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Long-term recording from single lumbar motoneurons of intact cats revealed activation patterns fundamentally different from those seen in decerebrate preparations. In intact cats, motoneuron bursts showed marked rate modulation without initial doublets. Each unit's frequencygram generally resembled the envelope of the gross electromyogram simultaneously recorded from the corresponding muscle. Average and peak discharge rates increased for faster gaits. These findings suggest that, in cat locomotion, rate modulation is a more important contributor to force regulation than was previously thought.

Publisher Summary Oculomotor research is characterized by accurate quantitative methods, well-defined performance criteria, mathematical models of control and feedback regulation, and a highly evolved theoretical basis in engineering control systems. Skeletomotor research has been and continues to be plagued by phenomenological description, intractable, unstable and inappropriate models, and an allied field of robotic engineering whose greatest contribution continues to be proofs of the impossibility of trajectory planning in real time. Oculomotor system represents an almost singular exception to the more general function and control of striate muscles. However, the primary process within the muscle that is modulated by changes in the neurally controlled state of activation is not length or even tendon strain. Rather, it is a complex, statistically distributed set of forces in the cross-bridges, which are highly dependent on the direction and magnitude of cross-bridge motion. In studying the function of any single muscle in such a dynamic system, the muscle should not be considered directly as a cause of the observed motion; rather, it is better considered as a generator of an “impulse” added to the system. A comparison of the control problems inherent in the biomechanics of the oculomotor versus skeletomotor systems reveals that the skeletal musculature must generally cope with much more diverse kinematic problems, including inertia, elasticity, and external loads. Because of the highly nonlinear properties of muscle, such factors can be expected to require very different and highly specialized structural and functional architectures in both the muscles and their neuronal control circuits.

Thirty different test patches of various thin film materials were chronically implanted in the subdural space of cats to determine their suitability as components for proposed neuroprosthetic devices. In particular, materials employed by the microelectronics industry were screened, and reactions were found to be quite dependent on specific formulations or surface preparations of otherwise similar materials. A nonspecific but severe complication of pressure necrosis under thin films that spontaneously roll and curl in vivo was noted.

The past ten years have witnessed the introduction of several new methods for chronic recording of electrical activity in peripheral nerves, single nerve fibers and muscles, and for monitoring mechanical events correlated with muscular action. Although these methods were developed as research tools in the study of movements of unrestrained, intact animals, such implantable interfaces with nerve and muscle also promise to be of value in clinical applications; some novel clinical efforts are already being pursued. A review is given of various new techniques that may be clinically applicable and the issues of surgical constraints, quality of signal isolation, and long-term reliability of implantable devices, which ultimately determine the usefulness of each method, are discussed. Experimental devices reviewed include nerve cuff electrodes, floating single-unit electrodes, EMG electrodes, length gauges, and force gauges. General considerations for potential clinical applications are then given.

This is a paper about sensory information, rather than motor control. It is common for motor control physiologists to proceed by stating what information the somatosensory system should provide to service their particular theories of motor control. This often leads to attempts to “find” the right sensory information, to interpret what is found in narrow and even misleading ways, and even to criticise when what should be there is manifestly not. I wish only to summarise what sensory information is likely to be present and to speculate briefly about what might be done with it.

The potential distribution within the internodal axon of mammalian nerve fibers is derived by applying known node potential waveforms to the ends of an equivalent circuit model of the internode. The complete spatial/temporal profile of action potentials synthesized from the internodal profiles is used to compute the node current waveforn, and the extracellular action potential around fibers captured within a tubular electrode. For amphibia, the results agreed with empirical values. For mammals, the amplitude of the node currents plotted against conduction velocity was fitted by a straight line. The extracellular potential waveform depended on the location of the nodes within the tube. For tubes of length from 2 to 8 internodes, extracellular wave amplitude (mammals) was about one-third of the product of peak node current and tube resistance (center to ends). The extracellular potentials developed by longitudinal and radial currents in an anisotropic medium (fiber bundle) are compared.

Sensory transduction in physiological systems faces the double dilemma of maximizing both sensitivity and dynamic range despite limitations inherent in the transmission of information via all-or-none action potentials. The thresholds for detection in senses such as vision and hearing are close to the theoretical physical limits of uncertainty whereas these same senses frequently must discriminate well among stimuli having five or six orders of magnitude greater amplitude. All of this information must be transmitted via digital pulse-rate encoding with a maximum dynamic range of 0–500 pps. Among the mechanisms known to assist in solving this encoding problem are logarithmic response curves in the transducers themselves, accomodative processes intrinsic to the transducers, gain control systems under efferent neural control, and mechanical gates (e.g. iris of the eye, tensor tympani muscle of the ear).

One of the major problems in the design of neurophysiologiv extracellular microelectrodes is the application and selective removal of the insulation. In addition to the usual problems of achieving pin-hole-free coatings and reproducible tip exposures and impedances, chronic electrode designshave the additional requirement of maintaining megohm levels of electrical isolation for months in vivo. A method of insulating finely tapered microelectrodes with complicated shapes by vapor condensation of Parylene-C is presented, along with a method for exposing controlled, reproducible lengths of their tips. In vivo and in vitro impedance tests and unit records obtained over 100 days in monkey motor cortex are presented. The electric arc process used to expose Parylene-covered iridium and tungsten microelectrodes is found to give cleaner recording surfaces with impedances lower than those obtainable with previously described methods. Chronic iridium microelectrodes so fabricated have recorded unit potentials and maintained constant impedances for over 4 months in vivo.

Once damaged, the adult mammalian nervous system is capable of little functional regeneration. Thus, clinical disorders such as deafness, blindness and paralysis have been treated primarily by substitution rather than correction (e.g. teletype for telephone, braille for print, wheelchair instead of walking). However, recent advances in the technology of miniature electronic implants and in the basic understanding of sensory and motor functions have made it possible to build neural prosthetic devices that work by exchanging information directly between computing devices and neurons. Such interfaces are already permitting thousands of otherwise deaf patients to hear sounds directly; some have appreciable speech comprehension without visual cues. There is active research on restoring many types of sensory, motor and autonomic function. Two interesting synergies have emerged. The first stems from the recognition that the biophysical processes involved in stimulating and recording from neurons are universal; thus, technical advances have broad implications in many clinical areas. The second stems from the unique opportunity that such prostheses present to conduct neurophysiological and psychophysical studies directly in conscious human subjects.

Cat sartorius has two distinct anatomical portions, anterior (SA-a) and medial (SA-m). SA-a acts to extend the knee and also to flex the hip. SA-m acts to flex both the knee and the hip. The objective of this study was to investigate how a "single motoneuron pool" is used to control at least three separate functions mediated by the two anatomical portions of one muscle. Discharge patterns of single motoneurons projecting to the sartorius muscle were recorded using floating microelectrodes implanted in the L5 ventral root of cats. The electromyographic activity generated by the anterior and medial portions of sartorius was recorded with chronically implanted electrodes. The muscle portion innervated by each motoneuron was determined by spike-triggered averaging of the EMGs during walking on a motorized treadmill. During normal locomotion, SA-a exhibited two bursts of EMG activity per step cycle, one during the stance phase and one during the late swing phase. In contrast, every recorded motoneuron projecting to SA-a discharged a single burst of action potentials per step cycle. Some SA-a motoneurons discharged only during the stance phase, whereas other motoneurons discharged only during the late swing phase. In all cases, the instantaneous frequencygram of the motoneuron was well fit by the rectified smoothed EMG envelope generated by SA-a during the appropriate phase of the step cycle. During normal locomotion, SA-m exhibited a single burst of EMG activity per step cycle, during the swing phase. The temporal characteristics of the EMG bursts recorded from SA-m differed from the swing-phase EMG bursts generated by SA-a.(ABSTRACT TRUNCATED AT 250 WORDS)

The available evidence about the behaviour of muscle spindle afferents during normal movements obtained in this and other laboratories is reviewed.

A new technique has been developed for stable, long-term recording from groups of individual primary afferent neurons in the freely walking cat. A number of fine, flexible wires are inserted into dorsal root ganglia via a small laminotomy in the lumbar spine. The cut end of each wire can record stable and separable action potentials from one to three dorsal root ganglion neurons; each unit has typically held for 1 to 4 days. A broad range of myelinated somatosensory afferent (conduction velocities of 30 to 120 meters per second) have been studies during locomotion. Most cutaneous and proprioceptive afferent studied have been sensitive monitors of complex combinations of step-cycle components, and their firing patterns would often have been difficult to predict from existing information.

The organization of the motor apparatus into anatomically and functionally defined pools of regularly recruited motor units derives from some of the earliest and most enduring observations of neurophysiology. In recent years, there has been much productive research concentrated on discovering the anatomical bases of this organization in the spinal cord circuitry and the properties of the final common pathway, the alpha motoneurons themselves (for review, see Burke, 1981a). However, almost all of the direct evidence for the function of this system has been derived from experiments on reduced or anesthetized animals and on human subjects performing highly constrained and artificial motor tasks.

Recurrent circuitry components are distributed widely within the brain, including both excitatory and inhibitory synaptic connections. Recurrent neuronal networks have potential stability problems, perhaps a predisposition to epilepsy. More generally, instability risks making internal representations of information unreliable. To assess the inherent stability properties of such recurrent networks, we tested a linear summation, non-spiking neuron model with and without a ‘dynamic leak’, corresponding to the low-pass filtering of synaptic input current by the RC circuit of the biological membrane. We first show that the output of this neuron model, in either of its two forms, follows its input at a higher fidelity than a wide range of spiking neuron models across a range of input frequencies. Then we constructed fully connected recurrent networks with equal numbers of excitatory and inhibitory neurons and randomly distributed weights across all synapses. When the networks were driven by pseudorandom sensory inputs with varying frequency, the recurrent network activity tended to induce high frequency self-amplifying components, sometimes evident as distinct transients, which were not present in the input data. The addition of a dynamic leak based on known membrane properties consistently removed such spurious high frequency noise across all networks. Furthermore, we found that the neuron model with dynamic leak imparts a network stability that seamlessly scales with the size of the network, conduction delays, the input density of the sensory signal and a wide range of synaptic weight distributions. Our findings suggest that neuronal dynamic leak serves the beneficial function of protecting recurrent neuronal circuitry from the self-induction of spurious high frequency signals, thereby permitting the brain to utilize this architectural circuitry component regardless of network size or recurrency.Author SummaryIt is known that neurons of the brain are extensively interconnected, which can result in many recurrent loops within its neuronal network. Such loops are prone to instability. Here we wanted to explore the potential noise and instability that could result in recurrently connected neuronal networks across a range of conditions. To facilitate such simulations, we developed a non-spiking neuron model that captures the main characteristics of conductance-based neuron models of Hodgkin-Huxley type, but is more computationally efficient. We found that a so-called dynamic leak, which is a natural consequence of the way the membrane of the neuron is constructed and how the neuron integrates synaptic inputs, provided protection against spurious, high frequency noise that tended to arise in our recurrent networks of varying size. We propose that this linear summation model provides a stable and useful tool for exploring the computational behavior of recurrent neural networks.

Psychophysical evidence strongly suggests that there is a central neural pattern of representation of sound frequencies; and that the overall pitch of any sound is a function of this central pattern. Physiological studies on the discharges of auditory nerve fibers as a function of cochlear location, and psychophysical studies employing controlled electrical stimulation of the auditory nerve array in man strongly indicate that the spectral composition of complex sounds is not extracted from either discharge rate versus position profiles across the auditory nerve array (the “cochlear place”), or by measuring input discharge periodicities per se. It is hypothesized that spectral components of sounds are extracted from the relative timing of discharges of auditory nerve fibers from separated locations along the auditory nerve array. To extract a spectral representational pattern with broad dynamic range, comparator neurons must detect the synchronicity of arrival of input from separated basilar membrane locat...

Single‐unit responses to intracochlear electrical stimulation were studied. Responses were highly phase‐locked to sinusoidal stimulation at frequencies less than 4 kHz. At any given stimulus frequency, synchronization indexes were greater than those that have been reported for eighth nerve responses to acoustic stimuli. At the onset of a low‐frequency (i.e., below 800 Hz) sinusoidal stimulus, firing rate gradually increased over many tens of milliseconds. Rise times were as long as 50–100 ms for low‐intensity, low‐frequency sinusoidal stimulus bursts. At greater intensities and/or higher stimulus frequencies, the response rise times were considerably shorter. Preliminary results indicate that responses to trains of short‐duration biphasic pulses do not exhibit such slow rise times. These results may relate to Eddington's [Ann. Otol. Rhinol. Laryngol. 87 (Suppl. 53), 1–37 (1978)] finding that behavioral threshold increases sharply as the burst duration of a sinusoidal stimulus is decreased. However, threshold is only a weak function of the duration of a train of short‐duration biphasic pulses. For sinusoids below 300 Hz, multiple discharges per cycle were common at levels considerably above threshold. [Work supported by NIH.]

An electrode array for implantation in a human cochlea. The array includes a spiral-shaped resilient carrier which generally conforms to the shape of the scala tympani of the cochlea. The carrier supports eight electrode pairs, with each electrode having an associated lead. The leads are arranged in the carrier to form a central rib structure which controls flexing of the array. The individual leads have an elongated cross-section and are individually vertically aligned in the rib structure, so that the array will readily flex in the plane defined by the array spiral. The structure limits flexing in the vertical direction. The restricted flexing serves to avoid injury to the basal membrane located at the upper surface of the scala tympani during implantation.