SUMMARY1. In the preceding paper (Crone, Hultborn, Kiehn, Mazieres & Wigstr6m, 1988) it was shown that a short-lasting synaptic excitation ('on' stimulus) of extensor motoneurones (primarily triceps surae) in the decerebrate cat often resulted in a maintained excitability increase, which could be reset by a short-lasting inhibitory stimulus train ('off' stimulus). In the present experiments intracellular recording from triceps surae motoneurones and the electroneurogram (ENG activity) from triceps surae nerve branches were performed in parallel.2. Sustained firing of individual triceps surae motoneurones was most often recorded in parallel with the maintained ENG activity following a synaptic 'on' stimulus. When the motoneurone was silenced, by a hyperpolarizing current through the microelectrode, there was no sign of on-going synaptic excitation during the maintained ENG activity following an 'on' stimulus. It was therefore suggested that voltage-dependent intrinsic properties of the motoneurones themselves could be responsible for the maintained firing.3. In confirmation of this hypothesis it was found that short-lasting depolarizing current pulses through the recording microelectrode could trigger a self-sustained firing in the motoneurone provided that the bias current (i.e. the holding potential) was kept within certain limits. Hyperpolarizing current pulses terminated the firing. When the spike-generating mechanism was inactivated (by long-lasting excessive depolarization) similar depolarizing and hyperpolarizing current pulses could initiate and terminate plateau potentials in the motoneurones. By grading the depolarizing current pulses it was found that the plateau potentials were of all-or-none character, typically around 10 mV in amplitude. The two levels of excitability which can be triggered by short-lasting excitation and inhibition of the motoneurones is referred to as 'bistable' behaviour of the motoneurones. 4. After an acute spinal transection, in the unanaesthetized cat, the bistable behaviour of the motoneurones disappeared. However, it reappears following intravenous injection of the serotonin precursor 5-hydroxytryptophan (50-120 mg/kg).* To whom reprint requests should be sent.

The likelihood of rupture of unruptured intracranial aneurysms that were less than 10 mm in diameter was exceedingly low among patients in group 1 and was substantially higher among those in group 2. The risk of morbidity and mortality related to surgery greatly exceeded the 7.5-year risk of rupture among patients in group 1 with unruptured intracranial aneurysms smaller than 10 mm in diameter.

SUMMARY1. Reciprocal inhibition between antagonist muscle groups at the ankle has been investigated in sixty healthy subjects. Hoffmann reflexes (H reflexes) in the soleus and tibialis anterior muscles were used to assess changes in reciprocal inhibition evoked by electrical stimulation of antagonist muscle nerves.2. Inhibition of the soleus H reflex was evoked by a single conditioning stimulus to the common peroneal nerve, and inhibition of the tibialis anterior H reflex was elicited by one conditioning stimulus to the posterior tibial nerve. Symmetrical central connections between the antagonist flexors and extensors were assumed and under this assumption the central delay for the inhibition, in addition to the delay for monosynaptic I a excitation, was calculated to be about 1 ms. The inhibition was evoked by weak stimuli to the nerves from antagonist muscle groups; the threshold for the inhibition was around 0-6 x threshold for a direct motor response (Mthreshold). Furthermore, tendon taps to the Achilles tendon facilitated the soleus H reflex and inhibited the tibialis anterior reflex at short latencies. The short central delay, the low electrical threshold and the. actions of Achilles tendon taps strongly suggest that the early reciprocal inhibition is homologous to the disynaptic Ia inhibition previously studied in animal experiments.3. With the test soleus H reflex kept at 15-25 % of the maximum directly evoked motor response (M-response) and the strength of the conditioning peroneal nerve stimulation kept at 1-0 x M-threshold, the inhibition from the peroneal nerve ranged between 0 and 40 % (mean, 14-9 %) at rest.4. Changes in the amount of reciprocal inhibition from the peroneal nerve were studied both during tonic and dynamic dorsi-and plantarflexion. During tonic dorsiflexion there was no significant change of inhibition as compared to rest, while inhibition decreased during tonic plantarflexion. However, during ramp-and-hold dorsiflexion there was a transient increase in reciprocal inhibition of the soleus H reflex. This increase in inhibition from the peroneal nerve could be seen 50 ms prior to the onset of contraction. The increase in inhibition before and at the very beginning of the contraction cannot be due to sensory feed-back during contraction, but must depend on a supraspinal control of the spinal cord.5. At conditioning-test intervals of 4-6 ms, the inhibition of the soleus H reflex from the peroneal nerve was considerably larger during tonic dorsiflexion than at rest.6-2 C. CRONE AND OTHERS Thus, tonic dorsiflexion revealed an inhibition with long latency from the peroneal nerve, which was not seen at rest. This long-latency inhibition was probably of group I origin since the threshold for the inhibition was very low (0-6 x M-threshold).6. The results are discussed in relation to the hypothesis that a-motoneurones and 'Ia inhibitory interneurones' projecting to antagonist motoneurones are controlled in parallel from the brain.

In decerebrate cats a train of impulses in Ia afferents may lead to a sustained increase in excitability of alpha-motoneurones of homonymous and heteronymous muscles. It was previously suggested that this long-lasting excitability increase reflects a maintained synaptic input to the motoneurones from excitatory interneurones. With intracellular recording from motoneurones we here demonstrate that the sustained increase of alpha-motoneurone activity is due to an all-or-none plateau depolarization. This plateau can be induced by a short train of excitatory synaptic potentials or a brief, intracellularly injected depolarizing current pulse and is terminated by a short train of inhibitory synaptic potentials or a hyperpolarizing current pulse. It is concluded that maintained motor unit firing triggered by a brief train of impulses in Ia afferent reflects an intrinsic bistable behaviour of alpha-motoneurones.

ObjectiveTo determine the diagnostic accuracy and clinical utility of electromagnetic source imaging (EMSI) in presurgical evaluation of patients with epilepsy.MethodsWe prospectively recorded magnetoencephalography (MEG) simultaneously with EEG and performed EMSI, comprising electric source imaging, magnetic source imaging, and analysis of combined MEG-EEG datasets, using 2 different software packages. As reference standard for irritative zone (IZ) and seizure onset zone (SOZ), we used intracranial recordings and for localization accuracy, outcome 1 year after operation.ResultsWe included 141 consecutive patients. EMSI showed localized epileptiform discharges in 94 patients (67%). Most of the epileptiform discharge clusters (72%) were identified by both modalities, 15% only by EEG, and 14% only by MEG. Agreement was substantial between inverse solutions and moderate between software packages. EMSI provided new information that changed the management plan in 34% of the patients, and these changes were useful in 80%. Depending on the method, EMSI had a concordance of 53% to 89% with IZ and 35% to 73% with SOZ. Localization accuracy of EMSI was between 44% and 57%, which was not significantly different from MRI (49%–76%) and PET (54%–85%). Combined EMSI achieved significantly higher odds ratio compared to electric source imaging and magnetic source imaging.ConclusionEMSI has accuracy similar to established imaging methods and provides clinically useful, new information in 34% of the patients.Classification of evidenceThis study provides Class IV evidence that EMSI had a concordance of 53%–89% and 35%–73% (depending on analysis) for the localization of epileptic focus as compared with intracranial recordings—IZ and SOZ, respectively.