Diffusion barrieres in the squid nerve fiber. The axolemma and the Schwann layer

The squid nerve barriers are formed by (a) the axolemma (membrane of the axon proper), a membrane 80 Å thick perforated by cylindrical pores 4.0 to 4.5 Å radius, and (b) the Schwann layer, constituted of numerous cells forming a layer one cell thick, crossed by 60 Å wide slit channels. If a molecule present in the axoplasm has to reach the extraneural space, it has to pass (a) the pores, and (b) the channels, in series, and the diffusion rate will depend on the effective diffusion areas per unit path length, A_pd/Δx for the axolemma, and A_cd/Δx for the Schwann layer. By addition, A_nd/Δx, the transneural effective area for diffusion per unit path length is obtained. The diffusion rates of C¹⁴-ethylene glycol (2.2 Å radius), and C¹⁴-glycerol (2.8 Å radius) were measured. The diffusion rate of H³-labeled water (1.5 Å radius) has been previously determined. The results expressed in terms of A_nd/Δx (mean values ± SD, referred to 1 cm² of nerve surface) are 5.3 ± 1.4 cm for water, 2.5 ± 0.4 cm for ethylene glycol, and 0.29 ± 0.03 cm for glycerol. Theoretical values for A_nd/Δx of 2.5 and 0.83 cm for ethylene glycol and glycerol have been calculated. The agreement between the theoretical values for A_nd/Δx and the experimental ones supports the diffusion barrier model described above.

Schwann cell and axon electrical potential differences. Squid nerve structure and excitable membrane location

In fifty-seven resting nerve fibers of the squid Sepioteuthis sepioidea impaled from outside to inside, three potential difference (PD) levels were recorded. In twelve nerve fibers the position of the tip of the micropipette, when it was recording one of the PD levels, was labeled with carmine, and the minute spot of dye was localized histologically. The first PD level of -10 to -26 mv, was located in the endoneurium cells; the second level, formed by a single PD of -33 to -46 mv, was located in the Schwann cell; and the third level, formed by a single PD of -50 to -65 mv, was located in the axon. In sixteen nerve fibers the independence among these PD levels was demonstrated as follows. One micropipette was inserted inside the axon for recording its PD; a second one was inserted inside one endoneurium cell or inside one Schwann cell for recording the PD; and a third micropipette was inserted inside the axon to supply current to depolarize the axon. When the axon was depolarized no significant change was observed in the PD of either the endoneurium cell or the Schwann cell. In all nerve fibers action potentials were registered from the axon only. This identifies the axolemma (axon plasma membrane) as the excitable membrane.

The effects of mechanical stimulation on some electrical properties of axons

Rapid, short duration mechanical compression of lobster giant axons by a crystal-driven stylus produces a depolarization and an increase in membrane conductance which develop immediately with compression but take several seconds to recover. The conductance increase occurs even when the depolarization is prevented electrically. If sodium is removed from the external medium or if procaine is added to it, compression produces almost no depolarization. Small bundles of myelinated frog fibers are depolarized by rapid compression but recover very rapidly (milliseconds); "off" responses are occasionally seen. The results are discussed in terms of the mechanoelectric transducer behavior of an axon membrane.

Permeability of squid axon membrane to various ions

The permeability of the squid axon membrane was determined by the use of radioisotopes of Na, K, Ca, Cs, and Br. Effluxes of these isotopes were measured mainly by the method of intracellular injection. Measurements of influxes were carried out under continuous intracellular perfusion with an isotonic solution of potassium sulfate. The Na permeability of the resting (excitable) axonal membrane was found to be roughly equal to the K permeability. The permeability to anion was far smaller than that to cations. It is emphasized that the axonal membrane has properties of a cation exchanger. The physicochemical nature of the "two stable states" of the excitable membrane is discussed on the basis of ion exchange isotherms.

Demonstration of increased permeability as a factor in the effect of acetylcholine on the electrical activty of venom-treated axons

D-Tubocurarine (curare) and acetylcholine (ACh) had been found to block electrical activity after treatment of squid giant axons with cottonmouth moccasin venom at a concentration which had no effect on conduction. It has now been demonstrated that this effect is attributable to reduction of permeability barriers. The penetration of externally applied C¹⁴-labeled dimethylcurare, ACh, choline, and trimethylamine into the axoplasm of the squid giant axon was determined in axons treated with either cottonmouth, rattlesnake, or bee venom, and in untreated control axons. The lipid-soluble tertiary nitrogen compound trimethylamine readily penetrated into the axoplasm of untreated axons. In contrast, after exposure of the axons to the lipid-insoluble quaternary nitrogen compounds for 1 hour their presence in the axoplasm was hardly detectable (less than 1 per cent). However, following 15µg/ml cottonmouth venom 1 to 5 per cent of their external concentration is found within the axoplasm while following 50µg/ml venom 10 to 50 per cent enters. The penetration of dimethylcurare is also increased by 10 µg/ml bee venom but not by 1 µg/ml bee venom nor 1000 µg/ml rattlesnake venom. The experiments show that when ACh and curare, following venom treatment, affect electrical activity, they also penetrate into the axon. Treatments which do not increase penetration are also ineffective in rendering the compounds active.

Localizing tritiated norepinephrine in sympathetic axons by electron microscopic autoradiography

Following intravenous infusion of tritiated norepinephrine, rat pineals were prepared for combined autoradiography and electron microscopy. Concentrations of photographic grains were observed only over regions of preterminal autonomic axons containing granulated vesicles, thereby directly demonstrating uptake of norepinephrine into these axons and strongly suggesting that their granulated vesicles contain norepinephrine.

Effect of temperature on the potential and current thresholds of axon membrane

The effect of temperature on the potential and current thresholds of the squid giant axon membrane was measured with gross external electrodes. A central segment of the axon, 0.8 mm long and in sea water, was isolated by flowing low conductance, isoosmotic sucrose solution on each side; both ends were depolarized in isoosmotic KCl. Measured biphasic square wave currents at five cycles per second were applied between one end of the nerve and the membrane of the central segment. The membrane potential was recorded between the central sea water and the other depolarized end. The recorded potentials are developed only across the membrane impedance. Threshold current values ranged from 3.2 µa at 267deg;C to 1 µa at 7.5°C. Threshold potential values ranged from 50 mv at 26°C to 6 mv at 7.5°C. The mean Q₁₀ of threshold current was 2.3 (SD = 0.2), while the Q₁₀ for threshold potentials was 2.0 (SD = 0.1).

Modes of initiation and propagation of spikes in the branching axons of molluscan central neurons

A study has been made of Aplysia nerve cells, mainly in the pleural ganglia, in which the main axon divides into at least two branches in the neighbourhood of the soma. Conduction between these branches was investigated by intracellular recordings from the soma following antidromic stimulation via the nerves containing the axonal branches. It has been shown that transmission between separate branches need not involve discharge of the soma but only of the axonal region between the soma and the origin of the branches. In some cells, the spike may fail to invade the other axonal branch, whereas transmission in the opposite direction is readily achieved. Often spikes in none of the branches are transmitted to the others, unless facilitated. Indications about the geometry of the neuron in the vicinity of the soma may be obtained from the study of the relative size of the A spikes originated in different branches. These observations, together with the presence of different sizes of A spikes, produced by orthodromic stimulation, provide evidence that spikes initiated at separate axonal "trigger zones" of Aplysia neurons may be conducted selectively to the effectors or other neurons innervated by the particular branch.

THEORETICAL POTASSIUM LOSS FROM SQUID AXONS AS A FUNCTION OF TEMPERATURE

Theoretical net ionic movements have been calculated for the propagated impulse of the squid axon from the Hodgkin-Huxley equations. The computed potassium movements agree approximately with the experimental data of Shanes, but vary too much with temperature (Q(10) = 1/2.75 from computation, 1/1.91 from experiment). Theoretical corrections providing higher ionic conductances increasing with temperature (according to J. W. Moore's experiments) give a Q(10) of 1/2.24, but the incorporation of the higher values of the maximum conductances, as observed under improved environmental conditions, leads to potassium movements that are considerably higher than Shanes's values.

AN ELECTRON MICROSCOPE STUDY OF CULTURED RAT SPINAL CORD

Explants prepared from 17- to 18-day fetal rat spinal cord were allowed to mature in culture; such preparations have been shown to differentiate and myelinate in vitro (61) and to be capable of complex bioelectric activity (14–16). At 23, 35, or 76 days, the cultures were fixed (without removal from the coverslip) in buffered OsO₄, embedded in Epon, sectioned, and stained for light and electron microscopy. These mature explants generally are composed of several strata of neurons with an overlying zone of neuropil. The remarkable cytological similarity between in vivo and in vitro nervous tissues is established by the following observations. Cells and processes in the central culture mass are generally closely packed together with little intervening space. Neurons exhibit well developed Nissl bodies, elaborate Golgi regions, and subsurface cisternae. Axosomatic and axodendritic synapses, including synaptic junctions between axons and dendritic spines, are present. Typical synaptic vesicles and increased membrane densities are seen at the terminals. Variations in synaptic fine structure (Type 1 and Type 2 synapses of Gray) are visible. Some characteristics of the cultured spinal cord resemble infrequently observed specializations of in vivo central nervous tissue. Neuronal somas may display minute synapse-bearing projections. Occasionally, synaptic vesicles are grouped in a crystal-like array. A variety of glial cells, many apparently at intermediate stages of differentiation, are found throughout the otherwise mature explant. There is ultrastructural evidence of extensive glycogen deposits in some glial processes and scattered glycogen particles in neuronal terminals. This is the first description of the ultrastructure of cultured spinal cord. Where possible, correlation is made between the ultrastructural data and the known physiological properties of these cultures.

THE MECHANISM OF DISCHARGE PATTERN FORMATION IN CRAYFISH INTERNEURONS

Excitatory and inhibitory processes which result in the generation of output impulses were analyzed in single crayfish interneurons by using intracellular recording and membrane polarizing techniques. Individual spikes which are initiated orthodromically in axon branches summate temporally and spatially to generate a main axon spike; temporally dispersed branch spikes often pace repetitive discharge of the main axon. Hyperpolarizing IPSP's sometimes suppress axonal discharge to most of these inputs, but in other cases may interact selectively with some of them. The IPSP's reverse their polarity at a hyperpolarized level of membrane potential; they sometimes exhibit two discrete time courses indicating two different input sources. Outward direct current at the main axon near branches causes repetitive discharges which may last, with optimal current intensities, for 1 to 15 seconds. The relation of discharge frequency to current intensity is linear for an early spike interval, but above 100 to 200 impulses/sec. it begins to show saturation. In one unit the current-frequency curve exhibited two linear portions, suggesting the presence of two spike-generating sites in the axon. Current threshold measurements, using test stimuli of different durations, showed that both accommodation and "early" or "residual" refractoriness contribute to the determination of discharge rate at different frequencies.

DDT: A NEW HYPOTHESIS OF ITS MODE OF ACTION

It is suggested that DDT and perhaps other chlorinated hydrocarbon insecticides owe their activity to the formation of a charge-transfer complex with a component of the nerve axon, with consequent disturbance of function. Experimental evidence is provided for the formation of two complexes with components of cockroach nerve; the complexes have been partially purified. Their formation is accompanied by an absorption in the 245- to 270-millimicron range.

A MOLECULAR STRUCTURAL BASIS FOR THE EXCITATION PROPERTIES OF AXONS

A structural model is suggested for axon membranes consisting of a double layer of lipid and phospholipid molecules in which the polar ends of certain phospholipids change their orientation and combining properties under the influence of an electric field. The phosphate groups act as ion exchange "gates" for the control of ion flow through the membrane. Expressions are developed for the calculation of membrane current components as functions of time, potential, and ionic environment. Approximate solutions show fairly good agreement with existing experimental data in a number of different respects such as steady-state current-voltage relations, the effect of calcium on steady-state current, potassium tracer flux ratios, initial current and rate of change of current, and the dependence of the time constants of current change on membrane potential.

"ELECTRICAL TRANSMISSION" AT AN EXCITATORY SYNAPSE IN A VERTEBRATE BRAIN

A type of excitatory synaptic transmission which is novel for the vertebrate brain has been found in the ner neuron (M-cell) by means of the passive spread of their action currents across the synaptic membrane. After stimulating the ipsilateral eighth cranial nerve, an excitatory postsynaptic potential (EPSP) appears in the M-cell with a latency which is very brief ( about 0.1 msec) and which proba; bly represents a negligible synaptic delay. This response is attributed to the club endings: there were steep gradients of potential along the lateral dendrite of the M-cell during activity and the early EPSP was maximal in the distal part of the dendrite where the club endings predominate. Potential changes in the M-cell spread (passively) backwards into certain eighth-nerve fibers (probably club endings) indicating the presence of special low-resistance connections between them and the M-cell.

TETRODOTOXIN BLOCKAGE OF SODIUM CONDUCTANCE INCREASE IN LOBSTER GIANT AXONS

Previous studies suggested that tetrodotoxin, a poison from the puffer fish, blocks conduction of nerve and muscle through its rather selective inhibition of the sodium-carrying mechanism. In order to verify this hypothesis, observations have been made of sodium and potassium currents in the lobster giant axons treated with tetrodotoxin by means of the sucrose-gap voltage-clamp technique. Tetrodotoxin at concentrations of 1 x 10^-7 to 5 x 10^-9 gm/ml blocked the action potential but had no effect on the resting potential. Partial or complete recovery might have occurred on washing with normal medium. The increase in sodium conductance normally occurring upon depolarization was very effectively suppressed when the action potential was blocked after tetrodotoxin, while the delayed increase in potassium conductance underwent no change. It is concluded that tetrodotoxin, at very low concentrations, blocks the action potential production through its selective inhibition of the sodium-carrying mechanism while keeping the potassium-carrying mechanism intact.