Circulatory responses to selective stimulation of medullated and non-medullated fibers of the aortic nerve

Circulatory responses to selective afferent stimulation of medullated and non-medullated fibers in the left aortic nerve were followed in terms of changes in arterial pressure, heart rate, contractile force of the left ventricular fragment and vascular resistance of the hind limb of cats with an opened thorax. Stimulation of the medullated afferents induced a reflex fall of blood pressure mainly by means of decrease in the peripheral vascular resistance, while activation of non-medullated fibers influenced the heart work, i. e. decreased heart rate and myocardial contractile force.     

Olfactory nerve fibers

Cross sections of olfactory nerves present a unique appearance. They indicate the presence of large numbers of very small nerve fibers, with a modal diameter of about 0.2 µ and a narrow range for their size variation. From one side of the nasal septum of a pig the yield of fibers was estimated at 6,000,000; the number arising from the turbinates would be considerably larger. The fibers are attached to the membranes of the Schwann sheaths in large bundles through mesaxons longer and more branched than those that have been seen in other nerves. Continuity of the axons between the nerves and the bipolar cells was traced in an examination of the olfactory mucous membrane; and the indication of a one-to-one relationship between cells and axons was reinforced by a comparative count. After the axons leave the bipolar cells they become incased in the central projections of the sustentacular cells. Where the latter come into contact with the basal cells the axons emerge to push back the plasma membranes of the basal cells in the first step in acquiring their nerve sheaths. Later steps are described. When the axons are delivered by the basal cells to the collecting Schwann tubes, they are already aggregated into small bundles with sheaths fundamentally the same as those they will possess until they are delivered to the glia in the olfactory bulb. Some of the aspects of the cytology of the bipolar cells and adjoining sustentacular cells are described. A survey of the physiological properties of olfactory nerve fibers was made in some experiments on the olfactory nerve of the pike. Almost all of the action potential is encompassed within a single elevation, manifesting at its front a conduction velocity of 0.2 m./sec. For a comparison, the last elevation in the C action potential in the sciatic nerve of the frog is cited as an example of conduction at the same velocity. Though expressed through long time constants, the properties of the pike olfactory fibers conform to the generalized schema for properties of vertebrate nerve fibers. This conformity signalizes that they differ from the exceptional properties of the unmedullated fibers of dorsal root origin. An afferent function for unmedullated nerve fibers does not imply that the fibers concerned are alike in their physiological properties.     

X-ray effects on single nerve fibers

Electrophysiological responses of median and lateral giant nerve fibers of the earthworm were recorded during exposure, in vitro, to x-rays. In response to external stimulation, during x-irradiation, these nerve fibers showed initially an increase in conduction velocity, a rise in spike amplitude, a decrease in relative refractory period, and an increase in sensitivity. These responses represent an enhancement of activity, attributable to bombardment with x-rays, rarely found in other biological systems. They were followed by deterioration of activity and eventual block, representative of the lethal action of x-rays, commonly observed in other biological systems. Several properties of the enhancement of activity were noted: The time at which maximum activity of one factor occurred did not coincide with the time at which the maxima of other activities occurred; spike amplitude, for example, was observed to increase while conduction velocity had already reached its maximum and was rapidly declining. The energy supplied by bombardment with x-rays did not act synergistically during the time of the enhanced response; that is, concomitant irradiation was not necessary in order to produce the enhanced response, once the nerve had been altered by x-irradiation. Once the nerve was responding in an enhanced manner, cessation of irradiation for short periods of time had no effect on the response. The phenomenon was therefore due to an irreversible alteration of the nerve fiber itself.