The complex structure of a simple memory

Effects of chronic nerve cuff and intramuscular electrodes on rat triceps surae motor units

In order to assess the long-term effects of implanted electrodes on motor unit properties, we studied triceps surae (TS) motor units in rats implanted for 3-10 months with a tibial nerve cuff electrode for H-reflex elicitation and intramuscular electrodes for recording TS electromyographic activity. Motor units with sag from implanted rats displayed greater tetanic force than those from unimplanted rats. Motor units without sag had shorter twitch contraction times. This disrupted the relationship between sag and contraction time that was always present in unimplanted rats. These differences were consistent with a small degree of muscle denervation and subsequent reinnervation. Further analyses ascribed this effect to the nerve cuff rather than to the intramuscular electrodes. Comparable changes in motor unit properties may occur in humans with implanted nerve cuffs.

Activity-dependent spinal cord plasticity in health and disease

Activity-dependent plasticity occurs in the spinal cord throughout life. Driven by input from the periphery and the brain, this plasticity plays an important role in the acquisition and maintenance of motor skills and in the effects of spinal cord injury and other central nervous system disorders. The responses of the isolated spinal cord to sensory input display sensitization, long-term potentiation, and related phenomena that contribute to chronic pain syndromes; they can also be modified by both classical and operant conditioning protocols. In animals with transected spinal cords and in humans with spinal cord injuries, treadmill training gradually modifies the spinal cord so as to improve performance. These adaptations by the isolated spinal cord are specific to the training regimen and underlie new approaches to restoring function after spinal cord injury. Descending inputs from the brain that occur during normal development, as a result of supraspinal trauma, and during skill acquisition change the spinal cord. The early development of adult spinal cord reflex patterns is driven by descending activity; disorders that disrupt descending activity later in life gradually change spinal cord reflexes. Athletic training, such as that undertaken by ballet dancers, is associated with gradual alterations in spinal reflexes that appear to contribute to skill acquisition. Operant conditioning protocols in animals and humans can produce comparable reflex changes and are associated with functional and structural plasticity in the spinal cord, including changes in motoneuron firing threshold and axonal conduction velocity, and in synaptic terminals on motoneurons. The corticospinal tract has a key role in producing this plasticity. Behavioral changes produced by practice or injury reflect the combination of plasticity at multiple spinal cord and supraspinal sites. Plasticity at multiple sites is both necessary-to insure continued performance of previously acquired behaviors-and inevitable-due to the ubiquity of the capacity for activity-dependent plasticity in the central nervous system. Appropriate induction and guidance of activity-dependent plasticity in the spinal cord is an essential component of new therapeutic approaches aimed at maximizing function after spinal cord injury or restoring function to a newly regenerated spinal cord. Because plasticity in the spinal cord contributes to skill acquisition and because the spinal cord is relatively simple and accessible, this plasticity is a logical and practical starting point for studying the acquisition and maintenance of skilled behaviors.

NO donor SIN-1 potentiates monosynaptic reflexes in the cat spinal cord

The effect produced by the nitric oxide donor SIN-1 on monosynaptic reflexes was examined. Experiments were performed on anesthetized, paralyzed and spinalized cats. Lumbar monosynaptic reflexes were produced by stimulation of Ia afferents. I.v. application of SIN-1 (500 microg/kg) produced a mean marked potentiation of 704% of pre-drug control (100%) in the amplitude of monosynaptic reflexes. In addition, in other experiments a concentration-dependent effect on the amplitude of monosynaptic reflexes was observed after microinjections of SIN-1 into the ventral horn (1 microl; 10(-12) - 10(-3) M), with a mean facilitatory effect of 355%. In both cases, the potentiation was reversible 45 min after i.v. or local application of SIN-1. These results provide the first evidence that monosynaptic reflexes can be potentiated by nitric oxide.

An approach to the complexity of the brain

The establishment of ordered neuronal connections is supposed to take place under the control of specific cell adhesion molecules (CAM) which guide neuroblasts and axons to their appropriate destination. The extreme complexity of the nervous system does not provide a favorable medium for the development of deterministic connections. Simon's [112] theorems offer a mean to approach the high level of complexity of the nervous system. The basic tenet is that complex systems are hierarchically organized and decomposable. Such systems can arise by selective trial and error mechanisms. Subsystems in complex systems only interact in an aggregate manner, and no significant information is lost if the detail of aggregate interactions is ignored. A number of nervous activities, which qualify for these requirements, are shown. The following sources of selection are considered: internal and external feedbacks, previous experience, plasticity in simple structures, and the characteristic geometry of dendrites. The role played by CAMs and other membrane-associated molecules is discussed in the sense that they are either inductor molecules that turn on different homeobox genes, or downstream products of genes, or both. These molecules control cellular and tissular differentiation in the developing brain creating sources of selection required for the trial and error process in the organization of the nervous tissue.

Time course of H-reflex conditioning in the rat

This study sought to define the course of operantly conditioned change in the rat soleus H-reflex and to determine whether, like H-reflex conditioning and spinal stretch reflex conditioning in the monkey, it develops in distinct phases. Data from 33 rats in which the right soleus H-reflex was trained up (i.e. HRup mode) and 38 in which it was trained down (i.e. HRdown mode) were averaged to define the courses of H-reflex increase and decrease. In HRup rats, the H-reflex showed a large phase I increase within the first 2 days followed by gradual phase II increase that continued for weeks. In HRdown rats, the H-reflex appeared to show a small phase I decrease and then showed a gradual phase II decrease over weeks. In combination with other recent work, the data suggest that H-reflex conditioning begins with a rapid mode-appropriate alteration in corticospinal tract influence over the spinal arc of the H-reflex, which causes phase I change, and that the continuation of this altered influence induces gradual spinal cord plasticity that is responsible for phase II change. The results further establish the similarity of H-reflex conditioning in primates and rats. Thus, they encourage efforts to produce a single coherent model of the phenomenon based on data from the two species and indicate the potential clinical relevance of the rat data.

Plasticity of motor systems after incomplete spinal cord injury

Although spontaneous regeneration of lesioned fibres is limited in the adult central nervous system, many people that suffer from incomplete spinal cord injuries show significant functional recovery. This recovery process can go on for several years after the injury and probably depends on the reorganization of circuits that have been spared by the lesion. Synaptic plasticity in pre-existing pathways and the formation of new circuits through collateral sprouting of lesioned and unlesioned fibres are important components of this recovery process. These reorganization processes might occur in cortical and subcortical motor centres, in the spinal cord below the lesion, and in the spared fibre tracts that connect these centres. Functional and anatomical evidence exists that spontaneous plasticity can be potentiated by activity, as well as by specific experimental manipulations. These studies prepare the way to a better understanding of rehabilitation treatments and to the development of new approaches to treat spinal cord injury.

Short-Term and medium-term effects of spinal cord tract transections on soleus H-reflex in freely moving rats

Spinal cord function is normally influenced by descending activity from supraspinal structures. When injury removes or distorts this influence, function changes and spasticity and other disabling problems eventually appear. Understanding how descending activity affects spinal cord function could lead to new means for inducing, guiding, and assessing recovery after injury. In this study, we investigated the short-term and medium-term effects of spinal cord bilateral dorsal column (DC), unilateral (ipsilateral) lateral column (LC), bilateral dorsal column ascending tract (DA), or bilateral dorsal column corticospinal tract (CST) transection at vertebral level T8-T9 on the soleus H-reflex in freely moving rats. Data were collected continuously for 10-20 days before and for 20-155 days after bilateral DC (13 rats), DA (10 rats), CST (eight rats), or ipsilateral LC (seven rats) transection. Histological examination showed that transections were 98(+/- 3 SD)% complete for DC rats, 80(+/- 20)% complete for LC rats, 91(+/- 13 SD)% complete for DA rats, and 95(+/-13)% complete for CST rats. LC, CST, and DA transections produced an immediate (i.e., first-day) increase in H-reflex amplitude. LC transection also produced a small decrease in background activity in the first few posttransection days. Other than this small decrease, none of the transections produced evidence for the phenomenon of spinal shock. For all transections, all measures returned to or neared pretransection values within 2 weeks. DA and LC transections were associated with modest increase in H-reflex amplitude 1-3 months after transection. These medium-term effects must be taken into account when assessing transection effects on operant conditioning of the H-reflex. At the same time, the results are consistent with other evidence that, while H-reflex rate dependence and H-reflex operant conditioning are sensitive measures of spinal cord injury, the H-reflex itself is not.

Resistance training enhances the stability of sensorimotor coordination

Strategies for the control of human movement are constrained by the neuroanatomical characteristics of the motor system. In particular, there is evidence that the capacity of muscles for producing force has a strong influence on the stability of coordination in certain movement tasks. In the present experiment, our aim was to determine whether physiological adaptations that cause relatively long-lasting changes in the ability of muscles to produce force can influence the stability of coordination in a systematic manner. We assessed the effects of resistance training on the performance of a difficult coordination task that required participants to synchronize or syncopate movements of their index finger with an auditory metronome. Our results revealed that training that increased isometric finger strength also enhanced the stability of movement coordination. These changes were accompanied by alterations in muscle recruitment patterns. In particular, the trained muscles were recruited in a more consistent fashion following the programme of resistance training. These results indicate that resistance training produces functional adaptations of the neuroanatomical constraints that underlie the control of voluntary movement.

Towards a network theory of cognition

For cognitive neuroscience to go forward a more explicit effort is needed to use neurophysiology to constrain how the brain produces human mental functions. This review begins with the suggestion that two fundamental features may be critical for this effort. The first is the connectivity of the brain, which occupies an intermediate position between complete redundant interconnections and independence. The term semniconnected is presented as a designation, which is an obvious derivation of the term semiconductors as used in engineering. The second is transient response plasticity where a given neuron or collection of neurons may show rapid changes in response characteristics depending on experience. Response plasticity is a ubiquitous property of the brain rather than a unique characteristic of "neurocognitive" regions. These two properties may be brought together when brain areas interact such that their aggregate function embodies cognition. Three examples are used to illustrate these general principles and to develop the idea that a particular region in isolation may not act as a reliable index for a particular cognitive function. Instead, the neural context in which an area is active may define the cognitive function. Neural context emphasizes that the particular spatiotemporal pattern of neural interactions may hold the key to bridge between brain and mind.

Activity-dependent plasticity of descending synaptic inputs to spinal motoneurons in an in vitro turtle brainstem-spinal cord preparation

An in vitro brainstem-spinal cord preparation from adult turtles was used to test the hypothesis that descending synaptic inputs to multifunctional spinal motoneurons (i.e., involved in respiration and locomotion) express activity-dependent depression or potentiation. The tissue was placed in a chamber that allowed for separate superfusion of the brainstem, spinal segments C(2)-C(4), and C(5)-D(1). Action potential conduction between the brainstem and spinal segments C(5)-D(1) was blocked by superfusing C(2)-C(4) with Na(+)-free solution. With C(5)-D(1) at [K(+)] = 10 mM, electrical stimulation at C(5) every 2 min evoked potentials in intact pectoralis (expiratory, inward rotation of shoulder) and serratus (inspiratory, outward rotation of shoulder) nerves that were stable for at least 2 hr. Application of conditioning stimulation (900 pulses at 1 or 10 Hz) at C(5) decreased pectoralis evoked potential amplitudes by approximately 40% initially and by 20% after 90 min; serratus evoked potentials were unaltered. Conditioning stimulation (100 Hz, 900 pulses) transiently depressed pectoralis evoked potential amplitude by <20% but produced a delayed 72% increase in serratus evoked potential amplitude after approximately 80 min. Conditioning stimulation (10 Hz) at C(5) also reduced the amplitude of sensory afferent evoked potentials in pectoralis produced by stimulating ipsilateral dorsal roots at C(8). Thus, long-lasting changes in descending synaptic inputs to multifunctional spinal motoneurons were frequency-dependent and heterosynaptic. We hypothesize that activity-dependent plasticity may modulate descending synaptic drive to spinal motoneurons involved in both respiration and locomotion.

Central control components of a 'simple' stretch reflex

The monosynaptic stretch reflex is a fundamental feature of sensory-motor organization in most animal groups. In isolation, it serves largely as a negative feedback devoted to postural controls; however, when it is involved in diverse movements, it can be modified by central command circuits. In order to understand the implications of such modifications, a model system has been chosen that has been studied at many different levels: the crayfish walking system. Recent studies have revealed several levels of control and modulation (for example, at the levels of the sensory afferent and the output synapse from the sensory afferent, and via changes in the membrane properties of the postsynaptic neuron) that operate complex and highly adaptive sensory-motor processing. During a given motor task, such mechanisms reshape the sensory message completely, such that the stretch reflex becomes a part of the central motor command.

A systems approach to the cellular analysis of associative learning in the pond snail Lymnaea

We show that appetitive and aversive conditioning can be analyzed at the cellular level in the well-described neural circuitries underlying rhythmic feeding and respiration in the pond snail, Lymnaea stagnalis. To relate electrical changes directly to behavior, the snails were first trained and the neural changes recorded at multiple sites in reduced preparations made from the same animals. Changes in neural activity following conditioning could be recorded at the level of motoneurons, central pattern generator interneurons and modulatory neurons. Of significant interest was recent work showing that neural correlates of long-term memory could be recorded in the feeding network following single-trial appetitive chemical conditioning. Available information on the synaptic connectivity and transmitter content of identified neurons within the Lymnaea circuits will allow further work on the synaptic and molecular mechanisms of learning and memory.

The operant and the classical in conditioned orientation of Drosophila melanogaster at the flight simulator

Ever since learning and memory have been studied experimentally, the relationship between operant and classical conditioning has been controversial. Operant conditioning is any form of conditioning that essentially depends on the animal's behavior. It relies on operant behavior. A motor output is called operant if it controls a sensory variable. The Drosophila flight simulator, in which the relevant behavior is a single motor variable (yaw torque), fully separates the operant and classical components of a complex conditioning task. In this paradigm a tethered fly learns, operantly or classically, to prefer and avoid certain flight orientations in relation to the surrounding panorama. Yaw torque is recorded and, in the operant mode, controls the panorama. Using a yoked control, we show that classical pattern learning necessitates more extensive training than operant pattern learning. We compare in detail the microstructure of yaw torque after classical and operant training but find no evidence for acquired behavioral traits after operant conditioning that might explain this difference. We therefore conclude that the operant behavior has a facilitating effect on the classical training. In addition, we show that an operantly learned stimulus is successfully transferred from the behavior of the training to a different behavior. This result unequivocally demonstrates that during operant conditioning classical associations can be formed.

Developmental tuning in a spinal nociceptive system: effects of neonatal spinalization

Recent studies indicate a modular organization of the nociceptive withdrawal reflex system. Each module has a characteristic receptive field, closely matching the withdrawal movement caused by its effector muscle. In the rat, the strength of the sensory input to each module is tuned during the first postnatal weeks, i.e., erroneous spinal connections are depressed, and adequate connections are strengthened. To clarify if this tuning is dependent on supraspinal structures, the effect of a complete neonatal spinal cord transection on the postnatal tuning of withdrawal reflexes was studied. The nociceptive receptive fields of single hindlimb muscles and compound withdrawal reflexes were examined in decerebrate unanesthetized and awake rats, respectively. Noxious thermal CO(2) laser stimulation was used to evoke reflex responses. Neonatal spinal cord transection resulted in a disrupted reflex organization in the adult rat, resembling that previously found in neonatal rats. The receptive fields of single hindlimb muscles exhibited abnormal distribution of sensitivity not matching the withdrawal action of the effector muscles. Likewise, the composite nocifensive movements, as documented in the awake rat, often resulted in erroneous movements toward the stimulus. It is concluded that withdrawal reflexes do not become functionally adapted in rats spinalized at birth. These findings suggest a critical role for supraspinal systems in the postnatal tuning of spinal nociceptive systems.

Mapping cognition to the brain through neural interactions

Brain imaging methods, such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), provide a unique opportunity to study the neurobiology of human memory. As these methods can measure most of the brain, it is possible to examine the operations of large-scale neural systems and their relation to cognition. Two neuroimaging studies, one concerning working memory and the other episodic memory retrieval, serve as examples of application of two analytic methods that are optimised for the quantification of neural systems, structural equation modelling, and partial least squares. Structural equation modelling was used to explore shifting prefrontal and limbic interactions from the right to the left hemisphere in a delayed match-to-sample task for faces. A feature of the functional network for short delays was strong right hemisphere interactions between hippocampus, inferior prefrontal, and anterior cingulate cortices. At longer delays, these same three areas were strongly linked, but in the left hemisphere, which was interpreted as reflecting change in task strategy from perceptual to elaborate encoding with increasing delay. The primary manipulation in the memory retrieval study was different levels of retrieval success. The partial least squares method was used to determine whether the image-wide pattern of covariances of Brodmann areas 10 and 45/47 in right prefrontal cortex (RPFC) and the left hippocampus (LGH) could be mapped on to retrieval levels. Area 10 and LGH showed an opposite pattern of functional connectivity with a large expanse of bilateral limbic cortices that was equivalent for all levels of retrieval as well as the baseline task. However, only during high retrieval was area 45/47 included in this pattern. The results suggest that activity in portions of the RPFC can reflect either memory retrieval mode or retrieval success depending on other brain regions to which it is functionally linked, and imply that regional activity must be evaluated within the neural context in which it occurs. The general hypothesis that learning and memory are emergent properties of large-scale neural network interactions is discussed, emphasising that a region can play a different role across many functions and that role is governed by its interactions with anatomically related regions.

Locomotor plasticity after spinal injury in the chick

Functional recovery after spinal cord injury likely depends, in part, on the reorganization of undamaged spinal circuitry. Segmental afferent input from the limbs remains largely intact after spinal injury and may provide an important source of activation and regulation of the spinal circuits that have lost descending input as a result of the injury. This purpose of this study was to investigate the contribution of cutaneous afferent inputs to the recovery of motor function after spinal injury in the chick. After lateral thoracic spinal hemisection, the motion of the ipsilateral limb was impaired during both walking and swimming. By 2 weeks postoperatively, limb motion recovered to preoperative values for walking but not for swimming. It was hypothesized that phasic afferent inputs experienced during walking, but not swimming, contributed to recovery of limb motion during walking. When a source of phasic cutaneous input was provided during swim training sessions, limb motion gradually improved to preoperative values. After 2 weeks of training, this improved motion was retained even after the source of cutaneous stimulation was removed. The proposed mechanism is an experience-dependent strengthening of the circuits activated during the improved limb motion, leading to a permanent change in limb action during swimming. Thus, the afferent inputs experienced during movement repetition are important during the acquisition of learned movements after spinal injury. These results are discussed in terms of behavioral, physiological, and anatomical evidence for spinal plasticity in other species. It is concluded that the spinal cord has significant plastic capabilities, and efforts should be directed toward maximizing the contribution of this plasticity to functional recovery after spinal cord injury.

Sag during unfused tetanic contractions in rat triceps surae motor units

Contractile properties and conduction velocity were studied in 202 single motor units of intact rat triceps surae muscles activated by intra-axonal (or intra-myelin) current injection in L5 or L6 ventral root to assess the factors that determine the expression of sag (i.e., decline in force after initial increase during unfused tetanic stimulation). Sag was consistently detected in motor units with unpotentiated twitch contraction times <20 ms. However, the range of frequencies at which sag was expressed varied among motor units such that there was no single interstimulus interval (ISI), with or without adjusting for twitch contraction time, at which sag could be detected reliably. Further analysis indicated that using the absence of sag as a criterion for identifying slow-twitch motor units requires testing with tetani at several different ISIs. In motor units with sag, the shape of the force profile varied with tetanic frequency and contractile properties. Simple sag force profiles (single maximum reached late in the tetanus followed by monotonic decay) tended to occur at shorter ISIs and were observed more frequently in fatigue-resistant motor units with long half-relaxation times and small twitch amplitudes. Complex sag profiles reached an initial maximum early in the tetanus, tended to occur at longer ISIs, and were more common in fatigue-sensitive motor units with long half-relaxation times and large twitch amplitudes. The differences in frequency dependence and force maximum location suggested that these phenomena represented discrete entities. Successive stimuli elicited near-linear increments in force during tetani in motor units that never exhibited sag. In motor units with at least one tetanus displaying sag, tetanic stimulation elicited large initial force increments followed by rapidly decreasing force increments. That the latter force envelope pattern occurred in these units even in tetani without sag suggested that the factors responsible for sag were expressed in the absence of overt sag. The time-to-peak force (TTP) of the individual contractions during a tetanus decreased in tetani with sag. Differences in the pattern of TTP change during a tetanus were consistent with the differences in force maximum location between tetani exhibiting simple and complex sag. Tetani from motor units that never exhibited sag did not display a net decrease in TTP during successive contractions. These data were consistent with the initial force decrement of sag resulting from a transient reduction in the duration of the contractile state.

In vitro analog of operant conditioning in aplysia. I. Contingent reinforcement modifies the functional dynamics of an identified neuron

Previously, an analog of operant conditioning in Aplysia was developed using the rhythmic motor activity in the isolated buccal ganglia. This analog expressed a key feature of operant conditioning, namely a selective enhancement in the occurrence of a designated motor pattern by contingent reinforcement. Different motor patterns generated by the buccal central pattern generator were induced by monotonic stimulation of a peripheral nerve (i.e., n.2,3). Phasic stimulation of the esophageal nerve (E n.) was used as an analog of reinforcement. The present study investigated the neuronal mechanisms associated with the genesis of different motor patterns and their modifications by contingent reinforcement. The genesis of different motor patterns was related to changes in the functional states of the pre-motor neuron B51. During rhythmic activity, B51 dynamically switched between inactive and active states. Bursting activity in B51 was associated with, and predicted, characteristic features of a specific motor pattern (i.e., pattern I). Contingent reinforcement of pattern I modified the dynamical properties of B51 by decreasing its resting conductance and threshold for eliciting plateau potentials and thus increased the occurrences of pattern I-related activity in B51. These modifications were not observed in preparations that received either noncontingent reinforcement (i.e., yoke control) or no reinforcement (i.e., control). These results suggest that a contingent reinforcement paradigm can regulate the dynamics of neuronal activity that is centrally programmed by the intrinsic cellular properties of neurons.

Operant conditioning of H-reflex increase in spinal cord--injured rats

Operant conditioning of the spinal stretch reflex or its electrical analog, the H-reflex, is a new model for exploring the mechanisms of long-term supraspinal control over spinal cord function. Primates and rats can gradually increase (HRup conditioning mode) or decrease (HRdown conditioning mode) the H-reflex when reward is based on H-reflex amplitude. An earlier study indicated that HRdown conditioning of the soleus H-reflex in rats is impaired following contusion injury to thoracic spinal cord. The extent of impairment was correlated with the percent of white matter lost at the injury site. The present study investigated the effects of spinal cord injury on HRup conditioning. Soleus H-reflexes were elicited and recorded with chronically implanted electrodes from 14 rats that had been subjected to calibrated contusion injuries to the spinal cord at T8. At the lesion epicenter, 12-39% of the white matter remained. After control-mode data were collected, each rat was exposed to the HRup conditioning mode for 50 days. Final H-reflex amplitudes after HRup conditioning averaged 112% (+/-22% SD) of control. This value was significantly smaller than that for 13 normal rats exposed to HRup conditioning, in which final amplitude averaged 153% (+/-51%) SD of control. As previously reported for HRdown conditioning after spinal cord injury, success was inversely correlated with the severity of the injury as assessed by white matter preservation and by time to return of bladder function. HRup and HRdown conditioning are similarly sensitive to injury. These results further demonstrate that H-reflex conditioning is a sensitive measure of the long-term effects of injury on supraspinal control over spinal cord functions and could prove a valuable measure of therapeutic efficacy.

Pseudo-inverse control in biological systems: a learning mechanism for fixation stability

The problem of redundancy in motor control is common to both robotics and biology. Pseudo-inverse control has been proposed as a solution in robotics and appears to be used by the oculomotor system for eye position. Learning mechanisms for implementing pseudo-inverse control using a distributed system of ocular motor units were investigated by modelling integrator calibration for horizontal eye movements. Ocular motoneuron (OMN) input weights were adjusted with a gradient-descent learning rule, using a retinal-slip estimate as an error signal. Firing-rate threshold only became related to motor-unit strength when a noise term was added to OMN firing rates. The learning rule suppressed those units making the largest contribution to the noise-related error, causing the strongest units to have the highest thresholds (size principle). Because the size principle and pseudo-inverse control are related, the trained system approximated pseudo-inverse control over the central +/-35 degrees of the oculomotor range.

Prolonged increase in micturition threshold volume by anogenital afferent stimulation in the rat

OBJECTIVE

To investigate whether anogenital afferent stimulation induces a prolonged increase in the micturition threshold volume of anaesthetized rats.

MATERIALS AND METHODS

Thirteen female rats, anaesthetized by alpha-chloralose and paralysed by pancuronium bromide were used for the experiments. The micturition threshold volume was determined by repeated cystometry. In two experiments, afferent activity was recorded from the exposed pudendal nerve; vaginal and anal afferents were stimulated electrically by ring electrodes. In one experiment, the dorsal clitoris nerves were exposed bilaterally and mounted for electrical stimulation. The afferents were stimulated continuously for 5 min at 10 Hz, using unipolar cathodic pulses of 0.5 ms duration with an amplitude of 10 mA (or 0.8 mA for the dorsal clitoral nerves).

RESULTS

Anogenital stimulation for 5 min induced a significant and prolonged increase in the micturition threshold volume (from a median value of 0.35 mL before to 0.45 mL after stimulation; P < 0.01). The increase in threshold volume was maintained for about 40 min after the end of stimulation. There was no obvious difference in effect between the stimulation sites nor with direct dorsal clitoral nerve stimulation. Neither the micturition threshold pressure nor the maximal contraction pressure were altered by stimulation. No tonic afferent after-discharge could be detected in the pudendal nerve recordings.

CONCLUSIONS

Artificial electrical stimulation of anogenital afferents induced a prolonged increase in the micturition threshold volume of anaesthetized rats. The change presumably involved the modulation of the synaptic transmission in the central micturition reflex pathway. It is proposed that the observed change represents the first step in the curative 're-education' process induced in patients with urge incontinence by electrical stimulation of anovaginal afferents.