A novel structure for online surgical undergraduate teaching during the COVID-19 pandemic

BACKGROUND

The Covid-19 pandemic necessitated the delivery of online higher education. Online learning is a novel experience for medical education in Sri Lanka. A novel approach to undergraduate surgical learning was taken up in an attempt to improve the interest amongst the students in clinical practice while maximizing the limited contact time.

METHOD

Online learning activity was designed involving medical students from all stages and multi consultant panel discussions. The discussions were designed to cover each topic from basic sciences to high-level clinical management in an attempt to stimulate the student interest in clinical medicine. Online meeting platform with free to use basic plan and a social media platform were used in combination to communicate with the students. The student feedback was periodically assessed for individual topics as well as for general outcome. Lickert scales and numeric scales were used to acquire student agreement on the desired learning outcomes.

RESULTS

A total of 1047 student responses for 7 questionnaires were analysed. During a 6-week period, 24 surgical topics were discussed with 51 contact hours. Eighty-seven per cent definitely agreed (highest agreement) with the statement 'students benefitted from the discussions'. Over 95% have either participated for all or most sessions. A majority of the respondents (83.4%) 'definitely agreed' that the discussions helped to improve their clinical sense. Of the total respondents, 79.3% definitely agreed that the discussions helped to build an interest in clinical medicine. Around 90% agreed that both exam-oriented and clinical practice-oriented topics were highly important and relevant. Most widely raised concerns were the poor Internet connectivity and limitation of access to the meeting platform.

CONCLUSION

Online teaching with a novel structure is feasible and effective in a resource-limited setting. Students agree that it could improve clinical interest while meeting the expected learning outcomes.

The blood-nerve barrier: structure and functional significance

The blood-nerve barrier (BNB) defines the physiological space within which the axons, Schwann cells, and other associated cells of a peripheral nerve function. The BNB consists of the endoneurial microvessels within the nerve fascicle and the investing perineurium. The restricted permeability of these two barriers protects the endoneurial microenvironment from drastic concentration changes in the vascular and other extracellular spaces. It is postulated that endoneurial homeostatic mechanisms regulate the milieu intérieur of peripheral axons and associated Schwann cells. These mechanisms are discussed in relation to nerve development, Wallerian degeneration and nerve regeneration, and lead neuropathy. Finally, the putative factors responsible for the cellular and molecular control of BNB permeability are discussed. Given the dynamic nature of the regulation of the permeability of the perineurium and endoneurial capillaries, it is suggested that the term blood-nerve interface (BNI) better reflects the functional significance of these structures in the maintenance of homeostasis within the endoneurial microenvironment.

Evidence of GLP-1-mediated neuroprotection in an animal model of pyridoxine-induced peripheral sensory neuropathy

Pyridoxine (vitamin B6) intoxicated rodents develop a peripheral neuropathy characterized by sensory nerve conduction deficits associated with disturbances of nerve fiber geometry and axonal atrophy. To investigate the possibility that glucagon-like peptide-1 (7-36)-amide (GLP-1) receptor agonism may influence axonal structure and function through neuroprotection neurotrophic support, effects of GLP-1 and its long acting analog, Exendin-4 (Ex4) treatment on pyridoxine-induced peripheral neuropathy were examined in rats using behavioral and morphometric techniques. GLP-1 is an endogenous insulinotropic peptide secreted from the gut in response to the presence of food. GLP-1 receptors (GLP-1R) are coupled to the cAMP second messenger pathway, and are expressed widely throughout neural tissues of humans and rodents. Recent studies have established that GLP-1 and Ex4, have multiple synergistic effects on glucose-dependent insulin secretion pathways of pancreatic beta-cells and on neural plasticity. Data reported here suggest that clinically relevant doses of GLP-1 and Ex4 may offer some protection against the sensory peripheral neuropathy induced by pyridoxine. Our findings suggest a potential role for these peptides in the treatment of neuropathies, including that associated with type II diabetes mellitus.

Schema-based learning of adaptable and flexible prey-catching in anurans I. The basic architecture

A motor action often involves the coordination of several motor synergies and requires flexible adjustment of the ongoing execution based on feedback signals. To elucidate the neural mechanisms underlying the construction and selection of motor synergies, we study prey-capture in anurans. Experimental data demonstrate the intricate interaction between different motor synergies, including the interplay of their afferent feedback signals (Weerasuriya 1991; Anderson and Nishikawa 1996). Such data provide insights for the general issues concerning two-way information flow between sensory centers, motor circuits and periphery in motor coordination. We show how different afferent feedback signals about the status of the different components of the motor apparatus play a critical role in motor control as well as in learning. This paper, along with its companion paper, extend the model by Liaw et al. (1994) by integrating a number of different motor pattern generators, different types of afferent feedback, as well as the corresponding control structure within an adaptive framework we call Schema-Based Learning. We develop a model of the different MPGs involved in prey-catching as a vehicle to investigate the following questions: What are the characteristic features of the activity of a single muscle? How can these features be controlled by the premotor circuit? What are the strategies employed to generate and synchronize motor synergies? What is the role of afferent feedback in shaping the activity of a MPG? How can several MPGs share the same underlying circuitry and yet give rise to different motor patterns under different input conditions? In the companion paper we also extend the model by incorporating learning components that give rise to more flexible, adaptable and robust behaviors. To show these aspects we incorporate studies on experiments on lesions and the learning processes that allow the animal to recover its proper functioning.

Schema-based learning of adaptable and flexible prey- catching in anurans II. Learning after lesioning

The previous companion paper describes the initial (seed) schema architecture that gives rise to the observed prey-catching behavior. In this second paper in the series we describe the fundamental adaptive processes required during learning after lesioning. Following bilateral transections of the hypoglossal nerve, anurans lunge toward mealworms with no accompanying tongue or jaw movement. Nevertheless anurans with permanent hypoglossal transections eventually learn to catch their prey by first learning to open their mouth again and then lunging their body further and increasing their head angle. In this paper we present a new learning framework, called schema-based learning (SBL). SBL emphasizes the importance of the current existent structure (schemas), that defines a functioning system, for the incremental and autonomous construction of ever more complex structure to achieve ever more complex levels of functioning. We may rephrase this statement into the language of Schema Theory (Arbib 1992, for a comprehensive review) as the learning of new schemas based on the stock of current schemas. SBL emphasizes a fundamental principle of organization called coherence maximization, that deals with the maximization of congruence between the results of an interaction (external or internal) and the expectations generated for that interaction. A central hypothesis consists of the existence of a hierarchy of predictive internal models (predictive schemas) all over the control center-brain-of the agent. Hence, we will include predictive models in the perceptual, sensorimotor, and motor components of the autonomous agent architecture. We will then show that predictive models are fundamental for structural learning. In particular we will show how a system can learn a new structural component (augment the overall network topology) after being lesioned in order to recover (or even improve) its original functionality. Learning after lesioning is a special case of structural learning but clearly shows that solutions cannot be known/hardwired a priori since it cannot be known, in advance, which substructure is going to break down.

Pyridoxine-induced toxicity in rats: a stereological quantification of the sensory neuropathy

Excess ingestion of pyridoxine (vitamin B6) causes a severe sensory neuropathy in humans. The mechanism of action has not been fully elucidated, and studies of pyridoxine neuropathy in experimental animals have yielded disparate results. Pyridoxine intoxication appears to produce a neuropathy characterized by necrosis of dorsal root ganglion (DRG) sensory neurons and degeneration of peripheral and central sensory projections, with large diameter neurons being particularly affected. The major determinants affecting the severity of the pyridoxine neuropathy appear to be duration and dose of pyridoxine administration, differential neuronal vulnerability, and species susceptibility. The present study used design-based stereological techniques in conjunction with electrophysiological measures to quantify the morphological and physiological changes that occur in the DRG and the distal myelinated axons of the sciatic nerve following pyridoxine intoxication. This combined stereological and electrophysiological method demonstrates a general approach that could be used for assessing the correlation between pathophysiological and functional parameters in animal models of toxic neuropathy.

GABA receptor-mediated inhibition of reflex deglutition in the cat

In anesthetized cats, swallowing elicited by electrical stimulation of the superior laryngeal nerves (SLNs) was inhibited by the GABA-mimetic muscimol and by diazepam, an action that was reversed by picrotoxin and bicuculline. This inhibition supports the involvement of GABA receptors, specifically those of the GABAA subtype which both antagonists have been shown to block in various areas of the central nervous system. The inhibition of reflex swallowing and its reversal were unaltered by a transection of the brainstem at a midcollicular level. Stimulation of the SLNs also caused a bradycardia that was inhibited by both muscimol and diazepam and was restored by both GABA antagonists. Data from these experiments provide suggestive evidence for a role of GABA-ergic transmission in the central control of the deglutitory reflex.

Perineurial permeability to sodium during Wallerian degeneration in rat sciatic nerve

In rat sciatic nerves, the effect of Wallerian degeneration on the rate of transperineurial passage of sodium between the endoneurium and the epineurial extracellular space was investigated. In nerves transected and ligated at the sciatic notch, an in situ technique was used to measure the permeability coefficient-surface area product (PS) of the mid-thigh portion of the perineurium to 22Na. Sampling times ranged from one day to sixteen weeks after the lesion. Additionally, endoneurial water content (an indicator of nerve edema) was also measured in transected, degenerating nerves at the same sampling times. Endoneurial water content increased significantly by the fourth day after transection, peaked at four weeks, and then remained elevated through 16 weeks of post-lesion measurement. The PS of the perineurium to 22Na on the 4th day after transection was significantly greater than that of control animals. This increase then declined to normal levels through the 2nd week, and finally increased to values that were 3-fold to 4-fold of control values for the remainder of the observation period. The earlier, short lasting increase in perineurial PS is probably associated with the inflammatory response to nerve section, and proliferation of perineurial layers and cells. The later increase in perineurial permeability is proposed to play a role in the dissipation of endoneurial hydrostatic pressure and clearance of myelin debris from the endoneurium. In view of the complex changes in perineurial permeability described herein, it would seem inappropriate to consider these phenomena merely as passive breakdowns of the barrier properties of the perineurium.

Endoneurial blood flow in rat sciatic nerve during development

1. Endoneurial blood flow (EBF) in the sciatic nerve of rats aged 2-12 weeks was studied using microelectrode H2 polarography. 2. EBF is highest in 2-week-old rats and progressively declines during development. Mean arterial pressure (MAP) is low at 2 weeks of age, gradually increases through the next 4 weeks, and is relatively constant thereafter. The decrease in EBF, in spite of an increase in MAP, occurs because the endoneurial vascular resistance is increasing faster than the MAP. 3. The higher EBF in younger rats is not due to the smaller diameter of their nerves. Sural and tibial nerves of 12-week-old rats, with diameters comparable to that of the sciatic nerve of a 3-week-old rat, have EBFs similar to that of the sciatic nerve of a 12-week-old rat. 4. There was no compelling evidence of autoregulation of EBF in 3-week-old rats over a MAP range from -40 to +30 mmHg of the normal value. 5. The increase of nerve vascular resistance with maturation is probably due to a decrease in capillary density and, to a lesser extent, to an increase in plasma viscosity and haematocrit. 6. The higher EBF in immature rats is likely to be a developmentally adaptive mechanism which permits greater blood-nerve exchange of material to accommodate the greater metabolic needs of rapidly elongating and myelinating axons and proliferating Schwann cells.

Responses of medullary neurons to moving visual stimuli in the common toad. I. Characterization of medial reticular neurons by extracellular recording

The concept of coded 'command releasing systems' proposes that visually specialized descending tectal (and pretectal) neurons converge on motor pattern generating medullary circuits and release--in goal-specific combination--specific action patterns. Extracellular recordings from medullary neurons of the medial reticular formation of the awake immobilized toad in response to moving visual stimuli revealed the following main results. (i) Properties of medullary neurons were distinguished by location, shape, and size of visual receptive fields (ranging from relatively small to wide), by trigger features of various moving configural stimulus objects (including prey- and predator-selective properties), by tactile sensitivity, and by firing pattern characteristics (sluggish, tonic, warming-up, and cyclic). (ii) Visual receptive fields of medullary neurons and their responses to moving configural objects suggest converging inputs of tectal (and pretectal) descending neurons. (iii) In contrast to tectal monocular 'small-field' neurons, the excitatory visual receptive fields of comparable medullary neurons were larger, ellipsoidally shaped, mostly oriented horizontally, and not topographically mapped in an obvious fashion. Furthermore, configural feature discrimination was sharper. (iv) The observation of multiple properties in most medullary neurons (partly showing combined visual and cutaneous sensitivities) suggests integration of various inputs by these cells, and this is in principle consistent with the concept of command releasing systems. (v) There is evidence for reciprocal tectal/medullary excitatory pathways suitable for premotor warming-up. (vi) Cyclic bursting of many neurons, spontaneously or as a post-stimulus sustaining event, points to a medullary premotor/motor property.

Developmental changes in blood-nerve transfer of albumin and endoneurial albumin content in rat sciatic nerve

The rate of entry of albumin into the endoneurial space and its content within that compartment during development were investigated by measuring the permeability coefficient-surface area product to 125I-albumin (PSA) of the blood-nerve interface (BNI), endoneurial residual plasma volume (Vp), blood-nerve interface index to albumin, and endoneurial water content in sciatic nerves of rats ranging in age from 1 to 24 weeks. There was a 30-fold reduction in PSA and a 4-fold decrease in Vp from 2 to 24 weeks, indicating that the endoneurial capillaries and perineurium become less permeable during development. On the other hand, the Alb-BNI index was relatively small at 1 week, increased to a peak value around 6-8 weeks, and then decreased to adult values by 13 weeks. The smaller Alb-BNI index in the neonatal period is consistent with endoneurial albumin being cleared across a permeable perineurium by epineurial lymphatics. Subsequently, as the perineurium becomes less permeable, endoneurial albumin content increases. It then decreases as the endoneurial capillary permeability also decreases. Additionally, metabolic clearance of albumin, especially during the first 2-3 weeks, by axons and glia to meet the nutritive requirements of rapid axonal growth and myelination could be partly responsible for a lower Alb-BNI index. It is suggested that in the developing nerve, the combination of epineurial lymphatics and a relatively permeable perineurium, together with axonal and glial uptake and protein catabolism aid in the clearance of plasma-derived osmolytes from the endoneurial space, and thus prevent the elevation of endoneurial hydrostatic pressure and onset of oedema that would have been seen in an adult nerve with a comparably permeable BNI.

Physiological changes in the sciatic nerve endoneurium of lead-intoxicated rats: a model of endoneurial homeostasis

The effects of inorganic lead (Pb) on peripheral nerves were studied by measuring the permeability coefficient-surface area product to albumin (PSA) of the blood-nerve interface (BNI), water and Pb content of endoneurium, and residual endoneurial plasma volume (Vp) of sciatic nerves of adult rats maintained on a 4% Pb diet for 2-12 weeks. Additionally, body weight, haematocrit, and blood Pb levels were also monitored. Within 1 week, both blood and endoneurial Pb levels had increased significantly above background levels and continued to increase up to the 6th week. Thereafter, over the next 6 weeks, the blood Pb levels were relatively stable, while the endoneurial Pb levels decreased sharply to a quarter of their peak values. The wet/dry weight ratio of the endoneurium, an indicator of nerve oedema, increased by about 30% from the 4th to 12th week, reaching a water content of 75% corresponding to a 24% increase of the total fascicular area. PSA increased only by the 8th week, without an accompanying increase in Vp. This suggests that the increase in PSA reflects an increase in the permeability of the BNI. Furthermore, the moderate 3-fold increase in PSA is more consistent with an adaptive response of the BNI to changes in the endoneurial microenvironment rather than a massive breakdown or disruption of the BNI. This is supported by the inconsistent or mild leakage of horseradish peroxidase from endoneurial capillaries observed in Pb-intoxicated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Patterns of change in endoneurial capillary permeability and vascular space during nerve regeneration

In frog sciatic nerve crushed and allowed to regenerate, an in situ perfusion technique was used to measure the permeability coefficient-surface area product (PS) of endoneurial capillaries to [14C]sucrose and endoneurial vascular space (V) at intervals of 3 days to 12 weeks post-crush. Additionally, the amplitude and latency of the compound action potential (CAP) of the regenerating nerve were also monitored. There was a delayed increase of both PS and V which peaked at 2-3 weeks after the crush and then declined. This is strikingly similar to the pattern seen in transected nerves. Whereas both PS and V reached near normal levels 6 weeks after transection, in regenerating nerves V continued toward normal levels but PS increased again to peak around 9 weeks post-crush. This second peak of PS coincided with a rapid increase of the CAP amplitude and a sharp decline of its latency. As is the case for transected nerves, the initial increase of PS is probably induced by breakdown products of axons or chemical signals from attendant Schwann cells and related to the clearance of debris from the endoneurium. The later increase of PS may be an endoneurial homeostatic mechanism to increase blood-nerve exchange during rapid axonal growth and remyelination.

Blood-nerve transfer of albumin and its implications for the endoneurial microenvironment

Blood-nerve transfer of plasma albumin was studied by measuring the permeability coefficient-surface area (PS) product of the blood-nerve barrier (BNB) to 125I-albumin in rat sciatic nerve using the i.v. bolus injection method. The calculated PS was 6.3 +/- 0.5 (S.E.M.) x 10(-7) ml.g-1.s-1. This value is smaller by more than an order of magnitude of that measured for sucrose and confirms the relative impermeance of the BNB to blood-borne solutes. From a review of the available evidence, it is concluded that normal blood-nerve exchange occurs predominantly across the endoneurial microvasculature, and the PS of the BNB reflects the permeability of capillaries to a greater extent than that of the perineurium. The only capillaries found to be less permeable than these are the cerebral capillaries. Proximo-distal differences (sciatic vs tibial) of the PS could not be detected. Blood-nerve albumin transfer was calculated at 1.2 mg.g-1.day-1, and the daily turnover of endoneurial albumin to be about 30%. It is postulated that small increases in PS of BNB to albumin lead to an elevation of endoneurial albumin concentration and, through the operation of Starling forces, subsequently produce endoneurial oedema. A major question posed by the results of this study is the identity of pathways for clearance of albumin and other macromolecules from the endoneurium.

Patterns of change in endoneurial capillary permeability and vascular space during Wallerian degeneration

In degenerating frog sciatic nerves an in situ perfusion technique was used to measure the permeability coefficient-surface area product (PA) of endoneurial capillaries to [14C]sucrose, and the endoneurial vascular space (V). Both PA and V started to increase after the 3rd day of degeneration. The increase in PA peaked around the 14th day of degeneration and then declined to reach near normal levels at 6 weeks post-transection. V increased until about the end of the 3rd week of degeneration and then declined to near normal levels at 6 weeks after transection. The delayed increase in capillary permeability may reflect an adaptive reorganization of endoneurial capillary structure and function in response to altered conditions of the endoneurial microenvironment, and it is suggested that this permeability increase is induced by breakdown products of axons or chemical signals from Schwann cells enveloping transected axons. Fluid extravasation from these leaking capillaries is probably responsible for the endoneurial oedema observed in Wallerian degeneration. The recovery of endoneurial capillary tone to near-normal levels at 6 weeks after transection leads to the intriguing conclusion that healthy nerve fibers are not essential for the maintenance of normal endoneurial capillary permeability.

Permeability of endoneurial capillaries to K, Na and Cl and its relation to peripheral nerve excitability

The permeability coefficient-surface area products (PA) of frog sciatic nerve endoneurial capillaries to K, Na and Cl were measured with an in situ perfusion technique and found to be 40.3, 24.6 and 32.8 X 10(-5) ml . g-1 . s-1, respectively. PAs to [14C]sucrose and 42K, when measured simultaneously, and their ratio were independent of perfusate K concentration (0.1-10.0 mM). Simultaneous measurements with 36Cl and 42K indicated that the Cl/K permeability ratio was significantly smaller than the mobility ratio of these two ions in free solution. On the other hand, comparable experiments with 22Na and 42K revealed that the K/Na permeability ratio was not significantly different from its respective mobility ratio. Thus, these results provide no evidence of facilitated transport of K by endoneurial capillaries, and suggest that K, Na and Cl traverse the endoneurial capillary wall by a paracellular route which is weakly selective for cations. The minimum extracellular K concentration (Ke) capable of producing a depolarization conduction block in frog sciatic nerve was between 12.5 and 15.0 mM. When the vasculature of this nerve was perfused with a hyperkalaemic (20.0 mM) Ringer solution, a conduction block developed in 7.9 min. Comparison of this time with the theoretically predicted rate of change of endoneurial Ke (induced by a comparable change of intravascular K concentration) indicated that an increase of endoneurial Ke is transmitted directly to the paranodal spaces of nerve fibres so as to immediately influence axonal excitability.(ABSTRACT TRUNCATED AT 250 WORDS)

Endoneurial capillary permeability to [14C]sucrose in frog sciatic nerve

Using an in situ perfusion technique, we measured the [14C]sucrose permeability-surface area product (PA) in endoneurial capillaries of 48 frog sciatic nerves, as 6.6 +/- 0.6 (S.E.M.) X 10(-5) s-1, and the vascular space as 1.31 +/- 0.10%. Assuming A = 30 cm2/g, P = 2.2 X 10(-6) cm/s. P for sucrose was greater than P in some barrier tissues with tight junctions, but was less than P in all capillaries examined so far except rat cerebral capillaries. These observations demonstrate that endoneurial capillaries are an effective part of the blood-nerve barrier to water-soluble non-electrolytes. The findings are consistent with capillary impermeability to microperoxidase and with capillary ultrastructure.

AC impedance of the perineurium of the frog sciatic nerve

The AC impedance of the isolated perineurium of the frog sciatic nerve was examined at frequencies from 2 Hz to 100 kHz. A Nyquist plot of the imaginary and real components of the impedance demonstrated more than 1 capacitative element, and a DC resistance of 478 +/- 34 (SEM, n = 27) omega cm2. Transperineurial potential in the absence of externally applied current was 0.0 +/- 0.5 mV. The impedance data were fitted by nonlinear least squares to an equation representing the generalized impedance of four equivalent circuits each with two resistive and two capacitative elements. Only two of these circuits were consistent with perineurial morphology, however. In both, the perineurial cells were represented by a resistive and capacitative element in parallel, where capacitance was less than 0.1 microF/cm2. The extracellular matrix and intercellular junctions of the perineurium were represented as single resistive and capacitative elements in parallel or in series, where capacitance exceeded 2 microF/cm2. Immersion of the perineurium in low conductance Ringer's solution increased DC resistive elements as compared with their values in isotonic Ringer's solution, whereas treatment for 10 min with a hypertonic Ringer's solution (containing an additional 1.0 or 2.0 mol NaCl/liter of solution) reduced DC resistive elements, consistent with changes in perineurial permeability. The results indicate that (a) perineurial impedance contains two time constants and can be analyzed in terms of contributions from cellular and extracellular elements, and (b) transperineurial DC resistance, which is intermediate between DC resistance for leaky and nonleaky epithelia, represents intercellular resistance and can be experimentally modified by hypertonicity.

Interaction between primary afferent nerves in the elicitation of reflex swallowing

In adult cats anesthetized with urethan, a number of observations were made that support the concept that some control over the rate of reflexly induced swallowing occurs in the intermediate network at the level of the nucleus of the solitary tract. It was shown that different nerves, e.g., the two superior laryngeal and glossopharyngeal nerves, when activated in sequence interact in such a manner as to facilitate reflex swallowing. It was also shown that the decrement in the reflex response observed to continuous stimulation of a primary afferent nerve, e.g., one superior laryngeal, can be reversed by switching the stimulation to another nerve either on the same or contralateral side. And finally, following discrete medullary lesions, it was observed that that portion of the nucleus of the solitary tract located 2 mm or more rostral to the rostralmost point of the dorsal medial sulcus appears to contain an integral component of the intermediate network that governs reflexly induced swallowing, whereas lesions restricted to the ventrolateral portion of the nucleus of the solitary tract have no effect on the reflex response.

Ionic permeabilities of the frog perineurium

Ionic permeation was investigated across the perineurium of the frog sciatic nerve, under normal conditions and following treatment by hypertonic Ringer, ouabain or amiloride. A cylindrical segment of perineurium removed from the nerve and mounted in vitro on two cannulae was continuously perfused. Permeation rates of 22Na and 42K across the perineurium were the same in either direction and were unaffected by the drugs. The mean 22Na permeability coefficient at the perineurium equaled 1.68 +/- 0.08 (S.E.M.) X 10(-6) cm/sec. Simultaneous measurement of transperineurial fluxes of 22Na, 42K and 36Cl indicated that the K/Na permeability ratio exceeded the ratio of limiting conductances of these ions in free solution, but that the Cl/K permeability ratio did not differ from the respective limiting conductance ratio. Immersion of the perineurial cylinder in Ringer, made hypertonic by addition of NaCl, increased the absolute permeability coefficients of the three ionic tracers but did not affect their permeability ratios. The flux ratio of 22Na/[14C]sucrose, however, was decreased by hypertonic treatment. It is concluded that there is no evidence of active Na or K transport across the perineurium and that the paracellular path in the perineurium exhibits size-dependent permselectivity properties. In addition, the low rates of transperineurial permeation of ions and water-soluble non-electrolytes (e.g. sucrose) are comparable to those in epithelia with tight junctions. These permeability coefficients provide quantitative estimates of the diffusion barrier properties of the perineurium.

Basal forebrain facilitation of reflex swallowing in the cat

In adult cats anaesthetized with urethane, electrical and chemical stimulation of the basal forebrain facilitated reflex swallowing elicited by electrical stimulation of the superior laryngeal nerve. A systematic stereotaxic mapping study using electrical stimulation revealed that the facilitatory sites were distributed along the course of the ansa peduncularis, specifically its rostral forebrain and hypothalamic components associated with the anterior amygdalar area, substantia innominata, lateral preoptic area, anterior hypothalamus and nucleus accumbens. By means of acute discrete radiofrequency lesions, the descending pathways mediating facilitatory influences from the nucleus accumbens and the amygdala to the brain stem were found to traverse the lateral hypothalamus. Ventral tegmental facilitatory sites in the midbrain are likely to be associated with these descending pathways; however, there is evidence for independent participation of this region of the brain in the control of swallowing. Chemical stimulation by means of microinjections of dopamine and apomorphine into the amygdala and nucleus accumbens also enhanced reflex swallowing. It is concluded that the results of this investigation implicate the basal forebrain as a site of integration of viscero-olfacto-gustatory information needed for the enactment of ingestive behaviour.

Modification of permeability of frog perineurium to [14C]-sucrose by stretch and hypertonicity

An in vitro method has been developed to determine quantitatively the permeability of the perineurium to radiotracers at room temperature. The permeability to [14C]sucrose of the isolated perineurium of the sciatic nerve of the frog, Rana pipiens, was measured at rest length, when the perineurium was stretched and after the perineurium had been subjected to hypertonic treatment. Mean permeability at rest length was calculated to be 5.6 +/- 0.27 (S.E.M., n = 45) X 10(-7) cm/sec, and both stretch and hypertonic treatment increased the permeability. A 10% stretch increased permeability reversibly, whereas a 20% stretch or immersion of the perineurium in a hypertonic bath increased permeability irreversibly. Altered permeability under these conditions might be related to changes in the ultrastruct of tight junctions in the perineurium.

The emetic action of L-dopa and its effect on the swallowing reflex in the cat

In adults cats anesthetized with urethane it was shown that facilitation of reflexly-induced swallowing by dopaminomimetics is caused by a central action independent of the emetic of such drugs. It is suggested that this modulatory influence is mediated by dopamine receptors associated with the amygdala and ventral basal striatum.

Nodal gap substance in diabetic nerve

Anoxia and KC1 have been used to inactivate peripheral nerves by depolarization conduction block. Investigation of the inactivation patterns in isolated sciatic nerves of healthy and alloxan-diabetic rats suggests that the paranodal gap substance of healthy nerve behaves as an effective periaxonal diffusion barrier. In diabetic nerve the permeability of this barrier is significantly increased. A marked reduction in the K' binding capacity of the nodal gap substance has been demonstrated in myelinated nerves of human diabetics and alloxan diabetic rats.

Effects of hyperkalaemia on the excitability of peripheral nerve

An experimental animal model has been developed for the study of excitability change in peripheral nerve during limb ischaemia. This model has been used to investigate the effects of hyperkalaemia on the sequence of excitability change that occurs during cuff-induced limb ischaemia and in the post-ischaemic recovery period. The results lend support to the hypothesis that the dynamics of K ion concentration in the periaxonal space play a critical role in determining these excitability changes and that the polyanionic mucopolysaccharide gap substance of the node of Ranvier is likely to constitute the diffusion barrier that defines the periaxonal space.