Biomedical subjects

H Querfurth

Publications and source records attributed to H Querfurth.

10 recordsLinked to original sources

PubMed2006

Action-potential initiation and maintained activity of the isolated frog muscle spindle.

The initiation of afferent action potentials represents the basic signalling process integrating and coding information of an external stimulus. There is also evidence in sensory receptor neurons of spontaneously generating action potentials that interfere with and modify the stimulus evoked activity. The present study investigates the generation of spontaneous action potentials in the isolated muscle spindle of the frog by recording receptor potentials, small subthreshold depolarizations, propagated impulses and interspike transients from the first Ranvier-node of the afferent stem fibre. The temporal pattern of spontaneous discharges at resting length depended on several variables interacting at the encoding site. In the absence of mechanical stimulation, a large resting receptor potential steadily depolarized the encoding node and provoked action potentials at irregular intervals. After each action potential, the hyperpolarizing afterpotential provided a slowly increasing depolarizing interspike transient of decreasing slope (time constant 128 ms), which evoked small subthreshold depolarizations (decay time > 5 ms; multimodal amplitude distribution) before the following action potential discharged. The probability of the small subthreshold depolarizations increased the longer the resting receptor potential stayed constant at its maximum amplitude. When increasing the static prestretch level encoding depended also on the stretch-evoked receptor potential as an additional parameter. The resulting depolarizing interspike transients were then larger and also more steeply rising, so that the afferent discharges increased in both rate and regularity. The experiments show dynamic threshold patterns that control action-potential initiation by the assessment of the actual amplitude of depolarization and its rate of rise.

Action Potentials↗

PubMed1990

Vaccine-associated paralytic poliomyelitis. Regional case series and review.

We present 10 cases of vaccine-associated paralytic poliomyelitis from Washington State to better acquaint physicians with the signs and symptoms of this condition. Diagnosis may be delayed, and milder cases may be overlooked, owing to the unfamiliarity of physicians with the condition. Based on these 10 cases, a rate of 6.4 cases (recipient plus contact) per million live births is estimated for the period 1966 through 1988 in Washington. This rate is higher than calculated for the nation as a whole. The explantation for this difference is unknown, although several possibilities exist, including case clustering.

Adult↗

PubMed1986

Transducer action of isolated frog muscle spindle evoked by pseudorandom noise stimuli.

The dynamic response properties of the isolated frog muscle spindle receptor were investigated by recording the receptor potential evoked by pseudorandom noise (PRN) stimuli. The entire dynamic range of the receptor was determined by measuring the sensory response either at different intensities of the PRN stimulus (sigma = 8-30 microns) around a constant mean length or at the same intensity while varying the mean length from resting length L0 up to L0 + 150 microns. The 3-dB bandwidth of the test signal was 130 Hz. Random stimuli often evoked brief receptor potentials with variable size but characteristic shape. This shape contained a fast depolarization transient of the receptor potential during the stretching phase of the stimulus and a slowly decaying repolarization transient during release of stretch. The depolarization transient rose faster in proportion to the increasing amplitude of the receptor potential, so that larger receptor potentials were more phasic in character than smaller ones. The repolarization transient exhibited two segments of different exponential decay: The first brief repolarization phase lasted for 5 ms; its decline (tau = 2-5 ms) was faster for larger receptor potentials. The second slowly decaying repolarization transient was the same for different receptor potential amplitudes (tau = 47 ms). Consequently, the slow repolarization transients of succeeding receptor potentials displayed temporal summation. Since the amplitude and shape of the receptor potential remained constant during repeated sequences of PRN stimuli, this test stimulus was the most appropriate for the investigation of dynamic response properties under stationary conditions. Long-term stimulation caused a small shift of the mean membrane voltage towards hyperpolarizing values. This finding together with the marked "off effect" after termination of the stimulus indicate the action of an electrogenic pumping mechanism. The dynamic range of the muscle spindle receptor extended from resting length L0 up to L0 + 100 microns. Within this range static prestretches placed a bias upon the transducing site and effectively enhanced the amplitude of the receptor potential. Further prestretch beyond the dynamic region kept the receptor potential constant at its maximum amplitude. The receptor potential amplitude distribution was not symmetrical about the mean but was skewed in favor of depolarization values responding to the stretch trajectories of the PRN stimulus. Variation of the operating point by increasing the static prestretch also shifted the mode of the response distribution towards depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1986

Encoder response of isolated frog muscle spindle elicited by pseudorandom noise stimuli.

The present experiments investigated the signal transfer in the isolated frog muscle spindle by using pseudorandom noise (PRN) as the analytical probe. In order to guarantee that the random stimulus covered the entire dynamic range of the receptor, PRN stimuli of different intensities were applied around a constant mean length, or PRN stimuli of the same intensity were used while varying the mean length of the spindle. Subthreshold receptor potentials, local responses, and propagated action potentials were recorded simultaneously from the first Ranvier node of the afferent stem fiber, thus providing detailed insight into the spike-initiating process within a sensory receptor. Relevant features of the PRN stimulus were evaluated by a preresponse averaging technique. Up to tau = 2 ms before each action potential the encoder selected a small set of steeply rising stretch transients. A second component of the preresponse stimulus ensemble (tau = 2-5 ms) opposed the overall stretch bias. Since each steeply rising stretch transient evoked a steeply rising receptor potential that guaranteed the critical slope condition of the encoding site, this stimulus profile was most effective in initiating action potentials. The dynamic range of the muscle spindle receptor extended from resting length, L0, to about L0 + 100 microns. At the lower limit (L0) the encoding membrane was depolarized to its firing level and discharged action potentials spontaneously. When random stretches larger than the upper region of the dynamic range were applied, the spindle discharged at the maximum impulse rate and displayed no depolarization block or "overstretch" phenomenon. Random stretches applied within the dynamic range evoked regular discharge patterns that were firmly coupled to the PRN. The afferent discharge rate increased, and the precision of phase-locking improved when the intensity of the PRN stimulus was increased around a constant mean stretch; or the mean prestretch level was raised to higher values while the intensity of the PRN stimulus was kept constant. In the case when the PRN stimulus covered the entire dynamic range, the temporal pattern of the afferent discharge remained constant for at least 10 consecutive sequences of PRN. A spectral analysis of the discharge patterns averaged over several sequences of PRN was employed. At the same stimulus intensity the response spectra displayed low-pass filter characteristics with a 10-dB bandwidth of 300 Hz and a high-frequency slope of -12 dB/oct. Increasing the mean intensity of the PRN stimulus or raising the prestretch level increased the response power.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1986

Facilitation effect of auxiliary noise stimuli on response of isolated frog muscle spindle to sinusoidal movements.

Receptor potentials and impulse patterns were recorded from isolated frog muscle spindles using sinusoidal and superimposed random stretches as stimuli with different sinus-to-noise ratios. The entire dynamic amplitude range of the spindle receptor was evaluated by measuring the sensory response at different levels of static stretch. Auxiliary random stimuli provoked rectified fast depolarizing receptor potential transients; their amplitude and slope grew larger with increasing intensity of the noise stimulus and with increasing prestretch level. Due to this strongly nonlinear behavior of the transducing site the frequency and size of the receptor potentials evoked by the auxiliary input signal increased during the stretching phase of the sinusoidal movement. Since the fast depolarizing receptor potential transients provided a powerful trigger for the action potential encoding site, auxiliary random stimuli effectively enhanced the afferent discharge rate, especially during the stretching phase of the sinusoidal movement. Auxiliary noise stimuli could even activate the afferent discharge to an otherwise subthreshold sinusoidal stretch. It is assumed that by the same mechanism the transfer characteristic of the receptor is broadened towards higher frequencies. Since auxiliary random stimuli increased the nonlinear properties of all receptor response components, a "linearizing" approximation technique only partially describes the receptor's transfer properties. The facilitation effect recorded in the differentiated muscle spindle when random stimuli were superimposed on sinusoidal displacements closely resembled the excitation of afferent firing when passive stretching interacted with active fusimotor innervation. A hypothesis is proposed to explain both effects by the same mechanism acting upon the transducing sensory endings: Since passive random stretches as well as active twitching of the intrafusal muscle fibers exhibited almost the same range of frequency components, we propose that both stimuli also generate the same kind of receptor potentials; namely, those fast-rising depolarization transients of the receptor potential, which vigorously drive the encoding site. In general, these experiments explain how the specific response of a neuron can be facilitated by an additional unspecific (noisy) input.

Acoustic Stimulation↗

PubMed1985

Processing vibratory stimuli in isolated frog muscle spindle.

Experiments were performed on isolated frog muscle spindle receptors to study the particular transducer and encoder mechanisms involved in the signal transfer of high frequency sinusoids (vibration). In order to systematically investigate the signal transfer over the entire dynamic range of the receptor, vibration stimuli were applied to the intrafusal muscle bundle at different prestretch levels, so that the isolated receptor potential or the afferent impulse train were recorded at different operating points. The vibration-induced receptor potential displayed severe distortion, because the depolarization during stretch rose steeply, whereas the repolarization transient during release of stretch declined more slowly. The positive peak velocity values of the depolarization transient increased with increasing stimulus frequency, although the ac-component of the receptor potential decreased. The negative peak velocity values of the repolarization transient remained constant throughout the frequency range. The amplitude of the receptor potential grew larger when vibration of constant amplitude was applied at increasing levels of prestretch, revealing another non-linearity of the transducer. These two types of non-linearity were influential in determining the afferent discharge pattern. Each fast depolarization transient facilitated the generation of a single action potential, which therefore could be firmly phase-locked to a small segment of the vibratory movement. Due to its short rise-time, the depolarization transient tended to prevent multiple firing during one stimulus cycle. The prolonged depolarizing afterpotential of the evoked action potential operated in the same direction. Increasing prestretch greatly enhanced the responsiveness of the spindle to vibration. Thus, under appropriate conditions, the afferent discharge was driven in 1:1 synchrony with the vibration. An analysis is given of the after-effects of repetitive activity at the receptor site. The progressive decline of the mean membrane voltage during long lasting stimulation and the "post-tetanic" hyperpolarization ("off-effect") on termination of the vibration suggest the action of an electrogenic pumping mechanism. As a consequence, the afferent impulse train possessed a complex structure segmented into several transient and steady states, which differed in impulse rate, phase response, and in the degree of phase-locking.

Action Potentials↗

PubMed1985

Information transmission by isolated frog muscle spindle.

Transmission of sensory information was calculated for the isolated frog muscle spindle receptor, using Shannon's information measure. Sinusoidal movements, random noise stretches, and sinusoids with superimposed auxiliary noise were applied as stimuli. In addition, the static prestretch level of the intrafusal muscle bundle was adjusted between resting length (L0) and L0 + 600 micron, so that the analysis of the information transmission properties covered the entire dynamic range of the sensory receptor organ. Sinusoidal stretches below 2 Hz evoked smoothly modulated cycle histograms, which were approximately linearly related to the stimulating sinewave. The transinformation rates under these conditions were generally low (5-17 bit X s-1), regardless of the amplitude of the applied movement. Increasing prestretch enhanced the modulation depth of the cycle histograms considerably, but increased the transinformation rates by less than 10 bit X s-1. By contrast, sinusoids above 2 Hz evoked clearly nonlinear cycle histograms, because each action potential was firmly phase-locked to a small segment of the stretch cycle. Under these conditions the transinformation rates grew larger with increasing stimulus frequency and approached 130 bit X s-1 at 60 Hz. Small amplitude sinusoidal stretches, however, evoked considerable transinformation rates in the high frequency region only then, when the spindle receptor was extended to higher prestretch levels. Random stretches evoked transinformation rates between 5 and 30 bit X s-1 depending on both the prestretch level and the intensity of the noise stimulus. The linear response components carried only about 25% of the transinformation rates transmitted by both the linear and nonlinear response components. Auxiliary noise stimuli greatly improved the information transmission of sinusoidal stretches. For example, a pure sinusoid evoked 5 bit X s-1. Adding a noise signal with equal energy to the sinusoidal movement elicited 20 bit X s-1. This facilitation effect of auxiliary noise was restricted to low frequency sinusoidal stimuli. The present results are discussed with respect to the information transmission properties of various sensory systems evaluated by either the same or different information processing procedure as that used in the present study. The functional significance of high transinformation rates sent by the muscle spindle to the central nervous system is discussed with respect to motor control.

Afferent Pathways↗

PubMed1985

Receptor potentials of isolated frog muscle spindle evoked by sinusoidal stimulation.

Receptor potentials in response to sinusoidal stimulation have been recorded from isolated muscle spindles of the frog. Sinusoidal displacements of different amplitudes (20-120 micron) and frequencies (0.1-100 Hz) were used. The mean static stretch level was adjusted between resting length (L0) and L0 + 400 micron, so that the amplitude and phase-response characteristics were measured at different operating points. Depending on the amount of static prestretch, there is a well-defined dynamic range, which limits the receptor potential by nonlinear compression of either its positive or negative half-cycle. For each point on the static operating curve there exists a dynamic operating curve with a sigmoidal shape. The range of each dynamic curve is approximately 80 micron, independent of the static displacement, and the maxima of all dynamic curves are the same. Therefore the dynamic curves are not symmetrical about their static operating point. The slope of the steepest portion is 10% of the maximum elicitable receptor potential per 10-micron dynamic displacement. For stimulus frequencies greater than 2 Hz the receptor potential deviates from a sinusoidal waveform, exhibiting a fast depolarization transient during stretch and a prolonged repolarization transient during release of stretch. The steepness of the depolarization transient increases with increasing stimulus frequency, amplitude, and prestretch level. As a result, the interval from trough to peak of the receptor potential shortens to less than 90 degrees instead of half a cycle. The repolarization transient has an exponential decay with a time constant of approximately 40 ms that remains constant during the various stimulus conditions. As a result of this slow decay time, individual receptor potentials summate, so that the response divides into a modulated receptor potential (AC component) and a maintained depolarization (DC component). The amplitude response characteristic of the stationary AC component increases with increasing stimulus frequencies up to a peak at 2 Hz, after which it declines with a slope of -3 dB/octave. Provided large sinusoidal stretches and/or extended prestretch levels are used, this high-frequency decline of the AC component is compensated for by the proportional increase of the DC component, so that the peak depolarization values remain constant from 2 to 100 Hz. Stimulus and response are in phase for stimulus frequencies less than 2 Hz and reverse to phase lag at higher stimulus frequencies.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1985

Action potential patterns of isolated frog muscle spindle in response to sinusoidal stimulation.

Signal transfer in the isolated frog muscle spindle is investigated using the linear frequency domain analysis technique. Sinusoidal stretches of different amplitudes (20-120 micron) and frequencies (0.1-120 Hz) were applied at different levels of static prestretch, ranging from resting length (L0) up to L0 + 400 micron, so that the frequency-response characteristics were measured at different operating points within the dynamic range. The neuronal responses were recorded from the first node of the afferent stem fiber with a modified air-gap technique. By this means, subthreshold receptor potentials, prepotentials preceding the impulse, and the propagated action potentials were recorded simultaneously, thus providing a detailed insight into the encoding process. There is a well-defined dynamic range of receptor responses. At L0, the encoding site is depolarized to its firing level and discharges spontaneous stimulus-independent impulses. The upper limit is given by the saturation of the receptor potential, which keeps the depolarization maximum below the level of sodium inactivation. Therefore a "depolarization block" or "overstretch" does not exist in the muscle spindle; i.e., the receptor retains its ability to encode information over a large range of dynamic and static displacements. Since the dynamic curves of the receptor potential are not symmetrical about their static operating point, the impulse pattern remains modulated throughout the dynamic range, even if small sinusoids are superimposed on a large static prestretch. The afferent discharge pattern is mainly regulated by the modulated AC component of the receptor potential. At low stimulus frequencies (less than 1 Hz) the receptor potential modulates almost linearly about the mean membrane voltage, so that the evoked discharge pattern displays a smooth analog signal, which is close to sinusoidal. Increasing the static prestretch increases both the peak response and the modulation depth of the impulse pattern. In the intermediate frequency range (1-10 Hz), the cycle histogram disintegrates into discrete peaks separated by empty bins, because the nonlinear receptor potential elicites firmly phase-locked action potentials during its fast depolarization transient. Raising the prestretch level improves the precision of phase locking and increases the number of spikes elicited per cycle.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1984

The transducer and encoder of frog muscle spindles are essentially nonlinear. Physiological conclusions from a white-noise analysis.

Nonlinear second order white-noise analysis has been applied to the isolated frog muscle spindle. Power (delta 2) of the Gaussian white noise (GWN) and the average prestretch level L were varied and the response of both the isolated receptor potential (transducer) and the action potential (encoder) level were analysed. The standard white-noise method is briefly presented. Particular emphasis, however, is put on the limitations in the range of validity of the method and, consequently, on the use and interpretation of the kernels as a Wiener model. Conclusions in the present paper are within this frame and are mainly of qualitative nature. The analysis reveals that the nonlinear contributions of the model are essential for approximating physiological results, thus ruling out purely linear modelling for this receptor organ. The dependence of the transducer kernels on delta are compatible with the behaviour of a rectifier. Rectification is represented by the lack of hyperpolarization within the isolated receptor potential and is enhanced by the substantial memory in the linear and nonlinear kernels as demonstrated by their extent in time. This is equivalent to low power in high frequencies of the response. Obviously, the hyperpolarizing potentials following each spike counteract the long transducer memory. At the encoder level the memory of the system is strongly reduced. This is achieved by using predominantly high frequency components of the receptor potential for triggering the process of impulse generation, and by the precise coupling and high frequency content of the impulses. This coupling precision is possible because of the sensitivity of the spike-generating mechanism to steep rising transients of the receptor potential and also owing to the reduction in transducer memory by the hyperpolarizing afterpotentials. The preference given to the high frequency components is also read from the structure of the second order transducer kernel and from both the linear and the second order encoder kernels, which allows the most effective input waveform for triggering action potentials to be determined. When the operating point is changed to higher prestretch values, kernel heights increase strongly implying higher response strength of the muscle spindle.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗