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The information transmission properties of single, de-efferented primary muscle-spindle afferents from the hind limb of the cat were investigated. The gastrocnemius medialis muscle was stretched randomly while recording spike trains from several muscle-spindle afferents in the dorsal root. Two classes of input stimuli were used: (i) Gaussian noise with band-limited flat spectrum, and (ii) Gaussian noise with a more "naturalistic" 1/f(n) spectrum. The "reconstruction" method was used to calculate a lower bound to the information rate (in bits per second) between the muscle spindles and the spinal cord. Results show that in response to the flat-spectrum input, primary muscle-spindle afferents transfer information mainly about high frequencies, carrying 2.12 bits/spike. In response to naturalistic-spectrum inputs, primary muscle-spindle afferents transfer information about both low and high frequencies, with "spiking efficiency" increasing to 2.67 bits/spike. A simple muscle-spindle simulation model was analyzed with the same method, emphasizing the important part played by the intrafusal fiber mechanical properties in information transmission.

The properties of membrane potential changes of skeletomotor neurons (S, FR, and FF) innervating triceps surae muscles during pseudorandom stretching of these muscles were studied in decerebrate cats. Peak amplitudes of pseudorandom muscle stretches ranged from 119 microns to 4.15 mm peak-to-peak. Sequences of ten identical stretching periods were applied for averaging. Shapes of membrane potential changes and probability density distribution of amplitudes of the input and output signals and power spectra suggest that the skeleto-motor neuron membrane has nonlinear properties. First- and second-order Wiener kernels were determined by applying the cross-correlation (Lee-Schetzen) method. The results suggest that the transfer function between muscle stretches and subthreshold membrane potentials is a Wiener-type cascade. This cascade is consistent with a linear, second-order, underdamped transfer function followed by a simple quadratic nonlinearity [linear (L) system followed by nonlinear (N) system, or LN cascade]. Including the nonlinear component calculated from the second-order Wiener kernel improved the model significantly over its linear counterpart, especially in S-type motoneurons. Qualitatively similar results were obtained with all types of motoneurons studied.

Spike discharges of skeletomotor neurons innervating triceps surae muscles elicited by white noise modulated transmembrane current stimulation and muscle stretch were studied in decerebrated cats. The white noise modulated current intensity ranged from 4.3 to 63.2 nA peak-to-peak, while muscle stretches ranged from 100 microns to 4.26 mm peak-to-peak. The neuronal responses were studied by averaging the muscle length records centered at the skeletomotor action potentials (peri-spike average, PSA) and by Wiener analysis. Skeletomotor spikes appeared after a sharp peak in PSA of the injected current, preceded by a longer-lasting smaller wavelet of either depolarizing or hyperpolarizing direction. The PSA amplitude was not related to the injected current amplitude nor showed any differences related to the motor unit type. The PSA amplitudes were virtually independent of the stretching amplitude sigma, after an initial increase with stretching amplitudes in the range of 15-40 microns (S.D.), or 100-270 microns peak-to-peak. Analyses of cross-spectra indicated a small or absent increase in gain with frequency in response to injected current, but about 20 dB/decade in the range 10-100 Hz in response to muscle stretch. The peaks of both Wiener kernels in response to current injection appear to decrease with the amplitude of injected current, but this decrease was not statistically significant. The narrow first-order kernels suggest that the transfer function between the current input and spike discharge is lowpass with a wide passband, i.e. there is very little change in dynamics. The values of the second-order kernels appear to be nonzero only along the main diagonal.(ABSTRACT TRUNCATED AT 250 WORDS)

Firing pattern of skeletomotor neurones innervating triceps surae muscles in response to pseudorandom muscle stretching and white noise modulated transmembrane current stimulation was investigated in decerebrate cats. Pseudo-random muscle stretching (upper cut-off frequency 60 Hz, amplitude ? (standard deviation) ranging from 18.5 μm to 40 μm) was applied to triceps surae muscles. Membrane potential changes and action potentials of skeletomotor neurones were recorded intracellularly. White noise modulated current was applied through the same (recording) microelectrode. Sequences of ten identical 5 s periods of either muscle stretching or transmembrane current stimulation were applied. Skeletomotor neurones belonging to slow motor units (rheobase less than 8.5 nA) generated action potentials in response to both pseudo-random muscle stretching and transmembrane current stimulation, while firing threshold of those belonging to fast motor units could not be reached by the muscle stretches applied. Peri-spike averaging of muscle length and injected current records showed that the action potentials appeared at the peak of either depolarizing current wave or muscle stretching both preceded by a change in opposite direction (the spikes coinciding with the peak in muscle length PSA being actually elicited by muscle spindle action potentials triggered at the moment of the peak stretching velocity). Time coupling of action potentials occurred during both muscle stretching and transmembrane stimulation, being more tight in the latter case as well as when larger amplitudes of the stimuli were applied. It is supposed that discharges from muscle spindle primary endings phase-locked to small pseudo-random muscle length changes may, due to the time coupling of skeletomotor action potentials, provoke a synchronous firing of skeletomotor neurones, mostly of those belonging to slow motor units. Possible effects of such a firing pattern on the resulting muscle reflex contraction and the stretch reflex stability as well as a possibility of it being provoked by fusimotor discharges are discussed.