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Objective The purpose of this study was to evaluate and compare the results of cervical vestibular evoked myogenic potential (cVEMP) and video head impulse test (p-vHIT) of posterior semicircular canal considered tools of inferior vestibular nerve function in vestibular neuritis. Study design Prospective cohort study. Setting Tertiary otology clinic. Patients and interventions Seventy-nine patients with vestibular neuritis participated in this study. We analyzed the interaural amplitude difference in cVEMP with a positive rate of p-vHIT according to gain and corrective saccade in the study population. Main outcome measure To evaluate the concordance rate of both tests, we analyzed Fleiss' Kappa value inter-test agreement of cVEMP with p-vHIT. Finally, we performed detailed analysis of the bilaterally absent response on cVEMP according to the p-vHIT results. Results The inter-test agreement between cVEMP and p-vHIT was 69.8% as we also considered the lesion side. This result indicated a statistically fair to good agreement in both tests. In mostly elderly patients with a bilaterally absent response (11 patients) on cVEMP, as a result of vHIT, nine patients with a bilaterally negative response on p-vHIT showed only canal paresis. Two patients showed canal paresis and a unilaterally positive response on p-vHIT. Conclusions Inter-test agreement between cVEMP and p-vHIT assessed in vestibular neuritis was relatively lower than we had predicted. Probably, p-vHIT can provide additional information on the differential diagnosis of dysfunction of the inferior vestibular nerve which is composed of the saccular nerve and the posterior ampullary nerve.

In humans, a positive followed by a negative waveform with short latency is reproducibly evoked in the sternocleidomastoid (SCM) muscle in response to intense sound. This sound-evoked vestibulocollic reflex is termed the vestibular evoked myogenic potential (VEMP). We investigated the characteristics of the VEMP as an otolith function test. The human VEMP has its best frequency located between 500 Hz and 1 kHz that resembles a V-shaped tuning curve of the saccular nerve in cats, recorded in the inferior vestibular nerve. The amplitude of the human VEMP was enhanced under microgravity during parabolic flight and was attenuated by the rostro-caudal linear acceleration on Earth. These findings suggest that the human VEMP is an otolith function test and originates from the saccule. After vestibular deafferentiation, unilateral VEMP dysfunction in the affected side appears and lasts for a long time. In patients with inner ear and retrolabyrinthine disorders, abnormality of the VEMP was not always coincident with that of the caloric test. These findings suggested that the VEMP, like the caloric test, is another important tool for the diagnosis of the affected side, and that a test battery of the VEMP and caloric test may help to increase the rate of abnormal findings among patients with the peripheral vestibular disorders. In the human VEMP, activation of acoustically responsive saccular afferents triggers an acoustic reflex in the SCM muscle. Future studies are needed to clarify in detail, the clinical usefulness of the VEMP as an otolith function test.

Von Oort first described the vestibulocochlear anastomosis in 1918, and there have been very few anatomical studies of it. It is located at the bottom of the internal auditory meatus and links the saccular nerve to the cochlear nerve. The aim of this study was to investigate the micro-anatomical characteristics of this anastomosis. We dissected 10 human temporal bones from five heads. We found the vestibulocochlear anastomosis in seven of the specimens, of which six were clearly visible. We were unable to uncover it in three specimens due to dissection problems. Its length was evaluated to be between 0.5 and 1 mm, with a diameter of 0.5 mm. The vestibulocochlear anastomosis could be the pathway for the nerve fibers of the cochlear efferent system, whose description remains incomplete. However, no anatomist, histologist or physiologist has demonstrated this function. From the results of this study, it seems likely that the vestibulocochlear anastomosis exists. Other studies are necessary to link it to the efferent system.

Because of technical obstacles in controlling current spread to adjacent peripheral nerve, eye movements evoked by activation of the otolith organs have not been investigated in detail compared to eye movements evoked by activation of the canal organs. We attempted to solve this problem by applying more sensitive methods using fine needle and strictly controlling stimulus current intensity compare with filed potential for selective stimulation.Eye movements evoked by selective, unilateral saccular (SAC) nerve stimulation were investigated using both electrooculography (EOG) and video recording in decerebrated cats in the presence or absence of anesthesia. Electrical stimulation was applied to the SAC nerve through implanted acupuncture needles.In the absence of anesthesia and with stimulus intensities less than (3.1 +/- 2.7) x N(1)T, we found supraduction in both eyes or in either the ipsilateral or contralateral eye of different cats. We observed downward eye movements using a stronger stimulus intensity ((6.2 +/- 2.9)) x N(1)T). The threshold for downward eye movements was significantly greater than that for upward eye movements (P0.05). In anesthetized cats, only downward eye movements were observed when stimulus intensities less than 10 x N(1)T ((7.8 +/- 2.3) x N(1)T) were used.These results confirm the known sacculo-ocular anatomical connections, which are involved predominantly in vertical eye movements. Because the sacculo-ocular connections are relatively weak, the normal supraduction evoked by SAC activation can be easily modified by factors such as level of anesthesia and the method of stimulation.

Objective : To determine the value of investigating the vestibular evoked myogenic potentials (VEMP's) induced by clicks and 500 Hz short tone burst (STB) for the diagnosis of acoustic neuromas. Methods : We studied the average responses to 100dB clicks and 500 Hz STB in the ipsilateral sternomastoid muscle. Ninety-five healthy subjects and 170 patients suffering from a unilateral acoustic neuroma were included in that study. Caloric and audiometric tests results were also analyzed. Results : Thirty-six/170 patients (21.2%) exhibited normal responses to clicks and to STB whereas 134/170 (78.8%) gave abnormally low or no responses. 78/170 (45.9%) showed no responses to both clicks and STB. In 56/170 patients (32.9%), VEMP's induced by high level clicks and STB were discordant: STB VEMP's were either normal ( n =32) or low ( n =24) in patients with an abnormal response to clicks (no response n =40 or low response n =16). In contrast, STB-induced VEMP's were always normal in cases of normal responses to clicks. No strong, systematic correlation could be found between saccular nerve dysfunction and either the degree of 4–8 kHz hearing loss or the extent of horizontal canalar impairment. Conclusions : These data indicate that high level clicks and STB provide complementary information about the functionality of the saccular nerve. Clicks are useful to detect a minor saccular nerve dysfunction. In cases in which there is no response to clicks, STB gives valuable information about a potential residual function of the saccular nerve.

The brainstem projection of the saccular nerve was investigated by transganglionic tracing with horseradish peroxidase in the cat. The labeled fibers were located most caudally and laterally (dorsolaterally) in the vestibular nerve root. This was unique when compared with the locations of the fibers from the nerves of the other vestibular-end organs that were studied by the present authors by the same approach previously. In addition, such a comparison revealed a specific location, from lateral to medial, for the fibers from each of the five divisions of the vestibular nerve. In the present study, labeled fibers from the sacculus, of fine caliber, were found close to the restiform body, both medially and laterally, some even penetrating through this structure. Labeled terminals were present in cell group "y". This was unique, compared with the nerves from the other end organs. Such terminals were also found in the four main vestibular nuclei, except for the medial vestibular nucleus, where no labeled terminals could be detected. No labeled terminals were found in the interstitial nucleus of the vestibular nerve. Together with the findings from our previous studies, this suggests that, in contrast to the ampullar nerves, the nerves from the maculae do not project to this structure. This study confirms, but also extends, findings reported from a previous investigation in the cat, using an experimental degeneration technique.

The aim of the study was to present a case report demonstrating that vestibular evoked myogenic potentials originate from the saccular nerve.Retrospective case review.Tertiary referral center.Vestibular evoked myogenic potentials are shown before and after operation in a patient with a surgically confirmed superior vestibular schwannoma (1.3 cm).Preoperative and postoperative audiometry, auditory brainstem response, caloric test, vestibular evoked myogenic potentials, and magnetic resonance imaging outcomes. RESULTS On preoperative audiometry, slight sensorineural hearing loss with normal auditory brainstem response could be detected. Caloric tests showed a reduced response of the right labyrinth. Vestibular evoked myogenic potentials were normal. The tumor was removed via a combined retrosigmoid and partial labyrinthectomy approach with the removal of the posterior semicircular canal. The facial, auditory, and inferior semicircular nerves were anatomically preserved. Postoperative audiometry revealed preservation of hearing and vestibular evoked myogenic potentials; caloric responses were now absent.The combined retrosigmoid and partial labyrinthectomy approach represents an ideal condition to study the influence of the saccular nerve on the origin of the vestibular evoked myogenic potentials. The maintenance of the vestibular evoked myogenic potential responses in our patient supports the concept that the vestibular evoked myogenic potential originates in the saccule and is transmitted in the saccular nerve.

While particle motion is thought to directly stimulate the inner ear of most fish species, it is difficult to measure and might not be predictable from pressure measurements in a small tank. It is therefore important to replicate experiments conducted relative to pressure measurements using stimuli of known particle motion, to ensure that unmeasured components of the stimulus field do not produce misleading frequency response profiles. The frequency sensitivity of the inner ear of the plainfin midshipman fish, Porichthys notatus, in response to isopressure stimuli has been described. This study now examines the frequency and directional response properties of midshipman saccular afferents in response to whole-body displacements simulating acoustic particle motion. Best frequencies were distributed bimodally, with peaks at 50 Hz and 100 Hz. Most units had cosinusoidally shaped directional response profiles in the horizontal and vertical planes, though some units showed slight deviations from this pattern. A few units (probably saccular efferents) had omnidirectional directional response profiles and did not phase lock to the stimulus waveform. These results are consistent with responses of the midshipman saccular nerve to isopressure stimuli, and strengthen the hypothesis that the frequency sensitivity of the midshipman ear matches the frequency content of behaviorally relevant vocalizations.

In contrast to the abundance of information available regarding the anatomy and physiology of afferents within the goldfish saccule, the efferent system of this auditory endorgan has been scarcely studied morphologically. In this study, acetylcholinesterase histochemistry with diaminobenzidine enhancement was used to describe the morphology of efferents. Under light microscopy, labeled fibers appeared in the distal portion of the saccular nerve, penetrated the basement membrane and formed a horizontal mesh-like plexus near the base of hair cells. Many vertical branchlets with terminal swellings protruded upward toward hair cells from the plexus. Under electron microscopy, dense extracellular labeling was present around efferent terminals, which often formed clusters on hair cells. Labeling was also present around unmyelinated fibers of passage within the sensory epithelium and the distal saccular nerve. These fibers contained coarse microtubules and small vesicles, and often ran in a bundle with other similar fibers. Based on their position within the epithelium, histochemistry and ultrastructural characteristics, these fibers were concluded to be efferents. These fibers became myelinated and unlabeled in the proximal saccular nerve. These results suggest that acetylcholinesterase can be a marker of entire distal unmyelinated portions of efferent fibers and demonstrated abundant efferent innervation in the goldfish saccule.

Although still the subject of discussion, vestibular-evoked myogenic potentials (VEMPs) have been considered to reflect the function of the saccular and, more recently, the cochlear tracts. To accurately determine the precise afferent pathway carrying VEMPs, we studied the outcomes of VEMPs and other examinations in patients with unilateral vestibular schwannomas. Eleven patients with unilateral vestibular schwannomas resected using a middle cranial fossa approach were included in the study. Patients underwent pure-tone threshold audiometry, caloric tests and analysis of auditory brainstem responses (ABRs) and VEMPs pre- and postoperatively. The results were compared with those obtained in patients with intact superior or inferior vestibular and cochlear nerves. Among the 11 patients studied, 4 retained their VEMPs postoperatively. Three of the 10 patients with inferior vestibular schwannomas exhibited normal VEMPs, preserved hearing levels (20 dB HL) and anatomically intact superior vestibular nerves. In all of these cases, ABRs more closely correlated with VEMPs than with caloric responses. In one of the cases with inferior vestibular schwannomas, VEMPs were preserved postoperatively and VEMP latencies were shortened, which indicates the preoperative presence of a conduction block in either the cochlear or superior vestibular nerve. VEMPs may be conducted in both the superior vestibular and cochlear nerves, as well as in the inferior vestibular nerve. Thus, evaluation of saccular nerve function should be performed carefully, especially in cases where hearing is preserved. It appears that cochlear conduction may proceed along two pathways, one direct and the other via the brainstem, but this remains to be verified.

Connections from the otolithic organs to sternocleidomastoid (SCM) motoneurons were studied in 20 decerebrate cats. The electrical stimulation was selective for the saccular or the utricular nerves. Postsynaptic potentials were recorded from antidromically identified SCM motoneurons; these muscles participate mainly in neck rotation and flexion. Partial transections of the brainstem at the level of the obex were performed to identify the possible pathway from the otolithic organs to the SCM motoneurons. Saccular or utricular nerve stimulation mainly evoked inhibitory postsynaptic potentials (IPSPs) in the ipsilateral SCM motoneurons. Some of the sacculus-induced IPSPs were preceded by small-amplitude excitatory PSPs (EPSPs). The latencies of the PSPs ranged from 1.8 to 3.1 ms after saccular nerve stimulation and from 1.7 to 2.8 ms after utricular nerve stimulation, indicating that most of the ipsilateral connections were disynaptic. In the contralateral SCM motoneurons, saccular nerve stimulation had no or faint effects, whereas utricular nerve stimulation evoked EPSPs in about two-thirds of neurons, and no visible PSPs in about one-third of neurons. The latencies of the EPSPs ranged from 1.5 to 2.0 ms, indicating the disynaptic connection. Thus, the results suggest a difference between the two otolithic innervating patterns of SCM motoneurons. After transection of the medial vestibulospinal tract (MVST), saccular nerve stimulation did not evoke IPSPs at all in ipsilateral SCM motoneurons, but some (11/40) neurons showed small-amplitude EPSPs. Most (24/33) of the utricular-activated IPSPs disappeared after transection, whereas the other 9 neurons still indicated IPSPs. In the contralateral SCM motoneurons, no utricular-activated EPSPs were recorded after transection. These MVST transection results suggest that most of the otolith-SCM pathways are located in the MVST at the obex level. However, the results also suggest the possibility that other otolith-SCM pathways exist at the obex level.

1. Single-channel recordings of potassium channel activity were made from two populations of primary-auditory neurons maintained in tissue culture. The saccular nerve, which is the auditory component of the eighth cranial nerve in goldfish, was separated into two branches according to its peripheral innervation pattern. Neurons which innervated the rostral saccular macula corresponded to a class of cells that showed spike frequency adaptation; whereas, neurons which innervated the caudal macula were consistent with another type of cell that demonstrated bursting spontaneous firing patterns in vivo. Both somatic and internodal axonal membranes from each of these neuronal classes were studied after acute removal of the myelin sheath by microdissection. 2. Dye injections were used to discriminate neuronal from myelin membrane. After successful removal of the myelin, patch electrodes containing Lucifer yellow were used to fill a neuron and reveal its morphology within the myelin sheath. Patches on myelin led to filling of Schwann cells that surrounded the neuron. 3. Four kinds of potassium channels were observed and characterized according to unitary conductance, inactivation, and sensitivity to internal calcium. Three voltage-dependent K+ channel types were found on the somatic and axonal membrane of the two neuronal populations. Two channel types showed voltage-dependent inactivation and had average conductances of 32 and 19 pS, each with distinctive subconductance states. The third type of channel activity had an estimated conductance of 12 pS and was noninactivating. 4. The fourth type of channel was the Ca2(+)-activated K+ channel (k(Ca)), which was classified by the dependence of its activity on the calcium concentration at its cytoplasmic surface. Unlike the other three potassium channel types, this kind of channel was found exclusively on neurons that innervated the caudal sensory epithelium. As with the other kinds of potassium channels, it was found on both somatic and axonal internodal membranes.

A clinical microwave device was used to heat and ear of the North American bullfrog in order to observe the temperature dependence of tuning in the sacculus, an organ known to posses the capability of electrical resonance in its hair cells. In tuning curves derived from reverse correlation analysis with noise stimuli, the temperature dependencies of the frequencies of tuning peaks and notches typically exhibited Q 10 s less than 1.2; whereas the frequencies of electrical resonances are expected to have Q 10 s of the order of 1.7. Therefore we conclude that electrical resonances are not significantly involved in tuning in the bullfrog sacculus.

SUMMARYIn goldfish and other otophysans, the Weberian ossicles mechanically link the saccule of the inner ear to the anterior swimbladder chamber (ASB). These structures are correlated with enhanced sound-pressure sensitivity and greater sensitivity at high frequencies (600–2000 Hz). However, surprisingly little is known about the potential impact of the ASB on other otolithic organs and about how auditory responses are modulated by discrete sources that change their location or orientation with respect to the ASB. In this study, saccular and lagenar nerve fiber responses and conditioned behaviors of goldfish were measured to a small, low-frequency (50 Hz) vibrating sphere (dipole) source as a function of its location along the body and its orientation with respect to the ASB. Conditioned behaviors and saccular nerve fiber activity exhibited response characteristics nearly identical to those measured from a hydrophone in the same relative position as the ASB. By contrast, response patterns from lagena fibers could not be predicted by pressure inputs to the ASB. Deflation of the ASB abolished the characteristic spatial response pattern of saccular but not lagena fibers. These results show that: (1) the lagena is not driven by ASB-mediated pressure inputs to the ear; (2) the ASB–saccule pathway dominates behavioral responsiveness, operating effectively at frequencies as low as 50 Hz; and (3) behavioral and neural (saccular) responses are strongly modulated by the position and orientation of the dipole with respect to the ASB.

An epithelial sheet isolated from the trout saccular macula, highly enriched in acousticolateralis receptor cells (hair cells), has been analyzed for primary amine-containing compounds. The hair cell preparation, compared to the saccular nerve, was found to contain elevated levels of the presumptive receptoneural transmitter, glutamate, as well as beta-alanine, and components eluting in the positions of the standards phosphoserine and phosphoethanolamine on cation-exchange HPLC. Saccular nerve contained a different spectrum of primary amines and was elevated specifically in carnosine/homocarnosine. Acid hydrolysis of perchlorate extracts of both hair cell and nerve fractions yielded large amounts of histidine. For the saccular nerve fraction, production of histidine by acid hydrolysis was matched by production of beta-alanine and gamma-aminobutyric acid (GABA) and disappearance of carnosine/homocarnosine. The dipeptides carnosine and homocarnosine have been chromatographically resolved by expanded HPLC and found to be present in saccular nerve in a ratio of approximately 10:1, respectively. Production of histidine in the hair cell extract was not coupled with production of beta-alanine and GABA. The hair cell histidine-containing unknown, present in millimolar concentration, has been identified as N-acetylhistidine by the hydrolysis and rechromatography of fractions from cation-exchange HPLC. The large and specific presence of N-acetylhistidine in the hair cell preparation, together with electrophysiological evidence for its facilitatory action on afferent fibers in the frog semicircular canal, is suggestive of a role for this molecule as well as glutamate in acousticolateralis receptoneural transmission.

In anesthetized toads, spontaneous activities were recorded from single afferent fibers of three semicircular canals and three otolith organs. Since some efferent fibers ramify within the eighth nerve and innervate two or more vestibular organs, a single branchlet of the eighth nerve was disconnected from its end organ, and was electrically stimulated to activate divergent efferent collaterals leading to other vestibular organs. The stimulation elicited an inhibitory effect on spontaneous activities of about one third of the afferent population, and a facilitatory effect on those of another one third. The remaining one third was unaffected. Whether or not the inhibitory or facilitatory effect was observed in an individual unit seemed to be related to its pattern and its rate of spontaneous activity. Most of the units showing relatively high and regular spontaneous firing were insensitive to the electrical stimulation, and units with a low firing rate and an irregular pattern of activity tended to be affected by the electrical stimulation. The activation of divergent efferent fibers elicited both inhibition and facilitation on the spontaneous afferent activities in all vestibular nerve branchlets, except in the saccular branchlet, where only inhibition was elicited. Electrical stimulation of the central stump of the saccular nerve branchlet, however, could produce both inhibitory and facilitatory effects in other vestibular nerve branchlets.

The suppression of background spike activity in the absence of deliberate acoustic stimulation occurs in fibers of the goldfish saccular nerve tuned in the region of 250 Hz. Suppression is most robust in the frequency range between 450 and 1050 Hz, the range of CF for the mid- and high-frequency saccular fibers. Suppression of background activity tends to occur following the suppressor tone offset ('off-suppression'), even though the spike response during the suppressor is below the background rate. This suggests that the suppressor tone is excitatory at the level of the hair cells and their synapses onto saccular afferents. Tones at the low- frequency edge of the suppression region may show net excitation at low intensity levels, and net suppression at higher levels. This suggests that the spike response observed is the result of the relative strengths of excitatory and suppressive effects which operate simultaneously. The magnitude and frequency of best suppression tends to increase with stimulus intensity. A suppressing tone produces transient excitation at onset. In fibers with high levels of spontaneous activity, a spike response 'rebound' often occurs 20 to 50 ms following the suppressing tone offset. These 'on' and 'off' effects are not due to energy 'splatter' in the stimulus domain. Suppression by tones can also be observed in non-spontaneous fibers when the background spike activity is evoked by noise. In these cases, however, off-suppression following a suppressed response and the 'rebound' seldom occurs. Possible sites of suppression are the hair cells and their synapses, the spike-initiation zones of the saccular afferents, and efferent inhibition. The most likely site seems to be the spike-initiation zones of saccular afferents. An important consequence of suppression for hearing is the sharpening of frequency response areas for low frequency fibers, and the partial preservation of frequency analysis in saccular fibers stimulated well above threshold.

In the otolith system, there are two types of neuronal circuitry that can enhance response sensitivity during linear acceleration and tilt of the head. One produces cross-striolar inhibition and the other commissural inhibition. Cross-striolar inhibition can be observed in over 50% of saccular-activated, second-order vestibular neurons. In contrast, it is seen in less than 33% of utricular-activated, second-order vestibular neurons. The majority of vestibular neurons that receive cross-striolar inhibition have axons that project to the spinal cord. Over 50% of the utricular-activated, second-order vestibular neurons received commissural inhibition from the contralateral utricular nerve. On the other hand, almost all the saccular-activated, second-order vestibular neurons exhibit no response to stimulation of the contralateral saccular nerve. The majority of vestibular neurons receiving commissural inhibition also have axons that project to the spinal cord. These results suggest that cross-striolar inhibition is important for increasing the sensitivity of the saccular system, whereas commissural inhibition is more important in the utricular system.

The central projections of the utricular and saccular nerve in macaques were examined using transganglionic labeling of vestibular afferent neurons. In these experiments, biotinylated dextran amine was injected directly into the saccular or utricular neuroepithelium of fascicularis (Macaca fascicularis) or rhesus (Macaca mulatta) monkeys. Two to 5 weeks later, the animals were killed and the peripheral vestibular sensory organs, brainstem, and cerebellum were collected for analysis. The principal brainstem areas of saccular nerve termination were lateral, particularly the spinal vestibular nucleus, the lateral portion of the superior vestibular nucleus, ventral nucleus y, the external cuneate nucleus, and cell group l. The principal cerebellar projection was to the uvula with a less dense projection to the nodulus. Principle brainstem areas of termination of the utricular nerve were the lateral/dorsal medial vestibular nucleus, ventral and lateral portions of the superior vestibular nucleus, and rostral portion of the spinal vestibular nucleus. In the cerebellum, a strong projection was observed to the nodulus and weak projections were present in the flocculus, ventral paraflocculus, bilateral fastigial nuclei, and uvula. Although there is extensive overlap of saccular and utricular projections, saccular inputs to the lateral portions of the vestibular nuclear complex suggest that saccular afferents contribute to the vestibulospinal system. In contrast, the utricular nerve projects more rostrally into areas of known concentration of vestibulo-ocular related cells. Although sparse, the projections of the utricle to the flocculus/ventral paraflocculus suggest a potential convergence with floccular projection inputs from the vestibular brainstem that have been implicated in vestibulo-ocular motor learning.

Activation maps of pre- and postsynaptic field potential components evoked by separate electrical stimulation of utricular, lagenar, and saccular nerve branches in the isolated frog hindbrain were recorded within a stereotactic outline of the vestibular nuclei. Utricular and lagenar nerve-evoked activation maps overlapped strongly in the lateral and descending vestibular nuclei, whereas lagenar amplitudes were greater in the superior vestibular nucleus. In contrast, the saccular nerve-evoked activation map coincided largely with the dorsal nucleus and the adjacent dorsal part of the lateral vestibular nucleus, corroborating a major auditory and lesser vestibular function of the frog saccule. The stereotactic position of individual second-order otolith neurons matched the distribution of the corresponding otolith nerve-evoked activation maps. Furthermore, particular types of second-order utricular and lagenar neurons were clustered with particular types of second-order canal neurons in a topology that anatomically mirrored the preferred convergence pattern of afferent otolith and canal signals in second-order vestibular neurons. Similarities in the spatial organization of functionally equivalent types of second-order otolith and canal neurons between frog and other vertebrates indicated conservation of a common topographical organization principle. However, the absence of a precise afferent sensory topography combined with the presence of spatially segregated groups of particular second-order vestibular neurons suggests that the vestibular circuitry is organized as a premotor map rather than an organotypical sensory map. Moreover, the conserved segmental location of individual vestibular neuronal phenotypes shows linkage of individual components of vestibulomotor pathways with the underlying genetically specified rhombomeric framework.

Noise has been commonly thought to degrade the performance of sensory systems. However, it is now clear that the detection and transmission of weak signals in sensory systems can be enhanced by noise via stochastic resonance (SR). In hair cells, the quality of mechanoelectrical transduction is enhanced up to twofold by nanometer level mechanical noise acting on the hair bundle. We wanted to know whether these gains could be preserved, perhaps even enhanced, as information flows across hair cell synapses, and into the stream of action potentials that in the frog conveys acoustic information to the central nervous system. To approach this question, we studied the effects of noise on the signal-to-noise ratio (SNR) of the 8th nerve's response to small mechanical stimuli directly applied to the amphibian sacculus. We found that approximately 2.5 nm of mechanical noise enhanced the response of the saccular nerve up to fourfold, suggesting that the positive effects of low-amplitude mechanical noise result in improved transmission of acoustic information.

We examined whether otolith-activated second- and third-order vestibular nucleus neurons received commissural inhibition from the contralateral otolithic macula oriented in the same geometric plane. For this purpose we performed intracellular recording in vestibular nucleus neurons after stimulation of the ipsi- and contralateral utricular and saccular nerves. More than half (41/72) of the utricular-activated second-order vestibular nucleus neurons received commissural inhibition from the contralateral utricular nerve. The remaining neurons (31/72) showed no visible response to contralateral utricular nerve stimulation. About half (17/36) of utricular-activated third-order neurons also received commissural inhibition from the contralateral utricular nerve. Approximately 10% (7/67) of saccular-activated second-order vestibular neurons received polysynaptic commissural inhibition, whereas 16% (11/67) received commissural facilitation. The majority (49/67) of saccular second-order vestibular neurons, and almost all (22/23) third-order neurons, showed no visible response to stimulation of the contralateral saccular nerve. The present findings suggest that many utricular-activated vestibular nucleus neurons receive commissural inhibition, which may provide a mechanism for increasing the sensitivity of vestibular neurons to horizontal linear acceleration and lateral tilt of the head. Commissural inhibition in the saccular system was less prominent than in the utricular system.

We studied the effects of stimulation of the utricular and saccular nerves on sympathetic nerve activity in decerebrated cats. Bipolar electrodes were fixed in place on the utricular and/or saccular nerve under visual observation; the other branches of the vestibular nerve were transected. Baroreceptors and vagus nerves were inactivated bilaterally so that inputs from baroreceptors and other visceral receptors did not influence the sympathetic nerve outflow. Postganglionic sympathetic nerve activity was recorded from the renal branch of the sympathetic nerve, which is known to be more sensitive to vestibular stimuli than other types of sympathetic fibers. With stimulation of either the saccular or utricular nerve at low stimulus intensity, a prominent inhibition followed by a rebound excitation was evoked on spontaneous renal nerve discharges. The latency of the inhibition ranged from 65 to 130 ms, and the duration of inhibitory responses was about 90-150 ms. An increase in stimulus intensity in both the saccular and utricular nerves induced inhibitory effects preceded by short-term excitation. The latency of this excitation, which was superimposed on the initial phase of the inhibitory responses, ranged from 55 to 90 ms.

Properties of otolith inputs to vestibulocerebellar neurons were investigated in 14 adult cats. In the vestibular nuclei, we recorded single-unit activities that responded orthodromically after stimulation of the utricular and/or saccular nerves and antidromically after stimulation of the cerebellum (uvula-nodulus and anterior vermis). Descending axonal projections to the spinal cord were also examined by antidromic stimulation of the caudal end of the C1 segment. Forty-seven otolith-activated neurons that projected to the uvula-nodulus were recorded. Thirteen (28%) of the 47 neurons received convergent inputs from the utriculus and sacculus. The remaining 34 (72%) vestibular neurons were non-convergent neurons: 18 (38%) received utricular input alone, and 16 (34%) received saccular input alone. Most (35/47) vestibulocerebellar neurons were located in the descending vestibular nucleus and only one of these projected to the spinal cord. Seven of the 47 vestibulocerebellar neurons were located in the lateral vestibular nucleus and most of these neurons projected to the spinal cord. The remaining neurons were located in group X (two neurons) and the superior vestibular nucleus (three neurons). In a different series of experiments, 37 otolith-activated vestibular neurons were tested to determine whether they projected to the uvula-nodulus and/or the anterior vermis. Nineteen of the 37 neurons projected to the anterior vermis, 13/37 projected to the uvula-nodulus, and 5/37 projected to both. The utricular and/or saccular nerve-activated vestibulocerebellar neurons projected to not only the uvula-nodulus, but also to the anterior vermis. In summary, the results of this study showed that vestibular neurons receiving inputs from the utriculus and/or sacculus projected to the cerebellar cortex. This indirect otolith-cerebellar pathway terminated both in the anterior lobe and in the uvula/nodulus.

The otolith system contributes to the vestibulo-ocular reflexes (VOR) when the head moves linearly in the horizontal plane or tilts relative to gravity. The saccules are thought to detect predominantly accelerations along the gravity vector. Otolith-induced vertical eye movements following vertical linear accelerations are attributed to the saccules. However, information on the neural circuits of the sacculo-ocular system is limited, and the effects of saccular inputs on extraocular motoneurons remain unclear. In the present study, synaptic responses to saccular-nerve stimulation were recorded intracellularly from identified motoneurons of all twelve extraocular muscles. Experiments were successfully performed in eleven cats. Individual motoneurons of the twelve extraocular muscles--the bilateral superior recti (SR), inferior recti (IR), superior obliques (SO), inferior obliques (IO), lateral recti (LR), and medial recti (MR) were identified antidromically following bipolar stimulation of their respective nerves. The saccular nerve was selectively stimulated by a pair of tungsten electrodes after removing the utricular nerve and the ampullary nerves of the semicircular canals. Stimulus intensities were determined from the stimulus-response curves of vestibular N1 field potentials in order to avoid current spread. Intracellular recordings were performed from 129 extraocular motoneurons. The majority of the neurons showed no response to saccular-nerve stimulation. In 17 (30%) of 56 extraocular motoneurons related to vertical eye movements (bilateral SR and IR), depolarizing and/or hyperpolarizing postsynaptic potentials (PSPs) were observed in response to saccular-nerve stimulation. The latencies of PSPs ranged from 2.3 to 8.9 ms, indicating that the extraocular motoneurons received neither monosynaptic nor disynaptic inputs from saccular afferents. The majority of the latencies of the depolarization, including depolarization-hyperpolarization, were in the range of 2.3-3.3 ms. Latencies of hyperpolarizations were typically longer than those of depolarizations. Only one contralateral SO motoneuron of 43 recorded oblique extraocular motoneurons (bilateral SO and IO) showed a depolarization-hyperpolarization in response to saccular-nerve stimulation at a latency of 2.5 ms. None of 30 recorded horizontal extraocular motoneurons (bilateral LR and MR) responded to stimulation of the saccular nerve. The neural linkage in the sacculo-ocular system is relatively weak in comparison to the utriculo-ocular and sacculo-collic systems, suggesting that the role of the sacculo-ocular system in stabilizing eye position may be reduced when compared with utriculo-ocular and semi-circular canal-ocular reflexes.

The hair cell orientation patterns present on the saccules of fishes may be important for encoding the direction of a sound source. This study was conducted to determine whether primary afferent projections to the medulla are organized by the best directions for the hair cells they innervate. The toadfish saccule has hair cells oriented primarily in the vertical plane: both the rostral and the caudal saccule have hair cell orientations sweeping from 0 degrees to 45 degrees, and the middle saccule has hair cells oriented at 90 degrees. Fluorescent dextran amines were used singly and in combination to label regions of the saccular nerve innervating rostral, middle, and caudal saccule. The projections of those afferents were examined in detail in the anterior and descending octaval nuclei, which are auditory nuclei in this species. There was no evidence of topographic projections based on location along the length of the saccule or based on hair cell orientation. There was some evidence that parallel inputs are present from each region of the saccule examined, which may be based on the 180 degrees opposition of hair cells found throughout the saccule; however, afferents from the rostral, middle, and caudal saccule appear to have overlapping projections to the anterior and descending octaval nuclei. These data suggest that in toadfish, calculations of the direction of the sound source may begin in either of these primary auditory nuclei by comparing afferent input from along the saccule.

Recordings were made from single afferent fibers in the inferior vestibular nerve, which innervates the saccule and posterior semicircular canal. A substantial portion of the fibers with irregular background activity increased their firing in response to moderately intense clicks and tones. In responsive fibers, acoustic clicks evoked action potentials with minimum latencies of < or = 1.0 msec. Fibers fell into two classes, with the shortest latency either to condensation clicks (PUSH fibers) or to rarefaction clicks (PULL fibers). Low- frequency (800 Hz) tone bursts at moderately high sound levels (> 80 dB SPL) caused synchronization of spikes to preferred phases of the tone cycle. PUSH and PULL fibers had preferred response phases approximately 180 degrees apart. These two response classes are consistent with fibers that innervate hair cells having opposite morphological polarizations, an arrangement found in the saccule. With low-frequency tone bursts, sound levels of > or = 90 dB SPL evoked increases in mean spike rate. Spike rates increased monotonically with sound level without saturating at levels < or = 115 dB SPL. Contraction of the middle-ear muscles decreased responses to sound, consistent with the sound transmission path being through the middle ear. Several fibers were labeled with biocytin and traced. All labeled fibers had bipolar cell bodies in the inferior vestibular ganglion with peripheral processes extending toward the saccular nerve and central processes in the vestibular nerve. Two fibers were traced to the saccular epithelium. One fiber was traced centrally and arborized extensively in vestibular nuclei and a region ventromedial to the cochlear nucleus. Our results confirm and extend previous suggestions that the mammalian saccule responds to sound at frequencies and levels within the normal range of human hearing. We suggest a number of auditory roles that these fibers may play in the everyday life of mammals.

Many teleost fish generate acoustic signals for vocal communication by the synchronized, high-frequency contraction of skeletal, sonic muscles. In midshipman, eight groups of brainstem neurons were distinguished after biocytin application to the sonic nerve that, we propose, represent the entire vocal motor circuit. Biocytin-filled terminals were ubiquitous within all areas containing labeled neurons and, together with ultrastructural evidence, suggested a serial, transneuronal transport at synaptic sites between at least three neuronal groups. The most intensely labeled neurons were positioned in the caudal brainstem and included a previously characterized pacemaker- motoneuron circuit and a newly recognized ventral medullary nucleus that itself gave rise to extensive commissural and lateral brainstem bundles linking the pacemaker circuitry to the rostral brainstem. Five additional groups formed a column rostrally within the medial brainstem adjacent to eighth nerve (octaval)-recipient nuclei largely presumed to be acoustic. This column extended dorsally up to the ventricular cell layer and as far anterior as midbrain isthmal levels. The best-defined group was in the octaval efferent nucleus that directly innervates the sacculus that is considered the auditory division of the inner ear. Saccular afferents and neurons throughout the medial column were also filled after biocytin application to the saccular nerve. This vocal- acoustic network overlaps low-threshold, electrical stimulation sites in the rostral brainstem that elicit vocalizations. The medial column must therefore be the origin of the descending pathway controlling activation of the vocal pacemaker circuitry and likely forms the basis for acoustically elicited vocalizations. We suggest this network, together with input from the pacemaker circuitry, is also the origin of a vocal-related, corollary discharge to acoustic nuclei. Direct links between vocal and acoustic brain regions are thus traits common to aquatic and terrestrial vertebrates.

The distribution of S-100-like immunoreactivity in the trout saccule (a presumed organ of hearing in fish) has been determined by means of immunohistochemistry. Within the sensory epithelium of the saccular macula, hair cells and myelinated saccular nerve fibers were found to be immunoreactive. Hair-cell immunoreactivity was relatively uniform throughout the macula except at the extreme periphery (rostral, caudal, ventral and dorsal), where staining was either decreased or absent. The immunoreactivity associated with myelinated nerve fibers was greatest at the peripheral edges of the nerve processes, a position corresponding to the location of Schwann cells. However, the nerve processes themselves (within and subjacent to the sensory epithelium), as well as cell bodies within the saccular nerve, were also immunoreactive. Thus, the immunoreactivity of the saccular nerve observed above the basal lamina can be attributed to the saccular nerve processes as well as to nerve-associated Schwann cells. Overall, the immunoreactivity displayed by hair cells was less intense than that associated with myelinated saccular nerve, as evidenced by a disappearance of signal in hair cells first, upon serial dilution of antibody. No S-100-like immunoreactivity was observed in supporting cells within the sensory epithelium or in epithelial cells in non-sensory regions. A concentration of S-100-like immunoreactivity in hair cells and saccular nerve is suggestive of the presence of S-100 calcium-binding protein-mediated activities in these cell types.

The dorsal medulla of anuran tadpoles undergoes significant anatomical reorganization over metamorphic development. To examine how functional properties alter in response to anatomical changes, we recorded single and multiple unit activity in the medulla of bullfrog tadpoles to a sinusoidal‐oscillating dipole source that traversed the length of the animal’s body. This dipole stimulus has been used previously to distinguish between pressure‐sensitive auditory responses in the saccular nerve of the goldfish and pressure‐gradient lateral line responses in the lateral line nerves of goldfish and other teleost fish. Recording sites in the developing dorsal lateral nucleus showed pressure‐type responses to dipole stimulation near the animal’s head and lung. Evidence for the role of the bronchial columella and the air‐filled lungs acting as a sound pressure transducer in mediating these responses was equivocal. Recording sites in the lateral line nucleus and surrounding neuropil showed pressure‐gradient‐type res...

1. In two series of cats all vestibular afferents except those innervating the saccular or utricular macula were transected and allowed to degenerate. Subsequent anatomical and physiological verification showed that the saccular nerve had been selectively spared in six cats. The utricular nerve was wholly or partly spared in five cats, but there was some possibility of contamination from other vestibular afferents. 2. The connections of the remaining nerve with dorsal neck motoneurons were studied 27-179 days after the initial surgery in preparations anesthetized with chloralose. 3. Stimulation of the saccular nerve usually evoked IPSPs in contralateral, EPSPs in ipsilateral neck motoneurons. Some of the potentials were disynaptic, many were later and probably trisynaptic. 4. The effects of utricular nerve stimulation were more complex, perhaps because of contamination by other vestibular, particularly canal, afferents. The predominant pattern consisted of ipsilateral inhibition and contralateral excitation, opposite to the effects of saccular nerve stimulation. Many potentials were disynaptic. 5. Various factors that may complicate interpretation of the results, including plastic changes following denervation, are considered. It is concluded that they are not a significant factor. 6. Our main result is a demonstration that the sacculus can contribute to vestibulospinal reflexes acting on the head.

The activity of single fibers from the saccular nerve has been recorded in isolated head preparations in the frog (Rana esculenta). Nearly all the fibers present an activation or a depression of their activity during the tilting movement in a given direction. This is considered to correspond to the existence of two anatomical populations of haircells in the saccular macula which differ in their functional polarization. The activity of only some units (about 40%) remained increased or decreased when the head was in a tilted position for 4-5 min. This shows that the sacculus is sensitive to the direction of gravity.

Electron microscopic observation was reported on the utricle and ampullae of a case, a 36-year-old male, with dizziness attack due to suspected saccular origin. After the removal of the utricle and ampullae, the dizziness recurred, and finally the saccular nerve section relieved the dizziness attack of this case. So the actual lesion was localized in the saccule and was not in the specimens obtained at the first surgery. Though most of hair cells appeared normal, there were a few with many vacuoles or shrinkage of the cytoplasm distributing sparsely throughout the sensory epithelia. The distribution of the pathological hair cells in the utricle was more general than in the ampullae. The sensory hair cells without pathology and the presynaptic structures were similar to those found in the experimental animals. Nerve and nerve endings showed normal structures. The findings may be physiological at this age group or an extension of the pathology of the dizziness-causing lesion.

The acoustic frequency selectivity of over 500 saccular nerve fibers of the goldfish was studied using automated threshold tracking based on spike rate increments defined statistically. Saccular fibers of the goldfish show great variation in (1) best sensitivity (-26 to + 35 dB re: 1 dyn/cm2), (2) best frequency (below 100 to 1770 Hz), (3) spontaneous rate (0 to over 200 spikes/s), (4) spontaneous type (silent, regular, irregular, burst), and (5) degree of tuning (Q 10 dB from less than 0.1 to 2). Saccular fibers may be grouped into four nonoverlapping categories based on tuning and best frequency: (1) untuned (less than 10-dB variation in sensitivity between 100 and 1000 Hz), (2) low frequency (BF from below 120 to 290 Hz), (3) midfrequency (BF between 330 and 670 Hz), and (4) high frequency (BF between 790 and 1770 Hz). Within each category, all spontaneous rates and types, and all degrees of tuning can be observed. The least sensitive fibers within each group have zero spontaneous rates. The goldfish is like all other vertebrates studied in that the peripheral auditory system is adapted for frequency selectivity throughout the animal's entire frequency range of hearing. Peripheral tuning most likely accounts for behavioral determinations of the "auditory filter" and for the detectability of signals masked by noise. The signal-to-noise ratio enhancement provided by these peripheral filters is likely to be of primary biological significance. A "place principle" of sound quality analysis based on lines "labeled" according to best frequency in the brain cannot be ruled out on the basis of the peripheral physiology.

The response characteristics of saccular nerve fibers in European grassfrogs (Rana temporaria) subjected to dorso-ventral, 10-200 Hz sinusoidal vibrations were studied. Only 4 fibers out of a total of 129 did not respond to the vibrations. 70 fibers had an irregular spontaneous activity of 2-48 spikes/s. These fibers were very vibration-sensitive. The synchronization thresholds at 10-20 Hz varied from below 0.005 to 0.02 cm/s2. In contrast to earlier results, all these fibers had low-pass characteristics (with respect to acceleration) and responded maximally at 10 and 20 Hz. 55 fibers had spontaneous activities from 0-2 spikes/s. These fibers were less sensitive than the fibers with higher spontaneous activity. The spike-rate thresholds varied from about 0.04 to above 1.28 cm/s2, giving a considerable range fractionation. Most of these fibers also had low-pass characteristics with respect to acceleration, but 8 fibers showed band-pass characteristics with maximal synchronizations and spike-rates occurring at 40-80 Hz. At high acceleration levels, most spikes fell within 5-10 degrees of the stimulus cycle. The phase-locking of the saccular fibers is therefore very acute at low frequencies. The phase angles preferred by the fibers at 10 Hz were bimodally distributed with the two peaks about 180 degrees apart. This finding probably reflects the morphological observation that the saccular macula contains two oppositely oriented hair-cell populations. The results also indicate that the actual motion of the otolith relative to the macula is complex. No behavioral role of a vibration receptor has been demonstrated in the grassfrog.(ABSTRACT TRUNCATED AT 250 WORDS)

1. Single unit recordings were made from neurons in the saccular branch of the eighth nerve in the catfish,Ictalurus punctatus. Spontaneous activity varied from zero to 184 spikes/s among the units recorded. Non-spontaneous units and three types of spontaneous activity were observed including regular, irregular and bursting. Non-spontaneous units were most frequently encountered. 2. Iso-intensity curves were used to examine the frequency response areas of the neurons. The best frequencies of different units were continuously distributed from below 160 Hz to over 1,250 Hz, with the most abundant units exhibiting best frequencies below 250 Hz. 3. Units having different best frequencies were, to a limited extent, tonotopically distributed within the saccular nerve, with low frequency units located more dorsally in the nerve than the high frequency units. 4. Phase locking was quantitatively assessed using a coefficient of synchronization (R). For each unit, the degree of phase locking varied with frequency and intensity of the stimulus. At any one intensity, the highest degree of phase locking was generally observed at the unit's best frequency. Increases in stimulus intensity resulted in enhanced phase locking ability up to a maximum value. The maximum R value was lower for units having best frequencies above 630 Hz, suggesting that these units were less capable of phase locking than lower frequency units.

Nerve fiber counts and the spectral distribution of the myelinated nerve fiber diameters were determined for the vestibular nerve of the White King pigeon. An analysis of variance indicated that the raw data for the utricular, saccular, neglectar, anterior ampullary, and horizontal ampullary nerve branches could be pooled to give an overall unimodal distribution having a mode in the range of 1–2 μm. this could not be done for the posterior ampullary nerve and an intraspecific variation was considered to be the cause of the discrepancy. The utricular nerve had more fibers smaller than 3 μm in diameter (84.9%) than the other branches. Overall, 63.8% of fibers (exclusive of posterior ampullary nerve) were less than 3 μm in diameter. For the individual nerve branches the following average fiber counts ± standard deviations were obtained: 1774 ± 88 for the utricular nerve, 111 ± 243 for the saccular nerve, 86 ± 5 for the neglectar nerve, 1716 ± 142 for the anterior ampullary nerve, 1554 ± 151 for the horizontal ampullary nerve, and 1679 ± 218 for the posterior ampullary nerve. When all branches were combined the average number of fibers was 8720 ± 847 in a diameter range of 1–17 μm.

Concerning the vestibular labyrinth, especially the function of the otolithic organ, many obscure problems have been remained. Conflicting statements appear in the medical literature concernigg the sacular function. The author carried out the experiment by giving the injury to the saccular nerve in the osseous labyrinth, without the influece on the labyrinthine fluid. On the other hand, each of other labyrinthine nerve was injured similarly and the saccular function was confirmed. Moreo- ver, the saccular macula was e5posed to D.C. stimulations. In the observation of these experiments, the author examined the relfex movement of eyeballs. The localization of the injury was confirmed by histological examination. The results obtained were summerized as follows: The saccule of rabbit has labyrinthine functions. 1) It performs the compensatory deviation of eyeballs in frontal plane, and the saccule moves the eyeball upward homolaterally and the downward contralaterally. 2) The saccule reacts on the movement of eyeball when the otolith is in suspended position. 3) The number of nystagmus changes by the stimulation on saccule, decreasing the number of mystugmus toward the opposite side.

THE auditory systems of fishes have been of particular interest in the development of auditory theory because a mechanical analysis of frequency had seemed unlikely on the basis of the ear's rather undifferentiated structural organisation1. Thus, at the level of the 8th nerve, information about stimulus frequency is thought to be coded in the temporal structure of neural activity rather than in across-fibre distributions of impulse rate2–4. However, recent neurophysiological data show that a limited peripheral frequency analysis occurs in several species5–7 and some workers have suggested that this may form the basis for psychophysical demonstrations of filtering8–10 and for frequency discrimination in the fish auditory system11,12. Such a ‘place principle’ of frequency analysis and pitch perception is presently generally believed to originate in the phyletic series among the amphibia and to reach the greatest degree of elaboration with the mammals13. We report here an experiment which helps to resolve this question of frequency codings by evaluating the adequacy of the temporal hypothesis to account for the behaviourally measured frequency difference limens for the goldfish11,12. A simple form of the temporal coding hypothesis states that the reciprocal of stimulus frequency (period) is represented in 8th nerve fibres as the distribution of time intervals between phase-locked impulses4. A discrimination between two different period lengths may thus entail a decision as to whether the two resulting distributions of neural interspike intervals are the same or different. Clearly, the performance on such a discrimination would be limited by the variability of the distributions, or the accuracy with which neural impulses are phase-locked to the stimulating waveform. We demonstrate here that the neurones which phase-lock best at any given frequency show degrees of phase-locking accuracy that would be expected if the central nervous system were processing the input optimally.

A protein has been detected that is associated with the saccular hair cell layer of the rainbow trout, Salmo gairdnerii R. By one- and two-dimensional SDS polyacrylamide gel electrophoresis, the molecular weight and isoelectric point of this protein are estimated to be 13.6 and 8.8 kDa, respectively. The 13.6 kDa protein cannot be detected electrophoretically in brain, gill, liver, and fractions containing the basal lamina, non-sensory epithelium, and saccular nerve. This protein does not bind antibodies to bovine myelin basic protein, while trout myelin basic proteins in the same molecular weight range do. In addition, the protein does not bind concanavalin A or react with the periodic acid-Schiff reagent. The 13.6 kDa band represents about 1% of the total protein in saccular sensory epithelium, and may be a marker protein for the hair cell layer.

Publisher Summary This chapter highlights the influences of semicircular canal and macular on neck motoneurons. Synaptic potentials were recorded in cat dorsal neck motoneurons in response to stimulation of individual ampullary nerves or macular nerves. Disynaptic excitatory postsynaptic potential (EPSP) and inhibitory postsynaptic potential (IPSPs) were seen in dorsal neck motoneurons by stimulation of individual ampullary nerves. The early disynaptic potentials were usually followed by later components. Dorsal neck motoneurons are excited by stimulation of the two anterior canals, inhibited from the two posterior ampullae. A majority of these motoneurons were inhibited from the ipsilateral, excited from the contralateral horizontal canal. Splenius motoneurons are influenced most consistently from the horizontal canal. Stimulation of the saccular nerve usually evoked IPSPs in contralateral, EPSPs in ipsilateral neck motoneurons. Many of the potentials were probably disynaptic, though some were trisynaptic. The effects of utricular nerve were complex, perhaps because of contamination by canal afferents. The predominant effects consisted of ipsilateral inhibition and contralateral excitation.

The cochlear and vestibular nerves rotate 90 degrees from the inner ear to the brain stem. Most of the rotation occurs within the internal auditory canal (IAC); only minimal rotation occurs in the cerebellopontine (CP) angle. At the labyrinthine end of the IAC, the cochlear nerve--which at first lies anterior to the inferior vestibular nerve (saccular nerve)--rapidly fuses with the inferior vestibular nerve. It then rotates to become inferior as the nerves leave the porus acousticus. The cochleovestibular (C-V) cleavage plane lies in a superior-inferior direction in the lateral IAC and rotates to become anterior-posterior in the CP angle. In 25% of patients in whom no C-V cleavage plane can be seen, it is not possible to completely transect all vestibular fibers. The surgical implications are that the most complete vestibular neurectomy can be done only in the lateral IAC, the cochlear and inferior vestibular nerves, because of their intimate association, should not be separated in the mid-IAC, in order to prevent damage to the cochlear nerve, and to create a complete denervation of the vestibular labyrinth, only the posterior ampullary nerve along with the superior vestibular nerve should be transected.