Transient changes in extrinsic fluorescence of nerve produced by electric stimulation

NERVES STAINED WITH ONE OF THE FOLLOWING FLUOROCHROMES WERE FOUND TO RESPOND TO ELECTRIC STIMULATION WITH TRANSIENT CHANGES IN THE FLUORESCENT LIGHT: acridine orange, acridine yellow, 8-anilinonaphthalene-1-sulfonate, auramine O, 1-dimethylaminonaphthalene-5-sulfonyl chloride, fluorescein isothiocyanate, LSD, pyronin B, pyronin Y, rhodamine B, rhodamine G, rose bengal, and 2-toluidinylnaphthalene sulfonate. The properties of covalently bonded fluorochromes in the nerve and the behavior of fluorochromes introduced into the axon interior were studied. By measuring fluorescence polarization, direct evidence was obtained showing a decrease in intramembrane viscosity during the action potential.

Acceleratory synapses on pacemaker neurons in the heart ganglion of a stomatopod, Squilla oratoria

The pacemaker neurons of the heart ganglion are innervated from the CNS through two pairs of acceleratory nerves. The effect of acceleratory nerve stimulation was examined with intracellular electrodes from the pacemaker cells. The major effects on the pacemaker potential were an increase in the rate of rise of the spontaneous depolarization and in the duration of the plateau. The aftereffect of stimulation could last for minutes. No clear excitatory postsynaptic potential (EPSP) was observed, however. On high frequency stimulation, a small depolarizing response (the initial response) was sometimes observed, but the major postsynaptic event was the following slow depolarization, or the enhancement of the pacemaker potential (the late response). With hyperpolarization the initial response did not significantly change its amplitude, but the late response disappeared, showing that the latter has the property of the local response. The membrane conductance did not increase with acceleratory stimulation. The injection of depolarizing current increased the rate of rise of the spontaneous depolarization, but only slightly in comparison with acceleratory stimulation, and did not increase the burst duration. It is concluded that the acceleratory effect is not mediated by the EPSP but is due to a direct action of the transmitter on the pacemaker membrane.

Fluorescence changes during conduction in nerves stained with Acridine Orange

Nerves from spider crabs anid squid fluoresce when staiined with Acridine Orange. The intenisity of fluorescence increases during nerse conduction. Prolongation of the electric response in the squid axon is associated with a fluorescence change of similar duration. These findings suggest that the physicochemical properties of the macromolecules around the dye molecules in the nerve membrane drastically change during the process of nerve conduction.

Inhibitory synapses on pacemaker neurons in the heart ganglion of a stomatopod, Squilla oratoria

The pacemaker neurons of the Squilla heart ganglion are innervated from the CNS through three pairs of extrinsic nerves. One of them, the alpha-nerve, is inhibitory to the heart beat. The effect of alpha-nerve stimulation on the pacemaker potential was examined with intracellular electrodes. Without extrinsic nerve stimulation the membrane potential of the pacemaker cell fluctuated spontaneously. On application of a tetanic train of stimuli to the alpha-nerve the membrane potential was shifted and fixed to a steady level, which with K(2)SO(4)-filled electrodes was near the peak of hyperpolarization after a spontaneous burst, but was less negative with KCl-filled electrodes. The shift of the membrane potential was due to the summated IPSP's. By changing the level of the membrane potential with injection of the polarizing current the IPSP could be reversed in sign, and the size of the IPSP was linearly correlated with the membrane potential level. During inhibition the membrane conductance increased. The increase depended on divalent cation concentrations in the outside medium. In Ca-rich saline the IPSP was greatly enhanced. In Mg-rich saline it was suppressed. The amplitude of antidromic spikes was reduced during inhibition especially when the spike frequency was high.

Cation interdiffusion in squid giant axons

Radiotracer techniques were used to study the influxes and effluxes of various univalent cations in internally perfused squid giant axons. Membrane currents and conductances were determined by the voltage-clamp technique under analogous internal and external conditions. Both sodium-containing and sodium-free internal and external media were studied. Membrane impedance was measured with an ac impedance bridge to determine the general magnitude and time course of the impedance loss which accompanied the excitation process in both varieties of external media. Maximum transmembrane currents were found to be of comparable magnitude to the charge transfer associated with the peak interdiffusion flux measured under the same conditions. The product of the membrane resistance and the interdiffusion flux remained constant over a wide range of resistance and flux values, both at rest and during activity, both in sodium-containing and sodium-free media. The implications of these findings for excitation theory are discussed.

Electrical properties of the pacemaker neurons in the heart ganglion of a stomatopod, Squilla oratoria

In the Squilla heart ganglion, the pacemaker is located in the rostral group of cells. After spontaneous firing ceased, the electrophysiological properties of these cells were examined with intracellular electrodes. Cells respond to electrical stimuli with all-or-none action potentials. Direct stimulation by strong currents decreases the size of action potentials. Comparison with action potentials caused by axonal stimulation and analysis of time relations indicate that with stronger currents the soma membrane is directly stimulated whereas with weaker currents the impulse first arises in the axon and then invades the soma. Spikes evoked in a neuron spread into all other neurons. Adjacent cells are interconnected by electrotonic connections. Histologically axons are tied with the side-junction. B spikes of adjacent cells are blocked simultaneously by hyperpolarization or by repetitive stimulation. Experiments show that under such circumstances the B spike is not directly elicited from the A spike but is evoked by invasion of an impulse or electrotonic potential from adjacent cells. On rostral stimulation a small prepotential precedes the main spike. It is interpreted as an action potential from dendrites.

Pacemaker potentials for the periodic burst discharge in the heart ganglion of a stomatopod, Squilla oratoria

From somata of the pacemaker neurons in the Squilla heart ganglion, pacemaker potentials for the spontaneous periodic burst discharge are recorded with intracellular electrodes. The electrical activity is composed of slow potentials and superimposed spikes, and is divided into four types, which are: (a) "mammalian heart" type, (b) "slow generator" type, (c) "slow grower" type, and (d) "slow deficient" type. Since axons which are far from the somata do not produce slow potentials, the soma and dendrites must be where the slow potentials are generated. Hyperpolarization impedes generation of the slow potential, showing that it is an electrically excitable response. Membrane impedance increases on depolarization. Brief hyperpolarizing current can abolish the plateau but brief tetanic inhibitory fiber stimulation is more effective for the abolition. A single stimulus to the axon evokes the slow potential when the stimulus is applied some time after a previous burst. Repetitive stimuli to the axon are more effective in eliciting the slow potential, but the depolarization is not maintained on continuous stimulation. Synchronization of the slow potential among neurons is achieved by: (a) the electrotonic connections, with periodic change in resistance of the soma membrane, (b) active spread of the slow potential, and (c) synchronization through spikes.

Effects of tetrodotoxin on excitability of squid giant axons in sodium-free media

The effect of tetrodotoxin on excitability of internally perfused squid giant axons immersed in various sodium-free media was examined. Action potentials were found to be suppressed by this substance, with or without sodium ion in the external medium. Tetrodotoxin showed a strong suppressive effect upon action potentials produced in media containing salts of only divalent cations (CaCl(2), CaBr(2), SrCl(2), or BaCl(2)). Our findings concerning the action of tetrodotoxin do not support the separate-channel hypothesis for excitable membranes.