Cross-correlation of medullary respiratory neurons in the cat

The brain stem of anesthetized cats was explored with two microelectrodes in the region of the nucleus retroambigualis group of respiratory neurons. A total of 36 simultaneously recorded pairs of neurons was obtained: 30 inspiratory pairs, 5 expiratory pairs, and 1 mixed pair. The detailed firing patterns of these pairs of neurons were analyzed for functional connectivity using the technique of cross-correlation. The small percentage (10%) of inspiratory pairs showing positive cross-correlation led to the conclusion that there are few strong correlations among inspiratory neurons in the region of the nucleus retroambigualis.     

Synaptic inputs to physiologically identified retinal X-cells in the cat.

The cat retina contains a number of different classes of ganglion cells, each of which has a unique set of receptive field properties. The mechanisms that underlie the functional differences among classes, however, are not well understood. All of the afferent input to retinal ganglion cells are from bipolar and amacrine cell terminals in the inner plexiform layer, suggesting that the physiological differences among cat retinal ganglion cells might be due to differences in the proportion of input that they receive from these cell types. In this study, we have combined in vivo intracellular recording and labeling with subsequent ultrastructural analysis to determine directly the patterns of synaptic input to physiologically identified X-cells in the cat retina. Our primary aim in these analyses was to determine whether retinal X-cells receive a characteristic pattern of bipolar and amacrine cell input, and further, whether the functional properties of this cell type can be related to identifiable patterns of synaptic input in the inner plexiform layer. We reconstructed the entire dendritic arbor of an OFF-center X-cell and greater than 75% of the dendritic tree of an ON-center X-cell and found that 1) both ON- and OFF-center X-cells are contacted with approximately the same frequency by bipolar and amacrine cell terminals, 2) each of these input types is distributed widely over their dendritic fields, and 3) there is no significant difference in the pattern of distribution of bipolar and amacrine cell synapses onto the dendrites of either cell type. Comparisons of the inputs to the ON- and the OFF-center cell, however, did reveal differences in the complexity of the synaptic arrangements found in association with the two neurons; a number of complex synaptic arrangements, including serial amacrine cell synapses, were found exclusively in association with the dendrites of the OFF-center X-cell. Most models of retinal X-cell receptive fields, because their visual responses share a number of features with those of bipolar cells, have attributed X-cell receptive field properties to their bipolar cell inputs. The data presented here, the first obtained from analyzing the inputs to the entire dendritic arbors of retinal X-cells, demonstrate that these retinal ganglion cells receive nearly one-half of their input from amacrine cells. These results clearly indicate that further data concerning the functional consequences of amacrine cell input are needed to understand more fully visual processing in the X-cell pathway.     

Dorsal medullary inspiratory neurons: effects of superior laryngeal afferent stimulation

In decerebrate paralyzed cats ventilated with a cycle-triggered pump, we examined the responses of inspiratory (I) neurons in the region of the ventrolateral nucleus tractus solitarius (NTS) to single electrical stimuli delivered to the ipsilateral superior laryngeal nerve (SLN). Sixty-five I neurons were classified as: I(-), I(0), I(+, early), I(+, late) or I(other) on the basis of responses to lung inflation, and as I(bulbophrenic) or I(non-bulbophrenic) on the basis of evidence of an excitatory projection to the contralateral phrenic motoneuron pool. The peristimulus histograms of contralateral phrenic activity showed an early peak of excitation with average latency of 4.9 +/- 0.1 ms (mean +/- S.E.M.), followed by depression at 7.3 +/- 0.2 ms, start of recovery from depression at 22.7 +/- 1.0 ms, and recovery to control levels at 28.4 +/- 1.1 ms. The peristimulus histograms of ipsilateral I unit activity showed an initial excitation (latency 2.9 +/- 0.3 ms), followed by spiking silence (latency 6.0 +/- 0.6 ms) and recovery to control discharge frequency at 38.8 +/- 3.6 ms. This time of inhibition was significantly longer than the time of phrenic depression, suggesting that other bulbophrenic excitatory projections are able to rapidly compensate for decreased NTS output. Subgroups of I neurons, as classified by lung inflation tests, did not differ significantly with respect to these timing variables. In contrast, latencies of excitation for I(bulbophrenic) neurons were significantly less than for I(non-bulbophrenic) neurons.     

Effects of contralateral superior laryngeal nerve stimulation on dorsal medullary inspiratory neurons

The effects of superior laryngeal nerve (SLN) stimulation on phrenic (PHR) nerve activity and on activity of dorsal respiratory group (DRG) inspiratory (I) neurons contralateral to the stimulus were examined in decerebrate, paralyzed cats. Stimulation caused bilateral PHR suppression followed by recovery at ca. 30 ms. Most DRG neurons (70%) contralateral to the stimulus were inhibited, but average onset of inhibition lagged that of PHR suppression. This contrasts sharply with the observation in an earlier study that inhibition of ipsilateral I neurons on the average preceded PHR suppression. The remaining neurons (30%) were not inhibited. Only 22% of contralateral neurons were excited by SLN stimulation, in contrast to 52% of ipsilateral neurons. Thus, contralateral DRG I neurons do not mediate the onset of bilateral PHR suppression by SLN stimulation and are probably inhibited through a longer pathway than that for the ipsilateral unit responses.     

Interactions of inputs from dorsal columns and ventral tracts with visual inputs on single neurons of cat superior colliculus

In cats with upper spinal lesions disconnecting the dorsal column (DC) from the spinocervicolemniscal and ventral tract (VT) systems, single unit responses in the superior colliculus could be evoked by DC and VT stimulation. Excitatory convergence, as well as facilitatory and inhibitory interactions, were observed between DC, VT, visual and auditory inputs.     

Synaptic interactions between thalamic inputs to simple cells in cat visual cortex.

We performed experiments in the cat geniculocortical pathway, in vivo, to examine how presynaptic spikes interact to influence the firing of postsynaptic targets. In particular, we asked (1) how do multiple spikes from a single presynaptic neuron interact to influence the firing of a postsynaptic target (homosynaptic interactions), (2) how do spikes from two different presynaptic neurons interact (heterosynaptic interactions), and (3) what is the time course of homosynaptic and heterosynaptic interactions? We found that both homosynaptic and heterosynaptic interactions increase the likelihood of driving a postsynaptic spike, although with different time courses. For two spikes traveling down a single geniculate axon, the second spike is more effective than the first for approximately 15 msec. For two spikes on separate axons, the interaction is faster ( approximately 7 msec duration, approximately 2.5 msec time constant). Thus changes in firing rate are perhaps best relayed by homosynaptic interactions, whereas heterosynaptic interactions may help detect coincident spikes from different thalamic inputs.     

Synaptic responses of neurons of the lateral medullary reticular formation to stimulation of the locomotor strip in the cat

Intracellular recording of neuronal responses in the lateral medullar reticular formation to stimulation of locomotor points located rostrally and caudally of the obex was made in mesencephalic uncurarized cats. Of 114 neurons with synaptic responses to the rostral point stimulation by current up to 30 microA 40 neurons were excited antidromically from the caudal point.     

Sound-induced laryngeal and respiratory reflexes originate from vestibular afferents

To determine whether the auditory or vestibular system causes the sound-induced laryngeal reflex, which has been considered to participate in the auditory feedback control of vocalization, click-induced laryngeal responses were compared before and after sectioning of the cochlear and/or vestibular nerves in cats. The sound-induced reflex modulation of respiratory muscle activity was also investigated, because respiratory movement is important for vocal control. Sectioning of the cochlear nerves had little influence on these responses. In contrast, sectioning of the vestibular nerves abolished these responses. It was concluded that the sound-induced laryngeal and respiratory reflexes are attributed to the vestibular system.     

Synaptic actions of vagal afferents on facial motoneurons in the cat

Synaptic potentials in facial motoneurons of cats were intracellularly recorded on stimulation of the vagal nerve, superior laryngeal nerve, solitary tract nucleus and spinal trigeminal tract nucleus. A possible disynaptic excitation was elicited in the facial motoneurons by stimulation of the vagal nerves and superior laryngeal nerves on both sides. Activation of the neurons in the solitary tract nucleus and/or trigeminal tract nucleus induced monosynaptic excitatory postsynaptic potentials (EPSPs) in the facial motoneurons.     

Synaptic distribution of afferents from reticular nucleus in ventroposterior nucleus of cat thalamus

This study was aimed at determining the synaptic circuitry that contributes to the alterations in thalamic function that accompany changes in behavioral states. The somatosensory sector of the thalamic reticular nucleus (RTN) was identified by microelectrode recording in cats and injected with Phaseolus vulgaris-leucoagglutinin (PHA-L). The axons of labeled RTN cells gave rise to collaterals within the RTN and continued into the dorsal thalamus where they terminated predominately in the ventral posterior lateral nucleus (VPL). After small injections in the upper limb representation of RTN, most labeled terminations in VPL were confined to its medial part, suggesting the presence of a topographic organization in the projection. Terminations were concentrated in localized, focal aggregations of boutons. Combined electron microscopic immunocytochemistry, using immunogold labeling for γ-aminobutyric acid (GABA), showed that the PHA-L labeled boutons were GABA-positive terminals that ended in symmetrical synapses. Eighty-two percent of these synapses were on dendrites of relay neurons, 8.5% on dendrites of interneurons, and 9.3% on somata. The terminals of RTN axons form the majority of axon terminals ending in symmetrical synapses in VPL. Their concentration on relay neurons probably underlies the capacity of the RTN projection to reduce background activity of VPL relay neurons in the awake state and to maintain oscillatory behavior of these neurons in drowsiness and early phases of Sleep. © 1995 Wiley-Liss, Inc.     

Synaptic organization of claustral and geniculate afferents to the visual cortex of the cat

Claustral and geniculate afferents to area 17 were labeled by anterograde axonal transport of peroxidase-conjugated wheatgerm agglutinin, and examined in the electron microscope. A peroxidase reaction protocol that led to labeling in the form of minute holes in the EM sections was used. Both types of afferents formed type 1 (presumed excitatory) synapses exclusively. In agreement with previous reports the great majority of geniculate afferents to layers 4 and 6 contacted dendritic spines. The claustral afferents to layers 1 and 6 also predominantly contacted spines. In layer 4, however, claustral afferents contacted spines and dendritic shafts about equally. The results suggest a substantial claustral input to smooth-dendrite cells in layer 4, which are thought to be inhibitory in function. This may be the circuit by which the claustrum helps to generate end-stopped cortical receptive fields (Sherk and LeVay, 1983).     

Synaptic detachment from hypoglossal neurons after different types of nerve injury in the cat.

Quantitative electron microscopy was used to examine synaptic numbers and total relative synaptic coverage on hypoglossal neuronal perikarya following transection or crush of the hypoglossal nerve in the cat. In cats subjected to nerve transection there was a statistically significant decrease in total synaptic coverage as well as in number of synaptic contacts/soma profile. After nerve crush there was a statistically significant decrease in total synaptic coverage but not in the number of synaptic contacts. These results indicate that the degree of synaptic detachment on motor neurons in the cat is influenced by the type of nerve injury.     

Lateral hypothalamic and peripheral cardiovascular afferent inputs to ventrolateral medullary neurons

Experiments were done in chloralose anesthetized, paralyzed and artificially ventilated cats to identify single units in ventrolateral medulla (VLM) projecting directly to the intermediate gray (IG) region of the upper thoracic cord and responding to inputs from pressor sites in the anterior lateral hypothalamus (Hla) and carotid sinus (CSN) and aortic depressor (ADN) nerves. Forty-eight units were antidromically activated in VLM to stimulation of the IG at the level of T2. Of these 48 units, 15 (31%) were orthodromically excited by stimulation of the Hla with a mean latency of 15.8 +/- 2.1 ms. In addition, 8 of the 15 units responding to Hla stimulation were also excited orthodromically by stimulation of either the CSN or ADN or both. Of the remaining 33 units, 15 responded to stimulation of only the buffer nerves and 18 were unresponsive to the tested inputs. These results provide electrophysiological evidence for the existence of neurons in VLM which receive hypothalamic and buffer nerve inputs and suggest that the VLM plays a role in integrating and relaying cardiovascular afferent information from peripheral baroreceptors and chemoreceptors and from supramedullary centers to provide effector signals to spinal autonomic neurons involved in the control of the circulation.     

Cross-correlation of medullary dorsomedial inspiratory neurons in the cat

The dorsomedial inspiratory nucleus in the medulla of cats was explored with pairs of microelectrodes with tip separations from 100 to 350 μm such that extracellular potentials of pairs of near neighbors were recorded simultaneously. Each neuron was identified as an Rα or Rβ type using lung volume manipulations, and cross-correlation histograms were computed. The cross-correlation histograms showed significant central peaks in four of seven Rα-Rα pairs, nine of nine Rβ-Rβ pairs, and four of six Rα-Rβ pairs. The cross-correlation histograms showed significant central peaks after bilateral vagotomy in two of two Rα-Rα pairs and three of three Rβ-Rβ pairs. These results demonstrate the existence of short-term synchronization in the firing patterns of these neurons. Such synchronization may be due to a common excitation of the neurons or to interactions between them.     

Cross correlation of medullary expiratory neurons in the cat

Near neighbors among medullary expiratory neurons might be expected to show short-term synchronization through the mechanisms of either self-reexcitation or shared afferents. Previous examinations that used the technique of cross correlation of pulse trains of near neighbors have yielded conflicting results. A study of 22 pairs of expiratory neurons showed no correlations and suggests that there is a lack of short-term synchronization between these neurons. Differences in recording techniques and experimental preparations may be responsible for the differences in the results of previous studies.