Frequency-to-voltage conversion in the pyramidal tract neuron: an important role of the inhibitory postsynaptic potential

Frequency-coded impulses are known to be converted into postsynaptic potentials (PSPs) at the synapse of a target neuron. This can be termed frequency-voltage (F-V) conversion. Studies on this problem in pyramidal tract neurons (PTNs) showed that not only the amplitude but also the duration of depolarizing PSPs was determined as a function of the input impulse frequency. Two opposite patterns of F-V conversion were observed following activation of two input systems to PTNs. Inhibitory postsynaptic potentials were found to play an important role in the regulation of the duration of PSPs by curtailing excitatory post-synaptic potentials.     

Movement-related potentials preceding voluntary movement are modulated by the mode of movement selection

In two experiments movement-related cortical potentials preceding voluntary movement were recorded. In experiment 1, subjects performed four motor tasks involving joystick movements. The four tasks differed in complexity (single vs sequential movements) and in the mode of movement selection, i.e., whether a movement or movement sequence was made in fixed or in self-determined directions. The choice of these tasks was based, firstly, on previous electrophysiological studies suggesting an effect of task-complexity on the amplitude of the readiness potential (RP) and, secondly, on previous positron emission tomography (PET) studies showing that activity of the supplementary motor area (SMA) is influenced by the mode of movement selection. The results show that, for single movements, RP amplitude is higher preceding freely selected movements than preceding movements in a fixed direction. In experiment 2 this effect was replicated using button presses instead of joystick movements. The results converge with PET evidence obtained in similar tasks and establish that the RP is modulated by the mode of movement selection. This modulation is probably related to differential involvement of the SMA.     

Development of action potentials and apamin-sensitive after-potentials in mouse vestibular nucleus neurones

The postnatal maturation of medial vestibular nucleus (MVN) neurones was examined in slices of the dorsal brainstem prepared from balb/c mice at specific stages during the first postnatal month. Using spike-shape averaging to analyse the intracellularly recorded action potentials and after-hyperpolarisations (AHPs) in each cell, all the MVN neurones recorded in the young adult (postnatal day 30; P30) mouse were shown to have either a single deep AHP (type A cells), or an early fast and a delayed slow AHP (type B cells). The relative proportions of the two subtypes were similar to those in the young adult rat. At P5, all the MVN cells recorded showed immature forms of either the type A or the type B action potential shape. Immature type A cells had broad spontaneous spikes, and the characteristic single AHP was small in amplitude. Immature type B cells had somewhat narrower spontaneous spikes that were followed by a delayed, apamin-sensitive AHP. The delayed AHP was separated from the repolarisation phase of the spike by a period of isopotentiality. Over the period P10–P15, the mean resting potentials of the MVN cells became more negative, their action potential fall-times became shorter, the single AHP in type A cells became deeper, and the early fast AHP appeared in type B cells. Until P15 cells of varying degrees of electrophysiological maturity were found in the MVN but by P30 all MVN cells recorded were typical adult type A or type B cells. Exposure to the selective blocker of SK-type Ca-activated K channels, apamin (0.3 μM), induced depolarising plateaux and burst firing in immature type B cells at rest. The duration of the apamin-induced bursts and the spike frequency during the bursts were reduced but not abolished after blockade of Ca channels in Ca-free artificial cerebrospinal fluid containing Cd²⁺. By contrast, in mature type B cells at rest apamin selectively abolished the delayed slow AHP but did not induce bursting activity. Apamin had no effect on the action potential shape of immature type A cells. These data show that the apamin-sensitive I _AHP is one of the first ionic conductances to appear in type B cells, and that it plays an important role in regulating the intrinsic rhythmicity and excitability of these cells. Received: 19 November 1996 / Accepted: 30 June 1997     

A model of electrical activity and cytosolic calcium dynamics in vascular endothelial cells in response to fluid shear stress

A mathematical model is proposed to describe the intracellularCa ²⁺ (Ca _i) transient and electrical activity of vascular endothelial cells (VEC) elicited by fluid shear stress (τ). The intracellularCa ²⁺ store of the model VEC is comprised of aCa _i-sensitive (sc) and an inositol (1,4,5)-trisphosphate (IP ₃)-sensitive compartment (dc). The dc [Ca ²⁺] is refilled by the sc whose [Ca ²⁺] is the same as extracellular [Ca ²⁺].IP ₃ produced by the τ-deformed mechanoreceptors discharges the dcCa ²⁺ into the cytosol. The increase of cytosolic[Ca ²⁺] inducesCa ²⁺ release (CICR) from the sc. The raisedCa _i activates aCa _i-activatedK ⁺ current (I _{K, Ca}) and inhibitsIP ₃ production. The cell membrane potential is determined byI _{K, Ca}, voltage-dependentNa ⁺ andK ⁺ currents. Steady τ>0.1 dyne/cm² elicits aCa _i varies sigmoidally withLog ₁₀(τ) with a maximal peakCa _i of 150 nM at τ=4 dynes/cm². Step increases of τ fail to elicit aCa ²⁺ response in cells previously stimulated by a lower shear. TheCa ²⁺ response gradually decreases with repetitive τ stimuli. Pulsatile shear elicits two to three times higherCa _i and hyperpolarizes the cell more than steady shear of the same magnitude. The simulatedCa ²⁺ responses to τ are quantitatively and qualitatively similar to those observed in cultured VEC. The model provides a possible explanation of why the vasodilating stimulus is greater for pulsatile flow than for nonpulsatile flow.     

Light and electron microscopic immunocytochemical localization of vasoactive intestinal polypeptide VIP-like activity in the rat small intestine

Vasoactive intestinal polypeptide nerve processes and cell bodies were identified by electron microscopic immunocytochemistry in the rat small intestine. Labeled nerve processes were numerous in the inner circular smooth muscle coat and mainly in the mucosa, but were absent in the longitudinal muscle layer. Submucosal blood vessels were often surrounded by immunoreactive vasoactive intestinal polypeptide positive nerves, in close associations (distance less than 40 mn) to blood vessel basement membranes and to smooth muscle cells. In the ganglia of the myenteric and submucous plexuses, labeled fibers surrounded unstained neural cell bodies. The synaptic vesicles of vasoactive intestinal polypeptide positive terminals were 35-40 nm in diameter and some dense core vesicles (80-120 nm in diameter) were also observed in the same profiles. These observations suggest that vasoactive intestinal polypeptide nerves may participate in regulating smooth muscle activity and local blood flow in the small intestine.     

CORRELATION OF SKIN POTENTIAL AND SKIN RESISTANCE MEASURES

Simultaneous measurements of skin potential (SP) and skin resistance (SR) obtained from 20 male and 20 female adult subjects during 2 sessions held 2 to 9 days apart were used in studying (1) the correlation of change measurements and prestimulus level in the two measures, and (2) the amount of correlation between SP and SR using both simple difference and residual change scores in which the regression of poststimulus values on initial level (prestimulus) has been controlled. Correlations within Ss and correlations among Ss showed large individual variability, correlation differences between males and females, and high correlation between SP and SR change scores. Although the law of initial value (LIV) seemed to have little applicability to the measurement of electrodermal responses, the results underscored the need to control for contamination of change measures by initial level regardless of direction.     

On Quantifying Surprise: The Variation of Event-Related Potentials With Subjective Probability

Two factors are known to determine the waveform of event-related potentials (ERP) elicited by task-relevant stimuli: the a priori probability of the stimuli and the sequence of immediately preceding stimuli. The relative contribution of these factors to the ERP waveform was assessed at nine levels of a priori probability (from .10 to .90). Random sequences of high (1500 Hz) and low (1000 Hz) tones were presented to 10 male subjects at each level of probability, both when the events were task-relevant and when the subjects were performing an alternate task to which the tones were irrelevant. The EEG was recorded from five midline electrode sites referred to linked mastoids. The amplitude of the P300 and Slow Wave components was inversely proportional to the a priori probability of task-relevant events. At every level of a priori probability, the magnitude of the P300 complex (N200-P300-Slow Wave) was diminished when the eliciting tone repeated the preceding tone, and was enhanced when it was preceded by the other tone. Thus, a priori probability and sequential structure appear to be independent determinants of the P300 complex.     

Two Autopsy Cases of Primary Leiomyosarcoma of the Liver Superiority of Muscle-specific Actin Immunoreactivity in Diagnosis

Two autopsy cases of leiomyosarcoma of the liver in a 49-year old female and 63-year-old male are reported. Both of the liver tumors showed electron microscopically dense patches in the cytoplasm and intermediate junctions between the tumor cells, suggesting a smooth muscle cell origin, irrespective of their different histological features. The nature of both tumors was confirmed by positive immunoreactivity for muscle-specific actin in the tumor cells, whereas desmin immunoreactivity was labile in both cases, showing a higher diagnostic value of the former compared with the latter in these leiomyosarcomas. Both cases, showed extensive distant metastases in spite of an evident difference in their mitotic indices, indicating that this index is not reliable for judging the metastatic potential of these tumors. Acta Pathol Jpn 41: 461–465, 1991.     

A planar slab bidomain model for cardiac tissue

A fully three-dimensional model of the ventricular or atrial free wall will involve a planar geometry of finite thickness. The governing equations for the interstitial and extracellular potential of a planar slab of cardiac tissue comprised of parallel fibers undergoing uniform plane-wave activation are presented. A comparison with a bidomain of cylindrical geometry with the same half-thickness shows that the potentials in the planar bidomain (as a function of depth) approach core-conductor behavior more quickly.     

Types of nerves in the enteric nervous system

The enteric nervous system is one of the three divisions of the autonomic nervous system, the others being the sympathetic and parasympathetic. In contrast to the other divisions, it can perform many functions independently of the central nervous system. It consists of ganglionated plexuses, their connections with each other, and nerve fibres which arise from the plexuses and supply the muscle, blood vessels and mucosa of the gastrointestinal tract. The enteric nervous system contains a large number of neurons, approximately 10⁷ to 10⁸. About ten or more distinct types of enteric neurons have been distinguished on electrical, pharmacological, histochemical, biochemical and ultrastructural grounds as well as on the basis of their modes of action. Both excitatory and inhibitory nerves supply the muscle and there are inhibitory and excitatory interneurons within the enteric plexuses. There are also enteric nerves which supply intestinal glands and blood vessels, but these receive less emphasis in this commentary.Correlations between groups of neurons defined on different criteria are poor and in many cases the physiological roles of the nerves are not known. The functions of noradrenergic nerves which are of extrinsic origin are reasonably well understood, but cholinergic nerves in the intestine are the only intrinsic nerves for which both the transmitter and to some extent the functions are known. In the case of non-cholinergic, non-noradrenergic enteric inhibitory nerves, the functions are understood but the transmitter is yet to be determined, both adenosine 5′-triphosphate and vasoactive intestinal polypeptide having been proposed. Other nerves have been defined pharmacologically (non-cholinergic excitatory nerves to neurons and muscle, intrinsic inhibitory inputs to neurons, and enteric, non-cholinergic vasodilator nerves) and histochemically (intrinsic amine-handling neurons and separate neurons containing peptides: substance P, somatostatin, enkephalins, vasoactive intestinal polypeptide, gastrin cholecystokinin tetrapeptide, bombesin, neurotensin and probably other peptides). Little is known of the functions of these nerves, although a number of proposals which have been made are discussed.