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The frequency-dependent effect of various anticonvulsant drugs on the conduction in central axons was studied in the corpus callosum of rat and guinea pig brain slices from the parietal region. Extracellularly recorded compound action potentials (CAPs) were evoked by either single stimulus or high frequency stimulation (40-80 Hz). The CAP in rats consisted of an early component (fast axons, 1.2-1.8 m/s) and a late component (slow axons, 0.5-0.7 m/s), while in the guinea pig only the slow phase was observed. Diphenylhydantoin increased the latency of a single response by 10%, and had no effect on the CAP amplitude. In contrast, both phenobarbital and pentobarbital reduced the amplitude of singly evoked CAPs. Stimulation at high frequency alone decreased the CAP amplitude by 10-20%. Identical stimulation in the presence of the drugs further suppressed the CAP amplitude by an additional 31%, with varying degree of drug efficacy. The depressant effect was significant for the slow axons but the fast axons were virtually unaffected by any of the drugs. The results are consistent with the hypothesis that the antiepileptic drugs DPH, Phe and Pnt may block axonal conduction from an epileptic focus into neighbouring areas of the brain.

A bifurcating crustacean motor neuron, which serves an integrative function by selectively controlling output to its daughter branches, was examined for its behavioral response to changes in pressure and in temperature when each was varied while the other was held constant, and when both were varied together. The neuron was exposed to helium pressure between 1 and 200 ATA and temperatures between 9 and 22 degrees C. The response of the neuron to pressure changes was biphasic and time dependent. Immediately following a pressure change, action potential amplitude and conduction velocity increased, and the ability of the branch point to pass high frequency trains improved; after 15-20 min at pressures above 35 ATA these measures were depressed below control values. The curve relating functional measures to temperature displayed a time-dependent hysteresis, fast warming leading to values for amplitude, velocity, and branchpoint capacity which corresponded to those made at a point 3-5 degrees C higher during slow cooling. The delayed depressant effects of compression and cooling were synergistic. Low temperature significantly enhanced the effects of pressure on amplitude and conduction velocity; high pressure increased the Q10 of both measures. However, slow cooling antagonized the transient compression-induced excitability increase, and prolonged exposure to hyperbaric pressure diminished the temperature hysteresis. The complex time-dependent changes in this branching axon's ability to conduct are related to previously described changes in membrane potential properties. The responses of this axon to pressure changes are different from responses of other axons studied at hyperbaric pressure. Thus, even within relatively stereotyped axon membrane the effects of pressure are not generalizeable among cells. The possible relevance of these findings to the high pressure nervous syndrome (HPNS) is discussed.

The impulse conduction capability of a crustacean bifurcating motor axon was studied under various physiological and physical conditions, and effects of several pharmacological agents were tested. The passive and active membrane properties were measured by intra- and extracellular recording, macropatch clamp and vaseline gap voltage clamp. Block of condition after high-frequency stimulation is caused by accumulation of K+ in the extracellular space, while its differential nature is attributed to early activation of the Na+-K+ electrogenic pump and increased intracellular Ca2+ in the thinner branch. Decreased Na+ and K+ conductance, or increased K+ conductance induced by various drugs, largely reduced the maximal following frequency of the branchpoint. High pressure initially increased the neuron excitability, and later decreased, to below control levels, the axon ability to conduct at high frequency. Cooling had a similar effect on the delayed effects of high pressure. The physiological significance of the frequency-dependent block and its possible role in anesthesia, epilepsy, high-pressure nervous syndrome and behavior are discussed.

The escape reflex of the lobster consists of a series of tail flips resulting from alternating activity of the abdominal flexor and extensor muscles. Electromyographic (EMG) activity was recorded from the medial (DEAM) and the lateral (DEAL1) deep abdominal extensor muscles during free swimming. During the escape response, the muscles were active either synchronously or separately, at frequencies of 100-120 Hz. This activity pattern could be generated either by central programming, or by a peripheral mechanism such as frequency-dependent differential conduction block into one of the two branches of the common excitor axon (C.Ex) innervating these muscles. In order to explore the latter possibility in a living animal, we left the DEAM and DEAL1 muscles innervated only by the C.Ex from the tested segment. This was accomplished by manually cutting all other axons in the nerve under visual control. During escape responses in six successfully dissected animals, we found 27 sudden failures of the DEAM responses and only three in DEAL1. The failures were usually preceded by an increase in the delay of the response. These findings strongly suggest that conduction block occurs in the M branch innervating the DEAM under physiological conditions.

1. Propagation of action potentials at high frequency was studied in a branching axon of the lobster by means of simultaneous intracellular recording both before and after the branch point. 2. Although the branching axon studied has a geometrical ratio close to one (perfect impedance matching) conduction across the branch point failed at stimulation frequencies above 30 Hz. 3. The block of conduction after high frequency stimulation occurred at the branch point per se. The parent axon and daughter branches continued to conduct action potentials. 4. Conduction block after high frequency stimulation appeared first in the thicker daughter branch and only later in the thin branch. 5. With high frequency stimulation there was a 10-15% reduction in amplitude of the action potential in the parent axon, a corresponding decrease in the rate of rise of the action potential, a 25-30% decrease in conduction velocity, marked increase in threshold and prolongation of the refractory period. In addition the membrane was depolarized by 1-3 mV. 6. Measurements of the membrane current using the patch clamp technique showed a large decrease in the phase of inward current associated with the action potential, before the branching point. 7. The small membrane depolarization seen after high frequency stimulation is not the sole cause of the conduction block. Imposed prolonged membrane depolarization (8 mV for 120 sec) was insufficient to produce conduction block. 8. In vivo chronic extracellular recordings from the main nerve bundle (which contains the parent axon) and the large daughter branch revealed that: (a) the duration and frequency of trains of action potentials along the axons exceeded those used in the isolated nerve experiments and (b) conduction failure in the large daughter branch could be induced in the whole animal by electrical stimulation of the main branch as in the isolated preparation. 9. Possible mechanisms underlying block of conduction after high frequency stimulation in a branching axon are discussed.

1. The ionic mechanisms involved in block of conduction of action potentials following high frequency stimulation were studied in a branching axon of the lobster Panulirus penicillatus. 2. A 2-3 mM increase in extracellular K concentration (normal concentration 12 mM) produced block of conduction into both daughter branches. 3. While conduction block induced by high frequency stimulation occurs first into the large daughter branch and only later into the smaller one, propagation into both branches is blocked simultaneously by increased extracellular K concentration. 4. Increasing extracellular K by 2-3 mM resulted in membrane depolarization, reduction in membrane resistance and reduced excitability. The latter two effects were larger than expected from the small depolarization. It appears that increase of extracellular K has direct effects on membrane excitability. 5. It is suggested that block of conduction after high frequency stimulation results from accumulation of K in the extracellular space. However, in order to account for differential conduction block in the two branches one must assume differential buildup of extracellular K concentration around the two branches during high frequency stimulation. 6. Ultrastructural studies using La and horseradish peroxidase as extracellular markers show that the space around the two branches is similar and is open to the extracellular space. Therefore differences in periaxonal volume cannot account for differential buildup of K around the two branches. 7. It is demonstrated that the lobster axon has a Na+/K+ electrogenic pump. After blocking this pump with ouabain, stimulation at high frequency resulted in a conduction block in the two branches almost at the same time. 8. Injection of Ca2+ intracellularly into the thick branch prevents or delays the appearance of conduction block after high frequency stimulation. 9. A mechanism based on these findings is suggested to explain the differential conduction block seen after high frequency stimulation in a branching axon with almost ideal impedance matching.

A crustacean bifurcating motor neuron that selectively controls output to its daughter branches was exposed to helium pressure of 1-200 atmospheres (atm) and temperatures of 9-22 degrees C. The membrane responses of this integrative axon were monitored by intra- and extracellular recording, macropatch clamp, and Vaseline gap voltage clamp. The response of the neuron to pressure changes was biphasic and time dependent. Initially there was an increase in action potential amplitude and rate of rise, in magnitude of the inward sodium current, in conduction velocity, and in the ability of the branch point to conduct at high frequency. After 10-20 min at a given pressure above 35 atm, these functional measures declined to levels below control. Action potential duration increased throughout. In addition, membrane resting potential was depolarized by 10-15 mV, and input resistance increased. Pressure-related depolarization was not seen in an axon pretreated with ouabain, a result consistent with pressure inhibition of the electrogenic sodium pump in this axon. Cooling induced changes opposite to the initial effects and similar to the delayed effects of pressure in all measures, including action potential amplitude, rise time, duration, membrane potential, membrane resistance, inward current amplitude, conduction velocity, and ability to conduct at high frequency. This axon differs from other axons that have been studied at hyperbaric pressure in the bimodal nature of its response and in the magnitude of pressure-related depression of membrane properties related to excitability.