Participation of cortical sensory areas in the elaboration of extrapyramidal motor responses

Levetiracetam suppresses long-loop reflexes at the cortical level

Posthypoxic coma is often associated with cortical and brainstem hyperexcitability. Five months following cardiopulmonary resuscitation after myocardial infarction and ventricular arrhythmia, a 47-year-old man presented with posthypoxic cerebral dysfunction, minimal responsiveness, severe spastic-dystonic tetraparesis, and stimulus-sensitive muscle spasms upon acoustic and sensory stimulation. Neurophysiological examination revealed increased long-loop reflexes in abductor pollicis brevis muscle following median nerve stimulation at the wrist, consistent with cortical hyperexcitability. Exaggerated startle responses provided evidence of concomitant brainstem disinhibition. Levetiracetam up to 3,000 mg per day suppressed transcortical long-loop reflexes in a dose-dependent manner without concomitant suppression of the H-reflex and only mild attenuation of the startle response. The present findings suggest a suppressive effect of levetiracetam on cortical neurons in the absence of a spinal effect on monosynaptic reflexes, and thus support the drug's efficacy in posthypoxic cortical hyperexcitability.

Exaggerated auditory startle responses in multiple system atrophy: a comparative study of parkinson and cerebellar subtypes

OBJECTIVE

Auditory startle responses (ASRs) have recently been reported to be exaggerated in cranial and peripheral nerve supplied muscles of patients with multiple system atrophy (MSA). ASRs displayed increased probability, amplitude and duration, shorter onset latency, and reduced habituation in comparison with healthy subjects. In order to investigate whether certain ASR features may differentiate MSA subtypes, the authors studied ASRs in 21 MSA patients (olivopontocerebellar type, MSA-C: n = 8, striatonigral type, MSA-P: n = 13), and 17 age-matched normal controls.

METHODS

ASRs were elicited by binaural high-intensity auditory stimuli which differed randomly in tonal frequency and intensity (250 Hz, 90 db; 500 Hz, 105 db; 750 Hz, 105 db; 1000 Hz, 110 db normal hearing level), presented through tubal insert phones. Reflex electromyographic activity was simultaneously recorded with surface electrodes from masseter, orbicularis oculi, sternocleidomastoid, biceps brachii, abductor pollicis brevis, rectus femoris, tibialis anterior, and soleus muscles.

RESULTS

Eighteen MSA patients (86%) had exaggerated ASRs as compared to normal subjects. At group level, indices of ASR disinhibition including increased ASR probability (in extremity muscles), shortened onset latency, and enlarged response magnitude were significantly more marked in MSA-P as compared to MSA-C patients. ASR probability showed habituation in normal subjects, less in MSA-P. and none in MSA-C patients. Three MSA-patients had no ASRs except in orbicularis oculi muscle.

CONCLUSIONS

Although absent ASRs may occur in some MSA patients, most of them exhibit exaggerated ASRs. This finding may reflect disinhibition of lower brainstem nuclei due to the degenerative disorder. ASRs were significantly more disinhibited in MSA-P versus MSA-C. suggesting involvement of different neural structures in the two MSA-subtypes.

The abdominal cutaneous reflex circuit in the rat: a spinobulbo-spinal reflex in phenobarbital-anesthetized animals

A description is given of an abdominal reflex (ACR) in the rat evoked by cutaneous stimulation. Latency of the ACR in response to stimulation of the contralateral hindlimb was 28.5 +/- 3.2 msec. Data from the analysis of discharges in dorsal and ventral roots and from bulbar reticular neurons triggered by ACR stimuli reveal that a spino-bulbo-spinal loop seems to be implicated. This is also supported by the chronic effects of pontile and spinal transections. ACR-concomitant discharges were also recorded in other axial (digastric m. and internal intercostal m.) and proximal limb muscles. The possible spino-bulbo-spinal mechanism of this cutaneous reflex is discussed.

Observations on chloralose-induced myoclonus in guinea-pigs

1 The physiological, biochemical and pharmacological features of alpha-chloralose-induced myoclonus in the guinea-pig have been studied. 2 EMG bursts in muscles jerking in chloralose-induced myoclonus are long, and are not time-locked to any cortical event recorded in the EEG, although they are evoked by auditory or peripheral nerve stimuli. 3 The efferent conduction velocity down the spinal cord of the signals generating the EMG bursts is fast but the afferent conduction velocity up the cord for stimulus-evoked jerks is slow, in distinction to the reverse characteristics of the spino-bulbo-spinal relfex arc. 4 alpha-Choralose did not cause any consistent change in 5-hydroxytryptamine (5-HT) or 5-hydroxyindoleacetic acid levels in any brain area, nor did it alter 5-HT turnover as judged by the depletion of 5-HT after p-chlorophenylalanine pretreatment. 5 Pretreatment of animals with drugs that increase brain 5-HT action (L-tryptophan with a monoamine oxidase inhibitor, or 5-hydroxytryptophan), or antagonize the action of 5-HT (cyproheptadine) did not abolish or obviously increase chloralose-induced myoclonus. 6 Chloralose-induced myoclonus is not similar to 5-HT-sensitive reticular reflex myoclonus in man.

Myoclonus developing after vermisectomy in photosensitive Papio papio

Two vermisectomized photosensitive baboons exhibited two different types of myoclonus, one induced by intermittent light stimulation (ILS) and the other occuring "spontaneously". The characteristics of these two types of myoclonus are described from a clinical and from an ECoG point of view. Myoclonus induced by ILS (ML) started at the eyelids and secondarily invaded the face and body; it was always preceded by frontorolandic spike-waves or polyspike-waves. The "spontaneous" myoclonus which followed vermisectomy (MV) was "massive", but involved firstly the trunk and secondarily the face and limbs; no ECoG paraoxysm accompanied this myoclonus, but we observed a parietal evoked potential of small amplitude, 10--15 msec after its onset. If MLs can be considered as consequences of the fronto-rolandic paroxysmal discharges, MVs seem to originate in the brain stem but appear similar to action myoclonus. This experimental situation showing two types of myoclonus resembles human hereditary degenerative syndromes (dyssynergic cerebellar myoclonus, progressive myoclonic epilepsy), without being exactly comparable. The conditions in which MVs were seen and their modifications during sleep and by different drugs are described. The relationships between MVs and MLs and myoclonic epilepsy are discussed.

Spino-spinal mechanisms in the chloralose anaesthetized cat

1. In chloralose anaesthetized cats the events in the lumbosacral cord that follow stimulation of a forepaw or neck nerve have been examined.2. Four events take place in the lumbosacral cord after forepaw or neck muscle nerve stimulation. These are: (1) a brief period of generalized enhanced motoneurone excitability, (2) a prolonged and as yet unexplained enhancement of monosynaptic reflexes, (3) a period of presynaptic inhibition in muscle and skin afferent fibres, and (4) a prolonged inhibition of spino-bulbo-spinal reflexes.3. A variable and sometimes selective reduction of one or another descending effects follows the placing of small lesions in the anterior portion of the suprasylvian gyrus.