SUMMARY1. Much current thinking about voluntary movement assumes that the segmental y loops can function as a servomechanism operated by the brain. However, the a motoneurones of the baboon's hand receive a powerful monosynaptic (CM) projection from the precentral gyrus. If servo-driving from the same cortical area is to be possible, it must project independently to the fusimotor neurones and have sufficient power to increase the afferent signalling from the muscle spindles. The cortical thresholds for contraction of m. extensor digitorum communis and for acceleration of the discharges of its muscle spindles have therefore been compared.2. Significant results in this context require that the spindles studied be coupled in parallel with the responding extrafusal muscle fibres. Many spindles were not unloaded by the submaximal contractions evoked by cortical stimulation, although all so tested were unloaded by maximal motor nerve twitches. Reasons are given for thinking that such apparent lack of parallel coupling is an artifact of complex intramuscular anatomy and limitation of shortening by 'isometric' myography.3. A brief burst of corticospinal volleys at 500/sec, which is specially effective in exciting ac motoneurones over the CM projection, failed to excite spindle afferents at or below the threshold for a cortical 'twitch'.4. In a few epileptiform discharges, bursts of spindle acceleration occurred independently of the clonic contractions. A relatively direct and independent cortico-fusimotor (CF) projection may therefore exist.5. Prolonged near-threshold stimulation at 50-100/sec, which allows time for temporal summation in the less direct projections (e.g. corticointerneuronal, cortico-rubro-spinal) and does not cause frequency-* Present address:
A new operation is described in which a neorectum and neoanal sphincter mechanism have been constructed to restore gastrointestinal continuity and continence in a patient who had previously undergone proctocolectomy and a permanent ileostomy. The neorectum was constructed by forming a triplicated pouch from the distal ileum. The neoanal sphincter was fashioned from a transposed gracilis muscle and was activated electrically by an implanted stimulator. A period of chronic low frequency stimulation altered the muscle characteristics and enabled the neosphincter to contract continually without fatigue. Complete continence was achieved by the neosphincter gripping a Silastic plug inserted within the efferent spout of the pouch. The patient was able to void completely when the stimulator was switched off and the plug removed.
The mechanism by which vagal nerve stimulation (VNS) exerts an anticonvulsant effect in humans is unknown. This study used (99m)Tc-HMPAO single photon emission tomography (SPECT) to examine the effects of VNS on regional cerebral activity in thalamic and insular regions. Seven subjects with epilepsy who had been receiving vagal nerve stimulation for at least 6 months underwent SPECT scanning with simultaneous scalp electroencephalographic (EEG) recording. Subjects were studied in two states; during VNS activity and during a comparison condition of VNS inactivity. A region of interest analysis demonstrated that rapid cycling stimulation (7 seconds on, 12 seconds off) was associated with relatively decreased activity in left and right medial thalamic regions. No systematic stimulation-related changes were observed on visual or spectral analysis of EEG data. The thalamus is involved in modulation of ongoing cortical EEG activity in animals. Our results support the hypothesis that VNS may exert an antiepileptic action by an effect on thalamic activity.
1. The response of thirty spindle afferents was studied during cortical stimulation of the ;best point' for eliciting a contraction of tibialis anticus in baboons anaesthetized with N(2)O/O(2) mixtures supplemented by small doses of barbiturates.2. No evidence of fusimotor activity before a contraction of the muscle was found. Evidence of fusimotor activity during the contraction and after the cortical stimulus was found. The various problems associated with interpretation of this activity are discussed.3. Depression of the spindle afferent response by tetanic cortical stimulation, which produced no detectable tension change, was found for four spindle afferents. This depression was particularly well documented for one spindle afferent. The depression could be explained by a decrease of static fusimotor activity.4. The spindle afferent response to muscle stretch was studied during light anaesthesia. The effect of barbiturates on this response suggested a depression of static fusimotor activity.
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