Disynaptic inhibition of spinal motoneurones from the motor cortex in the monkey.

1. The neuronal mechanism of disynaptic inhibition of spinal motoneurones by the corticospinal tract was investigated in Macaca irus. Surface stimulation or weak intracortical stimulation was used in order to evoke the inhibition. Intracellular records were taken from motoneurones in lumbar segments. 2. Both the disynaptic i.p.s.p.s evoked from group Ia afferents and the disynaptic i.p.s.p.s evoked from corticospinal fibres were found to be depressed by conditioning stimulation of motor axons to antagonistic muscles. Mutual facilitation of the actions from these two fibre systems occurred when nerve impulses set up in them reached the explored spinal segment synchronously. These observations led to the conclusion that disynaptic i.p.s.p.s from group Ia afferents and from the motor cortex are mediated by common interneurones. 3. No evidence either for or against projections of the same pyramidal tract cells to motoneurones of one motor nucleus and to interneurones interposed between group Ia afferents and motoneurones of an antagonistic muscle could be obtained by comparing cortical areas from which monosynaptic e.p.s.p.s and disynaptic i.p.s.p.s were evoked in the different motor nuclei. 4. The areas from which the disynaptic i.p.s.p.s were evoked in individual motoneurones appeared to be similar in size to the areas of cortical monosynaptic projections to motoneurones and showed similar degrees of overlap, indicating that the projections of pyramidal tract cells to Ia inhibitory interneurones are as extensive as to motoneurones and that they are similarly organized.     

Integration in descending motor pathways controlling the forelimb in the cat

A further analysis has been made of inhibitory pathways to motoneurones via C3-C4 propriospinal neurones (PNs). Intracellular recording was made from triceps brachi motoneurones and effects from higher centres and forelimb afferents on corticospinal IPSPs were investigated after transection of the corticospinal tract at the C5/C6 border. The shortest latencies of the IPSPs evoked by stimulation of the pyramid were as brief as those of the pyramidal EPSPs (Illert et al. 1977). It is postulated that the minimal linkage of the pyramidal IPSPs is disynaptic via inhibitory C3-C4 PNs projecting directly to motoneurones. It was confirmed that pyramidal IPSPs usually are depressed by volleys in forelimb motor axon collaterals (Illert and Tanaka 1978). A quantitative comparison was made of the recurrent depression of pyramidal IPSPs and of IPSPs caused by activation of the Ia inhibitory interneurones. The result support the hypothesis of two parallel inhibitory cortico-motoneuronal pathways via C3-C4 PNs, one disynaptic via the inhibitory PNs and the other trisynaptic via excitatory PNs and Ia inhibitory interneurones. Pyramidal volleys also evoked late IPSPs which in some cases were not depressed from forelimb motor axon collaterals. It is postulated that the late IPSPs are partly due to activation of inhibitory C3-C4 PNs. Disynaptic pyramidal IPSPs were effectively facilitated by volleys in rubro-, tecto- and reticulospinal fibres - but not from vestibulospinal fibres - showing a convergence from the former descending tracts on common inhibitory C3-C4 PNs. Projection from forelimb afferents and corticospinal fibres on common inhibitory C3-C4 PNs was revealed by strong facilitation of disynaptic pyramidal IPSPs from cutaneous forelimb afferents. No corresponding effect was evoked from C2 neck afferents. Stimulation in the lateral reticular nucleus (LRN) evoked monosynaptic IPSPs in some motoneurones. The results of threshold mapping in and around the LRN suggest that the IPSPs are caused by antidromic stimulation of ascending collaterals of inhibitory neurones also projecting to motoneurones, possibly the inhibitory C3-C4 PNs.     

The mode of activation of pyramidal tract cells by intracortical stimuli.

1. Direct and indirect effects of intracortical stimulattion on pyramidal tract cells were compared in the monkey and in the cat under barbiturate or chloralose anaesthesia. The hind-limb motor areas were explored, that in the monkey only within the convex part of the precentral gyrus. The intracortical stimuli were applied in the nearest vicinity of pyramidal tract cells, where antidromic spike potentials of single cells were recorded. 2. Average records of descending volleys in corticospinal tract fibres were taken from the surface of the lateral funiculus or from its dissected fascicles. The sensitivity of the recording was sufficient to detect responses in single fibres. 3. The latencies of the earliest descending volleys evoked by weak intracortical stimuli were compared with the latencies of the antidromic spike potentials of pyramidal tract cells evoked by stimulation of the lateral funiculus at a low lumbar level (same conduction distance). Only in about one third of cases these latencies were similar and compatible with a direct activation of pyramidal tract cells. In the remaining cases they indicated mono- or polysynaptic activation of pyramidal tract cells. 4. Latencies of the later components of the descending volleys indicated that they were due to indirect activation of pyramidal tract cells in practically all cases. 5. The components of the descending volleys attributable to the indirect activation of pyramidal tract cells were greatly increased when repetitive intracortical stimuli were applied instead of single ones. 6. The investigation leads to the conclusion that a weak intracortical stimulation is relatively ineffective in a direct excitation of pyramidal tract cells and that the effects of such a stimulation are mainly indirect, especially when repetitive stimuli are used.     

Integration in descending motor pathways controlling the forelimb in the cat. 13. Corticospinal effects in shoulder,elbow, wrist,and digit motoneurones

Summary The effect of corticospinal volleys evoked by stimulation of the contralateral pyramid was investigated using intracellular recordings from motoneurones to forelimb muscles. Confirming and extending previous observations (Illert et al. 1977, lllert and Wiedemann 1984), short latency EPSPs within a disynaptic range were evoked by a train of pyramidal volleys in all varieties of shoulder, elbow, wrist and digit motoneurones. The amplitude of pyramidal EPSPs was sensitive to the stimulus repetition rate. Maximal amplitudes were observed around 2–4 Hz, while at 10 Hz the early EPSP was markedly reduced and the long latency EPSP abolished. The persistence of disynaptic EPSPs after a corticospinal transection in C5/C6 suggested that, for all types of forelimb motor nuclei, disynaptic EPSPs are relayed by C3–C4 propiospinal neurones (PNs) (c.f. Illert et al. 1977). The transection, however, caused a clear reduction in the EPSP of all motoneurone types. After a ventral lesion of the lateral funicle in C5/C6 interrupting the axons of the C3–C4 PNs, disynaptic (and possibly trisynaptic) EPSPs were evoked by a short train of pyramidal volleys. It is postulated that intercalated neurones in a disynaptic cortico-motoneuronal pathway also exist in the forelimb segments. Disynaptic pyramidal IPSPs were observed in most types of forelimb motor nuclei both before and after a corticospinal transection in C5/C6. At all joints, pyramidal excitation dominated in motoneurones to physiological flexors, while in extensor motoneurones mixed excitation and inhibition or dominant inhibition was common. Comparison of pyramidal effects in slow motoneurones (classified according to the after-hyperpolarization duration) to the long head of the triceps and anconeus revealed dominant excitation in the former and inhibition in the latter. It is suggested that the slow motor units in these muscles differ in their function although both muscles are elbow extensors.This work was supported by the Swedish Medical Research Council (project no. 94 and 6953)     

Comparison of human motor cortical projections to abdominal muscles and intrinsic muscles of the hand

Summary Percutaneous electrical stimulation of the motor cortex was used to activate rapidly conducting corticofugal pathways to human abdominal muscles. Following cortical stimulation the response latencies for the abdominal muscles were similar to those for limb muscles which are a similar distance from the motor cortex. Cortically evoked responses recorded from the abdominal muscles had the same latency and similar amplitude during several voluntary tasks including expiration, expulsive manoeuvres and trunk flexion. Responses could also be evoked when the chemical drive to breathe was increased by rebreathing. In addition, the properties of the cortical projection to muscles of the abdominal wall were directly compared with those of the projection to the intrinsic muscles of the hand. The latencies of responses in abdominal muscles and intrinsic muscles of the hand were measured during static contractions over a range of strengths in the same subjects (0–100% maximal voluntary contraction, MVC). For both muscle groups, cortically evoked muscle responses of minimal latency occurred when background contractions reached 10–20% MVC with responses of maximal amplitude at 60% MVC. The variability in latency of fifty consecutive responses were similar for the two muscle groups. Furthermore, post-stimulus time histograms for 4 rectus abdominis motoneurones revealed a brief initial excitatory peak of 1.15ms duration (range 0.96–1.34ms) following cortical stimulation. The characteristics of this peak are the same as reported for motoneurones of intrinsic hand muscles. These findings demonstrate a powerful rapidly conducting pathway from the motor cortex to the human abdominal muscles. This pathway has many of the same properties as the monosynaptic corticospinal projection to the distal muscles of the upper limb.     

Pyramidal actions in identified radial motornuclei of the cat

This study aimed to establish the projection from the corticospinal tract (CST) to the motoneurones innervating the deep radial (DR) forelimb muscles. In the anaesthetized cat stimulation of the contralateral pyramid and intracellular recording from identified forelimb motoneurones was used. A train of pyramidal stimuli evoked disynaptic EPSPs in DR motoneurones. The effects were very similar in the different nuclei. Pyramidal IPSPs had a slightly longer latency and occurred in most cases together with disynaptic EPSPs. It is suggested that the inhibitory actions to the distal forelimb are predominantly relayed in a trisynaptic pathway, but that a disynaptic linkage seems possible as well. The disynaptic pyramidal EPSPs remained after CST transection in C5. They were abolished after CST transections in C2. It is concluded that disynaptic corticospinal excitation of distal DR motornuclei is relayed in a short midcervical propriospinal system. Transection experiments at different cervical levels suggest that the majority of the propriospinal neurones is located in C3-C4. The CST facilitated a variety of reflex pathways to motoneurones innervating distal forelimb muscles. Disynaptic excitatory and inhibitory effects from cutaneous and low threshold group I muscle afferents were common. They were present in all investigated nuclei and powerfully facilitated from the CST. It is suggested that this allows the brain to adapt the reflex mechanisms of the distal forelimb to the synergistic-antagonistic relations between the muscles, which are changing according to the performed movement.     

Bilateral cortical control of the human anterior digastric muscles

Transcranial magnetic stimulation (TCMS) was used to determine the organisation of cortical motor projections to the anterior digastric muscles in 12 normal human subjects. Two distinct types of potentials were evoked in anterior digastric with a figure-of-eight coil. A short-latency (3 ms) response appeared bilaterally on the surface electromyogram (EMG), but only ipsilaterally on intramuscular recordings: this was the result of direct stimulation of the ipsilateral trigeminal motor root. Motor evoked potentials (MEPs) were elicited in the anterior digastric muscles at variable onset latencies of around 10 ms by stimulation of scalp areas antero-lateral to the area for the first dorsal interosseous muscle of the hand. These were evoked bilaterally in relaxed anterior digastric muscles in six of the seven subjects. In the other subject, the responses in the relaxed muscle were exclusively ipsilateral. However, when the anterior digastric muscles were contracted, the responses were bilateral in all subjects. TCMS and spike-triggered averaging revealed that the bilateral responses were not due to the branching of axons from individual digastric motoneurones to muscles on each side. Because the digastric motor nucleus may contain separate populations of ipsi- and contralateral projecting motoneurones, it was necessary to study single motor-unit responses to TCMS to demonstrate a bilateral corticobulbar projection. The responses of 17 single motor units in the anterior digastric muscle to TCMS were recorded. All were activated by contralateral stimulation. Approximately 80% were also activated by ipsilateral TCMS, although one well-characterised motor unit was inhibited by ipsilateral TCMS. When bilateral activation was present, the ipsilateral responses were more secure than the contralateral responses, which may indicate an additional interneurone in the pathway to the contralateral motoneurone. The major conclusions from this study are that (1) the cortical representation of the anterior digastric muscle is antero-lateral to hand muscles; (2) the cortical projection to the anterior digastric muscles is bilateral; (3) the corticobulbar projection is stronger contralaterally than ipsilaterally but may involve at least one additional synapse; and (4) anterior digastric motoneurones do not branch to innervate the muscles bilaterally. Received: 8 March 1999 / Accepted: 16 June 1999     

Evidence for corticospinal excitation of presumed propriospinal neurones in man.

1. The possibility that stimulation of the motor cortex facilitates transmission in the pathway mediating non-monosynaptic ('propriospinal') excitation from low-threshold afferents to upper limb motoneurones was investigated. 2. Convergence between peripheral afferent volleys (from the ulnar or musculo-cutaneous nerve) and corticospinal volleys (evoked by magnetic stimulation of the motor cortex) was investigated using the spatial facilitation technique. Thus the effects of these volleys on the flexor carpi radialis H reflex were compared when applied separately and together. When cortical stimulation was optimal for the muscle from which the conditioning volley originated the facilitation of the reflex on combined stimulation was significantly larger than the algebraic sum of the effects of separate stimuli. 3. The extra facilitation on combined stimulation had all the characteristics of 'propriospinal' excitation (low threshold, long central delay, brief duration and depression when the afferent input was increased), and it is suggested that this reflects corticospinal excitation of 'propriospinal' neurones. 4. When varying the time interval between cortical and test stimulations, it was shown that extra facilitation on combined stimulation began 1 ms later than the onset of the control reflex facilitation. Assuming that the latter onset reflects the arrival of the monosynaptic corticospinal volley at the motoneurone pool, this 1 ms delay suggests a disynaptic pathway for the cortical excitation of motoneurones through 'propriospinal' neurones. 5. As at the onset of voluntary movement, the pattern of the cortical excitation of 'propriospinal' neurones was quite specific: extra facilitation of the reflex on combined stimulation only occurred when the cortical volley was preferentially directed to motoneurones supplying the muscle from which the afferents used for the peripheral volley originated. 6. It is concluded that corticospinal axons activate human 'propriospinal' neurones and thereby produce disynaptic excitation of the motoneurone pool. Given temporal summation with the monosynaptic excitation, this 'propriospinally mediated' disynaptic excitation might make a significant contribution to the evoked EMG potential.     

Probing the corticospinal link between the motor cortex and motoneurones: some neglected aspects of human motor cortical function

This review considers the operation of the corticospinal system in primates. There is a relatively widespread cortical area containing corticospinal outputs to a single muscle and thus a motoneurone pool receives corticospinal input from a wide region of the cortex. In addition, corticospinal cells themselves have divergent intraspinal branches which innervate more than one motoneuronal pool but the synergistic couplings involving the many hand muscles are likely to be more diverse than can be accommodated simply by fixed patterns of corticospinal divergence. Many studies using transcranial magnetic stimulation of the human motor cortex have highlighted the capacity of the cortex to modify its apparent excitability in response to altered afferent inputs, training and various pathologies. Studies using cortical stimulation at ‘very low’ intensities which elicit only short-latency suppression of the discharge of motor units have revealed that the rapidly conducting corticospinal axons (stimulated at higher intensities) drive motoneurones in normal voluntary contractions. There are also major non-linearities generated at a spinal level in the relation between corticospinal output and the output from the motoneurone pool. For example, recent studies have revealed that the efficacy of the human corticospinal connection with motoneurones undergoes activity-dependent changes which influence the size of voluntary contractions. Hence, corticospinal drives must be sculpted continuously to compensate for the changing functional efficacy of the descending systems which activate the motoneurones. This highlights the need for proprioceptive monitoring of movements to ensure their accurate execution.     

Integration in descending motor pathways controlling the forelimb in the cat

Summary A previously described disynaptic pathway from cortex to forelimb motoneurones whose intercalated neurones were excited both from other descending pathways and from forelimb afferents (Illert et al., 1976a, b) has been further analysed, mainly with respect to the location of the relay cells and their axons.Disynaptic EPSPs evoked in forelimb motoneurones by stimulation of the pyramid remained after complete transection of the corticospinal tract in C5 rostral to the forelimb segments but were abolished after a more rostral transection of the tract in the C2 segment. Corresponding findings were made with disynaptic rubral EPSPs after transection of the rubrospinal tract in these segments. It is concluded that disynaptic cortico-motoneuronal and rubro-motoneuronal excitation is relayed by propriospinal neurones originating in the C3–C4 segments. Other lesion experiments revealed that the axons of these propriospinal neurones descend to forelimb motoneurones in the ventrolateral part of the lateral funicle.Spatial facilitation of transmission from the corticospinal and rubrospinal tracts after transection of them in C5 occurred with a time course showing monosynaptic convergence from these pathways on common propriospinal neurones.Facilitation of disynaptic pyramidal EPSPs from the dorsal tegmentum remained after transection of the corticospinal tract at C5 but was abolished after a transection at C2. It is postulated that corticospinal and presumed tectospinal fibres converge onto common neurones in the propriospinal relay but evidence is also given for a more rostral relay (probably bulbar) with a similar convergence.The oligo- (probably mono-)synaptic facilitation of the disynaptic pyramidal EPSP evoked by volleys in cutaneous and group I muscle afferents from the forelimb likewise remained after a C5 transection of the corticospinal tract but was abolished after an additional C5 lesion in the dorsal column. It is concluded that propriospinal relay cells receive excitatory action from forelimb afferents ascending in the dorsal column. Spatial facilitation experiments using three tests revealed that propriospinal neurones monosynaptically excited from both corticospinal and rubrospinal fibres also receive excitation from cutaneous forelimb afferents.It is postulated that the propriospinal relay provides an important route for fast activation of forelimb motoneurones from the brain. The convergent monosynaptic excitation from several important motor centres in the brain is considered in relation to the general problem of the functional relationship between higher motor centres. The convergent action from forelimb afferents is taken to suggest that a descending command for a forelimb movement can be modified from the forelimb while on its way to the motoneurones.Supported by the Deutsche Forschungsgemeinschaft     

Effects of motor cortex stimulation on spinal interneurones in intact man

Summary The effect of a descending corticospinal volley on a spinal inhibitory pathway, has been studied in five intact human subjects. Approximately 63% inhibition of the H-reflex evoked in wrist and finger flexor muscles, was produced by motor threshold stimulation of the radial nerve. When a submotor threshold cortical shock was given 2 to 4 ms before the H-reflex, this inhibition was reduced to approximately 38%. The timing of this effect is compatible with either a monosynaptic or disynaptic corticospinal tract projection onto the spinal inhibitory interneurone.     

Short latency inputs to phrenic motoneurones from the sensorimotor cortex in the cat

Summary Short latency responses were recorded from C₅ phrenic roots and intracellularly from phrenic motoneurones following stimulation of the pericruciate cortex or medullary pyramids in cats anaesthetized with Nembutal or chloralose-urethane. Focal stimulation of the cortical surface (single pulses, 0.5–2 ms, 0.3–8 mA) during inspiration evoked EPSPs (latency 4.7 ± 1.7 ms, rise time 1.9 ± 1.1 ms, amplitude 0.22 to 3.94 mV) in 42% of motoneurones studied (n = 107). The EPSPs were absent, or on average 60% smaller, following stimulation during expiration. In all but two motoneurones, during both inspiration and expiration, hyperpolarizing potentials were observed either following the initial depolarization or alone. They could be reversed by hyperpolarizing current or chloride injection. Stimulation of the pyramidal tract at mid medullary level (1 to 3 pulses, 0.2 ms) evoked short latency excitation in phrenic motoneurones only with currents of more than 200 A. Smaller stimuli applied to the medial reticular formation above the pyramidal tract evoked excitation (onset latency 1.5–3.2 ms) in which the earliest part was probably monosynaptic. These results show that the corticospinal responses in phrenic motoneurones are both excitatory and inhibitory. They are not transmitted through the pyramidal tract and are at least disynaptic. Excitation evoked from the medullary pyramidal tract can be explained by current spread beyond the pyramidal tract fibres.     

Projection from area 3a to the motor cortex by neurons activated from group I muscle afferents

Two receiving areas in the pericruciate cortex are known for inputs from group I muscle afferents of forelimb nerves. One focus is near the postcruciate dimple of area 3a, and the other in the lateral sigmoid gyrus of the motor cortex (area 4gamma). The cortico-cortical projection of area 3a to 4gamma, and the relay by this projection of group I muscle afferent input to the motor cortex were investigated in cats. The following results were obtained. 1. Seventy-four neurons within area 3a were antidromically activated by intracortical microstimulation of the motor cortex. 2. Although excitation evoked by stimulation of group I muscle afferents could be demonstrated for only a few (8 of 48) cortico-cortical neurons in extracellular recordings, due to the methodological limitations discussed, this input evoked EPSPs in 8 of 9 cortico-cortical neurons recorded intracellularly. Therefore, it is likely that the majority of neurons projecting from area 3a to the motor cortex have an excitatory synaptic input from group I afferents. 3. Neurons projecting from area 3a to the motor cortex were most commonly found in cortical layer III, although some were found in layer V. 4. Five of nine pyramidal tract neurons of area 3a had a strong excitatory synaptic input from group I muscle afferents. 5. A new type of pyramidal tract neuron was found which has cortico-cortical axon collaterals connecting the two cytoarchitectonic regions. These various neurons may be part of a feedback system from muscle afferents to the motor cortex.     

Post-synaptic effects of cortical stimulation on forelimb motoneurones in the baboon

1. The arm area of the baboon's precentral motor cortex was stimulated by brief surface-anodal pulses, and the post-synaptic potentials elicited in contralateral forelimb motoneurones were studied by intracellular recording.2. Strong cortical stimuli elicited a rapid series of excitatory and, in some cells, inhibitory post-synaptic potentials (EPSPs and IPSPs respectively). Comparisons with the simultaneously recorded response of the pyramidal tract indicated that these post-synaptic potentials were due to a repetitive discharge of fast pyramidal fibres. Thus, the later synaptic events were mostly due to a repetition of the early monosynaptic EPSP and early IPSP respectively.3. Inhibition was seen more often in cells whose monosynaptic EPSP had a small maximal size than in those whose monosynaptic EPSP was larger. The net depolarization produced by a strong cortical stimulus was related to the maximal size of the early monosynaptic EPSP.4. In the Discussion, an interpretation is suggested for previous findings concerning the spinal distribution of late synaptic effects elicited by cortical stimulation.     

Integration in descending motor pathways controlling the forelimb in the cat

Summary Recording was made in the C3-C4 segments from cell bodies of propriospinal neurones identified by their antidromic activation from more caudal segments. Monosynaptic excitatory effects from descending motor pathways and primary afferents were investigated by electrical stimulation of higher motor centres and peripheral nerves in the forelimb and neck.The cell bodies were located mainly laterally in Rexed's layer VII. Threshold mapping for single axons showed that they descend in the lateroventral part of the lateral funicle. Antidromic stimulation at different spinal cord levels showed that some neurones terminated in the forelimb segments, others in the thoracic cord or in the lumbar segments. Terminal slowing of the conduction velocity suggested axonal branching over some segments.Monosynaptic EPSPs were evoked in the neurones by stimulation of the contralateral pyramid, red nucleus and dorsal tegmentum-superior colliculus. It is concluded that corticospinal, rubrospinal and tectospinal fibres project directly to both short and long propriospinal neurones. There was marked frequency potentiation in tectospinal synapses. Convergence from two descending tracts was common and in half of the tested cells all three tracts contributed monosynaptic excitation. Experiments with collision of descending volleys and antidromic volleys from the brachial segments demonstrated that the corticospinal and rubrospinal monosynaptic projection to the propriospinal neurones is by collaterals from fibres continuing to the forelimb segments.Stimulation of cervical primary afferents in the dorsal column gave monosynaptic EPSPs in somewhat less than half of the tested propriospinal neurones. The further analysis with stimulation of forelimb nerves and C2-C3 dorsal rami showed that monosynaptic EPSPs may be evoked from low threshold cutaneous and group I muscle afferents in the forelimb and from C2-C3 neck afferents entering close to the spinal ganglia, possibly from joint receptors. Convergence from cervical afferents and at least two of the above descending tracts was common.It is postulated that the propriospinal neurones previously indirectly defined by their action on motoneurones as relaying disynaptic excitation from higher motor centres to forelimb motoneurones (Illert et al., 1977) belong to those neurones of the C3-C4 propriospinal systems which terminate in the cervical enlargement. The function of the neurones projecting beyond the upper thoracic segments is discussed.Supported by the Deutsche ForschungsgemeinschaftIBRO/UNESCO Fellow     

Relative contribution from different nerves to recurrent depression of Ia IPSPs in motoneurones

1. The pattern of depression of Ia IPSPs by volleys in recurrent motor axon collaterals was investigated in motoneurones supplying hind-limb muscles in the cat. The test IPSPs were evoked by stimulation of dorsal roots and the conditioning antidromic volleys by stimulation of motor fibres in different peripheral muscle nerves.2. In all motor nuclei investigated the strongest depression of Ia IPSPs is evoked from motor fibres to muscles whose Ia afferents produce the IPSPs. For example, the Ia IPSP from the knee extensor recorded in motoneurones to a knee flexor is most effectively depressed by antidromic stimulation of motor fibres to the knee extensor.3. The origin of recurrent inhibition of alpha-motoneurones and of Ia inhibitory interneurones with the same Ia input display a striking similarity. This suggests that the same population of Renshaw cells mediates effects to motoneurones and to Ia inhibitory interneurones.4. The functional significance of impulses in motor axon collaterals was discussed and it was suggested that they have an important role in the control of the excitatory as well as inhibitory Ia actions to motoneurones. The recurrent inhibition may limit the Ia effects to excitation of homonymous motoneurones, which would provide optimal conditions for control of individual muscles via the gamma-loop.     

Facilitation of monosynaptic excitatory synaptic potentials in spinal motoneurones evoked by internuncial impulses

1. Descending tracts and primary afferent fibres were chronically degenerated in the lumbosacral cord of the cat, and attempts were made to evoke monosynaptic e.p.s.p.s in motoneurones by stimulation of interneurones with a pair of fine electrodes inserted into the cord.2. The reversal potential of monosynaptic e.p.s.p.s so produced was more negative than that measured for monosynaptic e.p.s.p.s produced by afferent impulses.3. Monosynaptic e.p.s.p.s evoked in motoneurones by internuncial impulses showed a significantly greater facilitation than those produced by afferent impulses.4. Monosynaptic e.p.s.p.s in a motoneurone produced by supramaximal intraspinal stimuli often revealed a fluctuation in amplitude. In such cases, when two successive stimuli were applied at a short interval, the mean amplitude of the second e.p.s.p.s was greater than that of the first e.p.s.p.s. This facilitation was associated with a decrease in the coefficient of variation of the e.p.s.p. amplitude fluctuation.5. The degree of facilitation of monosynaptic e.p.s.p.s evoked by internuncial impulses was not related to the amount of transmitter released by the preceding impulses.6. It is concluded that facilitation of monosynaptic e.p.s.p.s evoked by both afferent and internuncial impulses is based on the same mechanism and that the degree of facilitation of e.p.s.p.s is entirely determined by the nature of presynaptic elements.     

Cortical excitability and motor task in man: an investigation of the wrist extensor motor area

Task-related changes in the corticospinal excitation of the right extensor carpi radialis (ECR) muscle were investigated in 16 healthy human subjects. The subjects were asked to perform a tonic isometric wrist extension or to clench their hand around a manipulandum, thereby coactivating the antagonistic wrist muscles. At matched levels of background EMG in the ECR muscle, transcranial magnetic stimulation (TMS) was applied through a figure-of-eight coil at 20-30 sites spaced 1 cm apart over the hand area of the left motor cortex. The cortical maps of the representation of the ECR muscle constructed in this way did not change between the two motor tasks. Nevertheless, for all investigated cortical sites TMS evoked a smaller motor evoked potential (MEP) in the ECR muscles during hand clenching than during wrist extension. A similar decrease in the short-latency peak in the poststimulus time histogram (PSTH) of single ECR motor units to TMS during hand clenching was found in seven subjects (number of motor units = 35). In contrast, short-latency peaks in the PSTH evoked by electrical stimulation of the motor cortex had a similar size during the two tasks (number of motor units = 9; two subjects). Already the initial 0.5-1.0 ms of the short-latency peak evoked by TMS was depressed during hand clenching, which suggests that decreased excitability of corticospinal cells with monosynaptic projections onto ECR motor units was involved. This decreased excitability was not explained by increased intracortical inhibition, which was found to be of a similar size during hand clenching and wrist extension. The task-related changes in the efficiency of the motor cortex output are discussed in relation to the function of the wrist antagonist muscles in handling and gripping tasks.     

Tests for presynaptic modulation of corticospinal terminals from peripheral afferents and pyramidal tract in the macaque

The efficacy of sensory input to the spinal cord can be modulated presynaptically during voluntary movement by mechanisms that depolarize afferent terminals and reduce transmitter release. It remains unclear whether similar influences are exerted on the terminals of descending fibres in the corticospinal pathway of Old World primates and man. We investigated two signatures of presynaptic inhibition of the macaque corticospinal pathway following stimulation of the peripheral nerves of the arm (median, radial and ulnar) and the pyramidal tract: (1) increased excitability of corticospinal axon terminals as revealed by changes in antidromically evoked cortical potentials, and (2) changes in the size of the corticospinal monosynaptic field potential in the spinal cord. Conditioning stimulation of the pyramidal tract increased both the terminal excitability and monosynaptic fields with similar time courses. Excitability was maximal between 7.5 and 10 ms following stimulation and returned to baseline within 40 ms. Conditioning stimulation of peripheral nerves produced no statistically significant effect in either measure. We conclude that peripheral afferents do not exert a presynaptic influence on the corticospinal pathway, and that descending volleys may produce autogenic terminal depolarization that is correlated with enhanced transmitter release. Presynaptic inhibition of afferent terminals by descending pathways and the absence of a reciprocal influence of peripheral input on corticospinal efficacy would help to preserve the fidelity of motor commands during centrally initiated movement.     

Cortical control of presynaptic inhibition of Ia afferents in humans

The effect of transcranial magnetic stimulation was investigated on presynaptic inhibition of Ia terminals in the human upper and lower limb. Presynaptic inhibition of Ia afferents was assessed by three different and independent methods: (1) heteronymous Ia facilitation of the H-reflex (assessing ongoing presynaptic inhibition of Ia afferents in the conditioning volley); (2) long-lasting inhibition of the H-reflex by a group I volley (D1 inhibition, assessing presynaptic inhibition on Ia afferents in the test volley); (3) measurement of the monosynaptic Ia peak evoked in single motor units by a homonymous or heteronymous volley (post stimulus time histogram method). The first two methods were used on the lower limb; the last two on the upper limb. Provided that the corticospinal volley and the explored Ia volley were directed to the same target motoneurones, cortical stimulation evoked significant and congruent changes: (1) In the lower limb, transcranial stimulation provided increased heteronymous Ia facilitation and decreased D1 inhibition, both of which suggest a decrease in presynaptic inhibition of Ia afferents; (2) in the upper limb, transcranial stimulation provided an increase in the radial-induced inhibition of the wrist flexor H-reflex and a decrease in the peak of monosynaptic Ia excitation in single units, both of which suggest an increase in presynaptic inhibition. Selectivity of corticospinal effects was explored by testing presynaptic inhibition of Ia afferents to soleus motoneurones and focusing the transcranial stimulation to excite preferentially different motor nuclei (soleus, quadriceps and tibialis anterior). A cortical-induced decrease in presynaptic inhibition of Ia afferents was seen when, and only when, cortical and peripheral Ia volleys were directed to the same motor nucleus. Received: 18 July 1997 / Accepted: 10 November 1997