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A cylindrical multielectrode system specifically designed for intraspinal microstimulation was mechanically and electrically evaluated in the ventral horn of the feline lumbo-sacral spinal cord. Electrode insertions proved to be straight as evaluated from radiographs. Impedances were measured in situ and force recruitment curves from quadriceps muscles were collected over a wide range of stimulus parameters. For a given charge, higher current amplitudes produced greater forces than proportionally longer pulse durations, indicating that charge is not the sole indicator of evoked force in applications utilizing electrical stimulation. Overlap measurements for calculating current-distance constants were collected at a variety of current amplitudes, electrode pair separations, and pair orientations in the spinal grey matter. Forces obtained in the majority of these trials demonstrated an order effect, presumably due to asymmetric neuronal connectivity within the spinal cord. In the cases showing no order effect, the dorso-ventral electrode pair orientation yielded a higher average current-distance constant (278 /spl mu/A/mm/sup 2/) than either the medio-lateral or rostro-caudal electrode pair orientations (197 /spl mu/A/mm/sup 2/). Specifications of an array of cylindrical multielectrodes for use in future intraspinal microstimulation prostheses were updated.

Much has been studied and written about plastic changes in the CNS of humans triggered by events such as limb amputation. However, little is known about the extent to which the original pathways retain residual function after peripheral amputation. Our earlier, acute study on long-term amputees indicated that central pathways associated with amputated peripheral nerves retain at least some sensory and motor function. The purpose of the present study was to determine if these functional connections would be strengthened or improved with experience and training over several days time. To do this, electrodes were implanted within fascicles of severed nerves of long-term human amputees to evaluate the changes in electrically evoked sensations and volitional motor neuron activity associated with attempted phantom limb movements. Nerve stimulation consistently resulted in discrete, unitary, graded sensations of touch/pressure and joint-position sense. There was no significant change in the values of stimulation parameters required to produce these sensations over time. Similarly, while the amputees were able to improve volitional control of motor neuron activity, the rate and pattern of change was similar to that seen with practice in normal individuals on motor tasks. We conclude that the central plasticity seen after amputation is most likely primarily due to unmasking, rather than replacement, of existing synaptic connections. These results also have implications for neural control of prosthetic limbs.

Polymer-based longitudinal intrafascicular electrodes (polyLIFEs) were chronically implanted into the sciatic nerve of white New Zealand rabbits (n=8) for a period of 6 months (hereafter referred to as the long-term group). The impact of the implantation procedure, as observed 6 months post surgery, was evaluated in a sham-treated control group (n=9). The contralateral sciatic nerve served as the control for each animal. Nerve-fiber counts, fiber diameters, and myelin thickness were estimated at the level of the implant site, 1.5 cm proximally, and 1.5 cm distally for both nerves in sham-treated and long-term groups. Implantation of polyLIFEs had no significant effect on fiber counts, nerve-fiber diameter, or myelin thickness. A slight increase in connective tissue in the vicinity of the implant site was evident in the long-term group, including a thin but dense capsule immediately surrounding the implanted electrode.

Selective activation of muscle groups in the feline hindlimb by electrical stimulation of the ventral lumbo-sacral spinal cord was investigated. Spinal cord segments L5 to S1 were mapped using a penetrating tungsten needle electrode. Locations that produced isolated contraction of quadriceps, tibialis anterior or triceps surae/plantaris muscles when stimulated with a current of 40 /spl mu/A or less, and in which spread of activity to other muscles was not detected after increasing the stimulus to at least twice the threshold level, were defined as belonging to the target muscle's "activation pool." The quadriceps activation pool was found to extend from the beginning of L5 to the middle of L6. The tibialis anterior activation pool extended from the beginning of L6 to the middle of L7, and the triceps surae/plantaris activation pool extended from the caudal end of L6 to the beginning of S1. The three activation pools were located in Rexed motor lamina IX and their spatial organization was found to correspond well with that of the anatomically defined motor pools innervating the same muscles. The spatial and functional segregation of motor pools manifested at the spinal cord level can have direct applications in the areas of functional electrical stimulation and motor control.

We implanted polymer-based longitudinal intrafascicular electrodes (polyLIFEs) in feline dorsal rootlets acutely and for periods of two to six months to evaluate their electrical properties and biocompatibility. A total of 38 implanted electrodes were analyzed. Some 25 of the 38 electrodes were implanted with an insulative flexible polymer cuff, which was required for recording of afferent activity in situ. Electrode impedances remained stable for the duration of the experiments. The distributions of axons were measured at three levels of the implanted rootlets: the implant level, 1–2 mm proximal to the implant with respect to the cell body, and 1–2 mm distal to the implant with respect to the cell body. Similar measurements were made in five samples of fascicles neighboring an implant and six samples of control tissue from animals in which no implants were placed. The polyLIFEs demonstrated a high degree of biocompatibility, as no adverse effects on axon size were observed in either the implanted fascicle or neighboring neural tissue. However, the insulative cuffs were found to be a source of compression, resulting in necrosis of the neural tissue. © 1998 Biomedical Engineering Society.

The goal of the study was to provide specifications for a stimulating electrode array to be implanted in the lumbosacral spinal cord as part of a functional neuromuscular stimulation (FNS) system for control of lower extremity muscles in paralyzed individuals. Dual channel stimulation of the quadriceps activation pool in the feline ventral lumbo-sacral spinal cord was performed to measure electrode interactions and to explore the effect of various stimulation paradigms on muscle fatigue. There was no measurable overlap in the populations of motor neurons activated from 2 different electrodes for spacings /spl ges/1 mm with currents below 100 /spl mu/A. However, a statistically significant increase in the population of activated fibers due to current summation was observed when stimuli /spl ges/70 /spl mu/A. were simultaneously presented through pairs of electrodes within 3 mm of each other. Fatigue effects were studied with 3 paradigms: (1) stimuli were delivered through a single electrode, (2) stimuli were delivered through 2 electrodes with the stimulus to the second electrode presented during the refractory period of fibers stimulated by the first electrode, and (3) stimuli were interleaved between the 2 electrodes such that the stimulus to 1 electrode was presented midway between stimuli to the other electrode, and the rate of stimulation through a single electrode was half that used in the first 2 paradigms. During channel refractory and single channel stimulation did not differ from each other in the rate at which the muscle fatigued, in both cases the force decayed to 30% of its initial level within 2 min of the initiation of the stimulation regime, whereas the force with interleaved stimulation was still above the initial force at this time due to strong potentiation. Based on these results and on and activation pool dimensions obtained in an earlier study, preliminary specifications are presented for an electrode array to be implanted in the human spinal cord for functional neuromuscular stimulation.

This paper describes a closed-loop functional neuromuscular stimulation system that uses afferent neural activity from muscle spindle fibers as feedback for controlling position of the ankle joint. Ankle extension against a load was effected by neural stimulation through a dual channel intrafascicular electrode of a fascicle of the tibial nerve that innervated the gastrocnemius muscle. Ankle joint angle was estimated from recordings of tibialis anterior and lateral gastrocnemius spindle fiber activity made with dual channel intrafascicular electrodes. Experiments were conducted in neurally intact anesthetized cats and in unanesthetized decerebrate cats to demonstrate the feasibility of this system. The system was able to reach and maintain a fixed target ankle position in the presence of a varying external moment ranging in magnitude between 7.3 and 22 N-cm opposing the action of the ankle extensor, as well as track a sinusoidal target ankle position up to a frequency of 1 Hz in the presence of a constant magnitude 22- or 37-N-cm external moment.

The limits of present electrode technology are being reached in current motor prostheses for restoring functional movement in paralyzed people. Improved devices require electrodes and stimulation methods that will activate muscles selectively and independently with less implanted hardware. A practical functional neuromuscular stimulation (FNS) system may need to employ extraneural, intraneural, epimysial, or intramuscular electrodes or a combination of these types. The limitations of current muscle electrodes and the anatomy of peripheral nerve innervation of muscle have pointed to stimulation of peripheral nerve trunks as a promising area for investigation. Attempts to use conventional (extraneural) peripheral nerve electrodes for selective activation of muscles in chronic applications have met with only limited success. Intraneural (intrafascicular) electrodes offer the advantages of greater selectivity and lower power requirements, but these may be offset by the difficulty of inserting delicate electrodes through the collagenous epineurium and perineurium while avoiding unacceptable amounts of trauma. Cuff electrodes require more power than intrafascicular ones but may provide more stable recruitment patterns over time, and the opportunity for retrieval and replacement. >

A visual prosthesis for the blind using electrical stimulation of the visual cortex will require the development of an array of electrodes. Passage of current through these electrodes is expected to create a visual image made up of a matrix of discrete phosphenes. The quality of the visual sense thus provided will be a function of many parameters, particularly the number of electrodes and their spacing. We are conducting a series of psychophysical experiments with a portable "phosphene" simulator to obtain estimates of suitable values for electrode number and spacing. The simulator consists of a small video camera and monitor worn by a normally sighted human subject. To simulate a discrete phosphene field, the monitor is masked by an opaque perforated film. The visual angle subtended by images from the masked monitor is 1.7 degrees or less, depending on the mask, and falls within the fovea of the subject. In the study presented here, we measured visual acuity as a function of the number of pixels and their spacing in the mask. Visual acuity was inversely proportional to pixel density, and trained subjects could achieve about 20/26 visual acuity with a 1024 pixel image. We conclude that 625 electrodes implanted in a 1 cm by 1 cm area near the foveal representation of the visual cortex should produce a phosphene image with a visual acuity of approximately 20/30. Such an acuity could provide useful restoration of functional vision for the profoundly blind.

Psychophysical and electrophysiological techniques were used to study the encoding and processing of information about the frequency content of vibrational stimuli applied to glabrous skin in humans and cats. Trained human subjects were asked to discriminate changes in stimulus frequency and harmonic content for pairs of mono- and diharmonic sinusoidal vibrations applied to the fingertips. These psychophysical tests supplied data on what information is available to the central nervous system about the frequency components of vibratory stimuli. Electrophysiological recordings from nerves innervating the glabrous skin of the paw in cats during presentation of the same stimuli used in the psychophysical study provided data on how the peripheral nervous system encodes information about the physical parameters of cutaneous vibratory stimuli. The two sets of data indicated that the subjects derived information about the frequency of vibrotactile stimuli from the mean interval between action potentials in afferent nerve fibers activated by the stimulus.

We have studied muscle recruitment with Teflon-insulated, 25 microns diameter, Pt-Ir intrafasicular electrodes implanted in nerves innervating the gastrocnemius and soleus muscles of cats. The purpose of this study was to measure the performance of these bipolar electrodes, which had been designed to optimize their ability to record unit activity from peripheral nerves, as stimulating electrodes. Recruitment curves identified the optimal stimulus configuration as a biphasic rectangular pulse, with an interphase separation of about 500 microseconds and a duration of about 50 microseconds. The current required for a half-maximal twitch contraction was on the order of 50 microA. Current and charge densities needed for stimulation were well below levels believed to be safe for the tissue and electrode materials involved. When the spinal reflex pathway was interrupted by crushing the nerve, the force produced by a given stimulus changed in some cases, but not in others, implying that the spinal reflex contribution was not the same in all the implants. We conclude that intrafascicular recording electrodes are also a potentially valuable technology for functional neuromuscular stimulation, and warrant further development.

A biodegradable tube consisting of alternating laminae of aldehyde and heat-treated collagen and fibrin was tested as a nerve repair device. Tubes placed around intact nerves for 3 weeks showed a high degree of biocompatibility as evidenced by the absence of reactive or toxic alterations of epineurium contacting the implant and the presence of normal perineurial and endoneurial components. Tubes used to bridge a 5 mm gap in the rat tibial nerve showed good vascularization of the collagen laminae, a well-confined nerve regenerate, and advanced resorption of the fibrin component at 3 months. One year after being used to bridge 5-12 mm gaps in the cat radial and saphenous nerves, the tubes were replaced by well-vascularized connective tissue which surrounded a dense nerve regenerate. However, inadequate stabilization of the implants by the small anchoring sutures apparently caused some of the repairs to fail. In sum, the implants appear to promote regeneration of axons across a nerve gap by providing an oriented, well-vascularized physical substrate for neurite outgrowth.

The effects of spinal cord injuries are likely to be ameliorated with the help of functional electrical stimulation of the spinal cord, a technique that may benefit from a new style of electrode: the cylindrical multielectrode. This paper describes the specifications for, fabrication techniques for, and in vitro evaluation of cylindrical multielectrodes. Four tip shapes were tested to determine which shape required the lowest peak force and would, therefore, be expected to minimize dimpling during implantation. The impedance of the electrode interface was monitored for changes due to insertion as well as repetitive delivery of current pulses. The charge delivery capacity was determined by testing with safe (/spl ges/0.6 mC/cm/sup 2/) and damaging levels (/spl ges/0.8 mC/cm/sup 2/) of charge density. The results of these tests suggest that this electrode design could be used to stimulate neurons in the ventral horn of the spinal cord.

We are proposing to build a visual prosthesis based upon electrical stimulation of the visual cortex via an array of penetrating electrodes. To establish design specifications for the prosthesis we have conducted psychophysical experiments with a portable phosphene simulator. The simulator consists of a small video camera and monitor masked with an array of 63.5 pm diameter pinholes. The visual fizld of the monitor covers 1.7 degrees and falls within the fovea of the subject. The total number of phosphenes, their spacing, and the field magnification ratio can be varied. Subjects can achieve about 20130 visual acuity with a 1024 pixel image. Visual acuity is approximately an inverse linear function of pixel density.

The authors implanted 11 polymer-based intrafascicular electrodes into the peroneal and/or tibial branches of the right sciatic nerve in 8 New Zealand white rabbits for a period of 6 months where the contralateral limb served as the control. The fiber diameter distributions, myelin thickness, and axon counts were measured at the level of the implant, 1.5 cm proximally, and 1.5 cm distally. PolyLIFEs caused no significant effect on axon counts or diameter, but induced slight demyelination distal to the implant. Encapsulation of the implants was also minimal.

Describes a closed loop control system that uses afferent neural activity from muscle spindle fibers as feedback for controlling ankle position. The gastrocnemius muscle was stimulated through a dual channel intrafascicular electrode implanted in a fascicle of the tibial nerve. Dual channel intrafascicular electrodes were also used to record spindle fiber activity from the tibialis anterior and the lateral gastrocnemius muscles to estimate ankle joint angle. Experiments were conducted in neurally intact anesthetized cats and in unanesthetized decerebrate cats to demonstrate the feasibility of this control scheme. >

The goal of this study was to determine the feasibility of producing graded muscle contraction in individual muscles or muscle groups by electrically stimulating motor neurons in the lumbo-sacral spinal cord. Recruitment curves were obtained for quadriceps, tibialis anterior and triceps surae/plantaris by stimulating their activation pools in the ventral horn of the feline spinal cord. Mean twitch times-to-peak for quadriceps, tibialis anterior and triceps surae/plantaris were 33.0, 41.0, and 36.0 ms, respectively. Twitch duration as a function of stimulus strength demonstrated a mixed motor unit recruitment order, distinctively different from the inverse recruitment order exhibited by conventional methods of electrical stimulation of peripheral nerve. The recruitment curve slopes (expressed as a percentage of maximum force per nanocurrent of delivered charge) were shallow: 7.9 for quadriceps, 2.6 for tibialis anterior and 8.5 for triceps surae/plantaris. These results show that graded control of force in individual muscles or muscle groups can be obtained through spinal cord stimulation, and suggest that spinal cord stimulation could be used for functional neuromuscular stimulation applications.

Humans have a sense of static limb position derived primarily from the output of secondary muscle spindle endings. The features of finger pose these proprioceptors signal best were predicted by singular value decomposition of a kinematic model of the human long finger and the six muscles that actuate it. The analysis indicated that muscle spindles signal the location of the fingertip with less error than they signal angles of individual finger joints. The fingertip displacements for which proprioceptors have greatest sensitivity were also predicted. These fingertip displacements seem to correspond to the fine positioning of an object pinched between the fingertip and distal phalanx of the thumb. The analysis also predicted the directions in which subjects can displace the fingertip most rapidly. The directions seem to correspond to rapid closure of precision pinch or power grasp.

Studies have demonstrated that muscle spindle organs provide the majority of the proprioceptive information available to the nervous system about limb position. Other studies suggest that a sense of position may be lacking in the fingers, as subjects were unaware of rather large excursions of finger joints if the excursions were made slowly enough. We sought to investigate the basis for this unexpected finding with a biomechanical model of the human long finger and the forearm muscles which actuate it, in order to study potential contributions of spindle organs in the extrinsic muscles of the hand to a sense of position of the finger. The model, based on cadaver data, allowed us to determine how precisely estimates of the lengths of the extrinsic finger muscles can be transformed into estimates of: (1) the flexion/extension angles of the individual finger joints, and (2) the location of the fingertip in the flexion/extension plane. We found that, for some finger positions, length information from all three extrinsic muscles was not sufficient to precisely estimate the flexion angles of all finger joints. Precision of joint angle estimates could be as poor as +/- 18% of joint range of motion. However, length information from just two of the extrinsic muscles taken together could always provide information sufficient to estimate the location of the fingertip relative to the metacarpophalangeal joint within a reasonably small tolerance (+/- one-half thickness of the fingertip). Furthermore, it was possible to make this estimate without determining any of the finger joint angles. These results suggest that spindles in the extrinsic muscles alone can signal fingertip location, even though they may not provide sufficient information to estimate the individual joint angles that set the position of the fingertip. Thus, an absence of position sense for individual joints (the sense many studies have tried to measure) may say little about a sense of location of the tip of the finger.

A system for extracting single-unit activity patterns from multi-unit neural recordings was tested using real and simulated neural data. The system provided reliable estimates of firing frequency for individual units in simulated multi-unit data and allowed reliable determinations of the responses of individual cutaneous mechanoreceptor units to ‘natural’ stimuli such as brushing or pressing on the skin. An implementation of the system, which operated online and in real time, was used to obtain estimates of multiple, single-unit responses from multi-unit intrafascicular electrode recordings. The pattern of activity across the population of units in a given recording gave a reliable indication of the type of stimulus that had evoked the activity. It was concluded that this system, used in combination with intrafascicular peripheral nerve recordings, could be used to provide online, real-time information about peripheral stimuli.

The authors have studied activation of nerve fibers by pairs of Pt-Ir wire electrodes implanted within single fascicles of the nerve innervating the gastrocnemius muscle in cats. The purpose of this study was to determine if these intrafascicular electrodes can activate nerve fibers in different fascicles independently of each other and if they can also be used to activate separate subsets of axonal populations within a single fascicle. The average overlap of activated nerve fiber populations was 5.5% between fascicles and 27% within a fascicle, indicating that such selective activation is possible with these electrodes. >

The Automated Tactile Tester (ATT) was used to measure threshold values for trapezoidal skin indentation (light touch), low- and high-frequency vibration (50 and 150 Hz), pinprick (sharpdull transition point), warming (temperature awareness), and two-point discrimination in 61 patients with symptoms of median nerve compression at the wrist. We compared these data with values obtained in the same patients with manual monofilament tests, manual two-point discrimination measurements, and electrophysiologic nerve conduction studies. The ATT detected abnormal sensation in 71% of the hands tested, nerve conduction velocity was abnormal in 44% of the cases, and the manual tests indicated abnormality in 42% of the hands. The most indicative single test among those included in the present study for detecting sensory abnormality in these patients was threshold to a 50 Hz vibration administered by the ATT. We conclude that the ATT is a sensitive tool for the diagnosis and evaluation of compressive peripheral neuropathy and may allow objective documentation in a higher percent of patients than do more traditional testing methods.

A visual prosthesis, based on electrical stimulation of the visual cortex, has been suggested as a means for partially restoring functional vision in the blind. The prosthesis would create a pixelized visual sense consisting of punctate spots of light (phosphenes). The present study investigated the feasibility of achieving visually-guided mobility with such a visual sense. Psychophysical experiments were conducted on normally sighted human subjects, who were required to walk through a maze which included a series of obstacles, while their visual input was restricted to information from a pixelized vision simulator. Walking speed and number of body contacts with obstacles and walls were measured as a function of pixel number, pixel spacing, object minification, and field of view. The results indicate that a 25 × 25 array of pixels distributed within the foveal visual area could provide useful visually guided mobility in environments not requiring a high degree of pattern recognition.

A visual prosthesis based on electrical stimulation of the visual cortex with an array of penetrating electrodes is expected to produce pixelized visual images consisting of punctate spots of light (phosphenes). We measured reading speed in subjects viewing text with optically simulated phosphene fields in order to obtain estimates of the following design parameters for such an electrode array: pixel number, pixel spacing, and visual-field size. Comparisons were made between scanning the text with eye movements and scanning the text with head movements. The results indicate that a 25 x 25 array of pixels representing four letters of text projected on a foveal visual field of 1.7 degrees is sufficient to provide reading rates near 170 words/min with scrolled text and near 100 words/min with fixed text.

Multiunit recordings were made in anesthetized cats with chronically implanted intrafascicular electrodes over a period of six months. Neural signals recorded with these electrodes consisted of activity in sensory fibers innervating a variety of cutaneous mechanoreceptors. Mechanical stimuli were used to selectively activate individual nerve fibers, and the receptive field and receptor type were identified for each unit. Over a period of six months, there was a net shift in the recorded population, but the electrodes contained to provide a representative sample of the activity in the fascicle as a whole. The total number of units from which activity could be recorded remained roughly constant with time, and individual units persisted in the recordings for up to six months. These results indicate that intrafascicular electrodes could be used to sample information carried by individual somatosensory fibers on a long term basis. >

✓ The radial and sural nerves in cats were transected and either left unrepaired, repaired with 11-0 epineurial sutures, sutured and treated with topical application of Kenalog, or tubulated with hypoantigenic collagen. One year later the nerves were examined electrophysiologically, using single unit recording techniques, and histologically, by light microscopy. The results indicate that regeneration through the neuroma was comparable in all four groups, but that reinnervation of sensory structures in the skin was most successful in the tubulated nerves.

Dorsal column projection patterns and conduction velocities of regenerating myelinated sensory neurons were studied at intervals between 1 and 12 months after transection of the sural nerve. The neurons had significantly decreased conduction velocities, both in the fasciculus gracilis and in the periphery. Although dorsal root reflexes were temporarily abolished, there was no evidence of gross rearrangement of the neurons' ascending collaterals in the dorsal columns in response to transection of their peripheral processes or as a result of reestablishment of functional peripheral connections. These findings support the hypothesis that when a regenerating cutaneous sensory neuron reforms functional peripheral connections in tissue it originally innervated, its receptor properties are similar to those it had before the nerve was lesioned.

1. Type I sensory fibres in cat hairy skin innervate structures characterized by twenty to fifty specialized epithelial (Merkel) cells aggregated in a small dome-shaped elevation. Only one fibre enters each dome and it branches repeatedly to supply at least one terminal to each Merkel cell. After the nerve is cut, the Merkel cells and the dome ultimately disappear. 2. The distribution of domes on the posterior thigh was mapped before interruption of the femoral cutaneous nerve and after its regeneration. Regeneration after nerve crush was apparently complete, producing a coincidence pattern similar to those seen in control studies where the nerve was not damaged. After cutting the nerve fewer domes returned, but coincidence of regenerated femoral cutaneous domes with old sites generally was significantly greater than would be expected by chance alone. Non-femoral cutaneous fibres sprouting into the denervated femoral cutaneous field tended to form domes at old sites. Domes were also reformed on scars where domes had been excised. 3. Domes appearing at new locations and on excision scars were often small and close together (clustered). Individual domes in a cluster could be innervated by different Type I fibres. 4. Type I fibres are directed by some mechanism to sites formerly occupied by domes and to sites where domes are being induced.

As background for a report on our current selective adaptation experiments in decapod crustaceans, the various facts and hypotheses generally relevant to intraretinal sensitivity to polarized light in arthropods as well as cephalopods have been marshaled. On the basis of this review, the following working hypotheses have been made. 1) One ommatidium in the compound eye is the functional unit in image perception but contains in its component retinular cells subunits which can work independently in detecting other visual parameters, such as polarization. 2) Single retinular cells do respond differentially to light polarized in various planes. 3) Light sensitivity, including e-vector detection, is localized in the rhab domeres, which comprise closely packed arrays of microvilli protruding axially from retinular cells; the dichroism of the photopigment molecules, which are contained within the microvilli, provides the molecular basis of e-vector detection. 4) The visual pigment molecules have their major dichroic axis aligned predominantly parallel to the long axis of the microvillus containing them; typically all microvilli in a single rhab domere are closely parallel to one another, thus comprising at the cellular level a unit dichroic analyzer with maximum optical density to photons vibrating in the direction parallel to these microvillous protrusions. 5) In most decapod crustaceans, in cephalopods, and in some insects the microvilli in all rhabdomeres of a retinula are oriented in only two directions, perpendicular. to each other. Therefore, e-vector perception must depend at the retinular level on a two channel system consisting of a pair of dichroic analyzers with their major transmitting axes fixed at a 90 degrees angle determined by the two directions of microvillus orientation. Our new results on selective adaptation in the eye of Cardisoma provide direct experimental evidence for such a two-channel analyzer in which the pair of functional units have their maximum sensitivity to polarization in the same retinal directions as the rhab dom microvilli observed in electron micrographs. In turn, these directions correspond with the vertical and horizontal axes of the animal's normal spatial orientation. In e-vector detection the seven retinular cells of a single decapod ommatidium thus form two operational subgroups of four and three cells, respectively (39). The correspondence of the electrophysiological evidence for a dual polarization analyzer with the perpendicular directions shown by the microvilli in a single rhabdom strengthens the idea that one ommatidium is enough for detecting e-vector orientation. On this evidence we may conclude that the model developed above for a two-channel polarization analyzer effectively accounts for the relevant spectrophotometric, fine-structural, electrophysiological, and behavioral data currently available for a considerable number of arthropods and cephalopods.

Cutaneous type I receptor sites (Haarscheiben or tactile domes) were examined at intervals of 4 to 275 days after chronic denervation of the skin. The number of domes decreased with denervation time, and only about one-third of the domes originally present were still visible at 275 days. Most but not all of the Merkel cells from these domes were absent by 48 days, and the epithelium was significantly thinner than in nondenervated domes. Only a few of the examined domes appeared to be completely devoid of Merkel cells. It is concluded that after nerve transection, domes degenerate but do not always disappear entirely. The remmants may thus act as target sites which either attract regenerating type I nerve fibers or facilitate the formation of new dome structures after nerve regeneration.

Recordings made from individual type I cutaneous mechanoreceptors before and up to 9 months after neurectomy showed that surrounding, intact, low-threshold mechanoreceptive nerve fibers did not sprout to functionally innervate the denervated area of skin.

Publisher Summary This chapter presents a study in which regenerating sensory axons have been shown to form abortive terminals in the transition zone of the dorsal roots of the spinal cord after a lesion to the nerve at a distant site. The axons vigorously regenerated in the peripheral nerve, penetrated the basal lamina, and then abruptly ceased to grow as soon as they entered central nervous tissue and met processes of reactive astrocytes. These abortive terminals contained little cytoskeletal material and an abundance of vesicles, endoplasmic reticulum, and abnormal organelles. Membrane specializations indicative of cell–cell interaction were observed. Mechanisms whereby the astrocytic processes might inhibit axonal outgrowth are discussed in the chapter. Astrocytes have long been known to play a major role in the reactive processes that occur following an injury to the central nervous system. One particularly advantageous model for studying the interaction between astrocytes and regenerating sensory axons is provided by the transition zone between peripheral and central portions of the nervous system in the dorsal root of the spinal cord.

1. Micro-electrode recordings were made from normal and regenerating sural nerve fibres in cats. Increases in the excitability of the central terminals of these fibres after conditioning stimulation of other sural nerve fibres were taken as evidence for primary afferent depolarization. 2. At all recovery times studied the excitability changes seen were significantly less than those seen in control animals. Two factors contributed to the changes in primary afferent depolarization. First, the proportion of fibres that showed no evidence of primary afferent depolarization increased significantly. This proportion became smaller as recovery progressed. Secondly, where primary afferent depolarization was present, the magnitudes of the effects were slightly but significantly decreased compared with control values. 3. Excitability changes of the central terminals of sural nerve fibres were also measured after conditioning stimulation of the ipsilateral, unlesioned accessory sural nerve. One month after sural nerve transection there was a significant increase in the proportion of fibres showing no evidence of excitability changes following accessory sural nerve conditioning stimulation compared with control animals. Thus, the loss of primary afferent depolarization of regenerating sural nerve fibres was neither simply a consequence of desynchronization of the volley of impulses entering the spinal cord after conditioning stimulation of other regenerating sural fibres, nor due to fewer fibres being activated during conditioning stimulation of the lesioned nerves. 4. A possible explanation of these results is that after peripheral nerve crush or transection the central terminals of the damaged fibres retract or atrophy. Then as regeneration of the nerve proceeds, the central terminals of the fibres re-form.

The glabrous skin of the monkey's hand was stimulated with a waveform that indented the skin at a rate of 0.4 mm/sec, held the skin steadily or nearly steadily indented for 12 sec or longer, and then retracted back to the starting position. Recordings were made of activity in single afferent fibers in response to these stimuli. The average discharge frequency of 21 slowly adapting mechanoreceptors declined 38% during the first 12 sec of a steady indentation when the amplitude of the displacement was 0.65 mm and 36% when the displacement was 1.3 mm. When the plateau was not steady but the indentation depth gradually decreased by 15% during the 12-sec plateau period, the average decline was 47% for the 0.65-mm indentation and 46% for the 1.3-mm stimulus. When the indentation depth gradually increased by 15% during the 12-sec plateau, the discharge declined an average of 26% during the 0.65-mm indentation and 22% during the 1.3-mm displacement. To determine the effect of receptor adaptation on the perception of skin indentation depth, 13 human subjects had the skin of their fingertips indented 1 mm with similar trapezoidal waveform and were asked whether the indentation depth increased or decreased during the plateau portion of the stimulus. Ten of the 13 subjects thought that the indentation depth was increasing when the plateau was steady. The method of limits was then used to determine how much the stimulus had to change for the subject to feel the depth during the plateau as unchanging; i.e., a “perceptual zero.”(ABSTRACT TRUNCATED AT 250 WORDS)

After nerve transection, cutaneous type I mechanoreceptors (Haarscheiben or tactile domes) preferentially reappear at old loci, although some do appear at new locations. The mechanism by which this topological specificity is maintained was studied by transecting the femoral cutaneous nerve in cats in which about half of the Haarscheiben were removed by cauterization. Thirteen months after nerve transection, domes were found on uncauterized sites at a rate significantly greater than that expected by chance alone, but on cauterized old dome sites at a rate expected by chance alone. It is concluded the reappearance of type I receptors at old receptor sites following nerve transection is primarily due to intrinsic properties of the receptor sites, rather than to guidance of regenerating axonal sprouts to these sites by the endoneurial matrix of the distal stump of the lesioned nerve.

Electrical stimulation of the visual cortex causes subjects to see spots of light (phosphenes) in their visual fields. The authors have developed arrays of penetrating electrodes which can form the basis of a visual prosthesis centred around electrical stimulation of the visual cortex. These arrays have been constructed of inert materials, and have been implanted into cats to test array insertability and biocompatibility. Tests to determine electrode wire insertion force have also been performed. Preliminary acute histological results have shown little cortical damage due to the implants. >

Psychophysical and physiological experiments using low amplitude, high‐frequency vibrations which selectively activate Pacinian corpuscle mechano‐receptors have been employed to address the question: Can the vibrotactile system distinguish between pure and polyharmonic sinusoidal stimuli applied to the glabrous skin in the presence of concomitant changes in fundamental frequency and amplitude? Human subjects are presented with stimulus pairs in which a pure tone is paired with a tone having a significant second harmonic component. The fundamental frequency and amplitude of each member of the pair are varied independently so they typically differ in peak‐to‐peak amplitude and frequency composition as well as harmonic content. In addition, different phase relations between the fundamental and harmonic of the diharmonic tone are used. The subjects are asked to identify which member of the pair is diharmonic. Responses of Pacinian corpuscles to the same tone pairs are recorded from afferent fibers in the forelimb of the cat, using both on‐center and off‐center stimulation. The eventual goal is to collect similar data in primates in order to be able to describe the neural basis for the discrimination ability observed in the psychophysical tests.