A supervised machine learning approach to characterize spinal network function.

Spontaneous activity is a common feature of immature neuronal networks throughout the central nervous system and plays an important role in network development and consolidation. In postnatal rodents, spontaneous activity in the spinal cord exhibits complex, stochastic patterns that have historically proven challenging to characterize. We developed a software tool for quickly and automatically characterizing and classifying episodes of spontaneous activity generated from developing spinal networks. We recorded spontaneous activity from in vitro lumbar ventral roots of 16 neonatal [postnatal day (P)0-P3] mice. Recordings were DC coupled and detrended, and episodes were separated for analysis. Amplitude-, duration-, and frequency-related features were extracted from each episode and organized into five classes. Paired classes and features were used to train and test supervised machine learning algorithms. Multilayer perceptrons were used to classify episodes as rhythmic or multiburst. We increased network excitability with potassium chloride and tested the utility of the tool to detect changes in features and episode class. We also demonstrate usability by having a novel experimenter use the program to classify episodes collected at a later time point (P5). Supervised machine learning-based classification of episodes accounted for changes that traditional approaches cannot detect. Our tool, named SpontaneousClassification, advances the detail in which we can study not only developing spinal networks, but also spontaneous networks in other areas of the nervous system. NEW & NOTEWORTHY Spontaneous activity is important for nervous system network development and consolidation. Our software uses machine learning to automatically and quickly characterize and classify episodes of spontaneous activity in the spinal cord of newborn mice. It detected changes in network activity following KCl-enhanced excitation. Using our software to classify spontaneous activity throughout development, in pathological models, or with neuromodulation, may offer insight into the development and organization of spinal circuits.

Retracing your footsteps: developmental insights to spinal network plasticity following injury.

During development of the spinal cord, a precise interaction occurs between descending projections and sensory afferents, with spinal networks that lead to expression of coordinated motor output. In the rodent, during the last embryonic week, motor output first occurs as regular bursts of spontaneous activity, progressing to stochastic patterns of episodes that express bouts of coordinated rhythmic activity perinatally. Locomotor activity becomes functionally mature in the 2nd postnatal wk and is heralded by the onset of weight-bearing locomotion on the 8th and 9th postnatal day. Concomitantly, there is a maturation of intrinsic properties and key conductances mediating plateau potentials. In this review, we discuss spinal neuronal excitability, descending modulation, and afferent modulation in the developing rodent spinal cord. In the adult, plastic mechanisms are much more constrained but become more permissive following neurotrauma, such as spinal cord injury. We discuss parallel mechanisms that contribute to maturation of network function during development to mechanisms of pathological plasticity that contribute to aberrant motor patterns, such as spasticity and clonus, which emerge following central injury.

Modulatory and plastic effects of kinins on spinal cord networks.

KEY POINTS: Inflammatory kinins are released following spinal cord injury or neurotrauma. The effects of these kinins on ongoing locomotor activity of central pattern generator networks are unknown. In the present study, kinins were shown to have short- and long-term effects on motor networks. The short-term effects included direct depolarization of interneurons and motoneurons in the ventral horn accompanied by modulation of transient receptor potential vanilloid 1-sensitive nociceptors in the dorsal horn. Over the long-term, we observed a bradykinin-mediated effect on promoting plasticity in the spinal cord. In a model of spinal cord injury, we observed an increase in microglia numbers in both the dorsal and ventral horn and, in a microglia cell culture model, we observed bradykinin-induced expression of glial-derived neurotrophic factor. ABSTRACT: The expression and function of inflammatory mediators in the developing spinal cord remain poorly characterized. We discovered novel, short and long-term roles for the inflammatory nonapeptide bradykinin (BK) and its receptor bradykinin receptor B2 (B2R) in the neuromodulation of developing sensorimotor networks following a spinal cord injury (SCI), suggesting that BK participates in an excitotoxic cascade. Functional expression of B2R was confirmed by a transient disruptive action of BK on fictive locomotion generated by a combination of NMDA, 5-HT and dopamine. The role of BK in the dorsal horn nociceptive afferents was tested using spinal cord attached to one-hind-limb (HL) preparations. In the HL preparations, BK at a subthreshold concentration induced transient disruption of fictive locomotion only in the presence of: (1) noxious heat applied to the hind paw and (2) the heat sensing ion channel transient receptor potential vanilloid 1 (TRPV1), known to be restricted to nociceptors in the superficial dorsal horn. BK directly depolarized motoneurons and ascending interneurons in the ventrolateral funiculus. We found a key mechanism for BK in promoting long-term plasticity within the spinal cord. Using a model of neonatal SCI and a microglial cell culture model, we examined the role of BK in inducing activation of microglia and expression of glial-derived neurotrophic factor (GDNF). In the neonatal SCI model, we observed an increase in microglia numbers and increased GDNF expression restricted to microglia. In the microglia cell culture model, we observed a BK-induced increased expression of GDNF via B2R, suggesting a novel mechanism for BK spinal-mediated plasticity.

Diversification of intrinsic motoneuron electrical properties during normal development and botulinum toxin-induced muscle paralysis in early postnatal mice.

During early postnatal development, between birth and postnatal days 8-11, mice start to achieve weight-bearing locomotion. In association with the progression of weight-bearing locomotion there are presumed developmental changes in the intrinsic electrical properties of spinal -motoneurons. However, these developmental changes in the properties of -motoneuron properties have not been systematically explored in mice. Here, data are presented documenting the developmental changes of selected intrinsic motoneuron electrical properties, including statistically significant changes in action potential half-width, intrinsic excitability and diversity (quantified as coefficient of variation) of rheobase current, afterhyperpolarization half-decay time, and input resistance. In various adult mammalian preparations, the maintenance of intrinsic motoneuron electrical properties is dependent on activity and/or transmission-sensitive motoneuron-muscle interactions. In this study, we show that botulinum toxin-induced muscle paralysis led to statistically significant changes in the normal development of intrinsic motoneuron electrical properties in the postnatal mouse. This suggests that muscle activity during early neonatal life contributes to the development of normal motoneuron electrical properties.

Locomotor networks are targets of modulation by sensory transient receptor potential vanilloid 1 and transient receptor potential melastatin 8 channels.

It is well recognized that proprioceptive afferent inputs can control the timing and pattern of locomotion. C and Adelta afferents can also affect locomotion but an unresolved issue is the identity of the subsets of these afferents that encode defined modalities. Over the last decade, the transient receptor potential (TRP) ion channels have emerged as a family of non-selective cation conductances that can label specific subsets of afferents. We focus on a class of TRPs known as ThermoTRPs which are well known to be sensor receptors that transduce changes in heat and cold. ThermoTRPs are known to help encode somatosensation and painful stimuli, and receptors have been found on C and Adelta afferents with central projections onto dorsal horn laminae. Here we show, using in vitro neonatal mouse spinal cord preparations, that activation of both spinal and peripheral transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential melastatin 8 (TRPM8) afferent terminals modulates central pattern generators (CPGs). Capsaicin or menthol and cooling modulated both sacrocaudal afferent (SCA) evoked and monoaminergic drug-induced rhythmic locomotor-like activity in spinal cords from wild type but not TRPV1-null (trpv1(-/-)) or TRPM8-null (trpm8(-/-)) mice, respectively. Capsaicin induced an initial increase in excitability of the lumbar motor networks, while menthol or cooling caused a decrease in excitability. Capsaicin and menthol actions on CPGs involved excitatory and inhibitory glutamatergic mechanisms, respectively. These results for the first time show that dedicated pathways of somatosensation and pain identified by TRPV1 or TRPM8 can target spinal locomotor CPGs.

Modulation of AMPA currents by D(1)-like but not D(2)-like receptors in spinal motoneurons.

Dopamine can modulate and excite spinal locomotor networks, affect afferent transmission and increase motoneuronal excitability. One of the mechanisms whereby dopamine increases motoneuronal excitability is to potentiate AMPA channel-mediated glutamatergic transmission onto motoneurons. However, it is not known which dopaminergic receptor subtypes or the intracellular mechanisms contribute to these effects. In this study, we used whole-cell patch clamp techniques to record chemically evoked AMPA currents in neonatal mouse motoneurons. Bath application of D(1)-like receptor agonist (SKF 39383) increased the AMPA current amplitude and prolonged the decay time constant. In the presence of D(1) receptor antagonist LE300, the effects of DA on AMPA currents were blocked. In contrast, bath-application of the D(2)-like receptor agonist quinpirole did not modulate AMPA currents. In the presence of D(2) receptor antagonist L-741626, dopaminergic modulation of AMPA currents was unaffected. These results suggest that augmentation of AMPA transmission by dopamine is accomplished by D(1) receptor-based mechanisms. This short-term modulation does not appear to involve cycling of AMPA receptor into the membrane, since blocking insertion with botulinum toxin C did not affect the augmentation of AMPA currents after activating D(1) receptors. On the other hand, blocking protein kinase A (PKA) with H-89 completely abolished the effects of D(1) agonists. In addition, we used cell-attached single channel recording to demonstrate that stimulating D(1) receptors increased individual AMPA channel open probability and open duration. Our data demonstrate that dopamine increases the efficacy of glutamatergic transmission onto motoneurons by increasing AMPA conductances via a D(1) PKA-based signaling system.

Modulation of locomotor activity by multiple 5-HT and dopaminergic receptor subtypes in the neonatal mouse spinal cord.

Recently, it has been shown that bath-applied 5-HT can elicit fictive locomotion from perinatal mouse preparations. Since 5-HT acts on multiple receptor subtypes, the focus of this study was to examine which receptor families contribute to the genesis and modulation of locomotor activity. Blockade of 5-HT(2) (ketanserin or N-desmethylclozapine) or 5-HT(7) receptors (SB-269970) could reversibly block or modulate the locomotor-like pattern. A 5-HT(2) agonist (alpha-methyl-5-HT) was shown to be capable of activating the rhythm. Bath application of 5-HT(7) agonists (5-CT) generally led to a tonic increase in neurogram discharge, accompanied by bouts of rhythmic activity. Blockade of dopaminergic receptors (D(1) [R-(+)-SCH-23390 or LE 300]/D(2) [(+/-)-sulpiride or L-741,626] ) could reversibly disrupt the rhythm and most effectively did so when the D(1) and D(2) antagonists were added together. Conversely, 5-HT(2) and D(1)/D(2) agonists can interact to evoke locomotor activity. Overall, our data show that, in the neonatal mouse preparation, 5-HT evoked locomotion is partly dependent on activation of 5-HT(2), 5-HT(7), and dopaminergic receptor subtypes.

Tissue PO2 and the effects of hypoxia on the generation of locomotor-like activity in the in vitro spinal cord of the neonatal mouse.

The neonatal mouse en bloc spinal cord-brainstem preparation used in combination with advances in mouse genomics provides a novel strategy for studying the spinal control of locomotion. How well the mouse en bloc preparation is oxygenated however, is unknown. This is an important consideration given that (a) other superfused mammalian en bloc preparations have anoxic cores and (b) hypoxia can have profound effects on neuronal activity. Here we measure the level of tissue oxygenation in the mouse preparation and determine how neuronal activity within the spinal cord is influenced by poor superfusion and/or low oxygen. To measure tissue oxygenation, oxygen depth profiles were obtained (P0-1 and P2-3; Swiss Webster mice). At P0-1, spinal cords were oxygenated throughout under resting conditions. When fictive locomotor activity was evoked (5-HT 10 microM, dopamine 50 microM, NMA 5 microM), there was a substantial reduction in tissue PO(2) starting within 5 min of drug application. Following washout, the PO(2) slowly returned to control levels over a period of 30 min. The experiments described above were repeated using P2-3 preparations. In this older age group, the spinal cord preparations had a hypoxic/anoxic core that was exacerbated during metabolically demanding tasks such as drug-evoked rhythmic activity. To examine how an anoxic core affects neuronal activity within the spinal cord we either altered the flow-rate or manipulated superfusate PO(2). When the flow-rate was reduced a transient disruption in the rhythmicity of drug-induced locomotion occurred during the first 15 min (P0-1 preparations). However, the motor output adapted and stabilized. During prolonged superfusion with hypoxic artificial cerebrospinal fluid on the other hand, both the motor bursts in spinal nerves and the activity of most neurons near the center of the tissue were abolished.Overall, this study suggests that while oxygenation of P0-P1 preparations is adequate for studies of locomotor function, oxygenation of older preparations is more problematic. Our data also show that neonatal spinal neurons require oxygen to maintain activity; and the spinal locomotor rhythm generator continues to function providing the peripheral tissue of the cord is oxygenated. Together, these results are consistent with the results of a previous study which suggest that the locomotor pattern generator is located close to the surface of the spinal cord.

Adaptive locomotor plasticity in chronic spinal cats after ankle extensors neurectomy.

After lateral gastrocnemius-soleus (LGS) nerve section in intact cats, a rapid locomotor compensation involving synergistic muscles occurs and is accompanied by spinal reflex changes. Only some of these changes are maintained after acute spinalization, indicating the involvement of descending pathways in functional recovery. Here, we address whether the development of these adaptive changes is dependent on descending pathways. The left LGS nerve was cut in three chronic spinal cats. Combined kinematics and electromyographic (EMG) recordings were obtained before and for 8 d after the neurectomy. An increased yield at the ankle was present early after neurectomy and, as in nonspinal cats, was gradually reduced within 8 d. Compensation involved transient changes in step cycle structure and a longer term increase in postcontact medial gastrocnemius (MG) EMG activity. Precontact MG EMG only increased in one of three cats. In a terminal experiment, the influence of group I afferents from MG and LGS on stance duration was measured in two cats. LGS effectiveness at increasing stance duration was largely decreased in both cats. MG effectiveness was only slightly changed: increased in one cat and decreased in another. In cat 3, the plantaris nerve was cut after LGS recovery. The recovery time courses from both neurectomies were similar (p > 0.8), suggesting that this spinal compensation is likely a generalizable adaptive strategy. From a functional perspective, the spinal cord therefore must be considered capable of adaptive locomotor plasticity after motor nerve lesions. This finding is of prime importance to the understanding of functional plasticity after spinal injury.

In vitro resistance to bacterial biofilm formation on coated fluoroplastic tympanostomy tubes.

Bacterial biofilm formation has been implicated in persistent posttympanostomy otorrhea and irreversible tube contamination. The use of a tympanostomy tube with a resistance to biofilm formation by the most common organisms associated with persistent infection may decrease the incidence of chronic otorrhea and the need for tube removal. In this investigation, scanning electron microscopy was used to compare a phosphorylcholine-coated fluoroplastic tympanostomy tube to plain fluoroplastic and silver oxide-impregnated fluoroplastic for resistance to biofilm formation after in vitro incubation with Staphylococcus aureus or Pseudomonas aeruginosa. Only a biofilm from Pseudomonas formed on the untreated fluoroplastic tubes, whereas the silver oxide-impregnated tubes developed biofilms from both S aureus and P aeruginosa. In contrast, the coated fluoroplastic tube showed resistance to both staphylococcal and pseudomonal biofilm adhesion. This is the first study to demonstrate the effect of a surface treatment of fluoroplastic as a method to inhibit biofilm formation by both S aureus and P aeruginosa. This reinforces our previous studies showing that surface-adherence properties such as charge or slickness or both may be more beneficial than antibacterial treatments in preventing film adhesion.

Activity patterns and synaptic organization of ventrally located interneurons in the embryonic chick spinal cord.

To investigate the origin of spontaneous activity in developing spinal networks, we examined the activity patterns and synaptic organization of ventrally located lumbosacral interneurons, including those whose axons project into the ventrolateral funiculus (VLF), in embryonic day 9 (E9)-E12 chick embryos. During spontaneous episodes, rhythmic synaptic potentials were recorded from the VLF and from spinal interneurons that were synchronized, cycle by cycle, with rhythmic ventral root potentials. At the beginning of an episode, ventral root potentials started before the VLF discharge and the firing of individual interneurons. However, pharmacological blockade of recurrent motoneuron collaterals did not prevent or substantially delay interneuron recruitment during spontaneous episodes. The synaptic connections of interneurons were examined by stimulating the VLF and recording the potentials evoked in the ventral roots, in the VLF, or in individual interneurons. Low-intensity stimulation of the VLF evoked a short-latency depolarizing potential in the ventral roots, or in interneurons, that was probably mediated mono- or disynaptically. At higher intensities, long-latency responses were recruited in a highly nonlinear manner, eventually culminating in the activation of an episode. VLF-evoked potentials were reversibly blocked by extracellular Co2+, indicating that they were mediated by chemical synaptic transmission. Collectively, these findings indicate that ventral interneurons are rhythmically active, project to motoneurons, and are likely to be interconnected by recurrent excitatory synaptic connections. This pattern of organization may explain the synchronous activation of spinal neurons and the regenerative activation of spinal networks when provided with a suprathreshold stimulus.

Spontaneous network activity transiently depresses synaptic transmission in the embryonic chick spinal cord.

We examined the effects of spontaneous or evoked episodes of rhythmic activity on synaptic transmission in several spinal pathways of embryonic day 9-12 chick embryos. We compared the amplitude of synaptic potentials evoked by stimulation of the ventrolateral funiculus (VLF), the dorsal or ventral roots, before and after episodes of activity. With the exception of the short-latency responses evoked by dorsal root stimulation, the potentials were briefly potentiated and then reduced for several minutes after an episode of rhythmic activity. Their amplitude progressively recovered in the interval between successive episodes. The lack of post-episode depression in the short-latency component of the dorsal root evoked responses is probably attributable to the absence of firing in cut muscle afferents during an episode of activity. The post-episode depression of VLF-evoked potentials was mimicked by prolonged stimulation of the VLF, subthreshold for an episode of activity. By contrast, antidromically induced motoneuron firing and the accompanying calcium entry did not depress VLF-evoked potentials recorded from the stimulated ventral root. In addition, post-episode depression of VLF-evoked synaptic currents was observed in voltage-clamped spinal neurons. Collectively, these findings suggest that somatic postsynaptic activity and calcium entry are not required for the depression. We propose instead that the mechanism may involve a form of long-lasting activity-induced synaptic depression, possibly a combination of transmitter depletion and ligand-induced changes in the postsynaptic current accompanying transmitter release. This activity-dependent depression appears to be an important mechanism underlying the occurrence of spontaneous activity in developing spinal networks.

Plasticity in reflex pathways controlling stepping in the cat.

Previous studies have shown that stimulation of group 'I' afferents from ankle extensor muscles can prolong the cycle period in decerebrate walking cats and that the magnitude of these effects can be altered after chronic axotomy of the lateral-gastrocnemius/soleus (LGS) nerve. The effectiveness of LGS group I afferents in prolonging the cycle period decreases after axotomy, whereas the effectiveness of the uncut medial-gastrocnemius (MG) group I afferents is increased. The objectives of this investigation were to establish the time course of these changes in effectiveness and to determine whether these changes persist after transection of the spinal cord. The effects of stimulating the LGS and/or MG group I afferents on the cycle period were examined in 22 walking decerebrate animals in which one LGS nerve had been cut for 2 to 31 days. The effectiveness of LGS group I afferents declined progressively in the postaxotomy period, beginning with significant decreases at 3 days and ending close to zero effectiveness at 31 days. Large increases in the effectiveness of MG group I afferents occurred 5 days after axotomy, but there was no progressive change from 5 to 31 days. To test whether these changes in effectiveness were localized to sites within the spinal cord, the cord was transected in some decerebrate animals and stepping induced by the administration of L-DOPA L-3-4 dihydroxyphenylalanine (L-DOPA) and Nialamide. The effects of stimulating the MG and/or the LGS group I afferents on the cycle period were reexamined. In all four animals tested, stimulating the axotomized LGS group I afferents had a reduced effectiveness during locomotor activity in both the decerebrate and spinal states, whereas the increased effectiveness of the MG group I afferents was retained after transection of the spinal cord in two of five animals. Different mechanisms may be responsible for the changes in strength of the LGS and MG group I afferent pathways that project onto the rhythm generating sites in the spinal cord. This possibility follows from our observations of a linear relationship between the time after axotomy and decreased effectiveness of LGS group I afferents but no significant relationship between time postaxotomy and increased effectiveness of MG group I afferents; no significant relationship between the decreased effectiveness of LGS group I afferents and the increased effectiveness of MG group I afferents; and, after spinalization, consistent (4/4 cases) preservation of decreased LGS effectiveness but frequent (3/5 cases) loss of increased MG effectiveness.

Comparison of the effects of stimulating extensor group I afferents on cycle period during walking in conscious and decerebrate cats.

Previous studies have reported that stimulation of group I afferents from extensor muscles prolongs stance duration during walking in decerebrate cats. The main objective of this investigation was to determine whether this phenomenon occurs during walking in conscious cats. In conscious cats without lesions of the central nervous system (CNS), stimulation of group I afferents in the lateral gastrocnemius/soleus (LGS) nerve during stance prolonged extensor burst duration and increased the cycle period in five of seven animals. The mean increases in cycle period were modest, ranging from 6 to 22%. In five of six animals that walked both quadrupedally and bipedally at the same rate, the effects on cycle period were stronger during bipedal stepping (18% mean increase in cycle period compared with 9%). The stimulated nerves were transected and the experimental procedure was usually delayed in the conscious animals for 2-3 days following implantation of the stimulating electrodes. To assess whether chronic axotomy of the LGS nerve was a factor in the decreased effectiveness, four of the cats with chronic nerve section were decerebrated and their LGS nerves were stimulated after the animals began to spontaneously walk on a motorized treadmill. In all four of these animals, the effects of stimulating the chronically cut LGS nerve on the step cycle period became stronger following decerebration. However, these effects were not as strong as those produced when an acutely sectioned LGS nerve was stimulated. During both quadrupedal and bipedal walking, stimulation of the LGS nerve increased the amplitude of the medial gastrocnemius (MG) electromyogram. The augmented activity of the MG muscle contributed to an increased extension of the ankle during stimulated steps. The conclusion from these experiments is that stimulation of the group I afferents in extensor nerves can prolong stance in the conscious cat, but this effect is weaker than in decerebrate animals. It is likely that transmission in the polysynaptic group I pathways controlling stance duration is regulated in a complex fashion by descending signals from the brain in the conscious animal.

Myospherulosis of the upper eyelid as a complication of endoscopic sinus surgery.

Myospherulosis is a chronic inflammatory reaction to the mixture of red blood cells and petroleum based ointments. A literature review does not reveal any cases involving ophthalmic manifestations. We present the first reported case of a patient experiencing recurrent eyelid inflammation from myospherulosis after endoscopic sinus surgery. The pathophysiology and management of myospherulosis are discussed.

Control of locomotion in the decerebrate cat.

Many of the general concepts regarding the control of walking were described years ago by: Sherrington (1906) Integrative Actions of the Nervous System. Yale University Press: New Haven, CT; Sherrington (1910a) Remarks on the reflex mechanism of the step, Brain 33, 1-25; Sherrington (1910b) Flexor-reflex of the limb, crossed extension reflex, and reflex stepping and standing (cat and dog), J. Physiol. (Lond.) 40, 28-121; Sherrington (1931) Quantitative management of contraction in lowest level coordination, Brain 54, 1-28; Graham-Brown (1912) The intrinsic factors in the act of progression in the mammal, Proc. R. Soc. Lond. 84, 308-319; Graham-Brown (1914) On the nature of the fundamental activity of the nervous centres; together with an analysis of the conditioning of rhythmic activity in progression, and a theory of the evolution of function in the nervous system, J. Physiol. 49, 18-46; Graham-Brown (1915) On the activities of the central nervous system of the unborn foetus of the cat, with a discussion of the question whether progression (walking, etc.) is a 'learnt' complex, J. Physiol. 49, 208-215; Graham-Brown (1922) The physiology of stepping, J. Neur. Psychopathol. 3, 112-116. Only in recent years, however, have the mechanisms been analyzed in detail. Quite a few of these mechanisms have been described using the decerebrate cat. Locomotion is initiated in decerebrate cats by activation of the mesencephalic locomotor region (MLR) that activates the medial medullary reticular formation (MRF) which in turn projects axons to the spinal cord which descend within the ventrolateral funiculus (VLF). The MRF region regulates as well as initiates the stepping pattern and is thought to be involved in interlimb coordination. Afferent feedback from proprioceptors and exteroceptors can modify the ongoing locomotor pattern. Recently, the types of afferents responsible for signaling the stance to swing transition have been identified. A general rule states that if the limb is unloaded and the leg is extended, then swing will occur. The afferents that detect unloading of the limb are the Golgi tendon organs and stimulation of these afferents (at group I strengths) prolongs the stance phase in walking cats. The afferents that detect the extension of the leg have been found to be the length- and velocity-sensitive muscle afferents located in flexor muscles. Plasticity of locomotor systems is discussed briefly in this article. Descerebrate animals can adapt locomotor behaviors to respond to new environmental conditions. Oligosynaptic reflex pathways that control locomotion can be recalibrated after injury in a manner that appears to be functionally related to the recovery of the animal.

Contribution of hind limb flexor muscle afferents to the timing of phase transitions in the cat step cycle.

1. In this investigation, we tested the hypothesis that muscle spindle afferents signaling the length of hind-leg flexor muscles are involved in terminating extensor activity and initiating flexion during walking. The hip flexor muscle iliopsoas (IP) and the ankle flexors tibialis anterior (TA) and extensor digitorum longus (EDL) were stretched or vibrated at various phases of the step cycle in spontaneously walking decerebrate cats. Changes in electromyogram amplitude, duration, and timing were then examined. The effects of electrically stimulating group I and II afferents in the nerves to TA and EDL also were examined. 2. Stretch of the individual flexor muscles (IP, TA, or EDL) during the stance phase reduced the duration of extensor activity and promoted the onset of flexor burst activity. The contralateral step cycle also was affected by the stretch, the duration of flexor activity being shortened and extensor activity occurring earlier. Therefore, stretch of the flexor muscles during the stance phase reset the locomotor rhythm to flexion ipsilaterally and extension contralaterally. 3. Results of electrically stimulating the afferents from the TA and EDL muscles suggested that different groups of afferents were responsible for the resetting of the step cycle. Stimulation of the TA nerve reset the locomotor step cycle when the stimulus intensity was in the group II range (2-5 xT). By contrast, stimulation of the EDL nerve generated strong resetting of the step cycle in the range of 1.2-1.4 xT, where primarily the group Ia afferents from the muscle spindles would be activated. 4. Vibration of IP or EDL during stance reduced the duration of the extensor activity by similar amounts to that produced by muscle stretch or by electrical stimulation of EDL at group Ia strengths. This suggests that the group Ia afferents from IP and EDL are capable of resetting the locomotor pattern generator. Vibration of TA did not affect the locomotor rhythm. 5. Stretch of IP or electrical stimulation of TA afferents (5 xT) during the flexion phase did not change the duration of the flexor activity. Stimulation of the EDL nerve at 1.8-5 xT during flexion increased the duration of the flexor activity. In none of our preparations did we observe resetting to extension when the flexor afferents were activated during flexion. 6. We conclude that as the flexor muscles lengthen during the stance phase of gait, their spindle afferents (group Ia afferents for EDL and IP, group II afferents for TA) act to inhibit the spinal center generating extensor activity thus facilitating the initiation of swing.

Plasticity of the extensor group I pathway controlling the stance to swing transition in the cat.

1. This study examines whether the efficacy of polysynaptic group I excitatory pathways to extensor motoneurons are modified after axotomy of a synergistic nerve. Previously, it has been shown that stimulation of extensor nerves at group I strength can extend the stance phase and delay swing. Stimulation of the lateral gastrocnemius and soleus (LG/S) nerve prolongs stance for the duration of the stimulus train, whereas stimulation of the medial gastrocnemius (MG) nerve moderately increases stance. Our hypothesis was that after axotomy of the LG/S nerve the efficacy of the MG group I input would increase. 2. This idea was tested in 10 adult cats that had their left LG/S nerves axotomized for 3-28 days. On the experimental day the cats were decerebrated and the left (experimental) and right (control) LG/S and MG nerves were stimulated during late stance as the animals were walking on a motorized treadmill. A significant increase in the efficacy of the left MG nerve occurred 5 days after axotomy of the LG/S nerve when compared with the control response. By contrast, the previously cut LG/S nerve showed a reduction in efficacy after 3 days compared with the control limb. 3. Functionally, this plasticity may be an important mechanism by which the strength of the group I pathway is calibrated to different loads on the extensor muscles.

Stimulation of the group I extensor afferents prolongs the stance phase in walking cats.

Group I afferents in nerves innervating the lateral gastrocnemius-soleus (LG-Sol), plantaris (Pl), and vastus lateralis/intermedius (VL/VI) muscles were stimulated during walking in decerebrate cats. The stimulus trains were triggered at a fixed delay following the onset of bursts in the medial gastrocnemius muscle. Stimulation of all three nerves with long stimulus trains (> 600 ms) prolonged the extensor bursts and delayed the onset of flexor burst activity. LG-Sol nerve stimulation had the strongest effect; often delaying the onset of flex- or burst activity until the stimulus train was ended. By contrast, flexor bursts were usually initiated before the end of the stimulus train to the Pl and VL/VI nerves. The minimum stimulus strength required to increase the cycle period was between 1.3 x threshold and 1.6 x threshold for all three nerves. Simultaneous stimulation of the Pl and VL/VI nerves produced a larger effect on the cycle period than stimulation of either nerve alone. The spatial summation of inputs from knee and ankle muscles suggests that the excitatory action of the group I afferents during the stance phase is distributed to all leg extensor muscles. Stimulation of the group I afferents in extensor nerves generally produced an increase in the amplitude of the heteronymous extensor EMG towards the end of the stance phase. This increase in amplitude occurred even though there were only weak monosynaptic connections between the stimulated afferents and the motoneurones that innervated these heteronymous muscles. This suggests that the excitation was produced via oligosynaptic projections onto the extensor motoneuronal pool. Stimulation with 300 ms trains during the early part of flexion resulted in abrupt termination of the swing phase and reinitiation of the stance phase of the step cycle. The swing phase resumed coincidently with the stimulus offset. Usually, stimulation of two extensor nerves at group I strengths was required to elicit this effect. We were unable to establish the relative contributions of input from the group Ia and group Ib afferents to prolonging the stance phase. However, we consider it likely that group Ib afferents contribute significantly, since their activation has been shown to prolong extensor burst activity in reduced spinal preparations. Thus, our results add support to the hypothesis that unloading of the hindlimb during late stance is a necessary condition for the initiation of the swing phase in walking animals.

Suppression of the corrective response to loss of ground support by stimulation of extensor group I afferents.

1. When a hind leg of a walking cat fails to contact the ground at the end of swing, the limb is rapidly lifted and replaced in an attempt to seek support. In this investigation we tested the hypothesis that one factor in the initiation of this corrective response is the absence of signals from the group I afferents of extensor muscles. 2. Experiments were performed on decerebrate cats walking on a treadmill. The corrective response to loss of ground support occurred when one hind leg stepped into a hole cut into the treadmill belt. Group I afferents in various extensor nerves were stimulated via implanted cuff electrodes when the foot entered the hole. 3. Stimulation of extensor group I afferents suppressed the corrective flexion response. Instead of a flexor burst being generated soon after the foot entered the hole, extensor activity was maintained for a period exceeding the duration of the stimulus trains. The onset of the corrective response occurred at a relatively fixed latency of 70 ms after the end of the short stimulus trains (200-400 ms) provided that the contralateral limb was still in stance phase. For longer stimulus trains (500-1,000 ms) that terminated during or just prior to the swing phase of the contralateral leg, the onset of ipsilateral flexor activity occurred only after the contralateral leg had regained support. 4. We conclude that the absence of signals from group I afferents when a foot fails to contact the ground allows the locomotor rhythm generator to re-initiate a flexor burst to produce the corrective response.(ABSTRACT TRUNCATED AT 250 WORDS)