Ia Fibers Research Articles

Inhibitory effects on flexor reflexes in patients with a complete spinal cord lesion.

The inhibitory effects on flexors of electrical stimulation of a distal peripheral nerve were investigated in 7 paraplegic patients having a complete spinal cord section. The stimuli (3-50 mA) were applied to the sural nerve. Their effects were investigated on: 1) the ipsi- and contralateral H reflex of the Tibialis Anterior (TA); 2) the continuous EMG activity reflexly elicited in TA by a sustained pinch of the foot and 3) on the reflexes evoked in TA by contralateral sural nerve stimulation. Sural nerve stimulation induced two peaks of facilitation of the ipsilateral TA H reflex that could be replaced by inhibition as the stimulus intensity was increased. The comparison of the effect on H reflexes and the EMG activity suggests presynaptic inhibition of Ia fibres at time intervals longer than 300 ms. The stimulation could depress the sustained EMG reflex activity and induce a period of silence whose duration increased with the intensity of the stimulation. As shown in a previous study, a sural nerve stimulation induced a reflex in TA with a prolonged (more than 130 ms) latency. This late reflex could be selectively inhibited by a contralateral sural nerve stimulation, probably at an interneuronal level. These results confirm that the late reflex in TA is similar to the one observed after Flexor Reflex Afferent (FRA) stimulation in the acute spinal cat with DOPA. In addition, they show that at least some part of the "half centre" organization which has been described in the acute spinal cat with DOPA is also present in the human spinal cord chronically deprived of supraspinal control.

Identifying the afferents involved in movement-induced pain alleviation in man.

It has been clearly established that the perception of nociceptive stimulus decreases in intensity when movement is initiated in the part of the body to which the stimulus is applied. The pain alleviation is probably at least partly due to the activation of afferents which occurs during the movement. The present experiments were carried out with a view to investigating which groups of afferent fibres is mainly responsible for the gating of the nociceptive messages which occurs during movements. In eight volunteers we investigated the changes in the amplitude of the nociceptive leg flexion reflex (RIII) when the subjects were at rest, when they were performing active or passive ankle movements and when the spindle proprioceptive pathway was mobilized by applying vibratory stimulation to the Achilles tendon. Similar experiments were also carried out with eight other volunteers after anaesthetizing the ankle skin mechanoreceptors. This method was chosen because a clear-cut correlation is known to exist between the amplitude of the nociceptive motor reflex and the intensity of the pain perceived by the subject. The data obtained clearly show that anaesthesia of cutaneous mechanoreceptors connected to the large diameter afferent fibres prevented the decrease in the motor response which otherwise accompanied both active and passive movements. Activation of the Ia fibre group by tendon vibration resulted on the contrary in an increase in the amplitude of the motor response. Movement-induced pain alleviation therefore does not mainly involve the activation of the Ia group of fibres.

The H-reflex to magnetic stimulation of lower-limb nerves.

We elicited H-reflexes by magnetic and electrical stimulation of several different nerves in 10 healthy subjects and two patients with S-1 radiculopathy. The posterior tibial nerve at the popliteal fossa and the femoral nerve at the inguinal ligament were tested with both electrical and magnetic stimulation; the proximal sciatic nerve was tested only with magnetic stimulation. Muscle activity was recorded from the soleus muscle for posterior tibial and sciatic nerve stimulation and from the vastus medialis muscle for femoral nerve stimulation. No significant difference was found between the latency of H-reflexes evoked by magnetic or electrical stimulation. With magnetic stimulation, the mean (+/- SD) Ia sensory fiber conduction velocity in the proximal segment of the sciatic nerve was 72.4 +/- 3.3 m/s, while the motor nerve fiber conduction velocity in the same portion of the nerve was significantly slower, at 60.6 +/- 2.0 m/s. In two patients with unilateral S-1 radiculopathy, the latency of the H-reflex from the soleus muscle to both magnetic and electrical stimulation of the posterior tibial nerve was absent or prolonged on the affected side. Magnetic stimulation can be used to study the H-reflex and Ia fiber conduction velocity and is particularly advantageous when testing deeply located nerve trunks.

Percutaneous cervical stimulation: effects on intraspinal structures

Changes in the amplitude of motor evoked potentials (MEPs) from percutaneous cervical stimulation (PCS) obtained at rest in the thenar muscles, and smaller than 0.8 mV, were studied under 3 different experimental conditions. A significant enhancement was observed mainly with a conditioning subthreshold transcranial stimulus and when MEPs were obtained in coincidence with weak voluntary contraction of the target muscle. Subthreshold stimulation of Ia fibers of the median nerve seemed to have a smaller facilitatory effect. It is generally accepted that PCS excites the spinal motoneuron (SMN) axons at the spinal nerve. However, our results show that other SMNs, usually not recruited, may be triggered by PCS when they receive excitatory postsynaptic potentials from the pyramidal tract (PT) or Ia fibers. This behavior suggests that low intensity PCS also exerts subthreshold excitation of the PT fibers and, perhaps, of the incoming spindle afferents, which adds its effects to the conditioning stimuli.

The influence of contralateral primary afferents on Ia inhibitory interneurones in humans.

1. Contralateral influences on short latency reciprocal inhibition between wrist extensor and flexor muscles were investigated in twenty-two healthy volunteers. Reciprocal inhibition, probably mediated through the Ia inhibitory interneurone, was measured by conditioning the flexor carpi radialis (FCR) H reflex by weak stimulation of the ipsilateral radial nerve. Maximum reciprocal inhibition occurring at a precise delay between conditioning and conditioned stimulations was taken as the test level of inhibition. 2. Contralateral median or radial nerves were stimulated at short intervals before the onset of reciprocal inhibition. The latter was increased by 8.6% after median nerve stimulation and decreased by 16.5% after radial nerve stimulation. 3. The contribution of sensory fibres in the two nerves to contralateral effects was investigated by stimulating purely sensory branches of the nerves. No clear modification of the contralateral reciprocal inhibition was observed. The effects produced by mixed nerve stimulation are thus likely to have been mediated by Ia fibres. 4. In three hemiplegic patients where reciprocal inhibition was reduced unilaterally, stimulation on the spastic side produced contralateral effects similar to those observed in normal subjects. This result indicates that contralateral effects are not mediated through the Ia inhibitory interneurone ipsilateral to the conditioning stimulus. 5. Since contralateral effects occur after short delays (2 ms, median nerve; 3 ms, radial nerve), we suggest a functional scheme in which the excitability of Ia inhibitory interneurones is modified by contralateral primary afferents via the interneurones activated by group I fibres, probably Ia fibres. The short delays indicate that the interneurone transmitting primary afferent influences to the contralateral side is probably excitatory.

Effects of stimulation of group I afferents from flexor muscles on heterosynaptic facilitation of monosynaptic reflexes produced by Ia and descending inputs: a test for presynaptic inhibition

1. In the chloralose anesthetized cat, conditioning stimulation of group I flexor afferents depresses the monosynaptic potentials generated by Ia afferents in single spinal motoneurons or in populations of motoneurons without affecting the monosynaptic potentials produced by stimulation of descending fibers in the ipsilateral ventromedial fasciculus (VMF). 2. Heterosynaptic facilitation of monosynaptic reflexes was used to test changes in the presynaptic effectiveness of excitatory inputs with direct connections with motoneurons. We found that the heterosynaptic facilitation of Ia origin was reduced by conditioning stimulation of group I afferents from flexors, without affecting the heterosynaptic facilitation produced by stimulation of the VMF. 3. These results confirm and expand previous observations showing that the synaptic effectiveness of descending fibers synapsing with motoneurons is not subjected to a presynaptic control mechanism of the type acting on Ia fiber terminals, and provide further basis for the use of changes in heterosynaptic facilitation of monosynaptic reflexes of Ia origin as an estimate of changes in presynaptic inhibition of Ia fibers (Hultborn et al. 1987a).

Interaction of baseline synaptic noise and Ia EPSPs: evidence for appreciable negative correlation under physiological conditions

1. In the anesthetized cat, simultaneous intracellular recordings from pairs of spinal motoneurons were undertaken to see whether the amplitude of single-fiber excitatory postsynaptic potentials (EPSPs) in both cells fluctuated in a coordinated manner that would indicate correlative mechanisms at either pre- or post-synaptic level. Although these recordings revealed correlated fluctuations in the baseline, the single-fiber Ia/EPSPs recorded with the spike-triggered averaging technique exhibited no correlated fluctuations and, unexpectedly, virtually no increase in baseline variance associated with the EPSP. However, the fact that these experiments were carried out under conditions of high baseline synaptic noise (i.e., with muscle stretch) may have influenced the outcome because of interaction between EPSP and synaptic noise, and this possibility was evaluated explicitly. 2. A given connection was studied under low noise by electrically stimulating a single Ia fiber in the absence of muscle stretch. The same connection was analyzed under conditions of high noise by activating the fiber and all other stretch receptor afferents with muscle stretch and by using spike-triggered averaging to extract the EPSP. The differences in mean EPSP amplitude at a given connection under conditions of low noise and high noise were minimal. 3. Fluctuations in EPSP amplitude were then determined to see whether these were influenced by presence of baseline synaptic noise and whether the interaction was nonlinear. Two methods were used to measure EPSP fluctuations: measurement of the variance associated with the EPSP, and determination by the use of deconvolution methods of the discrete amplitude components associated with the EPSP. 4. An increase in baseline variance was observed during the EPSP evoked under low noise conditions at all six connections studied in this way. This increase disappeared at two of these connections when examined under high noise. This may help to explain the results obtained in pairs of motoneurons. 5. The deconvolution results were used to calculate the variance of the noise-free EPSP. This was found to differ from the variance of the EPSP amplitude distribution measured directly from the change in baseline variance associated with the EPSP. Analytic techniques suggested that this difference could be explained in most cases by negative correlation between the EPSP and baseline synaptic noise. These considerations led to an analytic method to assess the reliability of the deconvolution result. 6. Simulation studies revealed that the baseline variance increase associated with the EPSP is also highly dependent on the correlation between signal and noise.(ABSTRACT TRUNCATED AT 400 WORDS)

Cortical projection of putative group Ib afferent fibres from the human forearm

The possibility was investigated that impulses in group Ib afferents from fore-arm flexors have access to cerebral cortex in man. A long-lasting increase in the threshold (Th) of group Ia afferent fibres from flexor carpi radialis muscle (Fcr) was obtained after prolonged (100 Hz for 20 min) tendon vibration at the wrist. It was assumed that under this condition a weak electrical stimulation of the median nerve at the elbow, insufficient to reactive Ia fibres because of the rise in their threshold (as verified by the method of homonymous Ia facilitation of Fcr H reflex), engaged Ib fibres only. Peripheral volleys at Erb's point and cerebral cortical potentials to median nerve stimulation at the elbow were evoked before and after prolonged vibration of Fcr. During 10–30 min after the end of vibration, in which homonymous facilitation of Fcr H reflex was abolished, both the ongoing peripheral volley and the cortical responses were markedly reduced with respect to their control values. Recovery to pre-vibration control amplitudes coincided with recovery of Fcr H reflex homonymous facilitation. In order to verify if activity in afferents other than group Ib fibres might contribute to the cortical response after vibration, specially designed experiments were also performed. It is concluded that the cortical wave recorded after 20 min vibration represents the arrival of Ib impulses from flexor carpi radialis to the human cerebral cortex.

Crossed and uncrossed projections to cat sacrocaudal spinal cord: II. Axons from muscle spindle primary endings

The terminal fields of primary afferent fibers from tail muscle spindle primary endings were mapped within cat sacrocaudal spinal cord (S3-Ca7), using intra-axonal recording and horseradish peroxidase staining techniques. We sought to determine the ipsilateral and contralateral projection patterns and to relate these to the fibers' muscles of origin. Fifty-three group Ia fibers were successfully stained. Segmental collaterals originated from either the ascending or descending branch within the dorsal columns. Collaterals coursed rostromedially within the dorsal columns and traversed the medial aspect of the dorsal horn. Ipsilateral terminations were similar for all fibers. Within the ventral horn, boutons were consistently observed in the medial or central portions of lamina VII. In lamina VIII, a variable number of boutons was seen on fine branches emerging from larger fibers coursing ventrally. Clusters of terminals were plentiful in the regions of motoneurons, i.e., lamina IX and the nucleus commissuralis. Terminals were found in the adjacent white matter. In addition to ipsilateral terminations, some group Ia fibers (20 of 53) had collateral branches that crossed ventrally to the central canal, terminating within the midline ventral gray commissure and/or the contralateral ventral horn. Crossed projections always originated in medial (dorsal or ventral), but not lateral, muscles of the tail. These data suggest that ipsilateral projections of group Ia fibers make connections on sacrocaudal motoneurons, on neurons mediating segmental reflex functions and on neurons conveying ascending information. It is speculated that crossed and uncrossed connections between group Ia fibers from medial muscles and bilateral dendritic trees of motoneurons subserve synchronized co-contraction of synergistic muscles located on the two sides of the body, such as with dorsal or ventral flexion of the tail. Group Ia projections from lateral muscles, that are entirely ipsilateral, would be involved with lateral movements of the tail.

Convergence of Ia fibres from synergistic and antagonistic muscles onto interneurones inhibitory to soleus in humans.

1. The possibility that Ia afferent fibres from the gastrocnemius medialis (GM) and from the tibialis anterior (TA) muscle could converge on to a single interneuronal pool inhibitory to the soleus motoneurones was investigated. 2. The soleus H reflex, evoked by tibial nerve stimulation in the popliteal fossa, was conditioned by separate or combined stimulation of the nerves to the GM or TA muscles. Stimulus intensity was below the motor threshold (MTh), and the conditioning-test intervals were such as to evoke short-latency inhibition of the soleus H reflex. Care was taken to avoid current spread and artifacts connected with the closeness in time and space of the conditioning and test stimuli. 3. Separate stimulation of both GM and TA nerves was able to induce significant inhibitory effects on the H reflex amplitude at stimulus strengths larger than 0.75 x MTh, on the average. Combined stimulation of the two nerves was able to reduce the H reflex at lower stimulus strengths, at which either nerve was ineffective alone. 4. Conditioning stimulus strengths close to the MTh reduced the H reflex to approximately 80% of the control value, both on single and combined stimulation, i.e. saturation of the inhibitory effect was found. 5. By extrapolating the regression line through the normalized data from all subjects, it was assumed that the smallest stimulus strength necessary to drive the inhibitory interneurones to threshold was, on the average, 0.5 and 0.6 x MTh, on combined and separate nerve stimulation, respectively. 6. Tonic voluntary activation of the soleus abolished the inhibitory effects of both separate and combined stimulations. This was tested on the H reflex, on the rectified and averaged EMG, and on the peristimulus histogram of single motor unit discharge. 7. The findings strongly suggest the existence of spatial summation of the effects from GM and TA muscle at the level of a single interneuronal pool. Most probably, the responsible afferent fibres are group I spindle afferents, and the interneurones those mediating the reciprocal inhibition. The data do not support the notion of parallel pathways, exclusive to each nerve.

Single fiber studies of ascending input to the cuneate nucleus of cats: II. Postsynaptic afferents

The morphology of single postsynaptic afferent fibers terminating in the feline cuneate nucleus was investigated by using transport of Phasolus vulgaris leucoagglutinin from the cervical spinal cord and intraaxonal injections of horseradish peroxidase into identified postsynaptic fibers in the cuneate fasciculus. Injections of Phaseolus in C5 and C6 of both rhizotomized and non-rhizotomized cats gave similar results and confirmed previous observations with other techniques. In one animal with the smallest injection and the fewest labeled fibers in the cuneate nucleus, ten individual collaterals were reconstructed from serial sections. Most of these collaterals were at middle levels of the cuneate (from obex to about 4 mm caudal to it); they were largely confined to the rim and ventral regions of the nucleus, and their terminal fields were restricted rostrocaudally. Electrophysiologically identified fibers stained with horseradish peroxidase had large receptive fields on the ipsilateral forepaw, and latencies suggesting an oligosynaptic link to the periphery. Most of the collaterals from these fibers were also at middle cuneate levels and terminated mainly at the periphery of the nucleus but gave rise to larger terminal arbors, including sparse terminal branches to the core of the nucleus. Individual postsynaptic fibers differed in several respects from primary afferent fibers. While the spacing of collaterals of postsynaptic fibers was intermediate between that of G hair and Ia fibers, their arbors were larger than either, and could extend through the dorsoventral extent of the cuneate nucleus. The pattern of bifurcation of postsynaptic fibers resulted in stringier arbors which encompassed larger and less dense terminal fields than those of primary afferents. The number of boutons per collateral was intermediate between G hair and Ia fibers, but boutons of postsynaptic fibers were substantially smaller. These morphological differences are consistent with distinct functional roles for the two main ascending afferent systems, as suggested by electrophysiological data.

Single fiber studies of ascending input to the cuneate nucleus of cats: I. Morphometry of primary afferent fibers

The morphology of afferent fibers ascending to the cuneate nucleus has been examined in this and the subsequent paper in order to quantify the pattern of arborization and bouton arrangement of selected classes of primary afferents and to compare these data with data from postsynaptic fibers ascending to the cuneate nucleus. Electrophysiologically identified G hair and Ia muscle afferent fibers in the cuneate fasciculus were intraaxonally injected with horseradish peroxidase. Cutaneous afferents terminated dorsal to proprioceptive afferents, especially at middle levels of the cuneate nucleus. The spacing of collaterals along G hair fibers was variable, but averaged 1.46 collaterals per mm; collateral density was higher at middle cuneate levels than in the rest of the nucleus. Collateral density of Ia fibers was lower than for G hair fibers and was lowest at caudal levels of the nucleus. Branches of G hair collaterals, though often initially diverging, usually converged to terminate in a single focus in the dorsal part of the nucleus. The probability of bifurcation of Ia collaterals decreased steadily at successive branch points. These collaterals branched less symmetrically than G hair collaterals, and terminated in the ventral cuneate with less dense arbors, stretched mediolaterally, but of comparable cross-sectional area. Individual G hair collaterals gave rise to more boutons than Ia collaterals; in both cases they were mostly of the en passant type. Boutons were restricted to distal branches of G hair collaterals, whereas boutons of Ia collaterals were also located on proximal branches. Bouton size was similar for the two classes of collaterals. The data reported here, in combination with the published literature, suggest that the collaterals of roughly 300 G hair fibers overlap at any given point at middle levels of the cuneate nucleus. This high degree of anatomical convergence is not predicted by the functional segregation described with electrophysiological mapping, implying the presence of intrinsic nuclear mechanisms enhancing response specificity.

Sensitivity of monosynaptic test reflexes to facilitation and inhibition as a function of the test reflex size: a study in man and the cat

In parallel experiments on humans and in the cat it was investigated how the sensitivity of monosynaptic test reflexes to facilitation and inhibition varies as a function of the size of the control test reflex itself. In man the monosynaptic reflex (the Hoffmann reflex) was evoked in either the soleus muscle (by stimulation of the tibial nerve) or the quadriceps muscle (by stimulation of the femoral nerve). In the decerebrate cat monosynaptic reflexes were recorded from the nerves to soleus and medial gastrocnemius muscles; they were evoked by stimulation of the proximal ends of the sectioned L7 and S1 dorsal roots. Various excitatory and inhibitory spinal reflex pathways were used for conditioning the test reflexes (e.g. monosynaptic Ia excitation, disynaptic reciprocal inhibition, cutaneous inhibition, recurrent inhibition, presynaptic inhibition of the Ia fibres mediating the test reflex). It was shown that the additional number of motoneurones recruited in a monosynaptic test reflex by a constant excitatory conditioning stimulus was very much dependent on the size of the test reflex itself. This dependency had the same characteristic pattern whatever the conditioning stimulus. With increasing size of the test reflex the number of additionally recruited motoneurones first increased, then reached a peak (or plateau) and finally decreased. A similar relation was also seen with inhibitory conditioning stimuli. The basic physiological factors responsible for these findings are discussed. Finally, the implications for the interpretation of experiments in man with the H-reflex technique are considered.

Cutaneous nerve-evoked cholinergic inhibition of monosynaptic reflex in the neonatal rat spinal cord: Involvement of M 2 receptors and tachykininergic primary afferents

The mechanisms of a cutaneous nerve-evoked inhibition of monosynaptic reflex were investigated in an isolated spinal cord-peripheral nerve preparation of the neonatal rat. Conditioning stimulation of the saphenous nerve, with five pulses at 50 Hz and a strength sufficient to activate C fibers, evoked an inhibition lasting about 20 s of the monosynaptic reflex that was elicited by stimulation of the nerve branch to quadriceps femoris muscle and recorded from the L3 ventral root. This inhibition of monosynaptic reflex was potentiated by an anticholinesterase, edrophonium, and mostly blocked by atropine. Application of acetylcholine, muscarine, bethanechol, carbachol, arecoline and oxotremorine induced an inhibition of monosynaptic reflex. From the effects of muscarinic antagonists, pirenzepine, AF-DX 116, and 4-diphenylacetoxy- N-methylpiperidine on the agonist-evoked and primary afferent-evoked inhibition of monosynaptic reflex it was concluded that the muscarinic receptors involved in the cutaneous nerve-evoked inhibition of monosynaptic reflex are of M 2 type. When monosynaptic reflexes were evoked by two successive stimuli with intervals of 15 ms to 1 s, the second response was smaller than the first. This depression of monosynaptic reflex became less pronounced when the reflex was reduced by application of oxotremorine or arecoline or by conditioning stimulation of primary afférents, suggesting that the inhibition of monosynaptic reflex is presynaptic in nature. The late phase of the cutaneous nerve-evoked inhibition of monosynaptic reflex (5–20 s after conditioning stimulation) was markedly depressed by a tachykinin antagonist, spantide. Perfusion of the spinal cord with capsaicin (1 μM) for 1 h also abolished the late phase of the inhibition. Application of capsaicin or substance P to the spinal cord induced an inhibition of monosynaptic reflex, and the inhibition was antagonized by spantide and atropine. Measurements of acetylcholine content in the perfusate of isolated rat spinal cords demonstrated that bath-applied substance P evoked a release of acetylcholine from spinal neurons. These results suggest that tachykininergic primary afferent C fibers excite cholinergic spinal neurons and the released acetylcholine inhibits the monosynaptic reflex by acting on the presynaptic muscarinic receptors on group Ia fibers.

Pattern of projections of group I afferents from forearm muscles to motoneurones supplying biceps and triceps muscles in man.

1) Two independent methods were used, in man, to assess the modifications of the excitability of biceps and triceps brachii motoneurone pools following the stimulation of group I afferents coming from muscles acting at the wrist: a) the modifications of the excitability of a motoneuronal population were studied using a reflex technique, b) the modifications of the excitability of an isolated motor unit were estimated using a post-stimulus time histogram (p.s.t.h.) method. 2) The activation of group I afferents contained in the median nerve, originating from wrist flexors and pronators, resulted in a strong, short-latency facilitation of the biceps brachii motoneurones. A similar effect was also evoked by stimulation of group I afferents in the radial nerve, distally to the branch supplying the brachio-radialis muscle. The latency of both median and radial-induced facilitations is compatible with a monosynaptic linkage. 3) The stimulation of group I afferents in the median or the radial nerves produced inhibition of triceps motoneurones, with a latency compatible with a disynaptic linkage. 4) The prolonged vibration of the tendon of the flexor carpi radialis (FCR) or of the extensor carpi radialis (ECR) raised the threshold for both the facilitation of biceps and the inhibition of triceps motoneurones. The same pattern of excitatory and inhibitory convergence could also be obtained when the electrical conditioning stimulus to the median or radial nerves was replaced by a tap applied to the tendons of FCR or ECR respectively. Both results suggest that the conditioning fibres were Ia fibres. 5) The pattern of distribution of Ia afferents from muscles acting at the wrist onto motoneurones of muscles acting at the elbow has been compared to that described in the cat and monkey. A comparison has also been made between Ia connections of muscles acting at different joints in the upper and lower limb in man. The differences are discussed in relation to the manipulating capacity of the hand.

Gating of the afferent volley of the monosynaptic stretch reflex during movement in man.

1. The time course of the changes in presynaptic inhibition of Ia fibres to soleus motoneurones has been investigated during a voluntary ramp-and-hold plantar flexion. 2. Monosynaptic Ia facilitation of the soleus H reflex was evoked by stimulation of the inferior soleus (homonymous) and femoral (heteronymous) nerves. Changes in presynaptic inhibition of the Ia fibres mediating the conditioning volleys were inferred from changes in the amount of reflex facilitation evoked by such constant conditioning stimulations. 3. At the beginning of the voluntary contraction, both homonymous and heteronymous Ia facilitations were markedly increased with respect to their rest values. Then, they dropped abruptly in the middle of the ramp, whatever the ramp duration. 4. The stronger the contraction at the end of the ramp, the larger the initial increase in reflex facilitation. 5. These results indicate that presynaptic inhibition of Ia terminals on motoneurones of the contracting muscle is decreased during the first half of the ramp and then returns to its rest level. It is argued that these changes in presynaptic inhibition are centrally programmed, and it is suggested that, under certain conditions, presynaptic inhibition can be modulated according to both the strength and timing of the contraction. 6. The functional role of the resulting changes in the gain of the stretch reflex is discussed.

Spinal mechanisms in man contributing to reciprocal inhibition during voluntary dorsiflexion of the foot.

1. The inhibition of the soleus Hoffmann reflex (H reflex) during voluntary dorsiflexion of the foot--henceforth referred to as 'natural' reciprocal inhibition--was found to be initiated 50 ms before the onset of the EMG activity in the tibialis anterior muscle and to increase gradually during a ramp-and-hold dorsiflexion. There was a positive correlation between strength of tonic dorsiflexion and amount of 'natural' reciprocal inhibition. 2. The change of activity in the disynaptic and a long-latency group Ia inhibitory pathway and the change in presynaptic inhibition of the Ia fibres mediating the soleus H reflex were tested separately during ramp-and-hold dorsiflexion as well as during tonic dorsiflexion of the foot, and the results were compared with the development of the 'natural' reciprocal inhibition of the unconditioned soleus H reflex. 3. The disynaptic group I inhibition of soleus motoneurones was increased, as compared to rest, during the dynamic phase of a ramp-and-hold dorsiflexion movement, but the inhibition generally did not increase during tonic dorsiflexion of the foot. 4. The long-latency group I inhibition was seen only during dorsiflexion of the foot. It appeared around 50 ms before tibial anterior EMG activity and there was a positive correlation between strength of tonic dorsiflexion and amount of this long-latency inhibition. 5. Presynaptic inhibition of Ia afferents terminating on soleus motoneurones was estimated by an indirect method. The increase of presynaptic inhibition started soon after the onset of the ramp-and-hold dorsiflexion, and gradually became more pronounced during the ramp phase. The amount of presynaptic inhibition was positively correlated with strength of tonic dorsiflexion. 6. It is concluded that all investigated mechanisms may contribute to the 'natural' reciprocal inhibition and it seems that the different pathways are used differentially during different types of movement.

Carbonic anhydrase activity in the peripheral nervous system of rat: the enzyme as a marker for muscle afferents

The distribution of carbonic anhydrase (CA) activity was determined histochemically using Hansson's cobalt phosphate method in cross-sections of peripheral nerves from rats. As the studied nerves contain either efferent, proprioceptive or exteroceptive myelinated fibres, our survey particularly focused on the question whether CA-reactive nerve fibres are functionally related. Intense CA activity was detected in all large diameter (8–12 μm) muscle afferents. The amount of similarly reactive cutaneous afferents was negligibly low (3.6%). Efferent fibres displayed only weak CA activity, which was confined to the small myelinated fibres (3–6 μm). Moderate staining could be assessed in medium-sized (4–11 μm) proprioceptive fibres. The same reactivity occurred in a sizeable percentage (11.4%) of exteroceptive afferents. Their diameters ranged from 4 to 11 μm. These results indicated, that high enzyme activity is found predominantly in large-calibre proprioceptive afferents, which according to Hunt's classification were identified as group IA and IB fibres. Further confirmation for our observations was obtained by demonstrating high levels of enzyme activity in primary nerve endings of muscle spindles (IA fibres) and in axon terminals of Golgi tendon organs (IB fibres) constantly. Finally possible functions for neuronal CA are discussed with respect to its high activity in a functionally related subpopulation of nerve fibres.

Alteration of proprioceptive messages induced by tendon vibration in man: a microneurographic study.

The activities of single proprioceptive fibres were recorded from the lateral peroneal nerve using transcutaneously implanted tungsten microelectrodes. Unitary discharges originating from muscle spindle primary and secondary endings and Golgi tendon organs were identified by means of various physiological tests. The sensitivity of proprioceptors to mechanical vibrations with a constant low amplitude (0.2-0.5 mm) applied at various frequencies to the tendon of the receptor-bearing muscle was studied. Muscle spindle primary endings (Ia fibres) were found to be the most sensitive to this mechanical stimulus. In some cases their discharge could be driven in a one-to-one manner up to 180 Hz. Most of them also fired harmonically with the vibration up to 80 Hz and then discharged in a subharmonic manner (1/2-1/3) with increasing vibration frequencies. Muscle spindle secondary endings (II fibres) and Golgi tendon organs (Ib fibres) were found to be either insensitive or only slightly sensitive to tendon vibration in relaxed muscles. The effects of tendon vibration on muscle spindle sensory endings response to muscle lengthening and shortening induced by imposed constant velocity or sinusoidal movements of the ankle joint were studied. Modulation of the proprioceptive discharge frequency coding the various joint movement parameters was either completely or partly masked by the receptor response to vibration, depending on the vibration frequency. Moreover, vibrations combined with sinusoidal joint movements elicited quantitatively erroneous proprioceptive messages concerning the movement parameters (amplitude, velocity). The sensitivity of the Golgi tendon organs to vibration increased greatly when the receptor-bearing muscle was tonically contracted. These data confirm that vibration is able to preferentially activate the Ia afferent channel, even when the vibration amplitude is low. They define the frequency sensitivity of the muscle spindle primary and secondary endings and the Golgi tendon organs. They also show that the physiological messages triggered by ongoing motor activities undergo a series of changes during the exposure of muscles to vibration.

Evolution Of Motor, Ia And Cutaneous Nerve‐Fibre Conduction Velocities In Premature And Fullterm Newborns

The conduction velocities of motor, IA and non-nociceptive cutaneous sensory fibres have been studied in the ulnar and median nerves of 145 healthy fullterm neonates and in 77 preterm neonates (postconceptional age-range 28 to 42 weeks). The posterior tibial-nerve motor and IA fibres were studied: the sural nerve cutaneous-fibre conduction velocities were measured in fullterm neonates only. At each age the IA fibre conduction velocities were the highest. There was no difference between the conduction velocity increase per week of postconceptional age in motor and cutaneous fibres. Except for the cutaneous-nerve fibres, conduction velocities were higher in the upper than in the lower limbs.