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PubMed1995

Synaptic connectivity of local circuit neurons in laminae III and IV of hamster spinal cord.

The present study was undertaken to examine the morphological bases of local synaptic interactions between dorsal horn interneurons. Seven interneurons responding to innocuous mechanical stimuli were intracellularly recorded in lamina III/IV of an isolated preparation of hamster spinal cord with partially intact innervation from an excised patch of hairy skin. Axonal arborizations were stained with horseradish peroxidase (HRP) and examined with an electron microscope. Five cells had extensive synaptic terminations (375-1,785 boutons/axon) with localized distributions (rostrocaudal distance, 425-1,251 microns) overlapping the dendritic trees. Two cells gave rise to deep stem axons that bifurcated into rostrocaudal daughter branches with collaterals ventral to the parent cell bodies (79-661 boutons/axon). Axons of local interneurons were thinly myelinated and formed terminal and en passant enlargements (mean [+/- S.D.] diameter = 0.88 +/- 0.24 microns, n = 157) containing clear, round vesicles 20-60 nm in diameter. Collateral branches of deep axon cells produced round, vesicle-containing boutons comparable in diameter (0.93 +/- 0.22 microns, n = 31) to local axon cells. Both types of interneurons formed asymmetric synaptic contacts with dendritic profiles, but not with cell bodies or axon terminals. Postsynaptic profiles contained sparse ribosomes and had a mean diameter of 1.0 +/- 0.5 microns (n = 49), significantly smaller than a population of identified proximal dendrites (2.3 +/- 0.9 microns, n = 47). HRP-labeled boutons were rarely (5/45 or 11%) in synaptic contact with more than one profile. We conclude that lamina III/IV interneurons make axodendritic synapses predominantly with distal dendrites. Thus, terminations of deep dorsal horn interneurons appear to have a postsynaptic distribution overlapping with axodendritic contacts formed by several functional classes of cutaneous sensory fibers signaling innocuous mechanical stimuli. Such overlap suggests that local spinal networks selectively and strongly influence afferent signals at initial stages of somatosensory integration.

Adaptation, Physiological↗
PubMed1990

The lateral reticular nucleus in the cat. VIII. Excitatory and inhibitory projection from the bilateral ventral flexor reflex tract (bVFRT).

Intracellular recordings were obtained from 204 neurones in the lateral reticular nucleus (LRN). LRN neurones contacted by the bVFRT were identified by the responses evoked on stimulation of descending fibres in the contralateral ventral quadrant of the spinal cord (cVQ) at cervical (C5cVQ) and lumbar (L2cVQ) levels. Stimulation of the cVQ evoked excitatory or inhibitory responses in 124 of the 204 LRN neurones. EPSPs were evoked in 45, IPSPs in 52 and both EPSPs and IPSPs in 27 LRN neurones. The shortest latencies of the responses evoked from the cVQ indicated that both EPSPs and IPSPs were disynaptic. This finding was confirmed by direct stimulation of the ascending fibres in the ipsilateral ventrolateral funiculus at C3 (C3iVLF) or L1 (L1iVLF). In most LRN neurones activated or inhibited from the cVQ, stimulation of the iVLF evoked similar responses at a monosynaptic latency. These results indicate that the bVFRT consists of roughly equally large groups of excitatory and inhibitory neurones monosynaptically connected with the LRN. Excitatory and inhibitory bVFRT neurones had similar peripheral receptive fields and termination areas in the LRN. LRN neurones were divided into those contacted by cervical bVFRT neurones and lumbar bVFRT neurones. The former group consisted of LRN neurones responding to C5cVQ stimulation at latencies below 5 ms, whereas the latter group contained LRN neurones responding to stimulation of the L2cVQ. Cervical bVFRT neurones projected to most parts of the LRN whereas the projection of lumbar bVFRT neurones were confined to the ventrolateral part of the nucleus. Excitatory and inhibitory vVFRT neurones of each group had similar termination areas.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1978

Properties and connections of cat fastigiospinal neurons.

1. Neurons in the cat fastigial nucleus that project to the upper cervical spinal segments (fastigiospinal neurons) were fired by antidromic stimulation of the contralateral spinal cord. Dye ejection from the recording electrode was used to show that most neurons were in the rostral half of the fastigial nucleus. 2. Fastigiospinal neurons can be excited and/or inhibited by stimulation of forelimb and hindlimb nerves and by stimulation of the vestibular nerve. These inputs converge on many neurons. 3. Antidromic microstimulation was used to trace fastigiospinal axons to the vicinity of motor nuclei in in C2-C3. 4. The rostral fastigial nucleus was stimulated in preparations with the medial longitudinal fasciculus transected by a wide lesion that impinged on the medial reticular formation in the caudal medulla, to eliminate some potential axon reflexes. Short-latency EPSPs were recorded in some trapezius and biventer-cervicis motoneurons. In many cases there was little or no occlusion between these EPSPs and others evoked by stimulation of the vestibular nerve ipsilateral to the motoneurons. 5. Movement of the stimulating electrode and placement of this electrode lateral to the fastigial nucleus show that the zone from which low threshold EPSPs can be evoked is localized. 6. Latency measurements and lack of temporal facilitation with double shocks suggest that the EPSPs are monosynaptic. The evidence suggests that they are caused by fastigiospinal fibers terminating on motoneurons.

Animals↗
PubMed1994

The fixation area of the cat superior colliculus: effects of electrical stimulation and direct connection with brainstem omnipause neurons.

The superior colliculus has long been recognized as an important structure in the generation of saccadic displacements of the visual axis. Neurons with presaccadic activity encoding saccade vectors are topographically organized and form a "motor map." Recently, neurons with fixation-related activity have been recorded at the collicular rostral pole, at the area centralis representation or fixation area. Another collicular function which deals with the maintenance of fixation behavior by means of active inhibition of orientation commands was then suggested. We tested that hypothesis as it relates to the suppression of gaze saccades (gaze = eye in space = eye in head + head in space) in the head-free cat by increasing the activity of the fixation cells at the rostral pole with electrical microstimulation. Long stimulation trains applied before gaze saccades delayed their initiation. Short stimuli, delivered during the gaze saccades, transiently interrupted both eye and head components. These results provide further support for a role in fixation behavior for collicular fixation neurons. Brainstem omnipause neurons also exhibit fixation-related activity and have been shown to receive a direct excitatory input from the superior colliculus. To determine whether the collicular projection to omnipause neurons arises from the fixation area, the deep layers of the superior colliculus were electrically stimulated either at the rostral pole including the fixation area or in more caudal regions where stimulation evokes orienting responses. Forty-nine neurons were examined in three cats. 61% of the neurons were found to be orthodromically excited by single-pulse stimulation of the rostral pole, whereas only 29% responded to caudal stimulation. In addition, stimuli delivered to the rostral pole activated, on average, omnipause neurons at shorter latencies and with lower currents than those applied in caudal regions. These results suggest that excitatory inputs to omnipause neurons from the superior colliculus are principally provided by the fixation area, via which the superior colliculus could play a role in suppression of gaze shifts.

Animals↗
PubMed1980

Synaptic connections of buccal mechanosensory neurons in the opisthobranch mollusc, Navanax inermis.

Mechanical stimulation of various areas of the pharyngeal wall and lips can produce EPSPs and IPSPs, as well as abruptly rising impulses, in primary sensory cells. IPSP fields are generally larger than EPSP fields and these fields are distributed without obvious order around fields from which afferent spikes are evoked. Apparently monosynaptic excitatory and inhibitory contacts are formed between primary sensory neurons. These synapses are blocked by high Mg2+ indicating chemical transmission. IPSPs are inverted by Cl- injection. Excitatory inputs can be electrically far from the soma. Sensory cells form apparently monosynsptic excitatory or inhibitory contacts on motoneurons mediating pharyngeal expansion. Brief sensory excitation can initiate sustained firing within this neuronal population and sustained synaptic activity in motoneurons. Interactions of sensory neurons may be important in information processing and in generating motor paterns. These neurons serve both primary sensory and interneuronal functions.

Animals↗
PubMed1982

Connections of cat omnipause neurons.

Omnipause neurons (OPNs) are brainstem neurons that have been implicated in the generation of saccades. Anatomically demonstrated projections from the OPN region to the cerebellum and spinal cord originate from neighboring neurons, not from OPNs. OPNs are activated following stimulation of the optic chiasm and superior colliculus, but not following stimulation of the vestibular nerve.

Animals↗
PubMed2006

A computational model of visual marking using an inter-connected network of spiking neurons: the spiking search over time & space model (sSoTS).

In the real world, visual information is selected over time as well as space, when we prioritise new stimuli for attention. Watson and Humphreys [Watson, D., Humphreys, G.W., 1997. Visual marking: prioritizing selection for new objects by top-down attentional inhibition of old objects. Psychological Review 104, 90-122] presented evidence that new information in search tasks is prioritised by (amongst other processes) active ignoring of old items - a process they termed visual marking. In this paper we present, for the first time, an explicit computational model of visual marking using biologically plausible activation functions. The "spiking search over time and space" model (sSoTS) incorporates different synaptic components (NMDA, AMPA, GABA) and a frequency adaptation mechanism based on [Ca(2+)] sensitive K(+) current. This frequency adaptation current can act as a mechanism that suppresses the previously attended items. We show that, when coupled with a process of active inhibition applied to old items, frequency adaptation leads to old items being de-prioritised (and new items prioritised) across time in search. Furthermore, the time course of these processes mimics the time course of the preview effect in human search. The results indicate that the sSoTS model can provide a biologically plausible account of human search over time as well as space.

Action Potentials↗
PubMed2000

Local synaptic connections of basal forebrain neurons.

Single, biocytin filled neurons in combination with immunocytochemistry and retrograde tracing as well as material with traditional double-immunolabeling were used at the light and electron microscopic levels to study the neural circuitry within the basal forebrain. Cholinergic neurons projecting to the frontal cortex exhibited extensive local collaterals terminating on non-cholinergic, (possible GABAergic) neurons within the basal forebrain. Elaborate axon arbors confined to the basal forebrain region also originated from NPY, somatostatin and other non-cholinergic interneurons. It is proposed that putative interneurons together with local collaterals from projection neurons contribute to regional integrative processing in the basal forebrain that may participate in more selective functions, such as attention and cortical plasticity.

Animals↗