Flexor Nerves Research Articles

Postnatal development of node-paranode regions in two hind limb nerves of the cat

Node-paranode regions of large myelinated axons from the nerves to the lateral gastrocnemius muscle (ankle extensor) and the anterior tibial muscle (ankle flexor) were studied in the cat during postnatal development and examined with regard to the occurrence of paranodal Schwann cell Marchi-positive bodies and mitochondria. It was found, in newborn kittens with respect to both parameters, that paranodes of flexor nerve fibers, being part of the functionally more developed ankle flexor reflex arc [cf. Mellström, A. (1971). Acta Physiol. Scand., 82, 477–489], appeared more mature than did those of extensor nerve fibers, which are part of the less developed ankle extensor reflex arc. It is concluded that the maturation of large feline hind limb muscle nerve fibers runs through a “nodalization” process similar to that described earlier for feline lumbar spinal root fibers [cf. Berthold, C.-H. (1973). Neurobiology, 3, 339–352] and that this normally occurring, rather striking remodeling of the node-paranode regions is likely to be functionally significant.

Activity of lumbar interneurons during late long-lasting discharges in motor nerves of immobilized thalamic cats

Activity of lumbar spinal neurons was recorded extracellularly during late long-lasting discharges in efferent nerves in immobilized thalamic cats. Of the total number of cells tested, 70% changed their activity during late discharges. The activity of 35% of neurons was increased during late discharges in nerves to flexors, but inhibited during discharges in nerves to extensors. Responses of 27% of neurons were of the opposite character. Other neurons were found whose activity was increased (5%) and reduced (3%), respectively, during later discharges in both flexor and extensor nerves. Most interneurons which changed their activity during late discharges were located in lateral parts of the intermediate zone of gray matter and the ventral horn at a depth of 2.8 mm. The character of the afferent input to a neuron was found to depend on the late efferent discharges and activity of the neurons correlated with them. Neurons whose activity was unchanged during late discharges (30%) were mainly located rather more dorsally, at a depth of about 2.0 mm. The possible mechanisms of the participation of these groups of interneurons in the generation of late discharges are discussed.

The regression of synapses formed by a foreign nerve in a mature axolotl striated muscle

A study has been made of the factors which determine that the terminals of a foreign flexor nerve at synaptic sites in a urodele extensor muscle regress on return of the original extensor nerve. The quantal content (m¯) of the endplate potential (EPP) at flexor nerve terminal synapses, during innervation of a previously denervated extensor muscle, increased in about 8 weeks to reach the same size as at normal extensor nerve terminals; the same time was taken form¯to reach normal size at extensor nerve terminals when these reinnervated their own muscle. At flexor nerve terminals,m¯decreased eventually to zero if the extensor nerve terminals returned within about 6 weeks of synapse formation by the flexor nerve terminals to the same or an immediately adjacent synaptic site to that occupied by these terminals. During this decrease inm¯at flexor nerve terminals, stimulation of the flexor nerves in the presence of horseradish peroxidase (HRP) showed HRP-labelled flexor nerve terminals present only in those regions of the extensor muscle in which the electrical signs of flexor nerve terminals were observed, indicating that the decrease inm¯at regressing flexor terminals was accompanied by their vacating synaptic sites. However, flexor nerve terminals failed to regress from the extensor muscle on return of the original nerve supply if the flexor nerve was allowed to form synapses for more than about 10 weeks before the return of the extensor nerve.

Periodic changes in primary afferent depolarization during fictitious locomotion by thalamic cats

Dorsal root and cord dorsum potentials were recorded in lumbar segments L6, L7 during fictitious locomotion of thalamic immobilized cats. Negative waves in the dorsal root potential and respectively positive waves in the cord dorsum potential appeared during periodic discharges in the ankle flexor nerve. The conclusion is made that central locomotor generator activity modulates the level of primary afferent depolarization.

The ventral spino-oiivocerebellar system in the cat IV. Spinal transmission after administration of clonidine and L-Dopa

1. The transmission from the flexor reflex afferents (FRA) to the spino-olivocerebellar paths ascending through the ventral funiculus (VF-SOCPs; Oscarsson and Sjölund, 1977) was compared with the transmission from the FRA to segmental reflex arcs in cats with the spinal cord transected in the third cervical segment sparing only the ventral funiculus on one side. The climbing fiber responses evoked in Purkinje cells by electrical stimulation of limb nerves were monitored by recording the mass activity at the cerebellar surface simultaneously from several termination zones, while the flexion reflex was recorded from a flexor nerve and the primary afferent depolarization (PAD) from a dorsal root filament. 2. Changes in the segmental reflex responses were produced by i.v. injections of l-Dopa and Clonidine. L-Dopa produced the expected depression of the flexion reflex and the FRA-induced PAD in the FRA, whereas Clonidine depressed only the flexion reflex but did not affect or even enhanced the FRA-induced PAD in the FRA. 3. The changes in transmission to one of the ascending paths, the a-VF-SOCP, parallelled that in the flexion reflex arc. The transmission to another path, the b2-VF-SOCP, parallelled that in the pathway responsible for the FRA-induced PAD in the FRA. 4. The remaining hindlimb paths, the c1- and c3-VF-SOCPs, monosynaptically activated from primary afferents, were little influenced by the changes induced in the segmental reflex arcs. 5. It is suggested that the a- and b2-VF-SOCPs carry information related to interneuronal activity in segmental centers, whereas the c1- and c3-VF-SOCPs forward information mainly related to peripheral events.

Presynaptic and long-lasting postsynaptic inhibition during penicillin-induced spinal seizures.

In experiments on seven cats we tested the hypothesis that the epileptogenic effect of penicillin (PCN) on the spinal cord is mediated by a reduction of presynaptic inhibition. PCN-induced spinal hyperactivity was not associated with changes in either the presynaptic inhibition of extensor monosynaptic reflexes by conditioning volleys in flexor muscle nerves, or in evoked dorsal root potentials. Long-lasting inhibition of monosynaptic reflexes by repetitive cutaneous nerve volleys, shown by intracellular recording to be associated with prolonged inhibitory postsynaptic potentials (IPSPs), was also not changed by PCN. Antagonism of either pre- or postsynaptic spinal inhibition is not a necessary condition for induction of spinal seizures by PCN.

Peripheral excitatory input to substantia nigra

In chloralose-anesthetized cats, unitary responses were recorded from the substantia nigra after electrical and natural stimulation of the extremities. Forty-eight nigral neurons from a total of 67 in the pars compacta as well as in the pars reticulata responded to electrical stimulation of either paws or dissected peripheral nerves with a latency of about 15 to 20 ms. Bilateral convergent responses were generally observed with some preferential side for a given neuron. It was revealed that electrical stimulation of the flexor nerve or cutaneous nerve was far more effective in inducing nigral response. It was also demonstrated that passive flexion of the forearm increased the spontaneous discharge whereas passive extension suppressed it. These nigral reponses to peripheral stimulation were still recorded after extensive lesion of the diencephalon, thus interrupting all nigral afferent fibers from higher centers. The functional role of the flexor afferent projection of the substantia nigra is discussed.

The ventral spino-olivocerebellar system in the cat. V. Supraspinal control of spinal transmission

1. The transmission from the flexor reflex afferents (FRA) and from tracts running in the ipsilateral half of the spinal cord to the spino-olivocerebellar paths ascending through the ventral funiculus (VF-SOCPs; Oscarsson and Sjölund, 1977) was compared with the transmission from these sources to segmental reflex arcs. The climbing fibre responses evoked in Purkinje cells by electrical stimulation of limb nerves and spinal tracts were monitored by recording the mass activity at the cerebellar surface simultaneously from several termination zones, while the activity in flexor motoneurones was recorded from a flexor nerve and the primary afferent depolarization from a dorsal filament. 2. Changes in the segmental reflex response were produced by release from the tonic inhibition of transmission from the FRA in decerebrate preparations and by conditioning electrical stimulation of dissected spinal funiculi containing inhibitory descending tracts. 3. The changes of the transmission from the FRA to two of the paths, the a- and b2-VF-SOCPs, parallelled the changes of the transmission to the segmental reflex arcs. On the other hand, the monosynaptic transmission from the FRA to the c1- and c3-VF-SOCPs was not significantly influenced by the inhibitory descending control systems. 4. The a- and b2-VF-SOCPs but not the c1- and c3-VF-SOCPs received polysynaptic excitation from tracts running in the ipsilateral half of the spinal cord. 5. The suggestion that the a- and b2-VF-SOCPs carry information related to interneuronal activity in segmental centres, whereas the c1- and c3-VF-SOCPs forward information mainly related to peripheral events (Andersson and Sjölund, 1978) is supported by the present findings.

Formation and elimination of foreign synapses on adult salamander muscle

1. Synapses by flexor nerve were induced on denervated extensor muscle in adult salamander forelimbs. Excitatory potentials evoked by these ;foreign' synapses were at first small but increased to normal amplitude within several weeks, in the absence of correct nerve reinnervation.2. Upon return of the correct nerve the efficacy of foreign synaptic transmission began to decline. The time of initiation of this decline correlated well with the resumption of correct nerve transmission. The suppression of foreign transmission involved a reduction in mean quantal content of transmitter release.3. Suppression of foreign synapses was sufficiently thorough that most ceased transmitting entirely. Before reinnervation by the correct nerve 97% of the extensor muscle fibres received functional foreign synapses while 4-6 months after correct nerve return only 35% of the fibres retained foreign synapses, with weak transmission.4. Two lines of evidence indicate that suppressed foreign synapses are lost from the muscle: (a) a second correct nerve lesion 6-8 months after the initial denervation produced no significant increase in the proportion of fibres with foreign transmission and (b) four muscles which showed complete suppression of foreign transmission were bathed in medium containing horseradish peroxidase (h.r.p.) and the correct nerve was stimulated repetitively. Subsequent histochemical staining for h.r.p. and examination of synapses by electron microscopy revealed that 94% of the axon terminals had h.r.p. incorporated in vesicles. Thus at least that percentage of all identifiable synapses were from the correct nerve.5. This ability to eliminate incorrect synapses in favour of correct ones is speculated to be a general characteristic of embryonic nervous systems, which in adulthood is retained by salamanders but lost by most other animals.

A determination of excitability changes in dorsal spinocerebellar tract neurons from spike-train analysis.

1. Responses of DSCT neurons to random electrical stimulation of peripheral nerves of the hindleg at group I intensity were studied using cross-correlation analysis of the output spike train with the stimulus. Three types of response were found: type 1 was due to monosynaptic activation of DSCT cells, type 2 resulted from inhibition of those cells, and type 3 was due to a long-latency excitation that was probably polysynaptic. 2. Most of the units studied responded to stimulation of both proximal and distal flexor and extensor nerves. The extensive convergence of afferent input on DSCT cells is much greater than has been observed previously, with type 2 and type 3 responses totaling 80% of the observed responses. We attribute this to the sensitivity of the analysis in detecting small changes in postsynaptic excitability. 3. The results of the study, particularly the derivation of postsynaptic excitability changes, generally confirm those of earlier work employing intracellular recording. 4. By varying stimulus rate and stimulus intensity in the group 1 range and simulating the resulting correlations, we conclude that excitability changes in DSCT cells are the net result of complex interactions involving excitation and inhibition. A summary of these findings is presented as a model for the minimum circuitry necessary to account for the observed behavior.

Convergence on interneurones mediating the reciprocal Ia inhibition of motoneurones. II. Effects from segmental flexor reflex pathways.

Interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as "Ia inhibitory interneurones") were recorded in the lumbar spinal cord of the cat. Volleys in ipsilateral and contralateral high threshold muscle afferents, cutaneous and high threshold joint afferents evoked a mixture of polysynaptic excitation and inhibition. These effects were ascribed to pathways activated by flexor reflex afferents (FRA) and in addition a specific ipsilateral low threshold cutaneous pathway. Ia inhibitory interneurones excited monosynaptically from flexor nerves received stronger net excitation by volleys in ipsilateral FRA than did extensor coupled interneurones, while the opposite pattern was seen from the contralateral FRA. These patterns are similar to those found in flexor and extensor motoneurones respectivey. The FRA inhibition in Ia inhibitory interneurones was partly mediated by "opposite" Ia inhibitory interneurones, i.e. those which are mediating the Ia inhibition of Ia inhibitory interneurones. The extent to which the FRA inhibition is transmitted by Ia inhibitory interneurones was roughly estimated by its susceptibility to recurrent depression by antidromic ventral root stimulation. The main conclusion is that most segmental pathways seem to evoke their effects in parallel to motoneurones and Ia inhibitory interneurones which are monosynaptically linked to the same muscle. The functional importance of this conclusion is discussed in a following report.

Effects from the Vestibulospinal Tract on Transmission from Primary Afferents to Ventral Spino‐cerebellar Tract Neurones

Convergence of vestibulospinal and segmental effects onto spinal interneurones which project to the ventral spino-cerebellar tract (VSCT) neurones has been studied by intracellular recording in VSCT cells. The disynaptic Ia IPSPs evoked in a group of VSCT neurones from the quadriceps nerve are monosynaptically facilitated by the vestibulospinal tract while there was no facilitation of Ia IPSP evoked from a flexor nerve. These results support the view that Ia inhibition to VSCT cells and motoneurones is mediated by common interneurones. The disynaptic inhibition evoked in other VSCT cells from the vestibulospinal tract is facilitated by volleys in the contralateral flexor reflex afferents (FRA) or bilaterally from the FRA. It is postulated that these actions are mediated by collaterals of the interneurones responsible for the analogous effects in motoneurones. Findings are reported suggesting that the monosynaptic vestibulospinal EPSP in VSCT cells in most cases is collateral to the excitatory input to the last order interneurones of reflex pathways from the FRA to motoneurones and only exceptionally to the corresponding input to Ia inhibitory interneurones. In many VSCT cells the vestibulospinal tract evoked disynaptic EPSPs which are facilitated from the FRA; the functional significance of this action is uncertain. The results are consistent with the hypothesis that VSCT neurones signal information on interneuronal transmission to motoneurones.

LACERATIONS OF THE PLANTAR ASPECT OF THE FOOT

The absence of any known reference in the medical literature to lacerations of the plantar aspect of the foot prompted the author to send an questionnaire to members of the American Orthopaedic Foot Society and Association of Bone and Joint Surgeons asking each one to outline his treatment for various combinations of lacerations of the flexor tendons and plantar nerves in a group of patients with varied ages and occupations. To simplify results, only clean wounds satisfactory for immediate surgery were considered. A majority of surgeons preferred primary repair of a lacerated flexor hallucis longus tendon or extensive laceration of all flexor tendons and nerves but were equally divided between repair and non-repair of flexor tendons and plantar nerves of the middle three toes. A majority of surgeons preferred not to repair isolated lacerations of flexor tendons other than the flexor hallucis longus. Most surgeons electing to repair the structures preferred to approach them through the original plantar laceration rather than through an accessory incision in the non weight-bearing portion of the foot.

Area specific reflexes from normal and supernumerary hindlimbs of Xenopus laevis.

Two area specific reflexes elicited by natural stimulation of different regions of the hindlimbs of Xenopus laevis have been identified. Light or intense mechanical stimulation of the foot evokes reflex activity in the ipsilateral knee flexor nerve; moderate pressure applied to the calf evokes reflex activity predominantly in the ipsilateral knee extensor nerve. The reflex responses have been recorded electrophysiologically to overcome the limitations of behavioral observations for determining the presence of activity in particular muscles. Normal area specific reflexes are elicited in the normal ipsilateral hindlimb by stimulation of grafted supernumerary hindlimbs innervated either by hindlimb (lumbar) or by non-limb (thoracic) spinal cord segments. The area specific reflexes can be elicited only if the limb is grafted to a host younger than stage 54-55 of Nieuwkoop and Faber ('56), the stage at which reflex movements are first observed behaviorally. Abnormal reflex responses are evoked by stimulation of supernumerary limbs innervated by either thoracic or lumbar segments when the limb buds are grafted to older larvae. Supernumerary forelimbs grafted at early stages and innervated by either thoracic or lumbar spinal cord segments generally fail to elicit area specific reflex responses in the normal hindlimb. Single-unit recordings of afferent fibers supplying the normal and supernumerary hindlimbs show that each limb receives a separate nerve supply. No evidence for branched afferent fibers has been found. The implications of these results for theories of neuronal specification are discussed, particularly the hypothesis that peripheral tissues are able to specify the central actions of afferent fibers that innervate them.

The spinal course and distribution of fore and hind limb muscle afferent projections to the superior colliculus of the cat.

1. Projections to the superior colliculus from fore and hind limb muscle nerves have been examined. 2. Hind leg nerve stimulation at strengths sufficient to excite Group II and III fibres elicited unit discharge in all layers of the superior colliculus and underlying tegmentum. 3. Forelimb nerve stimulation excited units in all layers of the superior colliculus and underlying tegmentum. Twenty per cent were activated by stimulation below Group II threshold, the remainder by stimulation above Group II threshold. 4. Most units activated by limb afferent nerve stimulation were also activated by visual stimuli. There was considerable convergence from both flexor and extensor nerves and most units were excited by stimulation of nerves in more than one limb. 5. Units fired either with a short or long latency or with a paired discharge. Short latency units were most common in deep collicular layers and the tegmentum, long latency units were most common in superficial layers. 6. The spinal pathway of the afferents is mainly in the dorsolateral quadrant contralateral to the recording site. The anatomical characteristics of the pathway are similar to the lateral funiculus climbing fibrespinocerebellar pathway.

Effects of forward and backward classical conditioning procedures on a spinal cat hind-limb flexor nerve response

Previous studies assessing the effects of classical conditioning paradigms on spinal reflex responses have not assessed the possible consequences of simple stimulus overlap on any observed effects. The present study utilized both forward and backward stimulus pairings in a classical conditioning paradigm with the hind-limb flexor-nerve response in the cat to assess the effects of simple stimulus summation on response amplitude. Negative results in the backward groups and increased response amplitude in the forward groups suggested the necessity of CS-US forward pairings for the observed effect. The results are discussed in relation to the predictions of both “stimulus trace” and informational hypotheses for the spinal response system.

Functional organization of long ascending propriospinal pathways linking lumbo-sacral and cervical segments in the cat

In high spinal cats propriospinal pathways ascending from lumbo-sacral levels of the spinal cord can mediate strong excitatory and inhibitory changes in reflexes to different groups of motoneurones supplying muscles of the forelimb. Discharges evoked by electrical stimulation of hindlimb nerves could be evoked in 41% of experiments in the motoneurones of pectoralis major and minor. The latency of the discharge (8–18 msec) could be shortened by increasing the repetition frequency of the stimulus, the greatest reduction occurring in the range 1–4 Hz. Contralateral hindlimb nerves were less effective and the discharge generally occurred at a latency 1–2 msec longer than for ipsilateral nerves. Monosynaptic reflexes to pectoralis major and deep radial motoneurones supplying the physiological flexor muscles were strongly facilitated by hindlimb nerve stimulation, ipsilateral nerves being more effective than contralateral. Monosynaptic reflexes to latissimus dorsi showed a reciprocal pattern of conditioning, being depressed by ipsilateral and facilitated by contralateral hindlimb extensor nerves, the flexor nerves giving the reverse pattern. Monosynaptic reflexes to median and ulnar nerves supplying physiological extensor muscles were not significantly affected by hindlimb nerve conditioning. Polysynaptic reflexes to pectoralis major and deep radial motoneurones received initial strong facilitation followed by prolonged depression, ipsilateral hindlimb nerves being more effective than contralateral. In latissimus dorsi a reciprocal pattern similar to that for monosynaptic reflex testing was found. Polysynaptic reflexes to median and ulnar motoneurones received only prolonged depression. The hindlimb afferent nerves responsible for the discharge in forelimb motoneurones and for the facilitation and depression of forelimb reflexes include groups II and II muscle afferents and group II skin afferents, especially from quadriceps and sartorius muscles, and sural and superficial peroneal nerves, respectively. The ascending long propriospinal pathways are influenced bilaterally from hindlimb nerves and are located in the lower thoracic segments in the ventrolateral funiculus. The pathways mediate effects on ipsilateral and contralateral forelimb reflex systems, the ipsilateral projections being dominant. Part of the long ascending projection terminates monosynaptically on the motoneurones of pectoralis major. It is likely that group II afferents from ipsilateral quadriceps muscle activate the ascending tract monosynaptically and those from contralateral quadriceps disynaptically. The hypothesis is suggested that long propriospinal paths primarily represent intrinsic links between hindlimb and forelimb ‘motor centres’. The pattern of long ascending influences to groups of forelimb motoneurones corresponds closely to the sequences of hindlimb and forelimb stepping observed in normal cats. A functional role in stepping is therefore proposed for long ascending propriospinal pathways.

Field potentials in the lumbar ventral horn during combined extrapyramidal and cortical stimulation

Field potentials arising in the medial (Rexed's layer VIII) and lateral (region of the flexor motoneuronal nuclei) zones of the ventral horn during stimulation of the motor cortex or ventromedial funiculus in the cervical spinal cord were recorded in experiments on cats. Inhibition of the electrical response in the flexor nerve to stimulation of the contralateral cortex, an indicator of the cortical motor response, can be evoked by stimulation not only of the symmetrical point of the opposite cortex but also of the group of fibers of the ventromedial funiculus connected functionally with that point. In each experiment the fact that these fibers belonged functionally to the system conducting cortically evoked descending influences was verified in each experiment from a combination of specific criteria. Extrapyramidal inhibition of the cortical motor response is accompanied by the same changes in field potential in the medial and lateral zones of the ventral horn as cortical inhibition: by an increase in negativity in the first and by its suppression in the second. The differences between the two types of interaction are concerned chiefly with the temporal course of all the processes studied. The results are used as evidence of the extrapyramidal nature of cortical inhibition of the cortical motor response.

Effects of a classical conditioning paradigm on hind-limb flexor nerve response in immobilized spinal cats.

Effects of a classical conditioning paradigm on hind-limb flexor nerve response in immobilized spinal cats.

Evoked activity in the ventral horn of the lumbar segments of the spinal cord during cortical inhibition of cortical motor responses

Electrical activity in the flexor nerve and focal potentials (FP) in the medial and lateral zones of the ventral horn (VH) of segments L6 and L7 of the spinal cord, evoked by excitation of the contralateral motor cortex, were recorded in delicate experiments on cats. These focal potentials were studied during inhibition of the flexor response that developed as a result of prior excitation of the ipsilateral cortex ("cortical inhibition"). During the inhibition the FP's of the medial zone (layer VIII, according to Rexed) were greatly increased, mainly in their negative components, their time-characteristics being altered. When the interval between excitations was 50 msec (in that case the inhibition was most pronounced) the medial FP's arose against a negative background, which was a late component of the previous activity evoked by conditioning excitation. The appearance of this late component was correlated with the development of inhibition of the cortical flexor response. At the same time a positive condition developed in the lateral zone, in the region of the nucleus biceps-semitendinosus, which indicated orientation in a lateral direction of the interneurons discharging in the medial zone at late periods after the conditioning excitation. Inhibition of the flexor response was accompanied by depression of the lateral FP's without change in their sign or in the time-structure of their components. It is suggested that cortical inhibition of the cortical flexor response arises at the interneuron level. The functional structure of that inhibitory pathway is discussed.