Mechanisms of sensory adaptation in the isolated utricle

The occurrence of receptor adaptation in utricular afferent fibers is now widely recognized. The experiments reported here explored the basic mechanisms of adaptation at the level of the receptor organ. Spike discharges from single utricular afferent fibers were recorded in isolated labyrinths of an elasmobranch, during three types of stimulation: (a) tilts in the gravity field, (b) vibrations, and (c) electrical polarization delivered through the nerve filaments from which recordings were also made. Experimental evidence supported the conclusion that polarization affects the discharge by acting at the level of the spike triggering mechanism, the point of the afferent fiber at which impulses normally arise. Three types of afferent fibers have been described: Types I and II fire spontaneously and show phasic-tonic responses to tilts. Type III fibers do not have spontaneous activity and respond to tilts in a phasic manner. Adaptation to polarizing currents was observed in all afferent fibers. Type II fibers adapted slowly to vibrations whereas types I and III afferent fibers did not. The functional processes situated near the spike triggering site of the sensory axon is referred to as neural whereas those occurring at earlier stages of transduction are called preneural. Adaptation to tilts exhibited two successive components: an early, fast phase and a late, slow one. Our results suggested that these phases can be related to the mechanisms of preneural and neural adaptation, respectively. Because the time course of adaptation to polarizing currents was similar in different afferent fibers, we concluded that preneural adaptation was the origin of the differences among afferent fibers that allowed their classification into phasic, phasic-tonic, and tonic groups. No attempts were made to separate the influence of mechanical coupling and transduction in the production of preneural adaptation.     

Localization of a vocal pattern generator in the pontine brainstem of the squirrel monkey

Very little is known about the coordination of muscles involved in mammalian vocalization at the level of single neurons. In the present study, a telemetric single-unit recording technique was used to explore the ventrolateral pontine brainstem for vocalization-correlated activity in the squirrel monkey during vocal communication. We found a discrete area in the reticular formation just above the superior olivary complex showing vocalization-correlated activity. These neurons showed an increase in neuronal activity exclusively just before and during vocalization; none of them was active during mastication, swallowing or quiet respiration. Furthermore, the neuronal activity of these neurons reflected acoustic features, such as call duration or syllable structure of frequency-modulated vocalization, directly. Based on these findings and previously reported anatomical data, we propose that this area serves as a vocal pattern generator for frequency-modulated call types.     

Aftereffects of static or dynamic fusimotor activation on primary afferent discharge.

An analysis is presented of the effects of previous static or dynamic fusimotor activity on various phases of primary afferent discharge before and during ramp stretch of the soleus muscle of anesthetized cat. Short term effects (under 1 sec) following termination of fusimotor activity are generally opposite for static and dynamic fusimotor fibers. Compared to control levels, there is a pronounced depression of primary afferent discharge shortly after termination of static fusimotor activation, and a facilitation following termination of dynamic fusimotor activation. Facilitatory and depressive effects on afferent discharge at periods beyond 1 sec following cessation of fusimotor activation are similar for static and dynamic fusimotor fibers. Possible mechanisms of these changes are discussed in terms of changes in muscle spindle actin-myosin crossbridges, receptor adaptation, and the innervation of nuclear bag and nuclear chain spindle fibers.     

Discharge of group IV phrenic afferent fibers increases during diaphragmatic fatigue

The discharge of single unit group III (n=7) and group IV (n=8) phrenic afferent fibers was recorded during rhythmic diaphragmatic contractions before and after the onset of fatigue. Compared to pre-fatigue impulse activity, group IV, but not group III, phrenic afferent fibers discharged more (p<0.05) during rhythmic diaphragmatic contractions when the diaphragm was fatigued. This increase in group IV fiber discharge during diaphragmatic fatigue provides electrophysiological evidence consistent with the notion that group IV phrenic afferent fibers comprise the afferent arm of a fatigue-induced inhibitory reflex originating in the diaphragm.     

Audio-vocal interaction in the pontine brainstem during self-initiated vocalization in the squirrel monkey

The adjustment of the voice by auditory input happens at several brain levels. The caudal pontine brainstem, though rarely investigated, is one candidate area for such audio-vocal integration. We recorded neuronal activity in this area in awake, behaving squirrel monkeys (Saimiri sciureus) during vocal communication, using telemetric single-unit recording techniques. We found audio-vocal neurons at locations not described before, namely in the periolivary region of the superior olivary complex and the adjacent pontine reticular formation. They showed various responses to external sounds (noise bursts) and activity increases (excitation) or decreases (inhibition) to self-produced vocalizations, starting prior to vocal onset and continuing through vocalizations. In most of them, the responses to noise bursts and self-produced vocalizations were similar, with the only difference that neuronal activity started prior to vocal onset. About one-third responded phasically to noise bursts, independent of whether they increased or decreased their activity to vocalization. The activity of most audio-vocal neurons correlated with basic acoustic features of the vocalization, such as call duration and/or syllable structure. Auditory neurons near audio-vocal neurons showed significantly more frequent phasic response patterns than those in areas without audio-vocal activity. Based on these findings, we propose that audio-vocal neurons showing similar activity to external acoustical stimuli and vocalization play a role in olivocochlear regulation. Specifically, audio-vocal neurons with a phasic response to external auditory stimuli are candidates for the mediation of basal audio-vocal reflexes such as the Lombard reflex. Thus, our findings suggest that complex audio-vocal integration mechanisms exist in the ventrolateral pontine brainstem.     

Individual hemispheric asymmetry in vocal fold control of the squirrel monkey

In ten squirrel monkeys (Saimiri sciureus), electrodes were implanted into the left and right facial motor cortex at sites producing bilateral vocal fold adduction when electrically stimulated. All animals, in addition, had electrodes in the periaqueductal grey of the midbrain (PAG) at sites producing vocalization when electrically stimulated. In eight out of ten animals, motorcortical stimulation during periaqueductally elicited vocalization caused a change in vocal output. This change, in four cases, was more severe with left-sided ipsilateral motor cortex/PAG stimulation than with right-sided ipsilateral stimulation. In the other four cases, right-sided stimulation was more effective than left-sided stimulation. It is concluded that the majority of squirrel monkeys show a hemispheric asymmetry in vocal fold control with left- and right-sided dominance distributed about equally in the population.     

On the search for the vocal pattern generator. A single-unit recording study

In the squirrel monkey (Saimiri sciureus), single-unit activity was compared between the midbrain periaqueductal grey and the parvocellular and central nuclei of the medullary reticular formation during the production of species-specific vocalization. It was found that all three areas contain neurones with vocalization-related activity. The relative number of specific reactions types differed between areas, however. While the majority of periaqueductal cells fired just before, but not during vocalization, most cells in the reticular formation fired before and during vocalization. Modulation of discharge rate with changing fundamental frequency was only found in the reticular formation, not the periaqueductal grey. It is concluded that the parvocellular and central nuclei of the reticular formation, but not the periaqueductal grey are involved in vocal pattern generation.     

Non-rhythmical hippocampal units, theta rhythm and afferent stimulation

Physostigmine induced theta (θ) rhythm and unit activity were recorded from dorsal hippocampus in immobilized locally anesthetized rats. Two functionally different types of cells were identified, Type 1 which fired in rhythmical bursts phase-locked with γ cycles, and Type 2 which were non-rhythmical. Most of the units (80%) were Type 2 and either had bursting (40%) or non-bursting discharge patterns. Correlations between the θ rhythm and Type 2, Type 1 and Type 2, and between pairs of Type 2 cells were studied. Modifications of correlations by afferent stimulation were also analyzed. The principal findings was that in the presence of θ about half of the Type 2 cells revealed a degree of phase-locking with the rhythm. This finding suggests a caused relationship between both phenomena. Crosscorrelograms between Type 1 and 2 discharges, when positive, showed a symmetrical periodicity suggesting that Type 2 cells might function as an hippocampal output. The activity of Type 2 pairs, positively crossconelated in nearly two thirds of the cases, revealed excitatory interactions. Hippocampal afferent stimulation reset θ activity in phase, modified the temporal relationships between cells and with the θ rhythm, and changed the Type 2 discharge pattern. The above results indicate that θ-related Type 2 cells carry information representing θ rhythmicity. Phase-relationships between Type 2 cells and θ, and their modifications by hippocampal afferent activity may be necessary to establish time-relationships with other brain structures.     

Neural correlates of the lombard effect in primate auditory cortex

Speaking is a sensory-motor process that involves constant self-monitoring to ensure accurate vocal production. Self-monitoring of vocal feedback allows rapid adjustment to correct perceived differences between intended and produced vocalizations. One important behavior in vocal feedback control is a compensatory increase in vocal intensity in response to noise masking during vocal production, commonly referred to as the Lombard effect. This behavior requires mechanisms for continuously monitoring auditory feedback during speaking. However, the underlying neural mechanisms are poorly understood. Here we show that when marmoset monkeys vocalize in the presence of masking noise that disrupts vocal feedback, the compensatory increase in vocal intensity is accompanied by a shift in auditory cortex activity toward neural response patterns seen during vocalizations under normal feedback condition. Furthermore, we show that neural activity in auditory cortex during a vocalization phrase predicts vocal intensity compensation in subsequent phrases. These observations demonstrate that the auditory cortex participates in self-monitoring during the Lombard effect, and may play a role in the compensation of noise masking during feedback-mediated vocal control.     

Neurochemical transmission of baroreceptor input in the nucleus tractus solitarius

Baroreceptor activation has been found to produce different types of discharge patterns in neurons in the nucleus tractus solitarius (NTS). The contribution of different glutamate receptor subtypes, neuropeptide modulators and input from different baroreceptor subtypes to the generation of firing patterns in NTS barosensitive neurons was examined in a series of studies. Results from these studies indicate that both subtypes of ionotropic glutamate receptors contribute to discharge in barosensitive neurons, and the role of each subtype can vary for different neurons. The neuropeptide neurotensin was found to modulate baroreceptor control of BP and discharge of central barosensitive neurons, both through modulation of baroreceptor afferent input and possibly through release of neurotensin by baroreceptor afferent fibers in the NTS. Finally, selective modulation of input from baroreceptor subtypes indicates that there is some degree of divergent baroreceptor innervation of NTS neurons that could contribute to initiation of their different discharge patterns in response to baroreceptor input.     

Control of neuronal discharge timing by afferent fiber number and the temporal pattern of afferent impulses

We employ computer simulations to explore the effect of different temporal patterns of afferent impulses on the evoked discharge of a model cerebellar Purkinje cell. We show that the frequency and temporal correlation of impulses across afferent fibers determines which of four regimes of discharge activity is evoked. In the uncorrelated, here Poissonian, case, (i) cell discharge is determined by the total stimulation rate and temporal patterns of discharge are the same for different combinations of afferent fiber number and mean impulse rate per fiber giving the same total stimulation. Alternatively, if temporal correlations are present in the stimulus, (ii) for stimulation frequencies of 4 to at least 64 Hz there is a narrow range of afferent fiber number for which every stimulus pulse (composed of a single impulse on each afferent fiber) evokes a single action potential. In this case cell discharge is frequency locked to the stimulus with a concomitant reduction in discharge variability. (iii) For lower fiber numbers and thus discharge frequencies lower than the locking frequency, the variability of cell discharge is typically independent of afferent impulse timing, whereas, (iv) at higher fiber numbers and thus higher discharge frequencies, the reverse is true. We conclude that in case (iii) the cell acts as an integrator and discharge is determined by the stimulation rate, whereas in case (iv) the cell acts as a coincidence detector and the timing of discharge is determined by the temporal pattern of afferent stimulation. We discuss our results in terms of their significance for neuronal activity at the network level and suggest that the reported effects of varying stimulus timing and afferent convergence can be expected to obtain also with other principal cell types within the central nervous system.     

Anatomical and physiological evidence for a relationship between the ‘cingular’ vocalization area and the auditory cortex in the squirrel monkey

With the aid of the autoradiographic tracing technique the projections from cortical limbic vocalization areas to the auditory cortex in the superior temporal gyrus were studied in the squirrel monkey. The vocalization areas were identified by exploring the anterior limbic cortex with moving electrodes until a site was found where electrical stimulation yielded vocalization. Projections from the region around the cingulate sulcus and supracallosal anterior cingulate gyrus have their terminal fields in the lower part of the superior temporal gyrus (STG) and upper bank of the superior temporal sulcus. Injections just in front of the genu of the corpus callosum and in the subcallosal gyrus and gyrus rectus lead to terminal fields in the middle part of STG. No projections were found in the upper part of STG, i.e. the primary auditory cortex.To test the functional properties of this pathway, action potentials of single neurons in the auditory cortex were recorded during electrical stimulation of the cingular vocalization area. From a total of 135 STG neurons, an effect on spontaneous activity was seen in 27 cells. All except one of these neurons also reacted to acoustic stimuli. In most cases, stimulation of the cingular area caused a decrease in the discharge rate of the STG neurons. In 4 neurons, stimulation of the vocalization area had an influence on the acoustic reactivity of the STG neurons. The results provide evidence that during phonation the ‘cingular’ vocalization area exerts a predominantly inhibitory influence on auditory cortex neurons. This effect probably is mediated via the extreme capsule. Its possible function is discussed.     

Inhibition of nociceptive responses of wide-dynamic-range neurons by peripheral nerve stimulation

Of 107 neurons from the sacral and coccygeal levels of the spinal cord in anesthetized intact rats examined, 62 wide-dynamic-range (WDR) neurons that responded to noxious heating of the tail were recorded. On the basis of their inhibitory responses through A-beta or A-delta afferent fibers to noxious stimulation, these neurons were classified into one of the following three types: Type I--neurons inhibited only by A-beta afferent nerve impulses; Type II--neurons inhibited only by A-delta afferent nerve impulses; Type III--neurons inhibited by both. The present results are compared with previously reported behavioral results.     

Repetitive vocalizations evoked by electrical stimulation of avian brains. IV. Evoked and spontaneous activity in expiratory and inspiratory nerves and muscles of the chicken (Gallus gallus).

The activity in respiratory nerves and muscles in response to electrical stimulation of vocal substrates in the brain and to CO2 stimulation of the respiratory centers was studied in 28 adult chickens. It was found that the same nerves and muscles were active during both vocalization and respiration. Stimulation of vocal substrates resulted in short latency bursting in the expiratory nerves and muscles. As stimulation intensity increased, progressively longer duration bursts composed of numerous subbursts were produced. By relating muscle activity with sound production , such bursting was shown to underlie evoked vocalizations. Background activity in inspiratory nerves and muscles continued uninterruptedly past stimulus onset only stopping when expiratory activity began. Thereafter inspiratory bursting reciprocated with expiratory bursting and was shown to underlie the intervals between vocalizations. The pattern of activity which was evoked by stimulating vocal substrates was found to strongly interact with the pattern of activity evoked by CO2 stimulation of the respiratory system. Simultaneous records of respiratory and tracheal muscles demonstrated that the same information was sent to both groups of muscles during evoked vocalization. Activity in the respiratory muscles was recorded during spontaneous vocalization of a free-moving bird and was found to resemble that recorded from anesthetized birds. Finally the activity of single units in the obex region of the medulla was recorded during electrical stimulation of vocal substrates and during CO2 stimulation of the respiratory system. Rhythmically active units were found only in the medulla. Unit activity paralleled that found in the nerves and muscles. On the basis on the data accumulated, two models of the chicken vocal system are presented. The first is a model of the sound-producing structures of the chicken. The second is a model of the neural machinery which controls the sound-producing structures. The two models are used as a basis for an explanation of the production of voclizations by the chick of the same species.     

Generating sexually differentiated vocal patterns: laryngeal nerve and EMG recordings from vocalizing male and female african clawed frogs (Xenopus laevis).

Male and female African clawed frogs (Xenopus laevis) produce sexually dimorphic vocalizations; for males these include advertisement, amplectant, and growling calls, whereas female calls include ticking. Previous studies have shown that the vocal organ, the larynx, of the sexes differs in physiological properties that parallel vocal differences. However, it was not clear whether these characteristics are sufficient to explain sex differences in vocal behavior. To examine the contribution of the CNS to generating vocal patterns, we developed a preparation in which both laryngeal nerve activity and electromyograms can be recorded from awake, vocalizing frogs. Recordings reveal that the CNS of the two sexes produces patterned activity that closely matches each vocalization whereas the larynx faithfully translates nerve activity into sound. Thus, the CNS is the source of sexually differentiated vocalizations in Xenopus laevis. Furthermore, detailed analyses of compound action potentials recorded from the nerve lead us to hypothesize that neuronal activity underlying different male call types is distinct; some calls are likely to be generated by synchronous firing of motoneuron populations of either constant size or progressively larger sizes, whereas others are generated by asynchronous activity of motoneurons, a pattern shared with vocal production in females. We suggest that these distinct neuronal activity patterns in males may be subserved by two populations of motor units in males that can be distinguished by the strength of the neuromuscular synapse.     

Responses to somatosensory input by afferent and efferent neurons in the vestibular nerve of the frog

In the frog, we have recorded the activity of efferent and afferent fibers in the nerve of the horizontal semicircular canal in response to somatosensory stimulation. Recordings were made extracellularly by means of glass micropipettes filled with 2 M NaCl, and somatosensory stimulation was produced either by electrical stimulation of the sciatic nerve (ipsi- or contralateral to the recording side) or by vibratory stimulation of the gastrocnemius. The discharge frequency of 43% of the efferent fibers recorded was significantly increased by such stimulation, while the activity of the others was unaffected. The discharge rate of the afferent fibers was either significantly increased (in about 11% of the cases when the results were pooled together) or significantly decreased (in about 22% of the cases) by stimulation of the somatosensory system. The latencies of the responses ranged from 5 to 50 ms. These results show that: somatosensory input can influence the activity of the vestibular apparatus at the most peripheral level; modulation of the afferent discharge is mediated by the efferent vestibular system (EVS); the influence of the EVS on the vestibular afferent activity is both inhibitory and facilitatory, and the responses to somatosensory stimulation are mediated by both long-latency polysynaptic and short-latency oligosynaptic pathways. The functional significance of these two pathways is discussed.     

Neural activity of chorda tympani mechanosensitive fibers during licking behavior in rats

The chorda tympani nerve, supplying the anterior two-thirds of the tongue, contains gustatory and mechanosensitive afferent fibers. We have analyzed discharge patterns in rats of various fibers recorded from dissected nerve filaments during licking behavior of which 4 were taste-sensitive and 12 mechanosensitive. The incidence of these two types was estimated electrophysiologically under anesthesia and their conduction velocity measured. Recordings in freely moving animals showed that the mechanosensitive fibers innervating the dorsal part of the tongue gave two burst discharges per lick, suggesting that contact of the tongue with the upper incisors and/or lip occurred during tongue protrusion and retraction. The fibers from the tip of the tongue showed one burst discharge per lick, which was the response to contact with a drinking spout. No rhythmical discharges synchronized with lick signals were observed in the fibers from the lateral part of the tongue or the taste-sensitive fibers. Such mechanoreceptor discharges were difficult to detect in recordings from the whole chorda tympani nerve. This masking of responses was due mainly to activation of a small number of mechanosensitive fibers by licking-induced mechanical stimulation. The lubricating action of saliva also decreased mechanoreceptor sensitivity. Despite their small number, the mechanosensitive fibers had axons with faster conduction velocities (larger diameter) than the taste-sensitive fibers. This was probably the reason why dissected nerve bundles more frequently showed mechanical than taste responses in conscious rats.     

Feeding and contact call stimulation both induce zenk and cfos expression in a higher order telencephalic area necessary for vocal learning in budgerigars

Stimulation with natural contact calls and feeding were used to assess zenk and fos protein expression in budgerigars (Melopsittacus undulatus), a vocal learning parrot species in which feeding and physical contact often occur in conjunction with vocalization. Although only calls induced gene expression in Field L, the primary telencephalic auditory area, both calls and feeding induced gene expression in the frontal lateral nidopallium (NFl), a brain area in receipt of input from Field L which projects to areas afferent to vocal control nuclei and which is necessary for new call learning. NFl thus appears poised to provide both non-auditory as well as auditory feedback to the vocal system.     

Vocalization-related stapedius muscle activity in different age chickens (Gallus gallus), and its role in vocal development

The stapedius muscle activity associated with vocalization was analyzed in young and adult roosters. Our results show that remarkable differences in the behavior of vocalization-related stapedius muscle activity exist between these two ages. Unlike young roosters, electrical stimulation in the midbrain of adult cocks yields vocalizations associated with stapedius muscle EMG responses that always show a higher threshold and a longer latency than those of the vocalization induced. Moreover, the maximal amplitude of the stapedius muscle EMG response is consistently lower than that detected in young roosters, despite the fact that the maximal vocalization amplitude of the adult birds is much higher. On the whole our results demonstrate that vocalization-related stapedius muscle activity is strongly reduced in adulthood. The possibility that stapedius muscle may play a role during the vocal development was verified by comparing the crow of normal roosters with that of cocks from which the stapedius muscle had been removed shortly after hatching. Strong differences exist in the amplitude/frequency distribution of the crowing of normal and stapedectomized roosters, suggesting that the stapedius muscle exerts an important role in auditory feedback modulation, and that this feedback is used for normal vocal development.     

On the role of the reticular formation in vocal pattern generation

This review is an attempt to localize the brain region responsible for pattern generation of species-specific vocalizations. A catalogue is set up, listing the criteria considered to be essential for a vocal pattern generator. According to this catalogue, a vocal pattern generator should show vocalization-correlated activity, starting before vocal onset and reflecting specific acoustic features of the vocalization. Artificial activation by electrical or glutamatergic stimulation should produce artificially sounding vocalization. Lesioning is expected to have an inhibitory or deteriorating effect on vocalization. Anatomically, a vocal pattern generator can be assumed to have direct or, at least, oligosynaptic connections with all the motoneuron pools involved in phonation. A survey of the literature reveals that the only area meeting all these criteria is a region, reaching from the parvocellular pontine reticular formation just above the superior olive through the lateral reticular formation around the facial nucleus and nucleus ambiguus down to the caudalmost medulla, including the dorsal and ventral reticular nuclei and nucleus retroambiguus. It is proposed that vocal pattern generation takes place within this whole region.