Diphasic synaptic potentials and prolonged poststimulus hyperpolarization in Helix pomatia neurons

Synaptic activity of neurons giving diphasic excitatory-inhibitory potentials in response to orthodromic stimulation was recorded intracellularly. In response to stimulation of nerves by a single short pulse these neurons developed only the excitatory component of the diphasic potential, but with a longer stimulus a prolonged inhibitory phase, partly suppressing the initial excitatory component, was added. The excitatory phase appeared only when the resting potential reached a certain level. In their response to repetitive stimulation, neurons with a diphasic potential are divided into habituating and nonhabituating. The diphasic potential can also arise in response to application of acetylcholine to the soma of these neurons. It is postulated that this potential reflects the response of different receptors of the postsynaptic membrane to the same mediator. Prolonged poststimulus hyperpolarization can be obtained after repetitive orthodromic or direct stimulation of some neurons. However, as analysis of the results showed, poststimulus hyperpolarization is endogenous in origin and differs in its mechanisms from the diphasic potential.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 5, No. 2, pp. 193–200, March–April, 1973.     

Mechanism of anode break stimulation in the heart.

Anodal stimulation is routinely observed in cardiac tissue, but only recently has a mechanism been proposed. The bidomain cardiac tissue model proposes that virtual cathodes induced at sites distant from the electrode initiate the depolarization. In contrast, none of the existing cardiac action potential models (Luo-Rudy phase I and II, or Oxsoft) predict anodal stimulation at the single-cell level. To determine whether anodal stimulation has a cellular basis, we measured membrane potential and membrane current in mammalian ventricular myocytes by using whole-cell patch clamp. Anode break responses can be readily elicited in single ventricular cells. The basis of this anodal stimulation in single cells is recruitment of the hyperpolarization-activated inward current I(f). The threshold of activation for I(f) is -80 mV in rat cells and -120 mV in guinea pig or canine cells. Persistent I(f) "tail" current upon release of the hyperpolarization drives the transmembrane potential toward the threshold of sodium channels, initiating an action potential. Time-dependent block of the inward rectifier, I(K1), at hyperpolarized potentials decreases membrane conductance and thereby potentiates the ability of I(f) to depolarize the cell on the break of an anodal pulse. Inclusion of I(f), as well as the block and unblock kinetics of I(K1), in the existing Luo-Rudy action potential model faithfully reproduces anode break stimulation. Thus active cellular properties suffice to explain anode break stimulation in cardiac tissue.     

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.     

Unique after-hyperpolarization accompanying action potential inChara globularis

A unique after-hyperpolarization was found in internodal cells ofChara globularis. The cells generated an ordinary action potential due to regenerative depolarization induced by the outward electric current pulse larger than a threshold stimulus. After reaching a depolarizing peak, the membrane potential repolarized and overshooted the resting potential to a value which was somehow 40 mV more negative than the resting potential before stimulation (after-hyperpolarization). Since the membrane resistance increased during the after-hyperpolarization, the after-hyperpolarization is thought to be caused by an increase in the resistance (decrease in the conductance) of the passive diffusion channel.     

Analytical modeling of the hysteresis phenomenon in guinea pig ventricular myocytes

In the present study, we have demonstrated hysteresis phenomena in the excitability of single, enzymatically dissociated guinea pig ventricular myocytes. Membrane potentials were recorded with patch pipettes in the whole-cell current clamp configuration. Repetitive stimulation with depolarizing current pulses of constant cycle length and duration but varying strength led to predictable excitation (1:l) and non-excitation (1:0) patterns depending on current strength. In addition, transition between patterns depended on the direction of current intensity change and stable hysteresis loops were obtained in stimulus:response pattern vs. current intensity plots in 14 cells. Increase of pulse duration and decrease of stimulation rate contributed to a reduction in hysteresis loop areas. Changes in amplitude and shape of the subthreshold responses during the transitions from one stable pattern to the other, suggested that activity led to an increase in membrane resistance, particularly in the voltage domain between resting potential, and threshold. Therefore, we modelled the dynamic behaviour of the single cells as a function of diastolic membrane resistance, using previously published analytical solutions. Numerical iteration of the analytical model equations closely reproduced the experimental hysteresis loops in both qualitative and quantitative ways. In particular, the effect of stimulation frequency on the model was similar to the experimental findings. The overall study suggests that the excitability pattern of guinea pig ventricular myocytes accounts for hysteresis and bistabilities when current intensity is allowed to fluctuate around threshold levels.     

Responses of the Smooth Muscle Membrane of Guinea Pig Jejunum Elicited by Field Stimulation

Field stimulation of the jejunum elicited successively an action potential of spike form, a slow excitatory depolarization, a slow inhibitory hyperpolarization, and a postinhibitory depolarization as a rebound excitation. The slow depolarization often triggered the spike. The inhibitory potential showed lower threshold than did the excitatory potential. Both the excitatory potentials were abolished by atropine and tetrodotoxin. Effective membrane resistance measured by the intracellular polarizing method was reduced during the peak of the excitatory potential, but the degree of reduction was smaller than that evoked by iontophoretic application of acetylcholine. Conditioning hyperpolarization of the muscle membrane modified the amplitude of the excitatory potential. The estimated reversal potential level for the excitatory potenialt was about 0 mv. No changes could be observed in the amplitude of the inhibitory potential when hyperpolarization was induced with intracellularly applied current. Low [K]_o and [Ca]_o blocked the generation of the excitatory potential but the amplitude of the inhibitory potential was enhanced in low [K]_o. Low [Ca]_o and high [Mg]_o had no effect on the inhibitory potential.     

Increased efficacy of antidromic and orthodromic activation of cat alpha motoneurons upon arrival of coerulospinal volleys

The present study demonstrates the enhanced efficacy of impulse initiation among the hindlimb alpha motoneurons of flexor and extensor origins (n = 35) upon electrical stimulation of the locus coeruleus (LC) in decerebrate cats. When combined with the LC-evoked excitatory postsynaptic potential (EPSP), intracellular hyperpolarization-induced partial and total blocks of antidromic invasion were overcome, resulting in full-spike generation in all cells (n = 21). In three other cells, partial blocks, representing the motoneuron refractoriness resulting from double stimulation at close intervals, were relieved by the concomitant LC-EPSP. When an antidromic volley occurred at a time when the somadendritic (SD) membrane was near threshold, LC stimulation was shown to increase the probability of full-spike initiation as well as to shorten the initial segment (IS)-SD delay, suggesting a coerulospinal enhancement of the safety factor for IS-SD impulse conduction. When coincident with the LC-EPSPs, group Ia EPSPs of flexor and extensor origins were demonstrated to reach the threshold of discharging the cells (n = 4). In those cells exhibiting prominent depolarizing synaptic noise (n = 10), LC stimulation was sufficient to cause the cell to fire action potentials presumably by interacting with concomitant excitatory synaptic drive. The present results advocate that the descending LC excitatory drive has engaged in the action potential initiation process of the alpha motoneuron, facilitating its reaching the firing threshold during concurrent depressed membrane excitability as well as subthreshold converging inputs.     

Functional significance of the A-current

This work considers the response to simulated synaptic inputs of an excitable membrane model. The model is essentially of the Hodgkin-Huxley type, but contains an A-current in addition to sodium and delayed-rectifier potassium channels. The results were compared with previous simulations in which the stimulus was an injected current. These two types of stimuli give somewhat different results because synaptic stimuli directly change the membrane resistance, whereas injected current does not. The results of synaptic stimulation were similar to injected current in that very low frequencies of action potentials were elicited only where the stimulus was slightly above threshold. For most of the range of synaptic inputs that produced oscillatory behavior, the A-current had little effect on oscillation frequency. With synaptic stimuli as with injected current, the model membrane's spiking behavior does not begin immediately when an excitatory stimulus is imposed on a quiescent state. The delay before spiking is closely related to the inactivation time of the A-current. The synaptic results were different from the injected current results in that when substantial inhibition was present, the ability to produce very-low-frequency spiking was absent, even just above the excitatory threshold. The higher the degree of inhibition, the narrower the range of spike frequencies that could be elicited by excitation. At very high inhibition, no degree of excitation could elicit spiking.     

Effects of Temperature on the Generator and Action Potentials of a Sense Organ

Charge transfer through the receptor membrane of the nonmyelinated ending of Pacinian corpuscles is markedly affected by temperature. The rate of rise and the amplitude of the generator potential in response to a constant mechanical stimulus increase with temperature coefficients of 2.5 and 2.0 respectively. The duration of the falling phase, presumably a purely passive component, and the rise time of the generator potential are but little affected by temperature. The following interpretation is offered: Mechanical stimulation causes the conductance of the receptor membrane to increase and ions to flow along their electrochemical gradients. An energy barrier of about 16,000 cal/mole limits the conductance change. The latter increases, thus, steeply with temperature, causing both the rate of rise and the intensity of the generator current to increase. The membrane of the adjacent Ranvier node behaves in a distinctly different manner. The amplitude of the nodal action potential is little changed over a wide range of temperature, while the durations of its rising and falling phases increase markedly. The electrical threshold of the nodal membrane is rather constant between 40 and 12°C. Below 12°C the threshold rises, and the mechanically elicited generator current fails to meet the threshold requirements of the first node. Cold block of nerve impulse initiation then ensues, although the receptor membrane still continues to produce generator potentials in response to mechanical stimulation.     

Effect of non-symmetric waveform on conduction block induced by high-frequency (kHz) biphasic stimulation in unmyelinated axon

The effect of a non-symmetric waveform on nerve conduction block induced by high-frequency biphasic stimulation is investigated using a lumped circuit model of the unmyelinated axon based on Hodgkin-Huxley equations. The simulation results reveal that the block threshold monotonically increases with the stimulation frequency for the symmetric stimulation waveform. However, a non-monotonic relationship between block threshold and stimulation frequency is observed when the stimulation waveform is non-symmetric. Constant activation of potassium channels by the high-frequency stimulation results in the increase of block threshold with increasing frequency. The non-symmetric waveform with a positive pulse 0.4–0.8 μs longer than the negative pulse blocks axonal conduction by hyperpolarizing the membrane and causes a decrease in block threshold as the frequency increases above 12–16 kHz. On the other hand, the non-symmetric waveform with a negative pulse 0.4–0.8 μs longer than the positive pulse blocks axonal conduction by depolarizing the membrane and causes a decrease in block threshold as the frequency increases above 40–53 kHz. This simulation study is important for understanding the potential mechanisms underlying the nerve block observed in animal studies, and may also help to design new animal experiments to further improve the nerve block method for clinical applications.     

Single Versus Repetitive Spiking to the Current Stimulus of A-β Mechanosensitive Neurons in the Crotaline Snake Trigeminal Ganglion

1. Intrasomal recordings of potentials produced by current stimulation in vivo were made from 24 (A-) touch and 19 vibrotactile neurons in the trigeminal ganglion of 29 crotaline snakes, Trimeresurus flavoviridis. 2. Usually touch neurons responded with a single action potential at the beginning of a prolonged depolarizing pulse, whereas all vibrotactile neurons responded with multiple spikes.3. The electrophysiological parameters examined were membrane potential, threshold current, input resistance and capacitance, time constant, rebound latency, and its threshold current. Touch neurons had higher input resistance (and lower input capacitance) than vibrotactile neurons.4. In conclusion, current injection, which elicits a single or multiple spiking, seems a useful way to separate touch neurons from vibrotactile neurons without confirming the receptor response, and some membrane properties are also specific to the sensory modality.     

On a stochastic model for the firing sequence of a neuron

A stochastic model of a neuron with excitatories and inhibitories incident on it is studied. The excitatory and the inhibitory sequences are independent renewal processes. The effect of an excitatory is to increase the membrane potential by random amounts that are independently and identically distributed, while an inhibitory causes a reset of the potential to the rest level so that the accumulation must start anew. When the potential crosses a threshold level K, the neuron fires. Immediately after this, the membrane potential returns to the rest level. An expression for the probability density function of the interval between two successive firings is derived, and special cases worked out. Graphs of the mean and the mean − √variance versus the threshold level are presented and discussed.     

Investigation of the nature of electrical responses of horizontal cells of the fish retina

On the basis of the syncytial structure of the layer of horizontal cells of the fish retina, a method is developed which effectively shifts the membrane potential of cells by means of an electrical current. It is shown that the response of L-type horizontal cells to light and electrical stimulation of the retina is reversed when the membrane of the horizontal cells is depolarized by a direct current. The equilibrium potential of the cells was near the zero level. Consequently, the depolarization response of the horizontal cells to disconnection of the light and to electrical stimulation of the retina is an excitatory postsynaptic potential, whereas hyperpolarization of the horizontal cells to light is a decrease of this potential. It is shown that the membrane of fish horizontal cells have pronounced nonlinear properties: in the case of strong depolarization and especially in the case of hyperpolarization its impedance drops markedly. The latter probably occurs due to an increase of the permeability of the nonsynaptic membrane of the horizintal cells for K⁺. This can also explain the decrease of membrane impedance during the hyperpolarization response of the horizontal cells to bright light. The available data indicate the presence of regenerative properties of the membrane of horizontal cells.Institute of Problems of Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 89–98, January–February, 1971.     

Laser microirradiation of cerebellar neurons in culture

Electrophysiological and ultrastructural effects of focused laser radiation on neurons from neonatal rat cerebellum in tissue culture are reported. Action potentials were elicited by an extracellular current pulse train. The stimulator voltage required for half-maximum response frequency was measured as a function of the energy delivered by a single laser pulse. Above a “threshold” laser energy, the cell response to stimulation became negligible for all stimulator voltages. Electron micrographs of cells revealed that the mitochondria are preferentially damaged at an energy comparable to the electrophysiological threshold. The damaged mitochondria showed swollen matrix space and disrupted cristae membranes. Higher laser energies resulted in damage to other cytoplasmic structures. The results are consistent with a model that assumes that light interaction with the nerve cells proceeds by local heating of the mitochondria and nearby structures and leads to an increased conductance of the membrane to some ionic species.     

Physiological characteristics of the synaptic response of an identified sensory nonspiking interneuron in the crayfish Procambarus clarkii girard

Voltage-dependent variability in the shape of synaptic responses of the LDS interneuron, an identified nonspiking cell of crayfish, to mechanosensory stimulation was studied using intracellular recording and current injection techniques. Stimulation of the sensory root ipsilateral to the interneuron soma evoked a large depolarizing synaptic response. Its peak amplitude was decreased and the time course was shortened when the LDS interneuron was depolarized by current injection. When the cell was hyperpolarized, the peak amplitude was increased and the time course was prolonged. Upon large hyperpolarization, however, the amplitude did not increase further while the time course showed a slight decrease. The dendritic membrane of the LDS interneuron was found to show an outward rectification upon depolarization and an inward rectification upon large hyperpolarization. Current injection experiments at varying membrane potentials revealed that the voltage-dependent changes in the shape of the synaptic response were based on an increase in membrane conductance due to the rectifying properties of the LDS interneuron. Stimulation of the contralateral root evoked a small depolarizing potential comprising an early excitatory response and a later inhibitory component. Its shape also varied depending on the membrane potential in a manner similar to that of the synaptic response evoked ipsilaterally.     

Intracellular nonlinear frequency response measurements in the cockroach tactile spine neuron

The threshold of the cockroach tactile neuron increases strongly with depolarization by a process involving at least two time constants. This effect is probably responsible for the rapid and complete adaptation of the neuron's response to step inputs. A technique for intracellular recording and stimulation of the neuron has recently been established and this allows direct observation of the dynamic response of the neuronal encoder. A white noise stimulus was used to modulate the membrane potential of the neuron. The first-order frequency response function between membrane potential and action potential discharge could be explained by a variable threshold model with two time constants. Second-order frequency response functions could be accounted for by a Wiener cascade model. The dynamic nonlinear behavior of the encoder can therefore be explained by a unidirectional threshold which increases linearly and dynamically with membrane potential.     

Synaptic effects elicited in the Retzius cells of the leech Hirudo medicinalis by stimulation of the segmental roots

Electrical stimulation of the segmental roots of each ganglion of Hirudo medicinalis, elicits in both Retzius' cells inhibitory and excitatory effects. The IPSP and EPSP are chemical in nature, being dependent on the membrane potential, and suppressed by high Mg++. Selective inactivation of one RC shows that the responses of the contralateral RC are not due to electrotonic coupling between the two cells, but to synaptic actions impinging upon the membrane of both RCs. The two synaptic potentials appear to be mediated by two set of fibres with a different threshold to electrical stimulation. Their actions on the RCs appear to be polysynaptic on the basis of central latency. Simultaneous stimulation of two roots shows evidence for occlusion for IPSP and summation for EPSP, confirming the polysynaptic nature of the effects. The possible functional significance of the inhibitory and excitatory pathways, is discussed.     

Electrical properties of motoneurons in the spinal cord of rat embryos

Electrical properties of immature motoneurons were studied in vitro using isolated segments of spinal cords of rat embryos aged 14-21 days of gestation. Stable resting potentials and evoked synaptic potentials were recorded for more than 9 hr, indicating that motoneurons remain viable for many hours. Motoneurons are electrically excitable at 14 days of gestation and from the onset of excitability the action potentials are Na+-dependent but slow rising long-duration Ca2+-dependent action potentials can be evoked if K+ conductance is reduced. Thus, during embryonic development the regenerative potential inward current is Na+-and Ca2+-dependent. During motoneurons' differentiation there are some changes in their electrical properties: resting membrane potential increases, input resistance decreases, input capacitance increases, threshold for action potential decreases, and maximum rate of rise of action potential increases. Afferent motoneuron contacts are formed at 16-18 days of gestation when excitatory synaptic potentials can first be evoked in response to dorsal root stimulation. The changes in input capacitance and threshold for action potential occur at the onset of functional afferent motoneuron contacts, but it is not known whether these changes are autonomous or are influenced by the newly formed sensory inputs.