Facilitation and depression of synaptic transmission in amphibian sympathetic ganglia

There have been few reports concerning facilitation and depression in sympathetic ganglia9,17,40. In the present investigation, pairs of excitatory postsynaptic potentials (EPSPs) were recorded intracellularly from bullfrog paravertebral sympathetic ganglia for an analysis of the site and mechanism responsible for the phenomena of facilitation and depression of ganglionic transmission. The ratio of the amplitude of the second of a depression of ganglionic transmission. The ratio of the amplitude of the second of a pair of EPSPs divided by the first was compared to the time interval between each pair of EPSPs divided by the first was compared to the time interval between each pulse. These ratios demonstrated two phases: an earlier phase of facilitation (20-500 msec pulse intervals) and a later phase of depression (500 msec-10 sec). Additional parameters-rate of rise of synaptic potentials (dV/dt), synaptic currents (EPSCs), and synaptic conductances (Gtr)-were determined and all confirmed the results obtained with EPSPs. Furthermore, the degree of facilitation or depression could be modulated by altering the extracellular concentration of calcium. On the other hand, comparison of the amplitude of pairs of presynaptic terminal spikes did not show any variability over similar stimulus intervals, nor were the amplitudes of miniature EPSPs significantly different before or after an evoked EPSP. Therefore, the processes of facilitation and depression of ganglionic transmission occur as a result of normal nerve terminal activity. The processes are occurring simultaneously, such that one or the other may predominate depending upon the interval between pulses, as well as the relative concentration of extracellular calcium.

Environmentally Assisted Fatigue Crack Growth Rates in SAE 4340 Steel

Corrosion fatigue crack growth rates for SAE 4340 steel in distilled water environments were investigated utilizing fracture mechanics techniques. Crack growth rates, resulting from various forms of repeated loading, were compared to those induced by static load using small sample statistical theory. These comparisons indicate that corrosion fatigue crack growth may be either time or cycle dependent depending on the load profile, loading frequency, and temperature. Data are presented suggesting that whenever the maximum cyclic stress intensity in the fatigue cycle is below the static threshold stress intensity (KIscc), the environment no longer plays a major part in assisting the crack growth rate.

The quantitative determination of coronary flow with a positron (rubidium-84)

An objective method for the quantitative measurement of the coronary blood flow in ml./min. using a coincidence counting system with rubidium-84, a positron emitter, is described. Accidental coincidence counts arising from independent annihilation of positrons were greatly reduced by simple shielding.

The coronary blood flow was calculated by the extrapolation of the clearance values to zero time. At this time the myocardial extraction ratio of rubidium-84 is unity and clearance is equal to flow. The accuracy of this method was demonstrated on the isolated perfused dog heart.

It was found that the rate of change of the extraction ratio of rubidium-84 in the isolated perfused dog heart and in man was not altered with changes in flow.

The finding that nitroglycerin increases the coronary blood flow in patients without coronary artery disease while decreasing it in patients with coronary artery disease was confirmed.