Changes of somatosensory evoked potentials with increase of intracranial pressure in the rat's brain

Somatosensory evoked potentials (SEPs) to various combinations of two independent brain compression modalities (localized epidural pressure and intracerebral pressure evoked by an inserted balloon) were investigated in 24 rats. The SEP pattern in response to gradually expanding volume wihtout additional epidural pressure remained unchanged for a certain period. SEP changes occurred only shortly prior to death. On the other hand, remarkable SEP changes were observed in a gradually expanding intracerebral mass, when combined with epidural pressure application at about 50% of the lethal volume. SEP changes in response to intermittent and continuous epidural pressure, in addition to a small intracerebral mass, were investigated too. Intermittent application of minor epidural pressure led to specific P1 changes, which recovered after each pressure step. The same pressure, administered continuously, evoked SEP changes with only partial recovery in some instances. Severe epidural pressure, administered intermittently, gave rise to severe SEP changes with only partial recovery after each step. The same epidural pressure delivered continuously led to SEP changes with very small recovery. SEPs have proved to be a reliable method for signalling brain dysfunction corresponding to various modalities and degrees of intracranial pressure.     

Study of hemodynamics using a flow-pressure transducer

A method for simultaneous measuring the pressure and blood flow through the vascular wall is described. It is based on the application of a miniature pressure transducer combined with an ultrasonic blood flow transducer. The tests were performed in a Pitotube by means of pulsing flows. The flow-pressure transducer was applied to studying the hemodynamics of the cat thoracic aorta.     

A staphylococcal enterotoxin B magnetoelastic immunosensor

A magnetoelastic immunosensor for detection of staphylococcal enterotoxin B (SEB) is described. The magnetoelastic sensor is a newly developed mass/elasticity-based transducer of high sensitivity having a material cost of approximately $0.001/sensor. Affinity-purified rabbit anti-SEB antibody was covalently immobilized on magnetoelastic sensors, of dimensions 6 mm x 2 mm x 28 microm. The affinity reaction of biotin-avidin and biocatalytic precipitation are used to amplify antigen-antibody binding events on the sensor surface. Horseradish peroxidase (HRP) and alkaline phosphatase were examined as the labeled enzymes to induce biocatalytic precipitation. The alkaline phosphatase substrate, 5-bromo-4-chloro-3-indolyl phosphate (BCIP) produces a dimer, which binds tightly to the sensor surface, inducing a change in sensor resonance frequency. The biosensor demonstrates a linear shift in resonance frequency with staphylococcal enterotoxin B concentration between 0.5 and 5 ng/ml, with a detection limit of 0.5 ng/ml.     

Epidural Intracranial Pressure Measurement in Rats Using a Fiber-optic Pressure Transducer

Elevated intracranial pressure (ICP) is a significant problem in several forms of ischemic brain injury including stroke, traumatic brain injury and cardiac arrest. This elevation may result in further neurological injury, in the form of transtentorial herniation^1,2,3,4, midbrain compression, neurological deficit or increased cerebral infarct^2,4. Current therapies are often inadequate to control elevated ICP in the clinical setting^5,6,7 . Thus there is a need for accurate methods of ICP measurement in animal models to further our understanding of the basic mechanisms and to develop new treatments for elevated ICP.In both the clinical and experimental setting ICP cannot be estimated without direct measurement. Several methods of ICP catheter insertion currently exist. Of these the intraventricular catheter has become the clinical ''gold standard'' of ICP measurement in humans⁸. This method involves the partial removal of skull and the instrumentation of the catheter through brain tissue. Consequently, intraventricular catheters have an infection rate of 6-11%⁹. For this reason, subdural and epidural cannulations have become the preferred methods in animal models of ischemic injury. Various ICP measurement techniques have been adapted for animal models, and of these, fluid-filled telemetry catheters¹⁰ and solid state catheters are the most frequently used^{11,12,13,14,15}. The fluid-filled systems are prone to developing air bubbles in the line, resulting in false ICP readings. Solid state probes avoid this problem (Figure 1). An additional problem is fitting catheters under the skull or into the ventricles without causing any brain injury that might alter the experimental outcomes. Therefore, we have developed a method that places an ICP catheter contiguous with the epidural space, but avoids the need to insert it between skull and brain. An optic fibre pressure catheter (420LP, SAMBA Sensors, Sweden) was used to measure ICP at the epidural location because the location of the pressure sensor (at the very tip of the catheter) was found to produce a high fidelity ICP signal in this model. There are other manufacturers of similar optic fibre technologies¹³ that may be used with our methodology. Alternative solid state catheters, which have the pressure sensor located at the side of the catheter tip, would not be appropriate for this model as the signal would be dampened by the presence of the monitoring screw. Here, we present a relatively simple and accurate method to measure ICP. This method can be used across a wide range of ICP related animal models.     

Membrane Resonance and Stochastic Resonance Modulate Firing Patterns of Thalamocortical Neurons

We examined the interactions of subthreshold membrane resonance and stochastic resonance using whole-cell patch clamp recordings in thalamocortical neurons of rat brain slices, as well as with a Hodgkin-Huxley-type mathematical model of thalamocortical neurons. The neurons exhibited the subthreshold resonance when stimulated with small amplitude sine wave currents of varying frequency, and stochastic resonance when noise was added to sine wave inputs. Stochastic resonance was manifest as a maximum in signal-to-noise ratio of output response to subthreshold periodic input combined with noise. Stochastic resonance in conjunction with subthreshold resonance resulted in action potential patterns that showed frequency selectivity for periodic inputs. Stochastic resonance was maximal near subthreshold resonance frequency and a high noise level was required for detection of high frequency signals. We speculate that combined membrane and stochastic resonances have physiological utility in coupling synaptic activity to preferred firing frequency and in network synchronization under noise.     

Effect of sinusoidal forcing of ventilatory volume on avian breathing frequency

Awake chickens were unidirectionally ventilated at 3.6 l . min-1 with 3.2-4.8% CO2 in air. The air sacs on each side were made confluent and implanted with exit tubes connected to the following three devices: 1) a system of constant-flow generators which remove air at exactly the same rate that it entered the trachea, allowing no port for spontaneous volume changes; 2) a sinusoidal pump to force volume changes in the chicken; and 3) a pressure transducer to record air sac pressure, which reflected the sum of two pressure components, the passive pressure changes created by the pump and the active pressure changes due to breathing efforts. Over a range of pump frequencies, the amplitude of measured air sac pressure changes varied inversely with frequency. Above and below this range, pressure showed a beat pattern, indicating a difference in the frequencies of the two pressure components. Within the range lacking a beat pattern, breathing movements and the pump stroke had the same frequency. This range was greater at increased stroke volume. Breathing efforts worked with the pump at the high end of the range and against the pump at the low end. These findings show further evidence of the presence of a response to volume forcing and fit a previously described volume threshold model.     

An effective silencer design for artificially air conditioned environment

An effective silencer for an air conditioning duct is studied. A duct with an acoustically soft boundary is employed as an effective silencer. On the acoustically soft boundary the sound pressure is zero and it is impossible to realize such boundary in the air-borne sound field, because of the non-existence of a much lighter medium than the air. In this study, the arrangement of one-quarter wave-length acoustic tubes is employed as a soft boundary. This acoustic tube has frequency dependence, but the sound pressure becomes nearly zero at the tube mouth around the odd resonance frequency. The relation between the noise reduction efficiency and this acoustically soft boundary is examined experimentally and more than 40 dB noise reduction is obtained in a one-half octave band around the first resonance frequency. It is also made clear that more than one wave length of soft boundary is required to get enough reduction compared with the reduction obtained in the case of quite a long soft boundary.     

Characterization of the broad resonance in 31P NMR spectra of excised rat brain

31P NMR spectra of excised rat brain showed a broad resonance between-12 and -13 ppm. Subcellular fractions of brain, rich in membranes, exhibited the broad resonance and it was also present in isolated myelin, the major membrane component of brain. However, it was absent in brain cytosol (161,100 X g supernatant). Raising the temperature of the brain above 50 degrees C caused a gradual downfield chemical shift of the broad resonance, to about -1 ppm at 90 degrees C. An even larger downfield shift was produced by halothane or deoxycholate with concomitant narrowing of the line width of this resonance. Vesicles prepared from the phospholipids of excised brain or isolated myelin showed the broad resonance, and halothane produced the same downfield shift and peak sharpening in brain phospholipid vesicles as that in the intact brain. The chemical shift anisotropy was estimated to be 45 ppm for both myelin and the brain, as characteristic for biological membranes. The T1 and T2 relaxation times of the perpendicular 31P chemical shift tensor component of the broad resonance were 0.66 sec and 1.6 msec, respectively, in the same range as those for other biological membranes. Halothane-treatment of the brain increased both the T1 and T2 times considerably, as expected from the disruption of the phospholipid bilayer in a membrane. These data indicate that the broad resonance in the 31P NMR spectrum of excised rat brain originates exclusively from the phosphate head group of membrane bound phospholipids. Similar broad resonances were found in autopsied human brain and porcine spinal cord and to a lesser extent in excised rat liver and kidney.     

Mechanoptical transducer for quantifying activity in small insect muscles

Contraction strength, frequency, rate, relaxation rate, as well as transient and static muscle lengths were quantified for onion fly (Delia antiqua) oviducal muscle to demonstrate the application of a newly developed, sensitive, rapidly responding, stable and linear mechanoptical transducer system. Contractile patterns were also differentiated for the complex array of muscle segments in the oviduct. Computer-assisted analysis of analogue records showed that within-train contraction strength varied inversely as a function of contraction frequency in a train. Also, a tonic component, assumed to be a function of contraction frequency and the viscoelastic properties of the tissue, was superimposed on trains of phasic, longitudinal contractions. The transducer system described in this report provides opportunities to quantify contraction phenomena occurring at intervals approaching 1 ms in small (nom. 1 mm) tissue samples with resolution in the order of 1 μg of force and 10 μm of displacement.     

A high-precision automatic closed-circuit respirometer for small animals

An automatic apparatus for the continuous measurement of O2 consumption of small laboratory animals is described. By use of a high-sensitivity pressure transducer with associated circuitry together with a peristaltic O2 delivery system, the closed respirometer chamber is maintained at atmospheric pressure +/- 0.5 mmH2O. O2 delivery is measured to within 0.25 ml by recording rotations of the peristaltic pump, following calibration by the withdrawal of a preset volume of air from the chamber. Static trials (with the chamber empty) indicate a high degree of reproducibility of data with the chamber pressure remaining at atmospheric pressure +/- 0.5 mmH2O as a result of the proportional, as opposed to fixed-volume, delivery of O2. Trials with mice and rats have likewise produced data with a high degree of reproducibility.     

Nerve membrane electrical characteristics near the resting state

Summary An experimental analysis of the squid axon membrane impedance in the vicinity of the resting state and as a function of frequency is presented. Particular attention was devoted to the measurement of theresonance frequency, for which the absolute magnitude of the impedance attains its maximum value, in different, extracellular solutions, at various temperatures and in the presence of constant depolarizations or hyperpolarizations.The variations in the concentration of sodium, potassium and divalent ions and the addition of tetrodotoxin, changed markedly the maximum impedance but had little effect, at a fixed temperature, on the resonance frequency, whose temperature dependance is described by aQ ₁₀ variable from 3.7 (around 4 °C) to 1.9 (around 15 °C). Substitution of heavy water decreased the resonance frequency by a factor 1.25, fairly independent of temperature. Steady depolarizations or hyperpolarizations produced large variations of the resonance frequency, with strong temperature dependance.The results indicate that the resonance frequency is directly related to the membrane permeability changes which take place quite independently of the composition of the extra cellular solution and are governed by the electric field existing within the membrane structure rather than by the total membrane potential, to which membrane-solution boundary potentials can give a large contribution.     

Method for in-shoe shear stress measurement

A methodology is described for use of a shear transducer, based on a magneto-resistive principle, to measure stresses under the plantar surface of the foot in-shoe during walking. Particular attention is paid to a projected application for study of diabetic plantar ulceration and its management by footwear. The transducer has a disc construction, approximately 4 mm thick by 16 mm diameter, and measures two orthogonal axes of shear simultaneously; this disc is mounted into an inlay that can be inserted into any stock orthopaedic shoe of the type commonly prescribed for diabetic foot problems. The transducer is located in the metatarsal head region of the inlay; exact placement of the transducer is determined by reference to the direct pressure distribution, the common method of palpation shown to be imprecise. Pilot trials on normal subjects are presented to evaluate the method.     

Device for measuring soft tissue interface pressures

This paper describes the construction and performance of a simple pressure sensing device with a continuous electrical output. It was constructed utilizing a commercially available transducer, an electropneumatic sensor capsule and a 1 m long tube. The transducer used was a piezo-resistive pressure-sensitive device producing an output voltage proportional to the applied pressure. This low cost, high accuracy device is temperature compensated and shows good linearity and negligible hysteresis. The sensor cell has a good thickness-to-diameter ratio and is sufficiently flexible to conform to most contours of the body. The tubing that conveys the pressure transmitting fluid also serves as a means of keeping the transducer distant from the measuring site. The device showed a highly satisfactory performance under laboratory conditions and has proven to be robust and reliable when used for clinical studies.     

Characteristics of Resonance in Heart Rate Variability Stimulated by Biofeedback

As we previously reported, resonant frequency heart rate variability biofeedback increases baroreflex gain and peak expiratory flow in healthy individuals and has positive effects in treatment of asthma patients. Biofeedback readily produces large oscillations in heart rate, blood pressure, vascular tone, and pulse amplitude via paced breathing at the specific natural resonant frequency of the cardiovascular system for each individual. This paper describes how resonance properties of the cardiovascular system mediate the effects of heart rate variability biofeedback. There is evidence that resonant oscillations can train autonomic reflexes to provide therapeutic effect. The paper is based on studies described in previous papers. Here, we discuss the origin of the resonance phenomenon, describe our procedure for determining an individual's resonant frequency, and report data from 32 adult asthma patients and 24 healthy adult subjects, showing a negative relationship between resonant frequency and height, and a lower resonant frequency in men than women, but no relationship between resonant frequency and age, weight, or presence of asthma. Resonant frequency remains constant across 10 sessions of biofeedback training. It appears to be related to blood volume.     

Simultaneous positioning of cells into two-dimensional arrays using ultrasound

Contactless simultaneous positioning of micrometer-sized particles in suspension (e.g., copolymer beads, living cells, silicon microparts) can be performed using ultrasound. Current devices are capable of collecting particles into planes or lines by exciting a resonance in the fluid by means of a piezoelectric transducer located beneath the fluidic cavity and are designed such that a one-dimensional pressure field is created. The focus of this work is to collect cells in distinct point locations for potential drug screening array applications. A device to create two-dimensional arrays of cells within a micromachined chamber is described. The chamber is etched into a silicon wafer and sealed with glass; on the underside of the silicon layer a piezoelectric actuator is attached. A signal is applied to each of two orthogonally aligned strips electrodes defined on the surface of the piezoelectric plate. These two strip electrodes create independently addressable approximately one-dimensional pressure fields. It is shown that by applying the same signal to each electrode a diagonally aligned grid of cells can be produced. However, the independence of the two electrodes allows the application of two signals with slightly different frequencies to be applied which creates a grid of circular cell clumps highly suitable for the identified application. (c)     

Identification of a radical formed in the reaction mixture of rat brain homogenate with a ferrous ion/ascorbic acid system using HPLC–EPR and HPLC–EPR–MS

Identification of a free radical is performed for the reaction mixture of rat brain homogenate with a ferrous ion/ascorbic acid system using EPR, high performance liquid chromatography–electron paramagnetic resonance spectrometry (HPLC–EPR) and high performance liquid chromatography–electron paramagnetic resonance–mass spectrometry (HPLC–EPR–MS). EPR measurements of the reaction mixtures showed prominent signals with hyperfine coupling constants (α^N = 1.58 mT and α^Hβ = 0.26 mT). No EPR spectrum was detectable without rat brain homogenate, suggesting that the radical is derived from rat brain homogenate. An HPLC–EPR analysis of the reaction mixture showed a peak with retention time of 33.7 min. An HPLC–EPR–MS analysis of the peak gave two ions at m/z 224 and 137, suggesting that α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN)/ethyl radical adduct forms in the reaction mixture.     

Generators of the rhythmic alpha activity in the human EEG

An attempt to localize brain mechanisms of the rhythmic activity in the alpha-rhythm range was made using the equivalent dipole model. It is known that light flickering stimuli with the frequency close to that of the individual alpha rhythm induce an increase in its spectral power ("photic-driving" phenomenon). It was shown that the activity of the neuronal structures generating the alpha rhythm can be identified by specific frequency of the light stimulation and localized by means of construction of dipole models. Two sources of the alpha rhythm in the narrow-frequency bands with the maximal resonance responses in the frequencies of 10.1 and 10.5 Hz were localized in the thalamic structures.     

Apparatus for the automated measurement of the responses of humans to odorants

An automated system for the measurement of the psychophysicaland physiological responses of humans to odorant stimulationof the nose and eyes is described. All aspects of the generationand production of odor stimuli, the recording of physiologicaland psychophysical responses of the subjects and the storageof data are managed by an Apple He computer. Both the nasaland ocular olfactometers are based on electronic mass flow controllerswhich are used to control the ratios of volume flow rates ofclean and odorant-saturated air. The output of each olfactometeris measured by a photo-ionization detector. Odor stimuli aredelivered to custom-fitted face masks, that allow separate stimulationof the nose and eyes, through Teflon (rtm) flow valves. A videocamera records the responses of the eyes and a pneumotachograph,in combination with a pressure transducer, records changes inrespiratory behavior. An electronic mouse is used to enter thesubject's psychophysical responses directly into the computer.The advantages of this methodology and its current and potentialapplications are discussed.     

Identification of a radical formed in the reaction mixture of rat brain homogenate with a ferrous ion/ascorbic acid system using HPLC-EPR and HPLC-EPR-MS

Identification of a free radical is performed for the reaction mixture of rat brain homogenate with a ferrous ion/ascorbic acid system using EPR, high performance liquid chromatography-electron paramagnetic resonance spectrometry (HPLC-EPR) and high performance liquid chromatography-electron paramagnetic resonance-mass spectrometry (HPLC-EPR-MS). EPR measurements of the reaction mixtures showed prominent signals with hyperfine coupling constants (alpha(N) = 1.58 mT and alpha(H)beta = 0.26 mT). No EPR spectrum was detectable without rat brain homogenate, suggesting that the radical is derived from rat brain homogenate. An HPLC-EPR analysis of the reaction mixture showed a peak with retention time of 33.7 min. An HPLC-EPR-MS analysis of the peak gave two ions at m/z 224 and 137, suggesting that alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN)/ethyl radical adduct forms in the reaction mixture.     

Pulsatile flow in a catheterised artery

A model is presented for the physiological problem of a catheter which is inserted in a femoral artery to measure the pressure gradient. As the catheter will modify the pressure distribution in the artery, the pressure gradient which would be recorded by a perfect pressure transducer attached to it would differ from that in the uncatheterised artery. To estimate the magnitude of this error, it is assumed that the rates of flow of blood through the catheterised and the uncatheterised artery are described by the same known periodic function of time.