Electrical activity in the chemoreceptors of the blowfly. II. Responses to electrical stimulation

An analysis of the various parts of the electrical responses to the chemical and electrical stimulation of a single labellar chemosensory hair of the blowfly, Phormia regina, indicates that the recording conditions for the spike potentials approximate the intracellular recordings made in other types of sense cells. The large positive resting potential probably arises from the basement membrane of the hypodermal cells and neurilemma rather than from the neurons at the base of the chemosensory hair. The responses to polarizing currents passed through single chemosensory hairs support this analysis. The behavioral responses to similar polarizing currents are shown to result from the action of the current on the neurons at the bases of the adjacent chemosensory hairs. The reported neural interaction of the two chemosensory neurons associated with the chemosensory hair is probably due to the physical-chemical attributes of the stimulating solution rather than to any real neural interaction. Observations on the latency of the initial nerve impulse in response to chemical stimulation indicate that the chemosensory neurons are normally free from spontaneous spike activity.     

Reliable, responsive pacemaking and pattern generation with minimal cell numbers: the crustacean cardiac ganglion

Investigations of the electrophysiology of crustacean cardiac ganglia over the last half-century are reviewed for their contributions to elucidating the cellular mechanisms and interactions by which a small (as few as nine cells) neuronal network accomplishes extremely reliable, rhythmical, patterned activation of muscular activity-in this case, beating of the neurogenic heart. This ganglion is thus a model for pacemaking and central pattern generation. Favorable anatomy has permitted voltage- and space-clamp analyses of voltage-dependent ionic currents that endow each neuron with the intrinsic ability to respond with rhythmical, patterned impulse activity to nonpatterned stimulation. The crustacean soma and initial axon segment do not support impulse generation but integrate input from stretch-sensitive dendrites and electrotonic and chemically mediated synapses on axonal processes in neuropils. The soma and initial axon produce a depolarization-activated, calcium-mediated, sustained potential, the "driver potential," so-called because it drives a train of impulses at the "trigger zone" of the axon. Extreme reliability results from redundancy and the electrotonic coupling and synaptic interaction among all the neurons. Complex modulation by central nervous system inputs and by neurohormones to adjust heart pumping to physiological demands has long been demonstrated, but much remains to be learned about the cellular and molecular mechanisms of action. The continuing relevance of the crustacean cardiac ganglion as a relatively simple model for pacemaking and central pattern generation is confirmed by the rapidly widening documentation of intrinsic potentials such as plateau potentials in neurons of all major animal groups. The suite of ionic currents (a slowly inactivating calcium current and various potassium currents, with variations) observed for the crustacean cardiac ganglion have been implicated in or proven to underlie a majority of the intrinsic potentials of neurons involved in pattern generation.     

Augmentation and suppression of action potentials by estradiol in the myometrium of pregnant rat.

The purpose of this study was to investigate the actions of estradiol on spontaneous and evoked action potentials in the isolated longitudinal smooth muscle cells of the pregnant rat. Single cells were obtained by enzymatic digestion from pregnant rat longitudinal myometrium. Action potentials and currents were recorded by whole-cell current-clamp and voltage-clamp methods, respectively. The acute effects of 17beta-estradiol on action potentials and inward and outward currents were investigated. The following results were obtained. The average resting membrane potential of single myometrial cells was -54 mV (n = 40). In many cells, an electrical stimulation evoked a membrane depolarization, and action potentials were superimposed on the depolarization. In some cells, spontaneous action potentials were observed. Estradiol (30 microM) slightly depolarized the membrane (ca. 5 mV) and attenuated the generation of action potentials by reducing the frequency and amplitude of the spikes. Afterhyperpolarization was also attenuated by estradiol (30 microM). On the other hand, in 5 of 35 cells, estradiol increased the first spike amplitude and action potential duration, while frequency of the spike generation and afterhyperpolarization were inhibited. In voltage-clamped muscle cells, estradiol inhibited both inward and outward currents. Acute inhibition or augmentation of spike generation by estradiol is due to the balance of inhibition of inward and outward currents. Inhibition of both currents also prevented afterhyperpolarization, causing potential-dependent block of Ca spikes.     

The Spread of Excitation among Neurons in the Heart Ganglion of the Stomatopod, Squilla oratoria

Neurons in the heart ganglion of the mantis shrimp (a stomatopod crustacean) are functionally tightly linked together. The extracellular action potential from the whole trunk very often shows a complex form, but the response is all-or-none to the applied stimulus, indicating that the excitation in one neuron spreads very rapidly to all others. Application of isotonic MgCl₂ solution or repetitive stimulation sometimes separates the spike into its components. The resting potential of the soma membrane is 50 to 60 mv. External stimulation elicits a spike of 60 to 80 mv amplitude with a step on its rising phase. Hyperpolarization reveals one more inflection on the rising phase. These inflections divide the soma action potential into three parts, A₁, A₂, and B spikes in that order from the foot. The B spike disappears on increasing the hyperpolarization, but A₁ and A₂ remain, indicating that B originates from the soma membrane, whereas A₁ and A₂ originate from the two axons of the bipolar cell. Thus the impulse invades the soma from two directions, one from the stimulated side, the other from the other side via the "parallel axons" and the "side-connections;" the latter are presumed to interconnect the axons. When the parallel axons are cut, conduction takes place across the soma with a greatly reduced safety factor and a prolonged conduction time. Neuron-to-neuron transmission takes place in either direction.     

Electrophysiological effects of a neurotoxin extracted from the skin of the Australian frog Pseudophryne coriacea

1. The electrophysiological effects of a pumiliotoxin-B-like alkaloid extracted from the skin of the Australian frog Pseudophryine coriacea (PsC) have been studied in rat superior cervical ganglia at 37°C.2. PsC (50 mg/ml) elicits a broadening of the evoked compound action potential and, at rest, the appearance of spontaneous spike discharge at 10–20 Hz. Action potentials presumably originate far away from the soma, which is invaded in a typical ISSD sequence.3. The toxin effect is not related to any direct action on the preganglionic fibers of the sympathetic trunk, and does not involve synaptic mechanisms.4. Two-electrode voltage-clamp experiments showed that the main properties of the major voltage-dependent ionic currents are apparently unaffected by the toxin, while the cell input resistance is considerably reduced.5. The data are consistent with the hypothesis that PsC elicits a cationic permeability increase generating a pacemaker current in a region close to the cell soma.     

Electrophysiology of Supramedullary Neurons in Spheroides maculatus : I. Orthodromic and antidromic responses

This series of three papers presents data on a system of neurons, the large supramedullary cells (SMC) of the puffer, Spheroides maculatus, in terms of the physiological properties of the individual cells, of their afferent and efferent connections, and of their interconnections. Some of these findings are verified by available anatomical data, but others suggest structures that must be sought for in the light of the demonstration that these cells are not sensory neurons. Analysis on so broad a scale was made possible by the accessibility of the cells in a compact cluster on the dorsal surface of the spinal cord. Simultaneous recordings were made intracellularly and extracellularly from individual cells or from several, frequently with registration of the afferent or efferent activity as well. The passive and active electrical properties of the SMC are essentially similar to those of other neurons, but various response characteristics have been observed which are related to different excitabilities of different parts of the neuron, and to specific anatomical features. The SMC produce spikes to direct stimuli by intracellular depolarization, or by indirect synaptic excitation from many afferent paths, including tactile stimulation of the skin. Responses that were evoked by intracellular stimulation of a single cell cause an efferent discharge bilaterally in many dorsal roots, but not in the ventral. Sometimes several distinct spikes occurred in the same root, and behaved independently. Thus, a number of axons are efferent from each neuron. They are large unmyelinated fibers which give rise to the elevation of slowest conduction in the compound action potential of the dorsal root. A similar component is absent in the ventral root action potential. Antidromic stimulation of the axons causes small potentials in the cell body, indicating that the antidromic spikes are blocked distantly to the soma, probably in the axon branches. The failure of antidromic invasion is correlated with differences in excitability of the axons and the neurite from which they arise. As recorded in the cell body, the postsynaptic potentials associated with stimulation of afferent fibers in the dorsal roots or cranial nerves are too small to discharge the soma spike. The indirect spike has two components, the first of which is due to the synaptically initiated activity of the neurite and which invades the cell body. The second component is then produced when the soma is fired. The neurite impulse arises at some distance from the cell body and propagates centrifugally as well as centripetally. An indirect stimulus frequently produces repetitive spikes which are observed to occur synchronously in all the cells examined at one time. Each discharge gives rise to a large efferent volley in each of the dorsal roots and cranial nerves examined. The synchronized responses of all the SMC to indirect stimulation occur with slightly different latencies. They are due to a combination of excitation by synaptic bombardment from the afferent pathways and by excitatory interconnections among the SMC. Direct stimulation of a cell may also excite all the others. This spread of activity is facilitated by repetitive direct excitation of the cell as well as by indirect stimulation.     

Spike Potentials Recorded from the Insect Photoreceptor

Slow and spike potentials were recorded from single cells in the receptor layer of the compound eye of the drone of the honeybee. From electron microscopic observation of the drone ommatidium, it was concluded that the response had been recorded from the retinula cell. The following hypothesis is suggested for the initiation of spike potentials in the drone compound eye: Photic stimulation results in a decrease in the resistance of all or part of the retinula cell membrane, giving rise to the retinal action potential. The retinal action potential causes outflow of the current through the proximal process of the cell. This depolarizing current initiates spike potentials in the proximal process or axon of the retinula cell which are recorded across the soma membrane of the retinula cell.     

Steps in the production of motoneuron spikes

1. Spikes evoked in spinal motoneurons by antidromic stimulation normally present an inflection in their rising phase. A similar inflection is present in spikes evoked by direct stimulation with short pulses. 2. In either case the inflection becomes less prominent if the motoneuron membrane is depolarized and more prominent when it is hyperpolarized. Both antidromic and direct spikes may fall from the level of the inflection thus evoking a "small spike" only if sufficient hyperpolarization is applied. Similar events occur when antidromic or direct spikes are evoked in the aftermath of a preceding spike. 3. Spikes evoked by direct stimuli applied shortly after firing of a "small spike" may also become partially blocked at a critical stimulus interval. At shorter intervals, however, spike size again increases and no inflection can be detected in the rising phase. 4. When a weak direct stimulus evokes a small spike only, a stronger stimulus may evoke a full spike. Curves of the strength of the stimuli required for eliciting small or full spikes have been constructed in a number of conditions. 5. To explain the results it is assumed that threshold of the major portions of the soma membrane is higher than the threshold of the axon, the transition occurring over a finite area near the axon hillock. Following antidromic or direct stimulation, soma excitation is then initiated in the region of the axon hillock. Spread of activity towards the soma occurs at first slowly and with low safety factor. At this stage block may be easily evoked. Safety factor for propagation increases rapidly as the growing impulse involves larger and larger areas of the soma membrane so that, once the critical areas are excited, activation of the remaining portions of the soma membrane will suddenly occur.     

Further study of soma,dendrite, and axon excitation in single neurons

The present investigation continues a previous study in which the soma-dendrite system of sensory neurons was excited by stretch deformation of the peripheral dendrite portions. Recording was done with intracellular leads which were inserted into the cell soma while the neuron was activated orthodromically or antidromically. The analysis was also extended to axon conduction. Crayfish, Procambarus alleni (Faxon) and Orconectes virilis (Hagen), were used. 1. The size and time course of action potentials recorded from the soma-dendrite complex vary greatly with the level of the cell's membrane potential. The latter can be changed over a wide range by stretch deformation which sets up a "generator potential" in the distal portions of the dendrites. If a cell is at its resting unstretched equilibrium potential, antidromic stimulation through the axon causes an impulse which normally overshoots the resting potential and decays into an afternegativity of 15 to 20 msec. duration. The postspike negativity is not followed by an appreciable hyperpolarization (positive) phase. If the membrane potential is reduced to a new steady level a postspike positivity appears and increases linearly over a depolarization range of 12 to 20 mv. in various cells. At those levels the firing threshold of the cell for orthodromic discharges is generally reached. 2. The safety factor for conduction between axon and cell soma is reduced under three unrelated conditions, (a) During the recovery period (2 to 3 msec.) immediately following an impulse which has conducted fully over the cell soma, a second impulse may be delayed, may invade the soma partially, or may be blocked completely. (b) If progressive depolarization is produced by stretch, it leads to a reduction of impulse height and eventually to complete block of antidromic soma invasion, resembling cathodal block, (c) In some cells, when the normal membrane potential is within several millivolts of the relaxed resting state, an antidromic impulse may be blocked and may set up within the soma a local potential only. The local potential can sum with a second one or it may sum with potential changes set up in the dendrites, leading to complete invasion of the soma. Such antidromic invasion block can always be relieved by appropriate stretch which shifts the membrane potential out of the "blocking range" nearer to the soma firing level. During the afterpositivity of an impulse in a stretched cell the membrane potential may fall below or near the blocking range. During that period another impulse may be delayed or blocked. 3. Information regarding activity and conduction in dendrites has been obtained indirectly, mainly by analyzing the generator action under various conditions of stretch. The following conclusions have been reached: The large dendrite branches have similar properties to the cell body from which they arise and carry the same kind of impulses. In the finer distal filaments of even lightly depolarized dendrites, however, no axon type all-or-none conduction occurs since the generator potential persists to a varying degree during antidromic invasion of the cell. With the membrane potential at its resting level the dendrite terminals contribute to the prolonged impulse afternegativity of the soma. 4. Action potentials in impaled axons and in cell bodies have been compared. It is thought that normally the over-all duration of axon impulses is shorter. Local activity during reduction of the safety margin for conduction was studied. 5. An analysis was made of high frequency grouped discharges which occasionally arise in cells. They differ in many essential aspects from the regular discharges set up by the generator action. It is proposed that grouped discharges occur only when invasion of dendrites is not synchronous, due to a delay in excitation spread between soma and dendrites. Each impulse in a group is assumed to be caused by an impulse in at least one of the large dendrite branches. Depolarization of dendrites abolishes the grouped activity by facilitating invasion of the large dendrite branches.     

Processes of excitation in the dendrites and in the soma of single isolated sensory nerve cells of the lobster and crayfish

The stretch receptor organs of Alexandrowicz in lobster and crayfish possess sensory neurons which have their cell bodies in the periphery. The cell bodies send dendrites into a fine nearby muscle strand and at the opposite pole they give rise to an axon running to the central nervous system. Mechanisms of excitation between dendrites, cell soma, and axon have been studied in completely isolated receptor structures with the cell components under visual observation. Two sensory neuron types were investigated, those which adapt rapidly to stretch, the fast cells, and those which adapt slowly, the slow cells. 1. Potentials recorded from the cell body of the neurons with intracellular leads gave resting potentials of 70 to 80 mv. and action potentials which in fresh preparations exceeded the resting potentials by about 10 to 20 mv. In some experiments chymotrypsin or trypsin was used to make cell impalement easier. They did not appreciably alter resting or action potentials. 2. It has been shown that normally excitation starts in the distal portion of dendrites which are depolarized by stretch deformation. The changed potential within the dendritic terminals can persist for the duration of stretch and is called the generator potential. Secondarily, by electrotonic spread, the generator potential reduces the resting potential of the nearby cell soma. This excitation spread between dendrites and soma is seen best during subthreshold excitation by relatively small stretches of normal cells. It is also seen during the whole range of receptor stretch in neurons in which nerve conduction has been blocked by an anesthetic. The electrotonic changes in the cells are graded, reflecting the magnitude and rate of rise of stretch, and presumably the changing levels of the generator potential. Thus in the present neurons the resting potential and the excitability level of the cell soma can be set and controlled over a wide range by local events within the dendrites. 3. Whenever stretch reduces the resting membrane potential, measured in the relaxed state in the cell body, by 8 to 12 mv. in slow cells and by 17 to 22 mv. in fast cells, conducted impulses are initiated. It is thought that in slow cells conducted impulses are initiated in the dendrites while in fast cells they arise in the cell body or near to it. In fresh preparations the speed of stretch does not appreciably influence the membrane threshold for discharges, while during developing fatigue the firing level is higher when extension is gradual. 4. Some of the specific neuron characteristics are: Fast receptor cells have a relatively high threshold to stretch. During prolonged stretch the depolarization of the cell soma is not well maintained, presumably due to a decline in the generator potential, resulting in cessation of discharges in less than a minute. This appears to be the basis of the relatively rapid adaptation. A residual subthreshold depolarization can persist for many minutes of stretch. Slow cells which resemble the sensory fibers of vertebrate spindles are excited by weak stretch. Their discharge rate remains remarkably constant for long periods. It is concluded that, once threshold excitation is reached, the generator potential within slow cell dendrites is well maintained for the duration of stretch. Possible reasons for differences in discharge properties between fast and slow cells are discussed. 5. If stretch of receptor cells is gradually continued above threshold, the discharge frequency first increases over a considerable range without an appreciable change in the firing level for discharges. Beyond that range the membrane threshold for conducted responses of the cell soma rises, the impulses become smaller, and partial conduction in the soma-axon boundary region occurs. At a critical depolarization level which may be maintained for many minutes, all conduction ceases. These overstretch phenomena are reversible and resemble cathodal block. 6. The following general scheme of excitation is proposed: stretch deformation of dendritic terminals → generator potential → electrotonic spread toward the cell soma (prepotential) → dendrite-soma impulse → axon impulse. 7. Following release of stretch a transient hyperpolarization of slow receptor cells was seen. This off effect is influenced by the speed of relaxation. 8. Membrane potential changes recorded in the cell bodies serve as very sensitive detectors of activity within the receptor muscle bundles, indicating the extent and time course of contractile events.     

Axonal branching pattern and coupling mechanisms of the cerebral giant neurones in the snail,lymnaea stagnalis

The axonal branching pattern of the two cerebral giant neurones (CGCs) of Lymnaea stagnalis was studied with intrasomatically applied horseradish peroxidase. The cells are symmetrical. Each CGC projects to the ipsilateral n. labialis medius and n. arteriae labialis, the subcerebral commissure, and to all ipsi- and contralateral buccal nerves. The contralateral buccal nerves are reached via the ipsilateral cerebro-buccal connective and the buccal commissure. The CGC fire action potentials 1:1 in a driver-follower relationship. Each cell is capable of both driving and following. The relationship depends on the membrane potentials of the somata. In driving CGC spikes are initiated in a cerebral spike trigger zone located near the soma. In following cells spikes are initiated in a distal zone located in the buccal ganglia. The buccal zone is only affected by the partner CGC. CGC are synchronized by three coupling mechanisms: mutual excitatory chemical synapses, electrotonic coupling, and common input. The chemical and electrotonic connections are located in the buccal ganglia. All spikes are relayed to the partner cell via the chemical synapses. The electrotonic coupling improves the efficiency of the chemical synapses. The dual connection selectively synchronizes the CGC-axonal spikes from each side of the buccal mass. Common excitatory input affects the cerebral spike trigger zones and can initiate simultaneous spikes in both cells. This results in bilateral synchrony of spikes in the CGC-axons in both the buccal and the lip nerves.     

Electrical properties of the dendrite in an insect mechanoreceptor: Effects of antidromic or direct electrical stimulation

A study of the negative phase of the spikes recorded extra cellularly from insect mechanoreceptor has been performed in order to characterize some electrical properties of the dendrite which contains the transducing part of the sensory neuron. These properties have been investigated in mechanoreceptors of the metathoracic leg of the locust Schistocerca gregaria by firing antidromic action potentials both at rest and during mechanical or electrical stimulation. The amplitude of the negative phase of the spike appears to be correlated with the polarization of the dendritic membrane, although when bursts of action potentials are applied, the relation is more complex, including a depressive influence of a given spike on the following spike. The receptor potential and the antidromic dendritic spikes both originate in the same region of the dendrite but they involve different ionic processes. Our results indicate that the dendrite is electrically excitable. The spike which originates in the dendrite has an initial negative phase with a small superimposed positive component. A spike of this shape is never observed under natural stimulation. It is proposed that the negative phase of the antidromic impulse provides a suitable means for studying the variations in electrical polarization of the dendrite which cannot be recorded directly.     

Identification of Active Membrane Areas in the Giant Neuron of Aplysia

Intracellular and extracellular potentials were simultaneously recorded from the soma and different parts of the axon of the giant cell of Aplysia. Evidence was obtained that for all modes of stimulation the spike originates in the axon at some distance from the cell body. The conduction of the spike is blocked at a distance of 200 to 300 µ from the soma for the antidromic spike, closer to the soma for an orthodromic spike. This event is recorded in the soma as a small or A spike. After some delay, a spike is initiated in the resting part of the axon and in the axon hillock; the soma is invaded only afterwards. The response of these three parts of the neuron is recorded in the soma as the big or S spike.     

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.     

Hypertrophy of inferior olivary neurons: a degenerative, regenerative or plasticity phenomenon

After contralateral hemi-cerebellectomy, neurons in the cat inferior olive may either degenerate, appear unchanged (affected) or become hypertrophic. Morphological and physiological aspects of the latter two cell types are studied by means of intracellular recording and injection techniques and compared to normal olivary neurons. It is demonstrated that affected and hypertrophic olivary neurons can be activated by mesodiencephalic stimulation. Affected olivary neurons are morphologically very similar to normal cells. However, they may respond with long latency action potentials only to mesodiencephalic stimulation. Hypertrophic olivary neurons have an enlarged dendritic tree and soma. The soma and proximal dendrites are studded with spine-like processes. Their reaction to mesodiencephalic stimulation is very diverse and may consist of short and/or long latency action potentials that may or may not trigger dendritic spikes. It is argued that olivary hypertrophy does not present either a degenerative or regenerative state, but that both hypertrophic as well as affected olivary neurons can survive axotomy due to a strong and continuous electrotonic coupling, made possible by destruction of the GABAergic cerebellar afferents.     

Electrical Interaction of Paired Ganglion Cells in the Leech

The two paired giant ganglion cells (PGC's) found in each ganglion of the leech central nervous system fire synchronously in response to certain sensory input. Polarizing current passed into either of these cells is seen to displace the membrane potentials of both cells, the voltage attenuation between the two somata ranging from 2 to 5 times. This attenuation factor remains unchanged when the direction of the polarizing current is reversed, and remains about the same when the other cell of the pair is directly polarized. When one of the PGC's is partially depolarized with outward current, a repetitive train of impulses is generated. Each spike is followed by a spike in the other cell. Occasionally, a small subspike potential is seen in place of a follower spike. This potential appears to differ in shape and time course from synaptic potentials elicited by afferent input to these cells, and appears rather to be an electrotonic potential derived from the prejunctional impulse in the stimulated PGC. It is proposed that transmission between these cells is electrical, being accomplished by a flow of local circuit current across a non-rectifying junction or connection to the spike-initiating region of the other PGC.     

Impulse Origin and Propagation in a Bipolar Sensory Neuron

Intracellular recording techniques were used to study electrical activity in bipolar sensory cells associated with crayfish tactile receptors. Several lines of evidence indicate that spikes evoked by natural stimulation of the receptor originate at a dendritic locus. Although overshooting spikes are recorded in the soma in response to both natural and antidromic stimulation receptor potentials are observed only rarely, and, when present, their amplitude is less than 5 mv. Impulses propagating centrifugally into the soma following antidromic stimulation always exhibit an inflection in the rising phase of the spike; however, orthodromic spikes are usually uninflected. Occasionally, orthodromic responses (in the soma) exhibit rather unusual wave forms. Such spikes evoked by natural stimuli are indistinguishable from those elicited electrically in the dendrite, but they do not resemble antidromic impulses. Because the axonal and dendritic boundaries of the soma have a low safety factor for spike transmission, at high frequencies invasion of the soma by dendritic spikes is impeded and often blocked. The soma region can thus act as a low-pass filter. The significance of this self-limiting mechanism for the behavior of the animal is not known; it is suggested, however, that this impediment is a potentially critical one, and may, in other situations, have encouraged the evolution of alternative arrangements.     

Modulation of Activity of One Neuron by Subthreshold Slow Potentials in Another in Lobster Cardiac Ganglion

A direct demonstration is given of interaction between specific neurons without impulses, via graded slow potentials electrotonically spread from one cell to another. Repetitive polarizing or depolarizing current pulses of 50 to 200 msec. and subthreshold intensity were passed through an intracellular electrode in the soma of a follower cell of the isolated ganglion. When the frequency is near the natural rhythm of impulse bursts corresponding to heart beats and arising in a pacemaker cell 5 to 10 mm. posteriorly, the bursts rapidly become synchronized with the pulses. The effect disappears upon withdrawing the intracellular electrode. Brief pulses or full spikes in the follower are not effective. Hyperpolarizing long pulses attract the burst to a fixed period after the end of the pulse, depolarizations after the beginning of the pulse. The natural rhythm promptly reappears when the pulses are stopped and occasionally breaks through during weak repetitive pulses. Current pulses in postsynaptic cells also alter the threshold of a presynaptic neuron to externally applied stimuli. Some kind of direct, low resistance pathway for electrotonic spread, discriminating against spikes because of their brevity, is inferred, providing a basis for subthreshold interaction which is specific and not by way of a field effect. Due to the sensitivity of modulation of ongoing rhythms, electrotonic currents can be effective even after decrementing over several millimeters.     

Voltage-activated Ca2+ currents in insulin-secreting cells

Membrane voltage and voltage-clamped membrane currents have been investigated with the whole-cell patch clamp method in the insulin-secreting cell line RINm5F. The mean resting membrane potential of RINm5F cells was found to be -52 mV. Overshooting spike potentials could be evoked by depolarising voltage steps in the absence of a secretagogue. Inward membrane currents evoked by depolarising voltage steps were dependent upon extracellular Ca2+ and blocked by Co2+, nifedipine and verapamil. Outward membrane currents which were evoked by depolarising voltage steps to positive membrane potentials were reduced when Ca2+ entry was prevented. It is concluded that the voltage-activated Ca2+ currents underlie the voltage-activated spike potentials recorded from insulin-secreting cells.     

Magnesium-resistant excitatory synaptic potentials in the leech retzius cell

Postsynaptic potentials (PSPs) recorded from leech Retzius cells in response to stimulation of interganglionic connective could not be reversed by soma depolarization or abolished by 40 mM Mg ion, nor could input resistance changes be detected during them. Alteration of external Cl and K over a tenfold range provided no clear evidence that the PSPs involved a conductance change to either ion. The method of extrapolation yielded an apparent PSP equilibrium potential of about ?20 mV. The steep portion of the relationship between Retzius cell action potential amplitude and membrane potential extrapolated to an apparent reversal potential of ?13 mV. It is likely that the connective-to-Retzius cell PSPs were principally electrical events. Their apparent reversal potentials could have been in the range associated with chemical synapses because they traversed an electrical synapse with a variable coupling resistance, or because the polarizing currents, passing “backwards” across electrical synapses, changed the amplitude of the presynaptic action potentials.