9 of the cold fibers (characterized as A delta's) responded to menthol, none responded to nicotine, 8 of 16 were inhibited by hexanol, 9 of 19 responded to 2.5 M NH4Cl, 5 of 19 responded to 2.5 M KCl, and 1 of 19 responded to 2.5 M NaCl. In summary, lingual nerve fibers exhibit responsiveness to chemicals introduced onto the tongue. The differential responses of these fibers are potentially capable of transmitting information regarding the quality and quantity of chemical stimuli from the tongue to the central nervous system.

Influence of trigeminal nasal afferents on bulbar respiratory neuronal activity

This study examined the influence of nasal trigeminal afferents, the anterior ethmoidal nerve (AEN) and posterior nasal nerves (PNN) on the spike discharges of respiratory-related neurons recorded in the ventral respiratory group (VRG) (2.6-3.5 mm lateral to the midline, from 1 mm rostral to 3 mm caudal to the obex and at depth of 2-4 mm below the dorsal surface). Electrical stimulations to the AEN and PNN were administered to 10 pentobarbital anaesthetized cats and to 8 ketamine anaesthetized, vagotomized, curarized and ventilated cats. Single shock stimulations of either nerve evoked transient and total inhibition of inspiratory activities. Expiratory-related neurons of the VRG presented three patterns of activity in response to stimulation:excitation, inhibition or inhibition followed by excitation. More generally, expiratory units are activated with a short latency. In the course of repetitive stimulation of the AEN and PNN we observed a prolongation of the spontaneous inspiratory discharge which presented transient, short inhibition in response to each shock. Most expiratory units presented a short activation which was synchronous with the transient inhibition of inspiratory activities. When repetitive stimulation provoked a sneeze-like response, we observed a progressive increase in the duration of transient inspiratory inhibition first, associated with a progressive reinforcement of transient expiratory activation. Secondarily, just before the expiratory thrust, we noted a stronger inhibition of the inspiratory activity which preceded a high-frequency (400 Hz) expiratory discharge. Nasal afferents exert a forceful excitatory effect on bulbospinal (BS) and non-bulbospinal-non-vagal (NBS-NV) expiratory cells of the VRG. The effects due to vagotomy and curarization are discussed.

Effects of aging and food restriction on the trigeminal ganglion: a morphometric study

A quantitative morphometric study of the rat trigeminal ganglion was conducted to determine the changes that occur with aging. All measurements were tracked from young to old age in two rat groups simultaneously. One group was fed ad libitum, the other was maintained on restricted food intake from 6 weeks on. Immunocytochemical and radioimmunoassay techniques were used to study the neuron group that produces the peptide, CGRP and to compare it with the CGRP-negative neuron group. We observed that in the trigeminal ganglion, soma diameters and nucleus diameters of all neurons, whether CGRP positive or negative, increased modestly with age; so did total ganglion weight. Food restriction delayed, but did not prevent the increases in neuron diameters. No significant changes occurred as a function of age in the total number of neurons per ganglion, the ratio of CGRP positive to CGRP negative neurons and ganglion content of CGRP. Food restriction did not affect the parameters that remained constant with age. These findings are in contrast to the marked inhibitory effect of food restriction on age-related increase in thyroid calcitonin, a hormone that is encoded by the same gene as CGRP.

Axon populations in cat lingual and chorda tympani nerves

The lingual and chorda tympani nerves from five cats were examined so that normal axonal populations could be determined. After perfusion fixation, the chorda tympani and lingual nerves were removed and processed, and sections were taken from individual and combined nerves for both light and electron microscopy. The chorda tympani remained as a distinct group of smaller axons for at least 4 mm distal to its junction with the lingual nerve. The mean number +/- S.D. of myelinated axons in the chorda tympani central to the junction was 1322 (+/- 268) and in the lingual nerve central to the junction, 3227 (+/- 510). The counts were not significantly different distal to the junction, and there were no side-to-side differences. Mean myelinated axon circumferences were significantly smaller in the chorda tympani (12.86 +/- 0.87) than in the lingual nerve (22.79 +/- 1.99; p less than 0.01). The mean size of axons in the chorda tympani was slightly but consistently larger on the left (13.1 +/- 0.73) than on the right side (12.61 +/- 1.01; p less than 0.05). Distal to the junction, the average proportion of non-myelinated axons was 44% in both chorda tympani and lingual nerves.

A comparative HRP study of the neuronal supply to the inferior and superior nasal meatus in the cat

Neurons supplying the nasal mucosa in the cat were retrogradely labelled with horseradish peroxidase. Sensory trigeminal neurons to the inferior and superior nasal meati are somatotopically organized, according to the ophthalmic or maxillary origin of the afferents studied. Whatever their relative location, the cell bodies from nasal afferents were, on average, smaller than the overall cell population in the ganglion, in keeping with the high proportion of nasal receptors innervated by thin fibers. Postganglionic neurons from parasympathetic origin could be labelled in the sphenopalatine ganglion. These neurons probably supply mucosal secretory glands. They are in the same size range as the bulk of neurons in the same ganglia.

Trigeminal nasal receptors related to respiration and to various stimuli in cats

In twenty adult cats of either sex under nembutal anaesthesia, we aimed at delineating the sensitive territory of trigeminal nerves innervating the nasal mucosa. The different trigeminal nerves (anterior ethmoidal, posterior nasal and infraorbital nerves) were dissected in the orbit. Activity of these nerves was recorded during spontaneous nasal and tracheal breathing and in response to various stimuli: mechanical (manual probing and air jets) and irritants (ammonia vapours). Multiple and unitary activity recorded in nerve filaments enabled a classification of the receptors on the basis of their discharge pattern as rapidly-, intermediately- or slowly adapting receptors, and as drive or non-drive nasal receptors depending on whether or not the respiratory modulation was preserved during tracheal breathing.

Trigeminal nociceptive neurons in the subnucleus reticularis ventralis. I. Response properties and afferent connections

Trigeminal nociceptive neurons within the subnucleus reticularis ventralis medullae oblongatae (SRV), which lies ventral to the trigeminal subnucleus caudalis and subnucleus reticularis dorsalis medullae oblongatae, were studied in urethane/chloralose-anesthetized cats and monkeys. These neurons were called 'SRV neurons'. They were almost regularly excited by pressure to the ipsilateral cornea or to both corneas at a strength well above the human corneal pain threshold. Most of them were activated by noxious mechanical stimulation of the pinna, face and/or tongue. A significant fraction of SRV units was responsive to tapping of the ipsilateral dorsum of the nose and/or electrical stimulation of tooth pulp afferents. Evidence was obtained that responses to tapping of the dorsum of the nose were due to mechanical distortion of the nasal mucosa. Intracellular injection of HRP into SRV neurons demonstrated that injected neurons were large neurons characteristic of the SRV. Trigeminal tractotomy just rostral to the obex did not eliminate responses of SRV units to trigeminal inputs. Neurons relaying trigeminal inputs to SRV neurons were electrophysiologically identified in the nucleus reticularis parvocellularis which is ventromedially adjacent to the subnuclei oralis and interpolaris of the trigeminal spinal tract nucleus. These findings were supported by HRP injection into the SRV. Units having receptive fields similar to those of SRV neurons were found in lamina VII of the first cervical cord, suggesting that SRV neurons may be trigeminal lamina VII neurons.

Controlled noxious chemical stimulation: responses of rat trigeminal brainstem neurones to CO2 pulses applied to the nasal mucosa

The nasal mucosa of halothane-anesthetized rats was stimulated with defined CO2 pulses. Recordings were performed from single trigeminal brainstem neurons to characterize their responses to this controlled chemical irritation. All cells examined with this stimulus displayed graded discharges to increasing concentrations of CO2. Enhanced responses were obtained in a group of neurons when the duration of the interstimulus interval was increased. The application of chemical irritants, notably mustard oil or acetic acid induced vigorous ongoing discharges in all cells tested. The CO2 stimulation method described here thus provides an ideal model for the quantitative physiological and pharmacological examination of chemically induced nociception.

Electrophysiological responses to non-electrolytes in lingual nerve of rat and in lingual epithelia of dog

Epithelial and neural mechanisms underlying the trigeminal chemoreception of non-electrolytes were investigated in whole-nerve recordings from lingual nerve and in Ussing-chamber studies of isolated lingual epithelia. The non-electrolytes included menthol, amyl acetate, phenethyl alcohol, toluene, methanol, ethanol, propanol, butanol, hexanol and octanol. They produced different lingual nerve responses: methanol and ethanol only increased ongoing activity; longer-chain alcohols initially increased but then suppressed activity below baseline; phenethyl alcohol and toluene only suppressed activity. Their threshold concentrations for lingual nerve responses, with the exception of menthol, were proportional to the octanol:water partition coefficients of the stimuli. The threshold concentration for menthol was significantly lower than predicted by this coefficient. Calculation of the free energy of transfer from the threshold concentrations for the n-alcohols suggests that they undergo partition into a hydrophobic environment such as is found in lipid bilayers. Lanthanum chloride, which inhibited lingual nerve responses to hydrophilic compounds, presumably by blocking their diffusion across tight junctions, did not inhibit responses to these non-electrolytes. At high concentrations, hexanol acted as an anaesthetic in that the lingual nerve no longer responded to thermal and chemical stimuli whereas ethanol, which only increased lingual nerve activity, did not inhibit those responses. Epithelial transport, as indicated by the short-circuit current (Isc) measured across tongues bathed in symmetrical solutions of Krebs-Henseleit buffer, was reversibly inhibited by ethanol, hexanol, octanol, phenyl ethanol and menthol. The stimulus concentration necessary to inhibit 50% of the Isc decreased with increasing octanol:water partition coefficient.(ABSTRACT TRUNCATED AT 250 WORDS)

Central projections of trigeminal primary afferents innervating the nasal mucosa: a horseradish peroxidase study in the rat

The respiratory region of the nasal mucosa is innervated by the ethmoidal nerve. Chemical nociceptive stimulation of this area leads to upper airway reflexes that prevent access of noxious substances to the respiratory tract and the lungs. In the present study we examined the localization of the cell bodies of the respective primary afferent fibres within the trigeminal ganglion, as well as their central projections. In 25 rats a horseradish peroxidase-wheat germ agglutinin gel was applied to the right nasal cavity. The animals were killed after 48-72 h. For visualization of the tracer the tissue was processed according to the tetramethylbenzidine method. In the trigeminal ganglion almost all labelled cell bodies were localized in a medial band immediately caudal to the entrance of the ophthalmomaxillary branch. Transganglionic projections to the trigeminal brainstem nuclear complex were only localized in the superficial laminae of the subnucleus caudalis and in the subnucleus interpolaris, areas known to be involved in processing of nociceptive information. An additional labelled terminal field was observed in the interstitial subnucleus of the nucleus tractus solitarius, which is involved in respiratory control. These results are in favour of the hypothesis that the ethmoidal nerve in rat constitutes the afferent limb of protective upper airway reflexes since it transmits mainly nociceptive information.

Effect of experimentally elicited rhythmic oral activity on the linguodigastric reflex in the lightly anesthetized rabbit

The digastric reflex was elicited in lightly anesthetized juvenile rabbits by electrical stimulation of the tongue. The reflex was depressed by maintained intraoral mechanical stimuli and facilitated by transient intraoral mechanical stimuli applied using a vinyl tube in the buccal sulcus. Intraoral mechanical stimuli were also used to elicit rhythmic oral activities; these were of four types but only the two most easily distinguished types were studied further. One type, elicited by agitating a soft vinyl tube in the buccal sulcus (the tube was then taken further into the mouth by the animal and chewed), was categorized as "transport/chew"; another type, most easily elicited by fluid passed down the stationary tube (which was then removed), was categorized as "liquid." When initiated, the two rhythmic activities continued with no further external stimulation although the intraoral self-stimulation differed. When the digastric reflex was elicited during rhythmic movements, the response was phasically modulated. More than 80% of the digastric reflex responses elicited during jaw closure in "liquid" cycles were less than control values but less than 60% of responses elicited in the same phase of "transport/chew" cycles were less than control values; the difference was significant (P less than 0.001; chi 2). The findings were consistent with two hypotheses: that sensory input, generated by the animal during the rhythmic oral movements, modulates the reflex; and that stimuli initiating particular rhythmic oral movements select one of a number of potential modes of operation of the pattern generator, each with its characteristic pattern of modulation of the digastric reflex.

Quantitative diameter analysis of lingual nerve axons in man

Previous studies on axon counts and fiber-diameter spectra in lingual nerves have been carried out only on animal models. This study reports an histological investigation on a series of 20 lingual nerves removed post mortem from human subjects. The results show wide variation in the myelinated fiber counts--a variation which does not appear to be related to the ages of the subjects. When the results are compared with those of a previous study (Heasman and Beynon, 1983), it is seen that the lingual nerve:inferior dental nerve ratio of axon counts is not a consistent index. The fiber-diameter spectrum for the human lingual nerve is characterized by a bimodal curve with the more pronounced peak in the small-diameter fiber range.

Central projections of the ethmoidal nerve of the cat as determined by the horseradish peroxidase tracer technique

The ethmoidal nerve innervates the nasal mucosa and constitutes the afferent limb of several upper airway protective reflexes. Protective reflexes, such as sneezing, coughing, and apnea, are those reflexes that either expel foreign substances from the respiratory tract or stop them from gaining access to the lungs. The afferents for nasal receptors are thought to be a part of the trigeminal system rather than olfactory in nature. The objective of this study was to localize the cell bodies of these ethmoidal afferents and to trace the central projections of these neurons. Horseradish peroxidase was applied to the ethmoidal nerve in 11 adult cats. Following a survival period of 48-72 hours, the animals were killed and the tissue was processed according to the tetramethylbenzidine method. Reaction product was localized in cell bodies within the trigeminal ganglion, concentrated caudal to the entrance of the ophthalmic trunk of the trigeminal nerve. Transganglionic projections to the spinal trigeminal nucleus were localized primarily in the subnucleus interpolaris and in layers I and II of the subnucleus caudalis. There was also reaction product in cell bodies within the mesencephalic trigeminal nucleus. These results are in keeping with projections of other ophthalmic division receptor afferents, such as the cornea and the supraorbital nerve.

Pyramidal tract of the cat: axon size and morphology

The purpose of this work was to determine the number and morphology of pyramidal tract (PT) axons in the cat, using electron microscopy, modern methods of fixation, and computer-assisted morphometric analysis. Sections taken at the level of the medullary pyramids in three animals were fixed and magnified up to 10,000 X to produce photomicrographs. Morphological data were entered into computer files for analysis by tracing axon perimeters on micrographs mounted on a digitizer tablet. The number of axons per PT averaged 415,000, of which 88% were myelinated and 12% were unmyelinated. 90% of the myelinated axons fell in the diameter range 0.5-4.5 microns. Axons larger than 9 microns diameter accounted for 1% of the total; the largest were 20-23 microns. Myelinated axon mean diameter was 1.98 microns; because of the skewed distribution, with many small axons and a few very large axons, median diameter was 1.60 micron. Size distribution was relatively uniform throughout the PT cross section, with all sizes represented in all regions. However, the more medial regions had a higher proportion of small fibers than the more lateral regions: mean medial diameter was 1.85 micron while mean lateral diameter was 2.09 microns. Myelin sheath thickness averaged 7.9% of fiber diameter for axons up to 11 microns, but was constant at 0.9 micron for larger fibers. Myelinated fibers were distorted from the circular shape in cross section, with a mean circularity index (or form factor) of 0.85, which implies that the fibers could swell about 15% without rupture of the cell membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Corneal pain evoked by thermal stimulation

The thermal sensitivity of the eyelid and cornea was compared using an automated apparatus to produce stimulus pulses of known magnitude and duration over the range 33--45 degrees C. Subjects reported only temperature sensation when the skin of the upper eyelid was tested; however, corneal stimulation in the same subjects was always perceived as nociceptive. The possibility that other ocular tissues may be involved in the pain responses was shown to be unlikely by direct experimentation or by calculation of heat flow in those tissues. Cornea and eyelid thresholds were compared in relationship to the structural and physical properties of these tissues. It was found that the nerve endings of the corneal epithelium are less sensitive to temperature change when compared to the thermal receptors of the eyelid. It is concluded that the cornea is useful for the experimental study of pain.

Ouabain binding to renal tubules of the rabbit

It is well known that ouabain, a specific inhibitor of Na-K ATPase-dependent transport, interferes with renal tubular salt reabsorption. In this study, we employed radiochemical methods to measure the kinetics of [3H]ouabain binding to slices of rabbit renal medulla and high resolution quantitative autoradiography to determine the location and number of cellular binding sites. The kinetics obeyed a simple bimolecular reaction with an association constant of 2.86 +/- 0.63 SD x 10(3) M-1 min-1 and a dissociation constant of 1.46 x 10(-3) min-1, yielding an equilibrium binding constant of 0.51 x 10(-6) M. Binding was highly dependent upon temperature. At a concentration of 10(-6) M, the rate of accumulation between 25 degrees C and 35 degrees C exhibited a Q10 of 1.8. At 0 degree C the rate of ouabain dissociation was negligible. The specificity of binding was demonstrated with increasing potassium concentrations. At a concentration of 1 microM, 6 mM, and 50 mM K+ produced a 2.5- and 7-fold decrease, respectively, in the rate of ouabain accumulation observed at zero K+. Binding was completely inhibited by 1 mM strophanthin K. The major site of ouabain binding was the thick ascending limb; little or no binding was observed in thin limbs and collecting ducts. Moreover, binding was confined to the basolateral membranes. From autoradiographic grain density measurements, it was estimated that each cell contains over 4 x 10(6) ouabain binding sites or Na-K ATPase molecules. These results taken together with physiological and biochemical observations suggest that Na-K ATPase plays a key role in salt reabsorption by this segment.