FACILITATION OF TRANSMISSION FROM AUTONOMIC NERVE TO SMOOTH MUSCLE OF GUINEA-PIG VAS DEFERENS

Nerve-mediated excitation and inhibition of the smooth muscle cells of the avian gizzard

1. The electrical events evoked in smooth muscle cells of the chick and pigeon gizzards by vagal, perivascular sympathetic and transmural stimulation were recorded with intracellular micro-electrodes.2. Single stimulating pulses applied to the extrinsic nerves, or to intrinsic fibres, produced excitatory junction potentials (E.J.P.s) which were blocked by hyoscine. Repetitive stimulation caused facilitation of E.J.P.s and, at higher frequencies, summation. When the tissue was well oxygenated, maximal stimulation evoked action potentials and the tissue contracted. If the tissue was allowed to become anoxic, action potentials were blocked and thus junctional transmission could be observed uncomplicated by tissue contraction.3. Cholinergic E.J.P.s appeared to be due to stimulation of both post-ganglionic neurones and the pre-ganglionic input to post-ganglionic neurones, since ganglion-blocking drugs abolished the late phase of complex E.J.P.s while leaving unaffected their initial components.4. Repetitive perivascular sympathetic stimulation, after cholinergic E.J.P.s had been blocked by hyoscine, evoked a slow, long-lasting depolarization of the muscle cells. This depolarization was blocked by guanethidine (5 x 10(-6) g/ml.) and was mimicked by noradrenaline (10(-7) g/ml.).5. Under non-anoxic conditions, the tissue underwent rhythmical contractions which were associated with action potential firing. When the tissue was anoxic action potentials were not seen, but spontaneous membrane depolarizations resembling E.J.P.s were observed. Spontaneous miniature excitatory junction potentials (M.E.J.P.s) were observed only infrequently whether the tissue was well oxygenated or not.6. Single stimulating pulses applied to extrinsic or intrinsic nerves frequently evoked inhibitory junction potentials (I.J.P.s) in smooth muscle cells of the gizzards of both birds. I.J.P.s were blocked by tetrodotoxin (10(-7) g/ml.). Repetitive stimulation of inhibitory nerves gave rise to a membrane hyperpolarization; when stimulation was stopped the membrane potential showed rebound depolarization.7. These results are discussed in terms of the autonomic innervation of the avian gizzard, and possible explanations of the various types of activity observed are considered.

Excitation of Mytilus smooth muscle

1. Membrane potentials and tension were recorded during nerve stimulation and direct stimulation of smooth muscle cells of the anterior byssus retractor muscle of Mytilus edulis L.2. The resting potential averaged 65 mV (range 55-72 mV).3. Junction potentials reached 25 mV and decayed to one half maximum amplitude in 500 msec. Spatial summation and facilitation of junction potentials were observed.4. Action potentials, 50 msec in duration and up to 50 mV in amplitude were fired at a membrane potential of 35-40 mV. No overshoot was observed.5. Contraction in response to neural stimulation was associated with spike discharge. Measurement of tension and depolarization in muscle bundles at high K(+) indicated that tension is only produced at membrane potentials similar to those achieved by spike discharge.6. Blocking of junction potentials, spike discharge and contraction by methantheline, an acetylcholine antagonist, supports the hypothesis that the muscle is excited by cholinergic nerves. However, evidence of a presynaptic action of methantheline complicates this argument.

The innervation of the vas deferens of the guinea-pig

1. The compound action potential of the hypogastric nerve of the guinea-pig contained two main elevations. The low-threshold fibres had a range of conduction velocities from 1.5 to 10 m/sec. The high threshold fibres conducted action potentials at less than 1 m/sec. The hypogastric nerve contained small myelinated fibres and non-myelinated fibres.2. In the preparation in vitro, junctional potentials and contractions were elicited by stimulation of the rapidly conducting fibres alone. Trains of C fibre volleys were ineffective.3. In the preparation in vivo, conduction from the hypogastric nerve to the vas deferens nerve was unidirectional and abolished by hexamethonium. After the administration of hexamethonium, the contraction produced by stimulation of the vas deferens nerve was unaffected.4. The close arterial injection of acetylcholine (ACh) into the pelvic viscera caused centrifugal activity in the motor fibres of the vas deferens nerve, but no impulses were detected in the hypogastric nerve.5. Ganglion cells are present in the last 2 cm of the hypogastric nerve.6. It is concluded that there is a ganglionic relay between the hypogastric and vas deferens nerves.

Nervous factors influencing the membrane activity of intestinal smooth muscle

The effects of various chemical agents on the spontaneous membrane activities and those electrically elicited in the smooth muscles of small intestine were investigated.1. The effects of various chemicals on the spontaneously active membrane might be summarized as follows. (a) Cholinergic agents; atropine slightly hyperpolarized the membrane and reduced the amplitude of slow potential changes even in aged preparations. Prostigmine depolarized the membrane, and enhanced the amplitude and prolonged the duration of the slow potential changes. Atropine prevented the actions of prostigmine on the membrane. (b) Ba(2+) depolarized the membrane, and enhanced the amplitude and prolonged the duration of the slow potential changes. The spike frequency was initially increased, then reduced. Atropine and tetrodotoxin partially prevented the action of Ba(2+) on the membrane activities.2. Effects of chemical agents on the membrane activity elicited by electrical stimulation might be summarized as follows. (a) Short pulse stimulation (0.5-1 msec) generated the spike as a direct response of the muscle cell membrane, then it was followed by slow depolarization, delayed hyperpolarization, i.e. the ;inhibitory potential', and post-inhibitory rebound successively. (b) The slow depolarization and the post-inhibitory rebound were reduced in amplitude by treatment with atropine, and enhanced by treatments with prostigmine and Ba(2+). Tetrodotoxin blocked all activities except the spike.3. When repetitive stimulation (20 c/s) was applied to the membrane, the membrane hyperpolarized; then, after 3-5 sec, it gradually depolarized even if the stimulation was continued, and triggered spikes. The hyperpolarization always preceded depolarization. The duration and the amplitude of the delayed depolarization was proportionally increased by the increased intensity and duration of stimulation. Atropine and tetrodotoxin blocked the generation of the post-inhibitory rebound.4. Effects of repetitive stimulation on the stored tissues were observed. The responses to repetitive stimulation of the membrane of muscles which had been stored 50 hr at 4 degrees C, were the same as those observed in the fresh tissue. The response of the tissue which had been stored 100 hr was the same as that observed in the fresh tissue treated with tetrodotoxin, i.e. all activities except the spikes were blocked.

Properties of the inhibitory potential of smooth muscle as observed in the response to field stimulation of the guinea-pig taenia coli

1. The inhibitory potential evoked by field stimulation of the guineapig taenia coli was studied in hypertonic solution in which the spontaneous electrical and mechanical activity was slowed or abolished.2. There was no direct correlation between the inhibitory potential and the muscle membrane polarization in response to the stimulating current. The chronaxie for the inhibitory potential was less than 1 msec and that for the muscle spike 20-30 msec.3. The inhibitory potential decreased in size along the tissue when one end of the tissue was stimulated, but its spatial decay was slower than that of the electrotonic potential in the muscle bundle.4. Tetrodotoxin (5 x 10(-7) g/ml.) abolished the inhibitory potential without affecting the electrical properties of the muscle membrane. It was concluded that the inhibitory potential was the result of stimulation of intrinsic inhibitory nerves.5. The following properties of the intrinsic nerve fibres were deduced from the experimental results: The length of the nerve fibres is probably only a few mm and the space constant of the order of 0.1 mm. No repetitive firing is produced by a long current pulse. The absolute refractory period is 3-4 msec.

An analysis of the transmission of excitation from autonomic nerves to smooth muscle

1. An analysis has been made of the transmission of excitation from the hypogastric nerve to the smooth muscle cells of the guinea-pig vas deferens.2. Depolarization of single muscle cells with current pulses from an intracellular electrode gave local depolarizations of the cell membrane which were not propagated. The total membrane resistance after 100 msec of depolarization was 15 MOmega for depolarizations between 10 and 40 mV.3. Depolarization of some cell membranes with a current pulse during the excitatory junction potential (E.J.P.) decreased the amplitude of the E.J.P. from about 10 mV at 20 mV depolarization, to nearly zero at 60 mV depolarization. In some cells the E.J.P. was unchanged during depolarizations of 50 mV.4. The action of transmitter on the smooth muscle cell membrane continued for the duration of the E.J.P. Action potentials which occurred at various times during the E.J.P. failed to remove the remaining phases of the E.J.P.5. It was shown that the slow time course of the E.J.P. could not be due to the instantaneous and simultaneous release of transmitter from a number of relatively distant sources.6. It was shown that each smooth muscle cell was innervated by several axons. The serial sections examined with the electron microscope showed that a smooth muscle had either a single axon terminating within 200 A of the muscle or no axons terminating on it at all. Therefore transmitter must be released along the length of the axons as well as at the terminations of the axons.