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)

Regenerated and remyelinated nerve fibers have shorter internodes and thus more nodes than normal mature fibers. This requires either a decrease in the number of sodium channels per node or an increase in the number of channels per fiber or both. The purpose of this investigation was to determine what happens to sodium channel number, as estimated by 3H-saxitonin (STX) binding, in regenerated fibers and to relate this to nodal number. Five adult cats underwent cryoaxotomy of ventral root levels L5, L6, L7, and S1 on the left side. After regeneration for 16-45 weeks, binding parameters were determined. On the right (control) side, binding was consistent with that in unoperated animals (b = 1.3, Bmax = 10.2 +/- 0.4 fmol/mg wet, Kd = 0.6 +/- 0.1 nM). However, the regenerated nerves showed a 3.5-fold increase in maximal binding (b = 1.3, Bmax = 36.1 +/- 0.5, Kd = 0.45 +/- 0.4). Computer-aided histologic analysis of the regenerated roots revealed a decrease in fiber size; a significant decrease in internodal length for fibers in a given size class; and a 1.35-fold increase in total fibers per root. These factors account for a 2.36-fold increase in nodes per milligram (wet). The number of STX binding sites per regenerated node was calculated to be 1.95 X 10(6) (1.31, 3.07, 95% confidence limits), whereas it was 1.26 X 10(6) (0.78, 2.02) for the control roots. The difference was not significant (p greater than 0.05). It is concluded that, in regeneration, the increase in nodal number is accompanied by an increase in sodium channels, so that the number of channels per node is normal or slightly increased. There is a marked increase in channels per fiber and an even greater increase in channels per anterior horn cell. The implications of these data for nodal reorganization in remyelination are discussed.

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)

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)

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)