SUMMARY1. Motor unit synchronization has been studied in human first dorsal interosseous muscle.2. Two needle electrodes were inserted into the muscle and the activity of pairs of motor units recorded.3. Pre-and post-stimulus histograms of the firing of unit pairs showed a narrow central peak of duration 1-3-9-3 ms (88% of sample in the range 1-6 ms; mode 3-0 ms), together with a variable amount of synchronization of somewhat longer duration.4. For the duration of the whole synchronization peak (85% sample in range 5-15 ms; mode between 6-1 and 8-0 ms (31 % of sample)), units fired between 8 and 485% times more often than would have been expected had the units been firing independently of one another. Amplitudes of the peak of the recorded histograms expressed as a proportion of control ranged from 1-8 to 10-9 (mean 3*9; bin width 160 ,ts).5. The strength of synchronization between the firing of motor unit pairs was inversely related to differences in recruitment threshold. The largest amount of synchronization was observed for pairs of units in which both had recruitment thresholds < 0 5 N or > 1P0 N. Less synchronization was found between pairs of units in which one had a recruitment threshold < 0-5 N and the other a threshold > 10ON.6. The time course of synchronization was well matched by the predictions of a theoretical model based on the hypothesis that underlying the observed synchronization is the joint arrival of EPSPs from branched last-order input fibres.

Despite frequent reports of poor concentration following traumatic brain injury, studies have generally failed to find disproportionate time-on-task decrements using vigilance measures in this patient group. Using a rather different definition, neuropsychological and functional imaging research has however linked sustained attention performance to right prefrontal function--a region likely to be compromised by such injuries. These studies have emphasised more transitory lapses of attention during dull and ostensibly unchallenging activities. Here, an existing attention measure was modified to reduce its apparent difficulty or 'challenge'. Compared with the standard task, its capacity to discriminate traumatically head-injured participants from a control group was significantly enhanced. Unlike existing functional imaging studies, that have compared a sustained attention task with a no-task control, in study 2 we used positron emission tomography to contrast the two levels of the same task. Significantly increased blood flow in the dorsolateral region of the right prefrontal cortex was associated with the low challenge condition. While the results are discussed in terms of a frontal system involved in the voluntary maintenance of performance under conditions of low stimulation, alternative accounts in terms of strategy application are considered.

SUMMARY1. Motor unit firing has been studied during weak voluntary isometric contractions with pairs of needle electrodes in normal human subjects.2. Pre-and post-stimulus time histograms of the firing time of firing of one event unit before and after the time of firing of another reference (stimulus) unit showed a clear central peak, indicative of synchronization.3. Synchronization was seen in all the muscles studied. The mean strength of synchronization, expressed as the number of concomitant discharges of the two units as a proportion of the number of stimulus unit discharges, was 0 095 extra event unit spikes/reference unit spike (range 0 042-0{28) for first dorsal interosseous muscle, 0-016 extra event unit spikes per reference unit spike (range 0-4043) for medial gastrocnemius and 0-056 extra event unit spikes per reference unit spike (range 0-016-0-079) for tibialis anterior.4. The mean duration of synchronization was 11-3 ms (range 5-0-21-0 ms) for first dorsal interosseous, 10-3 ms (range 3-5-21-7 ms) for medial gastrocnemious and 13-5 ms (range 3 0-25 0) for tibialis anterior.5. Seven patients with radiographically and clinically identified central strokes were studied while they made weak voluntary isometric contractions. The duration of synchronization was significantly prolonged compared to that found in normal subjects. In these stroke patients the mean duration of synchronization on the affected side was longer than that seen in the normal subjects, and in first dorsal interosseous muscle was 35-4 ms (range 12-0-65-0 ms), in medial gastrocnemius was 21-3 ms (range 4 0-43 0 ms) and in tibialis anterior was 28-8 ms (range 14-0-49-0 ms).6. The mean strength of synchronization of motor unit discharge was found to be greater in the stroke patients than that seen in the normal subjects for first dorsal interosseous muscle (0 161 extra event unit spikes per reference unit spike, range 0-017-04391) and for medial gastrocnemius (0'030 extra event unit spikes per reference unit spike) but only significantly so when pooled data was compared. There was no difference in the strength of motor unit synchronization in tibialis anterior between stroke patients and normal subjects. * To whom correspondence should be addressed. 8073A. K. DATTA, S F. FARMER AND J. A. STEPHENS 7. Broad duration synchronization among first dorsal interosseous motor units was also found in a pateint with a rostral cervical spine lesion (total duration range 43-46 ms; n = 2), but not in a patient with a caudal (thoracic) spinal lesion. Broad duration 'synchronization was also found in a patient with untreated idiopathic Parkinson's disease (range 37-45 ms; n = 2) and in a patient with a cerebellar haematoma (range 19-28 ms; n = 2) but not in a patient with Huntington's chorea (range 9-20 ms; n = 2).8. Normal subjects were asked to isometrically co-contract two muscles; the firing of a motor unit in each muscle was studied. Synchronization of motor unit firing was always found between muscles defined as anatomical antagonists. The strengt...

The ventilatory responses to immersion and changes in temperature are reviewed. A fall in skin temperature elicits a powerful cardiorespiratory response, termed "cold shock," comprising an initial gasp, hypertension, and hyperventilation despite a profound hypocapnia. The physiology and neural pathways of this are examined with data from original studies. The respiratory responses to skin cooling override both conscious and other autonomic respiratory controls and may act as a precursor to drowning. There is emerging evidence that the combination of the reestablishment of respiratory rhythm following apnea, hypoxemia, and coincident sympathetic nervous and cyclic vagal stimulation appears to be an arrhythmogenic trigger. The potential clinical implications of this during wakefulness and sleep are discussed in relation to sudden death during immersion, underwater birth, and sleep apnea. A drop in deep body temperature leads to a slowing of respiration, which is more profound than the reduced metabolic demand seen with hypothermia, leading to hypercapnia and hypoxia. The control of respiration is abnormal during hypothermia, and correction of the hypoxia by inhalation of oxygen may lead to a further depression of ventilation and even respiratory arrest. The immediate care of patients with hypothermia needs to take these factors into account to maximize the chances of a favorable outcome for the rescued casualty.

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