Acute Electrophysiological Experiments Research Articles

Evoked thalamic neuronal activity following DRG application of two nucleus pulposus derived cell populations: an experimental study in rats

To investigate the effects on evoked thalamic neuronal activity of application of notochordal cells and chondrocyte-like cells derived from nucleus pulposus (NP) onto a dorsal root ganglion (DRG) and to compare these effects with a previously reported increased thalamic activity induced by NP. Nucleus pulposus was harvested from tail discs of adult rats and the disc cells were separated into two cell populations, notochordal cells and chondrocyte-like cells. The two cell populations were applied separately, or in combination, to the L4 DRG of anaesthetised female Sprague-Dawley rats during acute electrophysiological experiments. In control experiments, cell suspension medium was applied on the DRG. Recordings from the contralateral thalamus were sampled for 40min while electrically stimulating the ipsilateral sciatic nerve at above Aδ-fibre thresholds. Application of notochordal cells resulted in a decrease in evoked thalamic activity within 10min while chondrocyte-like cells did not induce any changes during the 40min of recording. The difference in evoked thalamic activity 40min after notochordal and chondrocyte-like cell application, respectively, was statistically significant. Neither an increased concentration of chondrocyte-like cells alone nor a combination of the two cell populations induced any changes in thalamic activity. Separate exposure of the DRG to the two NP-derived cell populations induced different effects on evoked thalamic activity, but none of the tested cell samples induced an increase in neuronal activity similar to that previously observed with NP. This indicates a high complexity of the interaction between NP and nervous tissue.

Plasticity of spatial hearing: behavioural effects of cortical inactivation

The contribution of auditory cortex to spatial information processing was explored behaviourally in adult ferrets by reversibly deactivating different cortical areas by subdural placement of a polymer that released the GABAA agonist muscimol over a period of weeks. The spatial extent and time course of cortical inactivation were determined electrophysiologically. Muscimol-Elvax was placed bilaterally over the anterior (AEG), middle (MEG) or posterior ectosylvian gyrus (PEG), so that different regions of the auditory cortex could be deactivated in different cases. Sound localization accuracy in the horizontal plane was assessed by measuring both the initial head orienting and approach-to-target responses made by the animals. Head orienting behaviour was unaffected by silencing any region of the auditory cortex, whereas the accuracy of approach-to-target responses to brief sounds (40 ms noise bursts) was reduced by muscimol-Elvax but not by drug-free implants. Modest but significant localization impairments were observed after deactivating the MEG, AEG or PEG, although the largest deficits were produced in animals in which the MEG, where the primary auditory fields are located, was silenced. We also examined experience-induced spatial plasticity by reversibly plugging one ear. In control animals, localization accuracy for both approach-to-target and head orienting responses was initially impaired by monaural occlusion, but recovered with training over the next few days. Deactivating any part of the auditory cortex resulted in less complete recovery than in controls, with the largest deficits observed after silencing the higher-level cortical areas in the AEG and PEG. Although suggesting that each region of auditory cortex contributes to spatial learning, differences in the localization deficits and degree of adaptation between groups imply a regional specialization in the processing of spatial information across the auditory cortex.

Functional Changes in Muscle Afferent Neurones in an Osteoarthritis Model: Implications for Impaired Proprioceptive Performance

BackgroundImpaired proprioceptive performance is a significant clinical issue for many who suffer osteoarthritis (OA) and is a risk factor for falls and other liabilities. This study was designed to evaluate weight-bearing distribution in a rat model of OA and to determine whether changes also occur in muscle afferent neurones.Methodology/Principal FindingsIntracellular recordings were made in functionally identified dorsal root ganglion neurones in acute electrophysiological experiments on the anaesthetized animal following measurements of hind limb weight bearing in the incapacitance test. OA rats but not naïve control rats stood with less weight on the ipsilateral hind leg (P = 0.02). In the acute electrophysiological experiments that followed weight bearing measurements, action potentials (AP) elicited by electrical stimulation of the dorsal roots differed in OA rats, including longer AP duration (P = 0.006), slower rise time (P = 0.001) and slower maximum rising rate (P = 0.03). Depolarizing intracellular current injection elicited more APs in models than in naïve muscle afferent neurones (P = 0.01) indicating greater excitability. Axonal conduction velocity in model animals was slower (P = 0.04).Conclusions/SignificanceThe present study demonstrates changes in hind limb stance accompanied by changes in the functional properties of muscle afferent neurones in this derangement model of OA. This may provide a possible avenue to explore mechanisms underlying the impaired proprioceptive performance and perhaps other sensory disorders in people with OA.

Excitability of Aβ sensory neurons is altered in an animal model of peripheral neuropathy

BackgroundCauses of neuropathic pain following nerve injury remain unclear, limiting the development of mechanism-based therapeutic approaches. Animal models have provided some directions, but little is known about the specific sensory neurons that undergo changes in such a way as to induce and maintain activation of sensory pain pathways. Our previous studies implicated changes in the Aβ, normally non-nociceptive neurons in activating spinal nociceptive neurons in a cuff-induced animal model of neuropathic pain and the present study was directed specifically at determining any change in excitability of these neurons. Thus, the present study aimed at recording intracellularly from Aβ-fiber dorsal root ganglion (DRG) neurons and determining excitability of the peripheral receptive field, of the cell body and of the dorsal roots.MethodsA peripheral neuropathy was induced in Sprague Dawley rats by inserting two thin polyethylene cuffs around the right sciatic nerve. All animals were confirmed to exhibit tactile hypersensitivity to von Frey filaments three weeks later, before the acute electrophysiological experiments. Under stable intracellular recording conditions neurons were classified functionally on the basis of their response to natural activation of their peripheral receptive field. In addition, conduction velocity of the dorsal roots, configuration of the action potential and rate of adaptation to stimulation were also criteria for classification. Excitability was measured as the threshold to activation of the peripheral receptive field, the response to intracellular injection of depolarizing current into the soma and the response to electrical stimulation of the dorsal roots.ResultsIn control animals mechanical thresholds of all neurons were within normal ranges. Aβ DRG neurons in neuropathic rats demonstrated a mean mechanical threshold to receptive field stimulation that were significantly lower than in control rats, a prolonged discharge following this stimulation, a decreased activation threshold and a greater response to depolarizing current injection into the soma, as well as a longer refractory interval and delayed response to paired pulse electrical stimulation of the dorsal roots.ConclusionsThe present study has demonstrated changes in functionally classified Aβ low threshold and high threshold DRG neurons in a nerve intact animal model of peripheral neuropathy that demonstrates nociceptive responses to normally innocuous cutaneous stimuli, much the same as is observed in humans with neuropathic pain. We demonstrate further that the peripheral receptive fields of these neurons are more excitable, as are the somata. However, the dorsal roots exhibit a decrease in excitability. Thus, if these neurons participate in neuropathic pain this differential change in excitability may have implications in the peripheral drive that induces central sensitization, at least in animal models of peripheral neuropathic pain, and Aβ sensory neurons may thus contribute to allodynia and spontaneous pain following peripheral nerve injury in humans.

Mechanical dissociation of retinal neurons with vibration

AbstractNeuromorphic devices that implement the functions of biological neural circuits by means of VLSI technology have been attracting great attention in engineering fields in the last decade. Concurrently, progress in neuroscience research has demonstrated nonlinear computations at the single neuron level, suggesting that individual neurons are not merely passive circuit elements but computational units. Thus, elucidating the properties of neuronal signal processing is an essential step in developing the next generation of neuromorphic devices. In this study, we developed a method of dissociating single neurons from specific sublayers of mammalian retinas without using proteolytic enzymes, instead combining tissue incubation in a low‐Ca2+ medium with a vibrodissociation technique previously developed for slices of brain and spinal cord. Our method takes less time and requires less manual skill than in the conventional enzymatic method, but yields a sufficient number of cells usable for acute electrophysiological experiments. Single retinal neurons dissociated by our method are useful for measuring nonlinear membrane conductances as well as spike firing properties in the perforated‐patch whole‐cell configuration. These neurons also enable us to investigate the effects of proteolytic enzymes on membrane excitability in such cells. © 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 92(9): 43–52, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10062

Tracheotomy improves experiment success rate in mice during urethane anesthesia and stereotaxic surgery

Urethane anesthesia is frequently used for acute experiments on small rodents in physiology and neuroscience. Severe respiratory distress is a common side-effect of urethane anesthesia in many strains of mice. Associated complications interfere with completion of experiments, and as a consequence more animals must be sacrificed. During experiments with stereotaxic brain surgery, we found that intubation by means of tracheotomy is an efficient way to maintain patent airways in these animals. Artificial ventilation of the animals is not required. In this paper we describe a simple, fast and reliable method for intubation of mice in experiments that involve a stereotaxic instrument. The method proved considerably easier to learn and apply than conventional intubation through the oral route. The incidence of breathing problems decreased from 77% in untreated mice to 9% in those that underwent tracheotomy. In addition, the success rate for our acute electrophysiological experiments increased from 24 to 77%. We conclude that tracheotomy reduces the number of sacrificed animals, and saves time and labor.

Design and fabrication of multichannel cochlear implants for animal research

The effectiveness of multichannel cochlear implants depends on the activation of perceptually distinct regions of the auditory nerve. Increased information transfer is possible as the number of channels and dynamic range are increased and electrical and neural interaction among channels is reduced. Human and animal studies have demonstrated that specific design features of the intracochlear electrode directly affect these performance factors. These features include the geometry, size, and orientation of the stimulating sites, proximity of the device to spiral ganglion neurons, shape and position of the insulating carrier, and the stimulation mode (monopolar, bipolar, etc.). Animal studies to directly measure the effects of changes in electrode design are currently constrained by the lack of available electrodes that model contemporary clinical devices. This report presents methods to design and fabricate species-specific customizable electrode arrays. We have successfully implanted these arrays in guinea pigs and cats for periods of up to 14 months and have conducted acute electrophysiological experiments in these animals. Modifications enabling long-term intracochlear drug infusion are also described. Studies using these scale model arrays will improve our understanding of how these devices function in human subjects and how we can best optimize future cochlear implants.

Mechanical Dissociation of Retinal Neurons with Vibration

The neuromorphic device, which implements the functions of biological neural circuits by means of VLSI technology, has been collecting much attention in the engineering fields in the last decade. Concurrently, progress in neuroscience research has revealed the nonlinear computation in single neuron levels, suggesting that individual neurons are not merely the circuit elements but computational units. Thus, elucidating the properties of neuronal signal processing is thought to be an essential step for developing the next generation of neuromorphic devices. In the present study, we developed a method for dissociating single neurons from specific sublayers of mammalian retinas with using no proteolytic enzymes but rather combining tissue incubation in a low-Ca2+ medium and the vibro-dissociation technique developed for the slices of brains and spinal cords previously. Our method took shorter time of the procedure, and required less elaborated skill, than the conventional enzymatic method did; nevertheless it yielded enough number of the cells available for acute electrophysiological experiments. The isolated retinal neurons were useful for measuring the nonlinear membrane conductances as well as the spike firing properties under the perforated-patch whole-cell configuration. These neurons also enabled us to examine the effects of proteolytic enzymes on the membrane excitability in those cells.

Effects of visual stimulation on the auditory responses of the HVC song control nucleus in anesthetized Bengalese Finches

ABSTRACT In songbirds, the auditory neurons of the telencephalic song control nuclei, especially those in the high vocal center (HVC), respond to the bird's own song (BOS) selectively. Since songs are elicited by the sight of conspecific females and interrupted by intense visual stimulation, such as strobe lights, visual input might modulate this auditory selectivity. This study used acute electrophysiological experiments using Bengalese Finches (Lonchura striata var. domestica) to examine whether strobe lights affect this auditory response. The results showed that visual inputs did not affect the neural activities in response to the BOS. When the visual stimulus was presented alone, we did not record comparable neural activities to auditory stimuli, although vague, weak electrical potential fluctuations were observed. This means that direct visual inputs do not reach all HVC neurons that have BOS selectivity, and the effects of visual information might be very limited in the song control system. Although...

Hippocampal (CA1) activities in Wistar rats from different vendors: Fundamental differences in acute ischemia

Two-vessel occlusion, a frequently used model of global cerebral ischemia in rats, results in a dysfunction predominantly within the CA1 field of the hippocampus; it induces many processes with different time-scales. However, the great divergence in the results of the studies reported in the literature suggests valuable differences in response to hypoperfusion-induced ischemia among the laboratory rats used in these studies. In the present work, the acute effects of two-carotid occlusion-induced global ischemia (2VO) on the CA3 stimulation-evoked population spike activity in the CA1 region of Wistar rats from different suppliers (Charles-River and Harlan) were compared. In the acute electrophysiological experiments, the hippocampal CA1 responses revealed that the Charles-River rats immediately compensated the 2VO much better than did the Harlan rats. However, 3 days later, no difference could be observed between the CA1 activities of these rats. The presented data show that the Wistar rats from different vendors represent an important source of variability in the results of acute experiments on the hippocampal ischemia. These observations draw attention to the importance of the careful choice of the laboratory rats (both strains and breeds) used in such experiments.

The impact of suppressive surrounds on chromatic properties of cortical neurons.

Stimulation of the suppressive surround of a cortical neuron affects the responsivity and tuning of the classical receptive field (CRF) on several stimulus dimensions. In V1 and V2 of macaques prepared for acute electrophysiological experiments, we explored the chromatic sensitivity of the surround and its influence on the chromatic tuning of the CRF. We studied receptive fields of single neurons with patches of drifting grating of optimal spatial frequency and orientation and variable size, modulated along achromatic or isoluminant color directions. The responses of most neurons declined as the patch was enlarged beyond the optimal size (surround suppression). In V1 the suppression evoked by isoluminant gratings was less than one-half that evoked by achromatic gratings. Consequently, many cells were most sensitive to achromatic modulation when patches just covered the CRF but were most sensitive to isoluminant modulation when patches were enlarged to cover the suppressive surround. Non-oriented neurons that were strongly chromatically opponent generally lacked suppressive surrounds. In V2 most neurons showed equal surround suppression from isoluminant gratings and achromatic gratings. This makes the relative sensitivity of V2 neurons to achromatic and isoluminant gratings mainly independent of the size of the grating. We also measured the chromatic properties of the CRF in the presence of differently colored surrounds. In neither V1 nor V2 did the surround alter the chromatic tuning of the CRF. Cortical mechanisms sensitive to chromatic contrast seem to provide little input to the suppressive surrounds of V1 neurons but substantial input to those of V2 neurons.

Neurotoxic effects of mercury on auditory cortex networks growing on microelectrode arrays: a preliminary analysis

Mercury is known to cause sensorineural hearing loss and impaired speech perception. However, there is still a lack of a quantitative description of mercury toxicity on central auditory structures. This is a preliminary study using the novel technique of microelectrode array (MEA) recordings to evaluate acute and chronic neurotoxic effects of mercury on auditory cortex networks (ACNs) in vitro. Morphological and electrophysiological effects of mercuric chloride (HgCl 2) were studied. Neurons dissociated from auditory cortices of 14-day-old mouse embryos were grown on photoetched MEAs containing 64 transparent indium–tin oxide (ITO) electrodes. For acute electrophysiological experiments, the spontaneous spiking and bursting activity from ACNs were compared before and after application of HgCl 2. For chronic electrophysiological experiments, auditory cortex cultures were treated with various concentrations of HgCl 2 from the day of seeding, and were tested 4 weeks later for the presence of spontaneous activity. Morphological analysis was conducted on 8-day-old ACNs treated with HgCl 2 for 3 days. Results of acute experiments indicated that <75 mM of HgCl 2 had an excitatory effect of variable magnitude on the spontaneous activity of ACNs; however, concentrations above 100 μM completely and irreversibly inhibited spike and burst activity. Chronic exposure of ACNs to 10 μM HgCl 2 completely blocked the spontaneous activity. Morphological analysis indicated that 10 μM HgCl 2 caused neuronal cell death in 3 days. It is concluded that HgCl 2 has a more toxic effect on auditory networks when exposed chronically, and the levels of mercury showing toxic effects on ACNs are within the dose range shown to cause neurologic symptoms in humans.

Changes in afferent impulse activity of small intestine mesenteric nerves in response to antigen challenge

Sprague–Dawley rats (weight 130–150 g) were sensitized by an intraperitoneal injection of 1 mg chicken egg albumin with 0.25 ml Freund's adjuvant to stimulate immunoglobulin E antibody production. Leukocyte migration inhibitory factor was used as an indicator of animal sensitization. In acute electrophysiological experiments on sensitized animals, an intra-arterial or intraluminal chicken egg albumin (100 μg) challenge evoked a 10% enhancement of the activity of mesenteric nerves of the small intestine, regardless of the injection site chosen. Afferent nerve activity in control animals was not changed during the chicken egg albumin challenge. Morphometry at the light microscope level showed activation of mast cell degranulation after the antigen challenge to presensitized rats. Intraluminal injections of a stimulator of mast cell degranulation, compound 48/80 (20–30 mg), were found to increase afferent discharges in intact rats. An antagonist of H 1 histamine receptors, clemastine, reduced the effect of compound 48/80. The results obtained provide direct evidence for the stimulation of sensory nerve endings by mast cell mediators released during mast cell degranulation.

A columnar model of somatosensory reorganizational plasticity based on Hebbian and non-Hebbian learning rules.

Topographical and functional aspects of neuronal plasticity were studied in the primary somatosensory cortex of adult rats in acute electrophysiological experiments. Under these experimental conditions, we observed short-term reversible reorganization induced by intracortical microstimulation or by an associative pairing of peripheral tactile stimulation. Both types of stimulation generate large-scale and reversible changes of the representational topography and of single cell functional properties. We present a model to simulate the spatial and functional reorganizational aspects of this type of short-term and reversible plasticity. The columnar structure of the network architecture is described and discussed from a biological point of view. The simulated architecture contains three main levels of information processing. The first one is a sensor array corresponding to the sensory surface of the hind paw. The second level, a pre-cortical relay cell array, represents the thalamo-cortical projection with different levels of excitatory and inhibitory relay cells and inhibitory nuclei. The array of cortical columns, the third level, represents stellate, double bouquet, basket and pyramidal cell interactions. The dynamics of the network are ruled by two integro-differential equations of the lateral-inhibition type. In order to implement neuronal plasticity, synaptic weight parameters in those equations are variables. The learning rules are motivated by the original concept of Hebb, but include a combination of both Hebbian and non-Hebbian rules, which modifies different intra- and inter-columnar interactions. We discuss the implications of neuronal plasticity from a behavioral point of view in terms of information processing and computational resources.

Structures mediating cholinergic reticular facilitation of cortical responses in the cat: effects of lesions in immunocytochemically characterized projections.

Cholinergic afferents to the neocortex controlled by the mesencephalic reticular formation (MRF) are known to transiently facilitate cortical excitability. In an attempt to identify the pathway mediating this effect in the cat visual cortex we combined retrograde tracing techniques with immunocytochemical methods to visualize the acetylcholine-synthesizing enzyme choline acetyltransferase (ChAT). In addition we examined, in acute electrophysiological experiments, whether local neurotoxin injections into nuclei of the basal forebrain interfered with the reticular facilitation of cortical evoked potentials. Cholinergic projections to area 17 originate from different centers in the homolateral substantia innominata/internal capsule, the septal nuclei, and the nuclei of the diagonal band of Broca. No direct cholinergic projection from the MRF to the visual cortex was observed. Retrogradely labelled cells intermingled with ChAT-positive neurons in the brainstem generally revealed immunopositivity for catecholaminergic markers. Local injections of neurotoxins in the substantia innominata blocked reticular facilitation, whereas local lesions of the septal nuclei and the nuclei of the diagonal band had no effect on MRF-induced facilitation. The blockage of the reticular facilitation of cortical evoked responses after unilateral lesions of the substantia innominata was bilateral, suggesting a cooperative interaction between basal forebrain structures of the two hemispheres. The anatomical and physiological data are discussed with respect to possible mechanisms of transient brainstem influences on cortical excitability.

Effects of neonatal deafening and chronic intracochlear electrical stimulation on the cochlear nucleus in cats

Four newborn kittens were deafened by daily intramuscular injections of neomycin sulfate, beginning the day after birth and continuing for 14–16 days. At 10–16 weeks of age the deaf kittens were implanted unilaterally with a four wire intracochlear electrode array. The animals were stimulated daily (starting at 13–18 weeks of age), for a period of one hour, at 6 dB above the electrically evoked auditory brainstem response threshold. After 3 months of chronic intracochlear electrical stimulation, animals were studied in acute electrophysiological experiments and euthanized for histological studies. This study compares the stimulated and control cochlear nuclei (CN) of these deafened animals to the CN of four normal adult cats. Statistical comparisons of spherical cell densities in the anteroventral cochlear nucleus (AVCN), cross-sectional spherical cell areas, and volumes of the cochlear nucleus subdivisions were included in the analysis. The results indicate that, by all of these measures, the cochlear nuclei in neonatally deafened animals were significantly different from the cochlear nuclei of control animals. As a result of deafening, the density of spherical cells was decreased by 30%, the cross-sectional areas of spherical cells were reduced by 20%. and the volume of the cochlear nucleus was reduced by 25%. These changes were observed in both cochlear nuclei (ipsilateral to both stimulated and unstimulated ears) of the deafened animals. With the measures employed, no significant difference was demonstrated in comparisons between the deafened/unstimulated and the deafened/stimulated cochlear nuclei. That is, no reversal of the profound effects of deafening was observed in the cochlear nuclei as a consequence of chronic intracochlear electrical stimulation which was begun 11 to 16 weeks after deafening.

Inhibition of amphetamine-induced locomotor activity by injection of carbachol into the anterior hypothalamic/preoptic area: Pharmacological and electrophysiological studies in the rat

The effects of carbachol injected into the anterior hypothalamic/preoptic area on locomotion initiated by intra-accumbens injections of amphetamine were investigated. Changes of locomotion following intracerebral injections were measured in an automated activity ☐ and the mean firing rate (m.f.r.) from neurons in the mesencephalic locomotor region (MLR) recorded in parallel acute electrophysiological experiments. Amphetamine (20.0 μg) injected to the nucleus accumbens caused a 2.5-fold increase in locomotion of rats. Subsequently, injections of carbachol (0.5 or 1.0 μg) into the hypothalamic/preoptic area reduced the amphetamine-induced locomotion. These effects were stronger with ipsilateral than with contralateral injections and were reversed by pretreating the hypothalamic/preoptic area with 1.5 μg of atropine before carbachol injection. In electrophysiological experiments, injecting carbachol into the hypothalamic/preoptic area reduced the m.f.r. of MLR neurons from8.3 ± 0.7to4.1 ± 0.5 per s and reduced the m.f.r. of 13 of 17 MLR neurons recorded continuously before and after injection. In contrast, injecting amphetamine into the nucleus accumbens increased the m.f.r. of units from 8.3 ± 0.7to12.8 ± 1.0 per s and increased the m.f.r. of 11 of 12 MLR neurons recorded continuously before and after injection. These results suggest that the hypothalamic/preoptic area contains muscarinic cholinoceptive areas which reduced locomotor activity by direct or indirect effects on the MLR.

Cold-sensitive mechanoreceptors with afferent C-fibres in the sheep duodenum

The effect of changes of temperature on the impulse activity of duodenal mechanoreceptors with afferent C-fibres was studied in acute electrophysiological experiments in chloralose-anaesthetised sheep. The impulse activity of 4 tension receptor units increased when the duodenal mucosa was cooled over the range 0-39 degrees C. The impulse activity of 10 tension receptors was reduced when the mucosa was cooled. Temperature changes between 39 degrees C and 52 degrees C did not alter the discharge pattern of either type of tension receptor response. An additional unit, which may have been associated with pyloric circular muscle, was excited by cooling over the temperature range 0-39 degrees C. This unit was insensitive to compression and was excited by close-intra-arterial injections of pentagastrin, nor-adrenaline, prostaglandin F2a and prostaglandin E2. The cold-sensitivity of mechanoreceptors in the duodenal muscularis externa may play a role in the reflexes regulating gastric emptying in experimental situations when duodenal chyme is cold.

Tension receptors with vagal afferent fibres in the proximal duodenum and pyloric sphincter of sheep.

Single-unit afferent activity was recorded from the hepatic-duodenal branch of the vagus nerve of chloralose-anaesthetized sheep during acute electrophysiological experiments. The impulse activity of sixty-seven slowly adapting mechanoreceptors situated in the muscularis externa of the proximal duodenum and pyloric sphincter was synchronous with alterations in electromyographic and tension records. Afferent units were excited during passive distension, compression and drug-induced increases in muscle tension, thus satisfying the criteria for 'in series' tension receptors (Iggo, 1955). From the responses to compression some evidence was found for the existence of separate populations of tension receptors with different mechanical thresholds. Two fibre populations were found: non-myelinated (0.70 +/- 0.26 S.D. ms-1) and myelinated (7.6 +/- 1.6 S.D. m s-1). Mucosal application of solutions of hydrochloric acid, volatile fatty acids, alkali and amino acids, and mucosal probing modified the activity of most units. These changes were reduced by anaesthesia of the mucosa. It is concluded that tension receptors in the sheep duodenum occupy a position 'in series' with longitudinal muscle, and that their activity can be modified by the particulate and chemical composition of chyme by a mechanism involving local nerve plexuses. The activity of tension receptors is compared with that of two other mechanoreceptor classes located serosally (five units) and in the lesser omentum (eleven units). Receptors in neither of these two classes were directly excited by active contraction of the duodenum.

Analysis of conduction of visual, somatic, and audiovibratory sensory information to the hippocamapal cortex in lizards

Acute electrophysiological experiments on lizards (Ophisaurus apodus) showed that electrical stimulation of the anterior dorsolateral thalamic nucleus and medial forebrain bundle evokes short-latency responses in the hippocampal (mediodorsal) cortex which coincides in distribution and configuration with responses in the same cortical area to sensory stimulation. Extensive destruction of these structures inhibits, or even completely blocks, the conduction of sensory (visual, somatic, audiovibratory) and tactile impulses to the hippocampal cortex. It is concluded that the anterior dorsolateral thalamic nucleus and medial forebrain bundle constitutes, if not the only, at least the principal pathway for transmission of these sensory impulses to the hippocampal cortex in lizards.