Efferent and antidromic vasodilator impulses to the tongue in the chorda-lingual nerve of the cat

Clinical anatomy of the chorda tympani: a systematic review

OBJECTIVE

The chorda tympani is at risk of iatrogenic injury throughout its course. This paper reviews the clinical anatomy of the nerve in adults.

DESIGN

Systematic literature review.

METHOD

Relevant English-language articles were identified using five electronic databases and one search engine. Data from approximately 70 scientific papers were supplemented with information from selected reference texts.

RESULTS

The anatomy of the chorda tympani differs from standard descriptions, particularly regarding its exit from the middle ear and area of lingual innervation. Whilst it is known to convey taste sensation from the anterior two-thirds of the tongue and parasympathetic innervation to the submandibular and sublingual salivary glands, the chorda tympani probably has additional sensory and secretomotor functions.

CONCLUSION

A detailed understanding of the anatomy of the chorda tympani may help to reduce the risk of iatrogenic injury during head, neck and middle-ear surgery, and to explain the variable consequences of such injury.

Does the chorda tympani nerve confer general sensation from the tongue?

OBJECTIVE

Patients often complain of numbness of the tongue after otologic procedures; this symptom may occur with or without taste alterations. The purpose of this study was to objectively assess possible changes in the general sensation of the tongue in patients undergoing middle ear surgery.

STUDY DESIGN AND SETTING

Thirty-three individuals were included in this prospective controlled clinical study conducted at a tertiary referral center. Fifteen patients underwent middle ear surgery and 18 volunteers served as a control group. Subjects underwent objective tests to assess general sensation of the tongue before and twice after surgery.

RESULTS

Forty-seven percent of the patients complained of numbness or tingling of the tongue shortly after surgery. In these patients, a significant reduction in sensitivities to light touch and 2-point discrimination on the operated side were noted at that time (P < 0.01; P < 0.009 respectively). Values returned to baseline on subsequent evaluations. No sensitivity change was noted for the contralateral hemi-tongue.

CONCLUSION

Objective changes in the general sensory function of the tongue correlated with post-operative clinical symptoms experienced by patients. The results of this study support the notion that the chorda tympani nerve confers general sensation from the tongue.

EBM RATING

A-1b.

The effect of nerve growth factor on functional recovery after injury to the chorda tympani and lingual nerves

Nerve growth factor (NGF) is known to ameliorate central changes and enhance the regeneration of damaged axons in the early stages after peripheral nerve injury. We have assessed the long-term outcome of placing NGF at a nerve repair site by determining the functional characteristics of several groups of sensory afferent and autonomic efferent fibres in the cat lingual nerve. Six months after entubulation repair, with or without the incorporation of NGF, the recovery of secretomotor and vasomotor efferents was determined by recording salivary flow from the submandibular gland and temperature changes on the tongue surface, each evoked by stimulation of the repaired nerve. Electrophysiological recordings from the lingual and chorda tympani nerves proximal to the repair allowed characterisation of mechanosensitive, thermosensitive and gustatory afferents. When compared with data from uninjured control animals, both repair groups showed changes in spontaneous discharge and persistent reductions in conduction velocity, receptor sensitivity, proportion of gustatory units, and rate of salivary secretion. Comparisons between the outcome of repair with or without NGF revealed few differences. In the NGF group the conduction velocity of afferents in the lingual nerve was lower, and the level of spontaneous activity was higher. However, NGF appeared to preferentially enhance the regeneration of thermosensitive afferents, suggesting that it may play a role in determining the phenotypic profile of the regenerating axonal population. This suggests that future therapeutic enhancement of regeneration after peripheral nerve injury may require a combination of factors to encourage regeneration of specific fibre groups.

The effect of brain-derived neurotrophic factor on sensory and autonomic function after lingual nerve repair

Brain-derived neurotrophic factor (BDNF) is important in the response to peripheral nerve injury and may enhance regeneration. We have assessed its role in the functional recovery of sensory afferents and autonomic efferents after repair of the chorda tympani and lingual nerves in the cat. Six months after entubulation repair, with or without the incorporation of BDNF at the repair site, the recovery of secretomotor and vasomotor efferents was determined by recording salivary flow from the submandibular gland and temperature changes on the tongue surface, each evoked by stimulation of the repaired nerve. Electrophysiological recordings from the lingual and chorda tympani nerves proximal to the repair were undertaken to characterise mechanosensitive, thermosensitive, and gustatory afferents. When compared with data from uninjured control animals, both repair groups showed changes in receptor sensitivity and spontaneous discharge, and persistent reductions in conduction velocity, proportion of gustatory and thermosensitive units, rate of salivary secretion, and vasomotor responses. Comparisons between the outcome of repair with or without BDNF revealed few differences. In the BDNF group, fewer units in the chorda tympani responded to gustatory or thermal stimuli and the sensitivity of the gustatory units was lower. The conduction velocity of afferents in the lingual nerve was also lower, but the mechanoreceptive field size was higher. Thus, despite its known trophic role in the gustatory system, BDNF had not enhanced recovery of these or other fibre populations. We conclude that the application of BDNF to a site of lingual nerve repair has a negative effect on the long-term outcome.

An electrophysiological study into the effect of neurotrophin-3 on functional recovery after lingual nerve repair

OBJECTIVE

Neurotrophin-3 (NT-3) is known to ameliorate central changes that result from peripheral nerve injury and may promote regeneration of myelinated axons. We have assessed its role in the functional recovery of sensory afferents and autonomic efferents after repair of the chorda tympani and lingual nerves in the cat.

DESIGN

Six months after entubulation repair, with or without the incorporation of NT-3 at the repair site, the recovery of secretomotor and vasomotor efferents was determined by recording salivary flow from the submandibular gland and temperature changes on the tongue surface, each evoked by stimulation of the repaired nerve. Electrophysiological recordings from the lingual and chorda tympani nerves proximal to the repair allowed characterisation of mechanosensitive, thermosensitive and gustatory afferents.

RESULTS

When compared with data from uninjured control animals, both repair groups showed persistent reductions in conduction velocity, receptor sensitivity, spontaneous discharge, proportion of gustatory and thermosensitive units, and rate of salivary secretion. Comparisons between the two repair groups revealed that in the NT-3 group, salivary secretion rate was lower and the activity evoked in the chorda tympani by gustatory or thermal stimuli was lower, but the spontaneous discharge rate was higher. Mechanosensitive units in the lingual nerve had slower conduction velocities but the mechanoreceptive field size, adaptation time and discharge frequency had increased.

CONCLUSIONS

Despite its known trophic role in the lingual somatosensory system, NT-3 did not enhance functional recovery from injury and had a negative effect on the long-term outcome for sensory and autonomic fibres.

Quantitative immunohistochemical investigation of the intrinsic vasodilator innervation of the guinea pig lingual artery

The vasculature of the guinea pig tongue is supplied by parasympathetic vasodilator nerve fibres of intrinsic origin. Here, we investigated first to what extent neuropeptides and the synthesizing enzymes of NO, CO and acetylcholine are contained and colocalized within periarterial lingual vasodilator axons of intrinsic origin. Then it was determined whether perivascular innervation by these fibre types changes with vascular diameter, in particular in comparison with the sensory substance P (SP)-positive and sympathetic noradrenergic vascular innervation. To this end, single, double and triple labelling histochemical techniques were performed on control tongues and tongues kept in short-term organotypic culture to induce degeneration of extrinsically originating nerve fibres. Cell bodies of intrinsic microganglia and their periarterial axons contained, simultaneously, NO synthase, vasoactive intestinal peptide and the acetylcholine-synthesizing enzyme choline acetyltransferase. Additionally, neuropeptide Y (NPY) was observed in a small percentage (12%) of neurons that increased to 39% after 36 h of organotypic culture. The CO synthesizing enzyme heme oxygenase-2 was detected only in perikarya but not in periarterial axons. Intrinsic vasodilator fibres were invariably present at arteries down to a luminal diameter of 150 microm, and reached 65% of section profiles of smallest arterioles, while noradrenergic and substance P-positive axons reached 80% of arteriolar profiles. These findings show that the intrinsic lingual vasodilator innervation of the guinea pig is far extending although slightly less developed than that by sensory and sympathetic axons, and differs both in this aspect and in patterns of colocalization from that reported for other organs, e.g. lung and pelvic organs.

The re-innervation of the tongue and salivary glands after lingual nerve repair by stretch, sural nerve graft or frozen muscle graft

The lingual nerve is sometimes injured during the surgical removal of impacted third molar teeth and may require repair. Removal of the damaged section of nerve prior to repair leaves a gap between the nerve ends, and we have investigated methods of closing the gap. THe characteristics of regenerated fibers in the chorda tympani have been recorded in cats 24 weeks after the removal of a segment of lingual nerve and repair of the defect by three methods. The nerve gap was closed either by stretching the nerve ends together and repairing under tension, or by the insertion of a sural nerve graft or freeze-thawed muscle graft. The properties of gustatory, thermosensitive, and mechanosensitive units and the return of the vasomotor and secretomotor responses were investigated by electrophysiological techniques and the data from each of the repair groups compared with those obtained from a series of normal control animals. After each method of repair, the integrated whole-nerve activity recorded from the chorda tympani during gustatory or thermal stimulation of the tongue was reduced when compared with controls, but there was little significant difference between the repair groups. Recordings made from single units in the chorda tympani revealed that conduction velocities were faster after stretch repair than after sural nerve graft or frozen muscle graft. In addition, 48% of the units had developed into principally gustatory units after stretch repair, indicating a better level of recovery than in the graft groups, which contained 33% and 32%, respectively. The secretomotor responses were also significantly greater after stretch repair than in either of the graft groups or the controls, but there was no difference in the vasomotor responses. These results reveal that repair of a short gap in the lingual nerve by stretching the ends together is followed by better overall recovery than after grafting, but where a graft is used, a similar level of recovery results from use of a frozen muscle graft or a sural nerve graft.

The effect of delayed nerve repair on the properties of regenerated fibres in the chorda tympani

The characteristics of regenerated fibres in the chorda tympani have been investigated in cats after nerve section without repair or after section followed by nerve repair twelve weeks later. In the unrepaired group the animals were allowed to recover for twenty four weeks and after delayed repair there was a further recovery period of twelve or twenty four weeks. The properties of gustatory, thermosensitive and mechanosensitive units and the return of vasomotor and secretomotor responses were then investigated and data compared with that from normal controls and from animals which had undergone immediate nerve repair. After nerve section, integrated whole-nerve activity recorded from the chorda tympani during gustatory or thermal stimulation of the tongue was reduced when compared to controls, but there were only small differences between the repaired and unrepaired groups. Recordings made from single units in the chorda tympani revealed that more units were spontaneously active after repair (P < 0.05) and the gustatory units produced more impulses when stimulated (P < 0.005). Twelve weeks after delayed repair the units had slower conduction velocities than those in the unrepaired nerves (P < 0.001), but by twenty four weeks after repair they were significantly faster (P < 0.05). There was little difference in the level of recovery twelve weeks after immediate or delayed repair. We conclude that delayed nerve repair results in better recovery than leaving the nerve unrepaired and that a twelve week delay before repair has little effect.

The reinnervation of the tongue and salivary glands after two methods of lingual nerve repair in the cat

The recovery of fibres in the chorda tympani after repair by epineurial suture or entubulation was investigated. The combined trunk of the chorda tympani and lingual branch of the trigeminal nerve was sectioned unilaterally, repaired using either epineurial sutures or entubulation, and allowed to recover for 12 weeks. The properties of gustatory, thermosensitive and mechanosensitive units, and the return of vasomotor and secretomotor responses were then investigated. After repair by epineurial suture, integrated whole-nerve activity recorded from the chorda tympani during stimulation of the tongue with gustatory or thermal stimuli was reduced in all areas of the tongue when compared to controls. After entubulation repair, little or no activity could be recorded. Recordings made from 57 single units in the chorda tympani after repair by epineurial suture revealed a greater proportion of purely mechanosensitive units and fewer gustatory units than in the controls. Fewer units were spontaneously active, they had lower maximum discharge frequencies, and produced fewer impulses when stimulated. Recordings made from 61 single units after repair by entubulation revealed receptor characteristics with greater differences from controls than after epineurial suture and there was only one gustatory and one thermosensitive unit. Vasomotor responses were completely restored after repair by epineurial suture, but some responses were smaller after entubulation repair. Secretomotor responses were significantly smaller after both methods of repair and there was no difference between the two groups.

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.

The reinnervation of the tongue and salivary glands after lingual nerve injuries in cats

The recovery of fibres in the chorda tympani and lingual nerves has been investigated in cats following nerve injury by recording the receptor properties of gustatory, thermosensitive and mechanosensitive units and the return of vasomotor and secretomotor responses. The combined trunk of the chorda tympani and lingual nerves was either crushed (4 animals) or sectioned (3 animals) unilaterally and recovery allowed for 12 weeks. After nerve crush, integrated whole nerve activity recorded from the chorda tympani during stimulation of the tongue with gustatory or thermal stimuli revealed a response profile which was similar to controls. After nerve section little or no activity could be recorded. Recordings made from 52 single units in the chorda tympani after nerve crush revealed that the proportions of gustatory, thermosensitive and mechanosensitive units were similar to those of controls. The units had slower conduction velocities, responded less vigorously and to a narrower range of stimuli. Recordings made from 46 units in the chorda tympani after nerve section revealed very few gustatory or thermosensitive units, the majority were purely mechanosensitive and the decrease in conduction velocity was greater than after nerve crush. Electrical stimulation of efferent vasodilator fibres in both the chorda tympani and lingual nerves, evoked a temperature rise on the dorsal surface of the tongue. This effect was completely restored after nerve crush but was significantly smaller after nerve section. The flow rate of saliva from the submandibular salivary gland was not significantly changed by nerve crush but was significantly smaller after nerve section. There was no evidence for functional reinnervation of gustatory or secretomotor terminals by inappropriate fibre types.

The characteristics and regional distribution of afferent fibres in the chorda tympani of the cat

1. Experiments were carried out on the cat's tongue to investigate regional variations in sensitivity to gustatory and thermal stimuli and to determine whether taste buds responding to particular stimuli are unique to specific areas. 2. In ten cats, integrated whole-nerve activity was recorded from the left chorda tympani during stimulation of the anterior, middle or posterior segments of the tongue. The level of activity produced by gustatory or thermal stimuli increased towards the posterior segment of the tongue but the relative activity evoked by each stimulus was similar for each segment. 3. Recordings were made from 108 single units dissected from the chorda tympani; fifty-five responded principally to gustatory stimuli, twenty were principally thermosensitive and thirty-three were purely mechanosensitive. 4. The gustatory units could be subdivided according to the stimuli which evoked the most vigorous discharge; there were sixteen salt units, eighteen acid units, nineteen salt-acid units and two quinine units. The salt units had a higher level of spontaneous activity than the other groups and the acid units supplied more fungiform papillae with a greater separation between them. 5. The two quinine units were both located near to the mid-line posteriorly but all other groups of gustatory, thermosensitive and mechanosensitive units were distributed over all regions of the dorsal surface of the tongue and did not have higher discharge rates in specific areas. 6. Individual stimulation of the fungiform papillae supplied by a single unit revealed that similar responses were evoked from each papilla but stimulation of a single papilla supplied by two or more units could evoke a different response in each unit. 7. This study revealed that the relative sensitivity to gustatory and thermal stimuli was similar in each region of the tongue and that taste buds responding to particular stimuli are not confined to specific areas.

Vasoactive intestinal peptide-like immunoreactivity in nerves associated with the cardiovascular system of guinea-pigs

The distribution of nerves with vasoactive intestinal peptide (VIP)-like immunoreactivity has been examined in the heart and vascular system of guinea-pigs. There was a very sparse supply of fibres to the heart. No immunoreactive cell bodies were found in the intrinsic cardiac ganglia; however, positive nerve cell bodies were seen along the superior vena cava near the right atrium. There were immunoreactive fibres with most arteries; these fibres were located at the media-adventitia junction. The supply to major distributing arteries, such as the aorta, subclavian, carotid and femoral arteries as well as to the pulmonary arteries, was sparse. Of the individual vascular beds, the most densely supplied arteries were the mesenteric and uterine (or in the male deferential) arteries. Arteries running to other organs or tissues, such as skeletal muscle, kidney, pancreas, spleen and heart were less densely supplied. There were clear differences in the innervation of different cerebral vessels. The greatest density was associated with the anterior and middle cerebral arteries. Fewer nerves accompanied the posterior cerebral, cerebellar and meningeal arteries. There was a sparse innervation of the rostral part of the basilar artery. Throughout the body, veins were sparsely supplied. The distribution of nerves with VIP-like immunoreactivity was not changed when noradrenergic nerves were degenerated by 6-hydroxydopamine or when substance P nerves were disrupted by capsaicin. It is concluded that VIP containing nerves innervating the heart and blood vessels form a population distinct from the substance P-containing and the noradrenergic nerves. It is suggested that the VIP fibres might be efferent vasodilator nerves to the blood vessels.

Distribution of choline acetyltransferase in cerebral and extracerebral cranial arteries of the cat. Its relationship to neurogenic atropine-sensitive dilation

Choline-acetyltransferase (ChAT) activity was surveyed in segments of cranial arteries--both cerebral and extracerebral--from the cat. High levels were found in pial arteries, both cerebral and cerebellar, and in the arteries to salivary glands, tongue, and nose. Intermediate levels were found in the external and internal maxillary arteries and many of their branches. Enzyme levels in the arteries supplying the head--common carotid, vertebral, and in several systemic arteries and veins and also the lingual vein--were probably not significant. Only those vessels that have higher ChAT contents show capacity for neurogenic vasodilation. The dilation of segments of a number of these arteries, the basilar, middle cerebral, lingual, and internal maxillary, is reduced significantly by atropine (5 X 10(-7) M). ChAT activity did not correlate with vessel norepinephrine content. The data may be interpreted as defining a functional vasodilator system to the head encompassing both cerebral and extracerebral arteries that depends in part on a functional cholinergic link involving a muscarinic receptor. It is separate from the adrenergic outflow. The tissues supplied by vasculature receiving this type of innervation are of ectodermal origin.

Vasomotor effects of facial nerve stimulation: noncholinergic vasodilation in the eye

The effect of facial nerve stimulation on ocular blood flow was studied in rabbits. The intracranial part of the facial nerve was stimulated electrically and the regional blood flow was measured with labelled microspheres. Effects on the intraocular pressure were determined in a separate series of experiments. Stimulation increased choroidal blood flow by about 200%. The blood flow of the iris and the ciliary processes also increased. The blood flow of the eyelids and the nictitating membrane increased by approximately 1000%, and the blood flow of Harder's gland increased by about 200%. The blood flow of the tongue and the submandibular gland also increased. The increase in the uveal blood flow could not be abolished by a cholinergic or ganglionic blockade. Ganglionic blockade abolished most of the increase in the blood flow of the eyelids, nictitating membrane and Harder's gland; the cholinergic blockade seemed less effective. The intraocular pressure increased with a mean value of 6 mmHg during optimal (20-30 Hz) stimulation. The increase could not be prevented by cholinergic blockade. Much of the increase in uveal blood flow seemed to be caused by stimulation of unconventional nerves. It is suggested that these nerves may be peptidergic using VIP as a transmitter and lacking a hexamethonium sensitive synapse between the site of stimulation and the eye. Their nature--afferent or efferent--remains unknown. A great part of the increase in blood flow of the eyelids, nictitating membrane and Harder's gland seemed to be due to other mechanisms involving nerves with a peripheral synapse.

Vasoactive intestinal polypeptide in cholinergic neurons of exocrine glands: functional significance of coexisting transmitters for vasodilation and secretion

By a combination of the indirect immunofluorescence technique with acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7) staining, it was shown that vasoactive intestinal polypeptide (VIP) is present in cholinergic (acetylcholinesterase-rich) neurons involved in control of secretion and vasodilation in exocrine glands of cat. The submandibular salivary gland was used as a functional model. Preganglionic nerve stimulation induced an atropine-resistant, hexamethonium-sensitive vasodilation and release of VIP into the venous outflow from the gland and an atropine- and hexamethonium-sensitive secretion. Infusion of VIP antiserum reduced both the vasodilation and secretion. Infusion of VIP caused vasodilation only, whereas acetylcholine caused both vasodilation and secretion. Simultaneous infusion of VIP and acetylcholine in low doses resulted in a marked potentiation of both vasodilation and secretion. The present morphological and functional data support the following hypothesis for regulation of vasodilation and secretion in exocrine glands. Preganglionic cholinergic nerves activate, via nicotinic receptors, postganglionic neurons, causing concomitant release from the same nerve endings of two coexisting putative transmitters, acetylcholine and VIP. Acetylcholine produces mainly secretion by a muscarinic action and VIP causes mainly vasodilation, but the two substances seem to cooperate directly or indirectly in both types of response. Thus, the coexistence of two putative neurotransmitters, VIP and acetylcholine, in one neuron may explain the dual effector response (i.e., the cholinergic secretion and the atropine-resistant vasodilation) caused by nerve stimulation in exocrine glands.

Assessment of the effects of vasoactive intestinal peptide (VIP) on blood flow through and salivation of the dog salivary gland in comparison with those of secretin, glucagon and acetylcholine

1. The vascular bed of the submandibular gland in situ was perfused with blood through the glandular artery at a constant pressure in anesthetized dogs. All drugs were administered intra-arterially. 2. Vasoactive intestinal peptide (VIP), secretin and acetylcholine produced a dose-dependent increase in blood flow through the artery (vasodilatation) but glucagon was almost ineffective. 3. Dose-blood flow response curves for VIP and secretin were parallel, and VIP was about 100 times as potent as secretin on a molar basis. Dose-blood flow response curves for acetylcholine were flatter than those for VIP and secretin. Acetylcholine was approximately as potent as secretin on a molar basis. 4. No tachyphylaxis developed to the vasodilator action of VIP. 5. The vasodilator responses to VIP and to electrical stimulation of the chordolingual nerve were scarcely modified by (-)-hyoscyamine in doses that fully antagonized the vasodilator response to acetylcholine. 6. VIP, secretin and glucagon were ineffective in eliciting salivary secretion. 7. The possibility that VIP is released from parasympathetic vasodilator nerves and mediates the atropine-resistant vasodilatation in the dog submandibular gland is discussed.

Vasodilatation by prostaglandin F2alpha in the canine tongue through a parasympathetic mechanism

1 The vascular bed of the tongue in situ was perfused with blood through the lingual arteries at a constant pressure in anaesthetized dogs. All drugs except for SQ 14,225 were administered intra-arterially.2 Prostaglandin F(2alpha) (PGF(2alpha)) produced a dose-dependent increase in blood flow through the lingual arteries (vasodilatation).3 Marked desensitization was observed on the vasodilator responses to repeated administration of PGF(2alpha).4 The vasodilator response to PGF(2alpha) was abolished by tetrodotoxin in doses that abolished the vasodilator response to electrical stimulation of the lingual nerve.5 The vasodilator response to PGF(2alpha) was not affected by hexamethonium in doses that almost abolished the vasodilator response to lingual nerve stimulation.6 The vasodilator responses to PGF(2alpha) and to lingual nerve stimulation were scarcely modified by (-)-hyoscyamine in doses that fully antagonized the vasodilator response to acetylcholine.7 Electrical stimulation of the vago-sympathetic trunk and noradrenaline produced a decrease in blood flow through the lingual arteries.8 These results indicate that the vasodilator response of the tongue to PGF(2alpha) is due exclusively to excitation of parasympathetic postganglionic neurones and that neuronal receptors involved are quite distinct from nicotinic receptors.9 Intravenous administration of SQ 14,225, an inhibitor of angiotensin I converting enzyme or kininase II, augmented the vasodilator responses to bradykinin and kallikrein but not that to lingual nerve stimulation.10 The results suggest that neither kallikrein nor*kinin (including bradykinin) is responsible for the parasympathetically induced vasodilatation in the tongue.

Vasodilator fibres to the tongue in the chorda tympani proper nerve

Electrical stimulation of the chorda tympani proper nerve (CT) causes vasodilatation in the constant-flow perfused tongue. It has been suggested, as an explanation for this, that the vasodilatation is the result of antidromic impulses in sensory fibres of the CT. However, after degeneration of the efferent fibres in CT in the rat it was found that electrical stimulation of the CT caused no vasodilatation. Since, by all criteria available, the sensory fibres remained, the result indicates that antidromic impulses could not be the cause of the vasodiltation observed in tongues with intact nerve supply. Further, since there are no glands in the part of the tongue innervated by the CT and the circulation of the large salivary glands is separate from the tongue in the rat, it is concluded that the vasodilatation was not a secondary effect of glandular activity, but was caused by vasodilator fibres in the CT.

Lingual blood flow and its hypothalamic control in the dog during panting

1. The effects of increased ambient temperature (Ta) on blood-flow and -temperatures of the tongue were studied in the unanaesthetized dog. At Ta of 20 degrees C and a relative humidity (rh) of 30% the mean lingual blood flow was 11 ml-min-1 (0.15 ml-g-1-min-1) and the temperature difference between the lingual artery and vein (deltaTLAV) was 1.0 degrees C. Accordingly, a heat loss of 48.6 J-min-1 was calculated even for the dog breathing with the mouth closed. When Ta was elevated to 38 degrees C at constant rh, panting ensued. In parallel fashion lingual blood flow increased to 60.4 ml-min-1 (0.81 ml-g-1-min-1) in mean and to 74.7 ml-min-1 (0.99 ml-g-1-min-1) at peak rate of thermal tachypnoea (272 breaths-min-1). This flow increase resulted from a decrease in the local vascular resistance since the driving systemic pressure remained constant. It was accompanied by an increase in TLAV to 1.5 degrees C equivalent to a heat loss of 400.7J-min-1 in mean and 496.2J-min-1 at maximum respiratory rate. 2. The preoptic/anterior hypothalamic (PO/AH) region was heated with a water perfused thermode in urethane anaesthetized dogs at constant body temperature in order to study the relationship in time between the increase in lingual blood flow and the onset of thermal panting. Lingual blood flow was found to be 20 ml-min-1 at a respiratory rate of 60 breaths/min. During hypothalamic heating both respiratory rate and lingual blood flow increased markedly. At maximum respiratory rates (244 breaths-min-1) lingual blood flow reached a level of 60 ml-min-1. When perfusion of the thermode was stopped, both respiratory rate and lingual blood flow synchronously returned to control values within 5 min. Similar changes did not occur in dogs in which a ventilatory response failed to be elicited during hypothalamic heating. 3. The results suggest that the tongue contributes to the evaporative heat loss mechanism and they confirm the concept that panting, associated with increased lingual blood flow, is induced by a common autonomic outflow pattern which is mediated by the central mechanism controlling thermal homeostasis.