Neuronal activity in the vestibular nuclei after trigeminal stimulation

The activity of 131 vestibular neurons was investigated in 40 guinea pigs during trigeminal stimulation. The influence of trigeminal sensory information was tested by means of electrical stimulation of the trunk of the trigeminal nerve and with nonnociceptive cutaneous stimulation of facial areas. The spontaneous activity of vestibular neurons was modified by trigeminal electrical stimulation with an increase or decrease of the discharge rate. Some units underwent rhythmic modulation. Forty-one percent of recorded units responded also to cutaneous trigeminal stimulation. An analysis of vestibular action potentials evoked by trigeminal stimulation revealed latency values from 1.2 to 6.2 ms. The anatomic-functional relationship between the trigeminal and the vestibular systems is discussed in light of the reported results.     

Periaqueductal gray stimulation: Effect on characterized nucleus gigantocellularis neurons in the rat

Although stimulation of the periaqueductal gray (PAG) and microinjection of opiates or opioids into the PAG was reported to affect the activity of nucleus gigantocellularis (NGC) neurons, it is not known whether a direct pathway exists between these brain regions. The purpose of this investigation was to electrophysiologically test this possibility by attempting to antidromically activate PAG neurons from the NGC. Seventy neurons in the PAG of urethane-anesthetized rats were activated by electrical stimulation in the NGC and classified on the basis of the antidromicity criteria of constant latency, high following frequency, and collision. Of these neurons, 50 (71%) were classified as being transsynaptically activated, and 20 (29%) were classified as being antidromically activated. Ten neurons demonstrated collision between spontaneous action potentials and stimulation-evoked action potentials. Although most of the responses could not be tested with all criteria, the data support the existence of a monosynaptic pathway from the PAG to the NGC.     

Auditory nerve fiber activity influenced by contralateral ear sound stimulation

Modulation of efferent olivo-cochlear bundle activity by heterolateral ear sound stimulation suggests that acoustic stimulation may modify contralateral auditory afferent activity. Therefore, changes in auditory nerve fiber activity elicited by contralateral ear sound stimulation were studied. Low sound intensities prevented cross talk between both ears, while response averaging enabled the demonstration of small effects. Heterolateral tonal stimulation decreased the homolateral sound-evoked activity of about 25% of the units studied. To elicit this effect heterolateral tones had to be at least 20 dB above the threshold for homolateral activation of the unit and had to last more than 100 ms. Frequency-response curves of the decrease in homolateral sound-evoked activity elicited by heterolateral stimulation were in most cases V-shape. Homolateral and contralateral frequency-response curves had similar characteristic frequencies. Occasionally W-shape curves were found. The effect also depended on the characteristics of the homolateral tone, being greater if it was at the characteristic frequency and of low intensity. Heterolateral white noise stimulation elicited an increase in the homolateral responses in about 88% of the units which showed decreased responses with heterolateral tone stimulation; other units were not influenced by low-intensity heterolateral white noise stimulation. Modifications of the “spontaneous” activity by heterolateral tones were observed in about 46% of the units which showed decreased responses with heterolateral tonal stimulation. The degree of contralateral modulation depended on the characteristics of the homolateral and heterolateral sounds. The described effects may act to enhance the perception of differences between sounds stimulating both ears.     

Effects of cerebellar and brain stem stimulation on hippocampal formation neurons in the monkey

Micro- and macroelectrode recording techniques were used to detect the presence of cerebellar influences on electrical activity in the hippocampal formation of the rhesus monkey. Only 2% of the cells studied responded to cerebellar stimulation. These responsive cells showed a decreased firing rate. No evoked potentials were detected after cerebellar stimulation in any of the monkeys. The effects of brain stem stimulation were tested in two monkeys. One electrode placement lateral to the central gray did not evoke any neuronal responses. The other, situated in the raphe nucleus near the border of the dorsal raphe and nucleus centralis superior, evoked inhibitory responses in 17% of the cells tested. These results (i) fail to provide support for the existence of an important pathway from the cerebellum to the hippocampal formation in monkeys, and (ii) preliminarily extend to a primate species previous results in rat and cat showing an inhibitory effect of raphe stimulation on hippocampal neurons.     

Properties of cells responding to visual stimuli in the rat ventral lateral geniculate nucleus

Single-unit recordings were made of the ventral lateral geniculate nucleus (LGv) in the albino rat anesthetized with urethane. Visual receptive field properties as well as the characteristics of responses elicited by electrical stimuli to the optic tract and to the visual cortex were examined. Compared with the relay cells of the dorsal lateral geniculate nucleus (LGd), LGv cells were characterized by the following properties. (i) They responded to visual cortex stimuli orthodromically as well as to optic tract shocks. (ii) The postexcitatory inhibition they showed after single optic tract or visual cortex stimuli was only short-lasting, at most 100 ms. (iii) Conduction velocities of the optic nerve afferent fibers were mostly in the range of slow fibers, 2 to 10 m/s. (iv) The receptive fields were essentially homogeneous in type; about 90% of the sample of 53 cells were On-tonic. (v) Receptive field sizes were substantially large, from 6.3 to 45.6° (mean, 22.3°). (vi) On-tonic cells revealed a regular maintained discharge whose level changed monotonically as a function of the luminous intensity of the stimulating light. The functional implications of these findings were compared with those of the relay cells in the LGd.     

Vestibular efferent fibers to ampulla of anterior, lateral and posterior semicircular canals in cats

Origins of vestibular efferent fibers to ampulla of semicircular canals in cats were examined using retrograde transport of horseradish peroxidase. The anterior canal was innervated from bilateral parvocellular reticular nucleus (PCRN), contralateral gigantocellular reticular nucleus and ipsilateral lateral reticular nucleus (LRN); the lateral canal, from ipsilateral PCRN and LRN as well as ipsilateral lateral vestibular nucleus; and the posterior canal, from bilateral PCRN and ipsilateral medial and lateral vestibular nuclei.     

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).     

Habituation of human limbic neuronal response to sensory stimulation

Hippocampal, parahippocampal gyrus, and amygdalar neuronal responses to visual and acoustic stimuli were analyzed during trains of several hundred stimulus repetitions as part of an investigation of sensory pathways to medial temporal lobe structures in complex-partial epilepsy patients who were being monitored with depth electrodes. Ten percent of more than 500 single and multiple units tested were responsive to simple sensory stimuli. The majority of the responsive units were recorded in the posterior parahippocampal gyrus (HG) during visual stimulation. Although neurons in pes hippocampi (PH; Ammons's horn) were also responsive to photic stimuli, no visually responsive units were found in amygdala. Very few units were responsive to acoustic stimuli, and these were found only in PH and amygdala, and not in HG. Significant trends of increase or decrease in response amplitude during trains of stimuli were found in all acoustically responsive units. Significant trends of visual response amplitude increase or decrease were found in 20% of PH units, and in 44% of HG units. Mean latencies of acoustically responsive units were longer than those of visually responsive units, and latencies of PH sensory units showing decremental response were longer than nondecremental PH units. Rate of response decrement was usually linear for acoustic responses and exponential for visual responses. The response dynamics of medial temporal lobe neurons are compared with those described in the animal limbic system and are related to habituation of human sensory evoked scalp potentials.     

Vestibular nerve input to neck and shoulder regions of lateral cuneate nucleus

Axonal transport of tritiated leucine by the vestibular nerve was observed in regions of the ipsilateral lateral cuneate nucleus (LCN) that were determined electrophysiologically to receive somatosensory input from the neck and shoulder and to respond to electrical stimulation of the vestibular nerve. Regions of the LCN receiving input from more distal musculature did not receive vestibular input.     

A study of the origin of brain stem projections to monkey spinal cord using the retrograde transport method.

We identified brain stem neurons projecting to cervical and lumbar levels of the spinal cord in young rhesus monkeys using the retrograde transport method. The somatotopic organizations of the red nucleus and lateral vestibular nucleus were clarified. In addition, the presence of bulbospinal neurons in the medial vestibular nucleus; the nucleus of the tractus solitarius; the medial and lateral reticular formations; the raphe nuclei magnus, obscurus, and pallidus; the hypothalamus; and the nuclei of the locus ceruleus and subceruleus was confirmed.     

Influences of locus ceruleus, raphe dorsalis, and periaqueductal gray matter on somatosensory-recipient thalamic nuclei

Spontaneous and evoked discharge of neurons in the nucleus ventralis posterolateralis (VPL) and spontaneous discharge of neurons in the posterior group and nucleus lateralis posterior (LP) were conditioned by brief trains of stimuli to the locus ceruleus (LC), raphe dorsalis (RD), and periaqueductal gray matter (PAG) in cats anesthetized with pentobarbital or ketamine. Stimulation of LC and RD was without effect on VPL neurons, but induced a long-latency, long-lasting inhibition of LP neurons. Stimulation of the PAG induced marked inhibition of the firing of neurons in all three thalamic nuclei. No differences were found between cats anesthetized with ketamine or pentobarbital.     

Norepinephrine and serotonin: Opposite effects on the activity of lateral geniculate neurons evoked by optic pathway stimulation

This study compares the responses of single units in the rat dorsal lateral geniculate nucleus (LGNd) to microiontophoretically applied norepinephrine (NE) and serotonin (5-HT). Most of the cells were identified physiologically as P-type (geniculocortical relay) neurons. At low iontophoretic currents (1 to 20 nA), NE caused a delayed increase in the spontaneous firing rate of these units, whereas 5-HT invariably slowed the discharge frequency. To compare the effects of the two monoamines on evoked activity, P-cells were driven by electrical stimulation of the afferent visual pathway at the level of the optic chiasm. NE caused a marked facilitation of both the short-latency (2 to 4 ms) and the delayed (70 to 230 ms) responses to such stimulation. The α-adrenoceptor antagonist phentolamine (10 nA), which by itself had no consistent effect on evoked activity, strongly diminished the response to NE. In contrast to NE, 5-HT was a powerful depressant of electrically evoked activity; neither phentolamine nor the 5-HT antagonist methysergide antagonized this response. Firing of LGNd units evoked by flashes of light was also facilitated by NE and depressed by 5-HT. We conclude that LGNd relay neurons exhibit the following unique features in their responsiveness to monoamines: (i) microiontophoretically applied NE facilitates, but 5-HT depresses, the spontaneous or synaptically evoked activity of virtually every cell; (ii) there is no dissociation between the actions of NE on spontaneous and evoked activity, as is the case in other brain regions.     

Abnormalities of cortical inhibitory neurons in amyotrophic lateral sclerosis

We have used peristimulus time histograms to study how paired, transcranial magnetic stimulation alters the firing of single motor units and the magnitude of unitary excitatory postsynaptic potentials (EPSPs) recorded from the extensor digitorum communis muscle. With stimulus intensity at threshold and an interstimulus interval of 30 ms, normal subjects (n = 20) demonstrated marked inhibition with a mean test/conditioning EPSP ratio of 13.8% (range 0–51%) and in 7 subjects the ratio was 0 (100% inhibition). In amyotrophic lateral sclerosis (ALS) the ratio was 133% (range 64–267%), P < 0.001. Fifty percent of patients had a test/conditioning EPSP ratio greater than 100% (0 inhibition). The abnormalities were independent of disease severity, bulbar versus spinal ALS, more prominent upper versus lower motor neuron findings, and disease duration. Normal inhibition occurred in 3 individuals, 1 each with multiple sclerosis, Kennedy's syndrome, and monomelic amyotrophy. We speculate that the marked loss of inhibition seen in all patients with ALS, which may be unique to this disorder, reflects loss of inhibitory modulation of the corticomotoneuron and could result in their chronic excitatory drive and eventual demise. © 1997 John Wiley & Sons, Inc.     

Changes in fusimotor outflow during vibration-induced contraction of triceps surae muscles in decerebrate cats

Changes in fusimotor outflow in medial gastrocnemius and lateral gastrocnemius plus soleus nerves were investigated during vibration-induced contraction of triceps surae muscles in decerebrate cats. A sustained increase in spike occurrence appeared in 33 of 58 investigated neurons, and a decrease in 13. Transient changes of an opposite sign occurred in 19 excited and 6 inhibited fusimotor neurons during the rising phase of reflex muscle contraction. The later changes appeared coincidently with silence in the electromyogram. Thus the discharge of one-third of the examined fusimotor population directed to the triceps surae muscles was modulated parallelly with that of the skeletomotor neurons. It is supposed that the changes in fusimotor spike occurrence during the rising phase of reflex muscle tension are induced mainly by changes in afferent input from muscle spindle primary endings and/or Golgi tendon organs. We could not ascertain whether the different responses (excitation vs inhibition) are related to fusimotor type or to changes in transmission through the reflex pathways. The possible influence of the changes in fusimotor outflow on muscle spindle pause and the silent period in skeletomotor discharge at the beginning of reflex muscle contraction induced by vibration is considered.     

Monosynaptic activation of long descending propriospinal neurons from the lateral vestibular nucleus and the medial longitudinal fasciculus

In decerebrate cats long descending propriospinal (LDP) neurons were recorded extracellularly in the cervical enlargement. They were identified antidromically by spinal cord stimulation at the L1–L2 level. Inputs to these cells were tested by stimulating the medial longitudinal fasciculus (MLF) 5 to 6 mm rostral to the obex, the lateral vestibular nucleus (LVN), the upper MLF 1 mm caudal to the trochlear nucleus, and the medial vestibular nucleus (MVN), all on the ipsilateral side. Action potentials were elicited in 44% ( ) of LDP neurons in the ventral horn (laminae VII, VIII) at a segmental latency of 1 ms or less following brain stem stimulation. This was considered to be a monosynaptic latency. The most effective stimulation sites were the MLF and the LVN. MLF stimulation accounted for about two-thirds of the monosynaptically elicited action potentials and LVN for about one-third. Another 22% of LDP neurons responded at longer latencies, but some of those responses may also have been monosynaptic. Stimulation of the upper MLF and the MVN were much less effective, indicating that the MLF input was predominantly from fibers originating in the medullary and/or pontine reticular formation.     

Evoked ventromedial hypothalamic activity: changes produced by drugs which modify female sexual behavior in the rat

The effect of several pharmacologic agents on the field potential and multiunit responses evoked in the ventromedial hypothalamic nucleus (VMN) by single shocks applied to the anterior part of the amygdaloid nucleus and the posterior part of the lateral amygdaloid nucleus was investigated in ovariectomized (OVX) and proestrous (PRO) rats using a combined infusion cannula and recording electrode. The local infusion of carbamylcholine and eserine incremented the multiunitary responses, whereas atropine, hexamethonium, and hemicholinium suppressed the multiunit responses and, in the case of atropine, the field response also. The onset of the suppressive effect of hemicholinium depended on the activation of the amygdaloid input. Phenoxybenzamine, methysergide, and picrotoxin increased the multiunitary evoked responses in OVX, but phenoxybenzamine and methysergide had no effect on PRO rats. On the contrary, propanolol suppressed the response. Because a good correlation can be observed between the effects of drugs on evoked responses and on sexual behavioral responses (lordosis) of the female rat as reported in the literature, the results are compatible with the hypothesis that (a) amygdaloid influences on sexual behavior are exerted through modifications of neuron activity in the VMN, (b) this influence involves the activation of a cholinergic synapse, and (c) catecholaminergic, GABAergic, and serotoninergic inputs modulate VMN neuron responses to activity originated in the amygdala.     

Cochlear nucleus unit responses to pure tones in the unanesthetized rabbit

Seventy-six units were studied in the cochlear nucleus of unanesthetized, paralyzed rabbits. Most were located in the dorsal cochlear nucleus. Fifty-eight units were characterized according to their response area by the scheme of Evans and Nelson (1973, Exp. Brain Res., 17, 402–427): 22.5% showed purely excitatory response areas (type ), 72.5% showed mixed excitatory and inhibitory response areas (types III and IV), and 5% showed purely inhibitory response areas (type V). Ten units had virtually no spontaneous activity so were unclassifiable. When characterized according to the scheme of Pfeiffer (1966, Exp. Brain Res., 1, 220–235), 41% of the units were primary-like, 24% were choppers, 20% were pausers, 3% were buildup, and 1% were on units. There was no overall correlation between the two methods of unit classification, but type units showed a high number of chopper patterns. Dorsal cochlear nucleus units in the unanesthetized rabbit showed response characteristics very similar to those which were reported in the decerebrate cat. However, there was a greater prevalence of primary-like, purely excitatory units in the rabbit dorsal cochlear nucleus. This may have reflected a species difference or have been due to technical considerations. It appears that the rabbit cochlear nucleus has very similar physiologic, as well as anatomic, characteristics as does that of the cat.     

Anesthetic-dependent excitability changes in the hypothalamic ventromedial nucleus of the rat

In acute experiments in rats anesthetized with urethane, the field potentials, population spike, and unit activity evoked in the hypothalamic ventromedial nucleus (HVM) by amygdaloid stimulation are significantly increased with respect to control when preceded by a conditioning volley at 20- to 100-ms intervals. Under pentobarbital anesthesia, in contrast, the evoked responses were inhibited by the conditioning stimulus for similar interstimulus intervals. In unanesthetized animals chronically implanted with stimulating and recording electrodes, a facilitation of responses by a conditioning stimulus was observed when they were awake or anesthetized with urethane. When the same animals were anesthetized with pentobarbital the HVM evoked response was inhibited by a conditioning pulse. Frequency facilitation and post-tetanic potentiation of HVM responses were markedly enhanced under urethane, whereas in pentobarbital-anesthetized animals inhibition predominated. Picrotoxin reversed the inhibition under pentobarbital to facilitation. These results suggest that the HVM neuron population is under both excitatory and inhibitory influences from the amygdala, the former being predominant in awake and urethane-anesthetized animals and the latter being expressed under pentobarbital anesthesia and is probably mediated by γ-aminobutyric acid.     

Slow rhythmic activity of caudate neurons in the cat: Statistical analysis of caudate neuronal spike trains

Spike trains of caudate neurons initially having mean interspike intervals of less than 4 ms were analyzed with progressive administration of pentobarbital (5 to 20 mg/kg). Among the neurons investigated, 77% (N = 79) showed evidence of a rhythmic basis of their activity in first-order interspike interval histograms and/or autocorrelation histograms in the course of becoming silent due to progressive administration of pentobarbital. Although the rhythmicies of given units varied depending on the level of anesthesia the most prominent cycle was almost always within the range of 200 to 320 ms; the majority were not discernable on visual inspection of the spike trains. Cortical stimuli reset the cycle. Cross-correlation histograms constructed from pairs of caudate neurons provided some evidence that their spontaneous firing was mutually inhibited. The possibility that the rhythmicities might arise from such mutual inhibition of spontaneously firing caudate neurons is discussed.