Central projections of primary vestibular fibers in the bullfrog. III. The anterior semicircular canal afferents

A study was made of the projection patterns in the vestibular nuclei of primary afferent fibers innervating the crista of the anterior semicircular canal (ASC) of the bullfrog. Individual neurons were intracellularly recorded to characterize their spontaneous activity and injected with horseradish peroxidase to visualize their central and peripheral projections. A quantitative evaluation was made of the lengths of the central projections of primary afferents in the various vestibular nuclei and of the number of branch terminals and their precise locations in each of the major vestibular projection areas in the brain stem. The differences between the central projection patterns of the two types of primary afferent fibers were documented: the thick fibers with their irregular spontaneous activity and the thin fibers with their more regular spontaneous activity. Each neuron sends many terminals to various vestibular nuclei. The ventral part of the ventral nucleus receives the greatest number of terminals from fibers of all sizes. The medial nucleus receives terminals from the thin fibers only, while the reticular formation receives terminals exclusively from the thick fibers. All the vestibular nuclei receive innervation from ASC afferents, with considerable overlap in areas of innervation between fibers of different caliber. Differences were found between the thick and thin fibers in their locations, their patterns of branching, and the appearance of their terminal boutons. These anatomic characteristics are interpreted to represent physiologic differences in the production of reflexes by thick and thin neurons. It is also postulated that these different neurons play different roles in vestibular system function: thick neurons contribute to the maintenance of equilibrium during movement while thin neurons participate in the maintenance of posture and vestibular tonus.

Central projections of primary vestibular fibers in the bullfrog. II. Nerve branches from individual receptors

The fibers from the nerves innervating each of the three semicircular canals and the saccule were labeled by injecting horseradish peroxidase extracellularly into these nerves. The projections into the various vestibular nuclei of each receptor were studied in transverse sections of the brain stem throughout the vestibular nuclear area. All five vestibular nuclei receive primary afferents throughout their areas. There are differences in the projection patterns of the canals. In the superior and ventral vestibular nuclei, the location of the projections depends on the crista injected. The anterior canal projects ventrally, the horizontal canal centrally, and the posterior canal more dorsally. Each canal, however, sends fibers to all areas, with overlap of fibers from the different cristae. The cerebellar nucleus receives uniform innervation from the three canals. The medial vestibular nucleus in the rostral and caudal areas receives only thin fibers from each canal, with considerable overlap. The descending nucleus in the rostral and caudal areas receives innervation from the cristae, also with considerable overlap, but with greater intensity in the ventral part of the caudal portion of the nucleus. Each crista sends fibers to the cerebellar granular layer and to the base of the cerebellar Purkinje cell layer. These fibers also innervate the reticular formation below the entry zone of the eighth nerve. The saccule innervates both the dorsal (acoustic) and the ventral nuclei, the latter in the most dorsal position. The innervation of the utricle could be ascertained only in the middle section of the descending and the medial nuclei, an area which does not receive significant innervation from the cristae. Primary afferent fibers course in the vestibular tract, forming a longitudinal bundle lateral to the vestibular nuclei. In the bundle the larger fibers are medially situated.

The vestibuloocular reflex of the adult flatfish. II. Vestibulooculomotor connectivity

The peripheral and central oculomotor organization of the adult flatfish presents no morphological substrates that suffice to explain adaptive changes in its vestibuloocular reflex system. The necessity for an adaptation occurs because of a 90 degrees displacement of the vestibular with respect to the extraocular coordinate axes during metamorphosis. Since a reorganization of vestibuloocular pathways must be hypothesized (12), the location and termination of electrophysiologically identified secondary vestibular neurons with focus on the horizontal canal system was studied with the intracellular horseradish peroxidase method in adult winter flounders. Pseudopleuronectes americanus. The oculomotor target sites of vertical canal related neurons were similar to those described in mammals. Presumed excitatory anterior canal neurons bifurcated after the main axon had crossed the midline. The descending branch headed toward the spinal cord. The ascending branch reached the oculomotor nucleus via the contralateral medial longitudinal fasciculus and terminated in the superior rectus and inferior oblique subdivisions. Presumed inhibitory posterior canal neurons ascended ipsilaterally in the medial longitudinal fasciculus and terminated mainly in the superior rectus and inferior oblique subdivisions. Horizontal canal neurons exhibited characteristics distinctly different from mammalian ones. Two types of second-order neurons were observed. In the first case, cell bodies were located in the anterior portion of the vestibular nuclear complex. After crossing the midline, the axon ascended in the contralateral medial longitudinal fasciculus. Major termination sites were found in the inferior oblique and superior rectus subdivisions of the oculomotor nucleus. Axonal branches then recrossed the midline and terminated in identical locations on the ipsilateral side. In the second case, cell bodies were located in the descending vestibular nucleus. Their axons crossed the midline and also ascended in the contralateral medial longitudinal fasciculus. Major termination sites were in the trochlear nucleus and in the inferior rectus subdivision of the oculomotor nucleus. As in the first case, axonal branches also recrossed the midline and terminated in identical motoneuron pools on the ipsilateral side. The above target sites were exactly those expected to be used in a reciprocal excitatory-inhibitory fashion during compensatory eye movements. Head-down movement would be excitatory for the lower horizontal canal producing contractions of both superior recti and inferior obliques as well as relaxation of the antagonistic inferior recti and superior obliques.(ABSTRACT TRUNCATED AT 400 WORDS)

Computer analysis of vestibular responses to putative neurotransmitters

An inexpensive microcomputer (Commodore 64K) based system was developed for the analysis of neural spike trains. The trains were recorded from single ampullary units in response to mechanical stimulation of the isolated semicircular canal of the bullfrog (Rana catesbeiana). A BASIC program provided a number of options while machine language subroutines generated interstimulus interval (ISI) and peristimulus time (PST) histograms. Up to thirty 5-s spike trains could be combined for analysis (0.1 ms resolution ISI, 100 ms bin width PST). Histograms and summary statistics were saved on floppy disks. The cost of adding this computer system to an existing neurophysiology laboratory is less than US $600 (printed, tape, and disk versions of these programs are available). The system was used to measure vestibular responses to putative vestibular neurotransmitters such as carbachol (an acetylcholine mimic) (Rossi et al., 1980) and gamma-aminobutyric acid (GABA) (Flock and Lam, 1974).

Physiological localization of the semicircular canal receptors in the crista

Isolated posterior semicircular canals from the bull frog model were used. The cupula was removed from the crista and the sensory cilia on the crista were depressed toward the canal side by using a glass micropipette. Seven different points were selected for stimulation on the ampullary wall. Ampullary nerve action potentials induced by depression were recorded and were converted into spike-density histograms. Response decremental time constants were measured on these histograms. The longest time constant was measured at the two lateral points of stimulation and progressively shortened toward the central point of stimulation. These results indicate that the origin of the adaptation is possibly sensorineural.

Two groups of secondary vestibular neurons mediating horizontal canal signals, probably to the ipsilateral medial rectus muscle, under inhibitory influences from the cerebellar flocculus in rabbits

Vestibular nuclear neurons that mediate horizontal canal signals to the ipsilateral medial rectus motoneurons were explored in anesthetized and decerebrate rabbits. These neurons were identified by four criteria: (1) they were activated monosynaptically by ipsilateral vestibular nerve stimulation and (2) antidromically from the oculomotor nucleus region, while they were inhibited by (3) direct floccular stimulation and (4) ipsilateral retinal stimulation that activated floccular Purkinje cells via a climbing fiber afferent pathway. Neurons fulfilling these criteria were found in two anatomically different regions, i.e. the rostrolateral part of the medial vestibular nucleus and in the ventral part of the lateral vestibular nucleus. In decerebrate rabbits, neurons in both loci responded to horizontal rotation of the whole body with the type I pattern (excited by ipsilateral rotation). These results suggest that horizontal canal signals are conveyed to ipsilateral medial rectus motoneurons by two separate groups of vestibular nuclear neurons which may play different roles in the vestibulo-ocular reflex.

Action of the efferent vestibular system on primary afferents in the toadfish, Opsanus tau

Spinalized toadfish were held in a lucite chamber and perfused through the mouth with running seawater. Primary vestibular afferents and vestibular efferent axons and somas were studied with glass microelectrodes. Vestibular semicircular canal afferent and efferent axons were visually identified and penetrated with glass microelectrodes. Afferents responded to pulses of injected current with trains of action potentials, whereas efferents responded with only a single spike. This differential response to injected current served to further distinguish these two classes of nerve fibers that share the same canal nerve for part of their course. When current pulses were injected into efferent somadendritic recording sites, cells responded with trains of action potentials similar to those seen in other central nervous system neurons. Semicircular canal afferents were spontaneously active and occupied the same spectrum of regularity as vestibular afferents recorded in other species. Behavioral arousal evoked by lightly touching the fish on the snout or over the eye resembled spontaneous arousal observed in the field and consisted of eye withdrawal, fin erection, and attempted swimming. Efferent vestibular neurons were spontaneously active and increased their frequency of discharge when the fish was behaviorally aroused. Most efferents were briskly activated by behavioral arousal, but the time constant of the decay of their responses was variable ranging from 100 to 600 ms. Not only touch, but multimodal stimuli were capable of increasing the level of spontaneous activity of efferent vestibular neurons. The shortest latency to behavioral activation was 160 ms. Vestibular primary afferents also manifested increase in neuronal activity with behavioral activation. Irregularly discharging afferents were much more responsive than regularly discharging afferents. One rare case of transient inhibition in a regularly discharging afferent is illustrated. Severing the efferent vestibular nerve blocked behavioral activation in vestibular primary afferents. Electrical stimulation of the efferent vestibular nerve produced excitatory postsynaptic potentials (EPSPs) at latencies within the monosynaptic range in vestibular primary afferents. These monosynaptic EPSPs could produce action potentials in primary afferents or could sum with subthreshold depolarizations produced by current passed through the microelectrode to initiate impulses.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous and driven responses of semicircular canal primary afferents in the unanesthetized pigeon

Spontaneous activity from 120 vestibular semicircular canal (SC) primary afferents and driven responses from 20 SC primary afferents to 2.3-decade bandwidth (0.029-6.152 Hz) sum of sinusoidal angular rotations were characterized in 10 unanesthetized pigeons. The results were compared with those previously gathered from barbiturate anesthetized pigeons. The average spontaneous mean firing rate (MFR) of SC primary afferents in unanesthetized pigeons was found to be 168 impulses per second (I/s), n = 120, and is approximately 80% higher than that for SC primary afferents in anesthetized pigeons [93 I/s, n = 149 (Ref. 9) and 92 I/s, n = 124 (Ref. 22)]. The spontaneous discharge of SC primary afferents from unanesthetized pigeons was classified according to the coefficient of variation (CV) of 512-1,024 interspike intervals (ISIs) into regular (CV less than 0.1), intermediate (0.1 less than or equal to CV less 0.4), and irregular (0.4 less than or equal to CV) categories. The percentages of SC primary afferents falling within each CV category were similar for unanesthetized and anesthetized pigeons (22). The relation between the spontaneous mean ISI and its standard deviation (SD) for SC primary afferents in the unanesthetized pigeon is best described by a power function model, which provides a significantly better fit than does a linear model. The parameters of this power function model are similar for SC primary afferents in unanesthetized and anesthetized pigeons (10). The spontaneous ISIs of individual SC primary afferents in the unanesthetized pigeon were found to be distributed either normally, log-normally, or according to the first passage time of the Wiener-Levy (WL) process. The gain of anterior SC primary afferents at 0.25 Hz is similar for anesthetized (2.93 I X s-1 X deg-1 X s-1, n = 14) (11) and for unanesthetized (3.01 I X s-1 X deg-1 X s-1, n = 14) pigeons. Bode plots constructed from the responses to sum sinusoidal angular accelerations reveal that SC primary afferents in unanesthetized pigeons display a phase lead and gain enhancement at frequencies between 0.6 and 6 Hz.(ABSTRACT TRUNCATED AT 400 WORDS)

Surgical section of the posterior ampullary nerve

Surgical section of the posterior ampullary nerve (PAN) has been used as a form of therapy for persistent benign paroxysmal positional vertigo (BPPV). Eighteen patients were reviewed. The PAN was identified in 16 patients, 15 of whom were completely cured and one of whom had improvement of symptoms. Severe sensorineural hearing loss (SNHL) occurred in three patients. This study suggests that surgical section of the PAN is effective. The procedure is facilitated by a postauricular incision and canalplasty.

Projections from the nuclei prepositus hypoglossi and intercalatus to the superior colliculus in the cat: an anatomical study using WGA-HRP

In view of the recognized role of the superior colliculus (SC) in eye and head movement, and following recent physiological studies, the presence of afferents to the SC from the vestibular complex and adjacent cellular groups were sought using axonal transport techniques. Injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) were made in the SC of 9 adult cats. Retrogradely labelled cells were observed in the nuclei prepositus hypoglossi (PH) and intercalatus (INT) predominantly contralaterally with only occasional cells in the medial (MVN) and descending (DVN) vestibular nuclei contralaterally. There appeared to be some topographical differences in anteroposterior distribution of labelled cells in the perihypoglossal nuclei after restricted SC injections. Two cases of intramedullary injection of WGA-HRP which involved the PH, INT, MVN and DVN revealed anterogradely labelled terminations in laminae IV and VI of the contralateral SC. A few additional labelled terminations were found in the medial portion of the contralateral medial geniculate body (MGB) and the nucleus of the optic tract-(NOT). The possible role of the perihypoglossal nuclear complex as a site of convergence of vestibular and neck afferent inputs destined for the SC is discussed.

An experimental study on cupular function: mapping of the cupula by direct stimulation

In the present study, the posterior semicircular canal of the frog was isolated and the entire cupula was exposed after cutting the ampullary wall. The surfaces of the utricle and the lateral cupula were depressed by a fine glass pipette. Action potentials were taken from the posterior ampullary nerve and were compared by using nine points of stimulation on the cupular surface of the utricular side and two points on the lateral surface. The maximum action potential was recorded at stimulus to the lowest point on the utricular side surface. This potential proportionally decreased as the stimulation point approached the top of the cupula. Depression of the lateral surface evoked only a small potential. In contrast to the stimulus applied to the utricular surface, a stimulus to the upper point of the lateral surface yielded a greater potential than did a stimulus to the lower point.

An experimental study on the physical properties of the cupula. Effect of cupular sectioning on the ampullary nerve action potential

The frog posterior semicircular canal (PSC) was isolated and a part of the ampullary wall was cut to allow removal of the cupula from the crista. The cupula was replaced on the crista and the PSC ampullary action potential was recorded. The cupula was again removed and was sectioned in half, either in the plane vertical to the crista (vertical sectioning), or in the plane parallel to the crista (horizontal sectioning). The sectioned half of the cupula was then replaced on the crista. The action potentials after replacement of the vertical or horizontal segments of the cupula were compared to those achieved when the entire cupula was replaced. After vertical sectioning, the action potentials were significantly reduced; they were 50.3% of the completely replaced cupula when a small stimulus was used and 79.1% when a large stimulus was used. A reduced attachment surface between the cupular base and the crista is possibly responsible for the decreased action potential in the vertically sectioned specimen. After horizontal sectioning, the action potentials were 64.5% for the small stimulus and 108.2% for the large stimulus. These results indicate that elicited responses are related to the height of the cupula and the deflection angle. This further suggests that the movement of the cupula is represented by that of the elastic system.

Effects of lesion of the interstitial nucleus of Cajal on vestibular horizontal canal neurons in the cat

Effects of procaine infusion into the interstitial nucleus of Cajal (INC) on vestibular nuclear neurons related to the horizontal canal were studied in cats anesthetized with nitrous oxide and paralyzed with gallamine. Neurons that responded to sinusoidal horizontal rotation (at 0.18 Hz) were recorded extracellularly in the medial and descending vestibular nuclei. Spontaneous activity of type I neurons increased, whereas that of type II neurons decreased following procaine infusion into the ipsilateral INC. Gain of the neuronal response to horizontal rotation decreased after the ipsilateral INC infusion, but there was no consistent effect on phase. Infusion into the contralateral INC seemed less effective. Similar effects were obtained with electrolytic lesions that were confined to the ipsilateral INC area. These results suggest that the INC influences type I neurons through inhibitory action of type II neurons and that it eventually controls the gain, but not the phase, of the horizontal vestibular reflexes.

The effect of glutamate on the frog semicircular canal

L-glutamate (Glu) has at least two sites of action in the frog semicircular canal: the hair cell (presynaptic) and the primary afferent nerve fibres (postsynaptic). Glu's action on the hair cell results in an increased release of the natural transmitter which is responsible for a substantial increase in the frequency of firing in primary afferents. The presynaptic action of Glu is antagonized by D-alpha-aminoadipate (D alpha AA). Glu produces a long-lasting depolarization on the afferent nerve fibres which does not by itself elicit any afferent discharge of impulses when the release of the natural transmitter is prevented. Glu-induced nerve depolarization is only partially antagonized by D alpha AA. The difficulty of reconciling some of the observations made of the effects of Glu in semicircular canals with its presumed role as an afferent transmitter in this organ is discussed, but this role is not definitely rejected.

Primary afferent projections of the octavus nerve to the inferior reticular formation and adjacent nuclei in the elasmobranch, Rhinobatos sp

Projections of primary afferent fibers from the octavus nerve to the inferior reticular formation were determined by nerve degeneration and HRP labeling. Descending afferents from the horizontal ampullary nerve exit the ventral border of the nucleus octavus descendens via arcuate fiber tracts, and project to a group of neurons adjacent to the spinal lemniscus; to the inferior reticular formation; and to the nucleus funiculi lateralis. The possible influence of these afferent projections on directed swimming motion is discussed.

An experimental study on a function of the cupula. Effect of cupula removal on the ampullary nerve action potential

We used a posterior semicircular canal that had been isolated from a frog. From the utricular side the ampulla was cut open at a position one third of the way along the long axis. The cupula was removed through this opening using a glass micropipette. The action potential from the posterior ampullary nerve was recorded before and after removal of the cupula. After removal, the action potential disappeared almost completely. When the cupula was put back on the crista, the action potential was restored. When the cupula was put back upside down, the action potential recovered, but to a lesser extent.

Floccular influence on excitatory relay neurones of vestibular reflexes of anterior semicircular canal origin in the cat

Floccular influence on excitatory vestibular reflex arcs of anterior semicircular canal origin was examined in the anaesthetized cat. Stimulation of the anterior semicircular canal nerve (ACN) evoked disynaptic excitatory postsynaptic potentials (EPSPs) in all sampled inferior oblique (IO), superior rectus (SR), and biventor cervicis (BIV) muscle motoneurones of the contralateral side. Conditioning stimulus to the flocculus depressed the amplitude of the EPSPs in both IO and SR motoneurones by 50% on the average but not in any BIV motoneurones. The excitatory vestibulo-ocular neurones identified by orthodromic and antidromic responses to stimulation of the ACN and the contralateral IO motoneurone pool, respectively, were classified as VOC (vestibulo-ocular neurones with axons descending to the cervical segment) or VO (vestibulo-ocular proper) neurones on the basis of whether or not they responded antidromically to stimulation of the spinal cord in the C1 segment. All of the VO neurones in the superior vestibular nucleus (n = 19) were inhibited from the flocculus while the activities of three-fourths of the VO neurones (36/48) in the other vestibular nuclei were not suppressed by floccular stimulation. In contrast, none of VOC neurones (n = 49) received floccular inhibition. Besides inhibition, floccular stimulation induced the antidromic or orthodromic responses in some VO and VOC neurones.

Axon collaterals of anterior semicircular canal-activated vestibular neurons and their coactivation of extraocular and neck motoneurons in the cat

We studied the ascending and descending axonal trajectories of excitatory vestibular neurons related to the anterior semicircular canal, by means of local stimulation and spike-triggered signal averaging techniques in anesthetized cats. More than 200 vestibular neurons related to the ampullary nerve of the anterior semicircular canal (ACN) were identified as vestibulo-ocular neurons by antidromic stimulation of the contralateral inferior oblique (IO) muscle motoneuron pool. In the descending, medial and ventral lateral nuclei, about 60% of these vestibulo-ocular neurons were also activated antidromically by upper cervical spinal cord stimulation (vestibulo-ocular-collic (cervical) = VOC). These VOC neurons produced unitary EPSPs in the majority of neck extensor motoneurons located at the C1 segment. None of the VOC neurons had axons descending as far as the thoracic level. Most of these VOC neurons were activated monosynaptically following stimulation of the ACN. The conduction velocity of the descending axons of VOC neurons was approximately 63 m/s, which was significantly faster than that of the ascending axons. The remaining 40% of the vestibulo-ocular neurons were not activated antidromically following spinal cord stimulation at intensities of 1 mA or more (vestibulo-ocular = VO). Most of the VO neurons were activated polysynaptically by ACN stimulation. The superior vestibular nucleus contained VO neurons that were activated mono- and polysynaptically following ACN stimulation.

Possible cholinergic neurotransmission in the cristae ampullares of the chick inner ear

The presence of choline acetyltransferase (ChAT) was investigated in the chick inner ear in order to assess the possible role of acetylcholine (ACh) in neurotransmission within the vestibular labyrinth. ChAT activity found in homogenates of isolated chick vestibular cristae ampullares is of the same order of magnitude and has similar properties as its homologous enzyme in nervous tissue from various regions. These findings are in accord with a probable neurotransmitter role of ACh in the chick vestibular labyrinth.

Elasmobranch oculomotor organization: anatomical and theoretical aspects of the phylogenetic development of vestibulo-oculomotor connectivity

The oculomotor organization of two elasmobranch species, smooth dogfish (Mustelus canis) and little skate (Raja erinacea), was studied by investigating the extraocular muscle apparatus and the oculomotor motoneuron distribution. The macroscopic appearance of the eye muscles was similar to any lateral-eyed vertebrate species (e.g., goldfish, rabbit). The size of extraocular muscles was expressed by counting single muscle fibers and comparing cross-sectional areas of the extraocular muscles. There were significant differences in the number of fibers in the six extraocular muscles in dogfish, but not in skate. Fiber sizes varied considerably; thus, the number of fibers did not relate to cross-sectional areas. In the dogfish, no one pair of agonist-antagonist extraocular muscles was larger than the others, suggesting that there was no preference for eye movements in a particular plane of space. However, the lateral rectus was more than twice the size of most of the other muscles. In the skate, cross-sectional areas of the horizontal eye muscles were smaller than those of the vertical eye movers. This may indicate a reduced utilization of horizontal eye muscles, which may reflect the bottom-dwelling habitat and mode of locomotion of the skate. The distribution of the extraocular motoneurons was determined by injecting horseradish peroxidase (HRP) into single eye muscles. Medial rectus, superior rectus, and superior oblique motoneuron populations were located contralateral to their respective muscles. Lateral rectus, inferior rectus, and inferior oblique motoneurons were located ipsilateral to their muscles. This distribution is in contrast to almost all other vertebrates studied thus far, where medial rectus motoneurons are located ipsilateral to the muscle which they innervate. The oculomotor arrangement in elasmobranchs is likely to have consequences for the circuitry responsible for the production of conjugate compensatory eye movements in the horizontal plane. We hypothesize that, in contrast to other vertebrates, the basic elasmobranch vestibulo-ocular reflex pathway consists of three identically structured three-neuron-arcs connecting the three semicircular canals to their respective extraocular muscles. This innervation pattern may constitute a special feature of the elasmobranch brain or a phylogenetically older arrangement of eye movement pathways.

Differences between cats in response properties of horizontal semicircular canal primary afferents

The response properties of cat horizontal canal afferents (N = 81) were characterized by three parameters: their long time constants (tau), low frequency gain constants (G1), and middle frequency gain constants (Gm). An average value of each of these parameters was calculated for each of eight animals and comparisons were made across animals. There were significant differences between individual animals in their average values of tau and Gm. There was also a significant negative correlation between tau's and Gm's. An animal with a larger average tau tended to have a smaller average Gm. We also used anatomic data on membranous canal duct diameter from the literature to independently estimate the potential effect of interanimal anatomic variability on the predicted range of tau and Gm values in a population. We then compared the data from our 81 afferents with the predictions from the anatomic data.

Pharmacology of the isolated semicircular canal: effect of GABA and picrotoxin

We have investigated the proposal that gamma aminobutyric acid (GABA) is the afferent transmitter in the vestibular system using the isolated posterior semicircular canal of the bullfrog (Rana catesbeiana). This was done by examining the effects of GABA and picrotoxin on the evoked response of neural units in the posterior ampullar nerve. GABA had no effect on evoked response up to 1 mM, and inhibited the evoked response at 10 mM. Picrotoxin, a GABA antagonist, had no effect on evoked response between 5 microM and 100 microM. These results indicate that GABA is not the afferent transmitter in the vestibular system.

Local mechanisms in vestibular receptor control. Effects of curare on the EPSPs and spike discharge recorded from single afferent fibres of the posterior canal nerve of the frog

In the frog, the afferent discharge of EPSPs and spikes has been recorded intracellularly from single fibres of the posterior ampullar nerve before and after administration of D-tubocurarine (5 X 10(-6) M) in the following experimental conditions: (a) in animals in which the labyrinth was merely exposed (intact frogs), (b) in isolated acute labyrinth preparations, (c) in isolated chronic labyrinth preparations after degeneration of the efferent fibres. In both (a) and (c), curare induced either facilitation or inhibition of the resting discharge in a large number of units. By contrast, after degeneration of the efferent fibres, the effects of the drug were almost completely suppressed. These results indicate the existence of peripheral mechanisms involving the efferent nerve terminals which can control the release of afferent transmitter even in the absence of any central influence.

Anatomical characteristics of the anterior vestibular nerve of the bullfrog

Study was made of the dimensions of the nerves to the receptor organs in the anterior branch of the bullfrog vestibular nerve. The number of fibers and their diameters and trajectories in the nerve and Scarpa's ganglion were studied. A correlation was made between the anatomical and physiological properties of neurons identified with intracellular injections of horseradish peroxidase. The data suggest a specific pattern of innervation of the organs, organization of fibers and cells in the vestibular nerve, and significant correlation between the anatomical and physiological properties of individual neurons. A comparison was made between the information obtained from the bullfrog and that from other animals, which suggests a similarity between species and supports the hypothesis of a differential physiological role of the neurons in each vestibular organ according to anatomical characteristics.