SUMMARYCross-correlation analysis of the naturally occurring discharges of y-motoneurones to gastrocnemius and soleus muscles has revealed a tendency towards synchronized firing in the decerebrated, spinal cat. The primary feature of the cross-correlation between two ymotoneurones is a symmetrical peak centred at time zero with reference to discharges at the level of the cell bodies in the ventral horn of the spinal cord. The peak has a half-width in the range 3-10 ms. Secondary features of the correlograms are frequently present, especially when the primary peak is large. They consist of dips to either side of the peak and are mirrored in the respective autocorrelation functions of the two contributory neurones. This suggests that they arise as a result of periodicity in firing patterns of the neurones. The degree of synchrony has been measured as the ratio (k') of the total counts contributing to the peak over the number expected by chance. The value of k' was found to be dependent upon the firing frequencies of the neurones. Plots of k' against the geometric mean frequency of discharge showed a falling curvilinear relation as frequency increased. There was a weak inverse relation between the width of the cross-correlation peak and firing frequency. These relations had to be acknowledged before any assessment of change in the degree of synchrony could be made. The correlation between discharges is absent or weak in the decerebrated cat with an intact spinal cord and in intact cats anaesthetized with pentobarbitone or chloralose. Anaesthetics administered to a spinal animal did not suppress synchrony. The synchrony between y-motoneurones is characteristic of the short-term synchrony (Sears & Stagg, 1976) which arises from activity in shared presynaptic axons. In a few instances a stronger, more-tightly coupled synchrony was observed. The cross-correlogram peak had a half-width of 0-5-l 0 ms and was superimposed upon the usual wider-based peak. We presume that this intense form of synchrony has a different origin. The properties of the short-term synchrony are discussed with respect to (a) the mechanism of supraspinal control, and (b) a plausible model of the membrane properties of y-motoneurones which could account for the dependency of k' on frequency of firing.

Twitch and tetanic contractions of single motor units of the cat peroneus tertius muscle were examined after application of a test allowing their identification as either fast fatigable (f.f.) or fast fatigue‐resistant (f.r.) or fast intermediate (f.i.) or slow units as established by Burke, Levine, Tsairis & Zajac (1973). The test was found to leave two kinds of after‐effects in f.f., f.r. and f.i. units whereas it did not affect slow units. The first after‐effect was an early and brief potentiation of twitch tension occurring in all f.r. and f.i. units and in most f.f. units. The second after‐effect, termed ‘delayed fatigue', was a prolonged depression of tension output, that developed slowly following the early potentiation in all f.f. and f.i. units and more than half of the f.r. units. One hour after the test, unfused tetanic contractions elicited by 20‐40/sec stimulation were deeply depressed in motor units that had been left without stimulation since the end of the test. Recovery took place in 3‐5 hr. Motor units affected by delayed fatigue could nevertheless be made to develop nearly normal tension by gradual build‐up upon prolonged stimulation at 30‐40/sec. Maximal tetanic contractions elicited by 200/sec stimulation were much less depressed during delayed fatigue than unfused tetanic contractions. These observations suggest that contractile mechanism were not impaired by delayed fatigue. Since absence of change in muscle action potential indicated that excitation of muscle fibres was not affected either, delayed fatigue might be due to a temporary failure of excitation‐contraction coupling.

Coronary artery translocation is the most important step in achieving a successful result in arterial switch operations. Although a few centers have reported excellent results, coronary artery transfer requires a high technical expertise. We report a new technique of arterial switch operation without coronary translocation. By creating flaps in the proximal great arteries, the coronaries are transferred to the neoaorta without distortion of their original anatomic position. This technique avoids problems related to coronary translocation. Because coronary buttons are not excised, there is no need for nonviable material to be used in reconstruction of neopulmonary artery. Arterial wall is sutured to arterial wall, so postoperative bleeding is lessened. This technique can be used for any type of coronary anomaly and great arterial relationship. Coronary perfusion is well maintained. Two patients with transposition variants and ventricular septal defects have been operated on successfully with this technique. Postoperative investigations showed good coronary perfusion, without right or left ventricular outflow obstruction or leakage through the semilunar valves. This technique achieves anatomic correction for transposition of the great arteries, just as a conventional arterial switch operation does, but it avoids problems related to coronary artery translocation. We believe that it is a much simpler, more reliable, and more reproducible operation than others in current use, and it can be carried out by many cardiac surgeons with acceptable results. The early results are encouraging, although longer follow-up and more cases are essential.

SUMMARY1. Physiological tests were used to identify skeletofusimotor or , axons to the cat peroneus tertius muscle in order to assess the proportion of f axons in the motor supply to this muscle.2. Static ft axons (,fS) were identified by: (a) observation of a delay between the complete block of extrafusal contraction and the failure of spindle activation upon prolonged stimulation, (b) increase of spindle excitation with stimulation frequencies above that eliciting maximal extrafusal contraction, (c) observation of 'unfused' frequencygram of spindle primary afferent discharge during stimulation of the axon at frequencies above that eliciting complete fusion of extrafusal contraction and (d) static action exerted on the response of the spindle afferent to ramp stretch.3. Dynamic ft axons (ftD) were identified by the persistence of spindle activation after selective block of extrafusal neuromuscular junctions and by their dynamic action on spindle primary endings. 4. The actions of1 16 motor axons (conduction velocity 56-104 m/sec) on ninety-five spindle afferents (fifty-seven from primary and thirty-eight from secondary endings) were examined in ten experiments. Thirty-six ft axons (31 % of the total sample) were identified: twenty-four /IS (conduction velocity 69-104 m/sec) and twelve ftD (conduction velocity 56-91 m/sec).5. Twenty (35 %) primary endings were activated by a ftS and sixteen (28 %) by apfD axon. Nineteen (45 %) secondary endings were activated by a ftS and five (13 %) by a ftD axon. Convergence of ftD and ftS axons on the same spindle occurred in 10 % of instances. ft-innervated spindles were also supplied by y axons.6. Most of the ftS motor units were of the fast-fatigue resistant (FR) type, with a few units of the fast-fatiguable (FF) type, and nearly all the ,fD motor units were of the slow (S) type.

No abstract