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H GRUNDFEST

Publications and source records attributed to H GRUNDFEST.

At least 37 records · Page 2Linked to original sources
PubMed1961

The ionic mechanisms of hyperpolarizing responses in lobster muscle fibers.

Lobster muscle fibers develop hyperpolarizing responses when subjected to sufficiently strong hyperpolarizing currents. In contrast to axons of frog, toad, and squid, the muscle fibers produce their responses without the need for prior depolarization in high external K(+). Responses begin at a threshold polarization (50 to 70 mv), the potential reaching 150 to 200 mv hyperpolarization while the current remains constant. The increased polarization develops at first slowly, then becomes rapid. It usually subsides from its peak spontaneously, falling temporarily to a potential less hyperpolarized than at threshold for the response. As long as current is applied there can be oscillatory behavior with sequential rise and subsidence of the polarization, repeating a number of times. Withdrawal of current leads to rapid return of the potential to the resting level and a small, brief depolarization. Associated with the latter, but of longer duration, is an increased conductance whose magnitude and duration increase with the antecedent current. Hyperpolarizing responses of lobster muscle fibers are due to increased membrane resistance caused by hyperpolarizing K inactivation. The oscillatory characteristic of the response is due to a delayed superimposed and prolonged increase in membrane permeability, probably for Na(+) and for either K(+) or Cl(-). The hyperpolarizing responses of other tissues also appear to result from hyperpolarizing K inactivation, on which is superimposed an increased conductance for some other ion or ions.

Animals↗
PubMed1961

Impulse propagation at the septal and commissural junctions of crayfish lateral giant axons.

Transmission across the septal junctions of the segmented giant axons of crayfish is accounted for quantitatively by a simple equivalent circuit. The septal membranes are passive, resistive components and transmission is ephaptic, by the electrotonic spread of the action current of the pre-septal spike. The electrotonic spread appears as a septal potential, considerably smaller than the pre-septal spike, but usually still large enough to initiate a new spike in the post-septal segments. The septal membranes do not exhibit rectification, at least over a range of +/- 25 mv polarization and this accounts for their capacity for bidirectional transmission. The commissural branches, which are put forth by each lateral axon, make functional connections between the two axons. Transmission across these junctions can also be bidirectional and is probably also ephaptic. Under various conditions, the ladder-like network of cross-connections formed by the commissural junctions can give rise to circus propagation of impulses from one axon to the other. This can give rise to reverberatory activity of both axons at frequencies as high as 400/sec.

Animals↗
PubMed1959

The electrophysiology and pharmacology of lobster neuromuscular synapses.

Effects of drugs on resting potential, membrane resistance, and excitatory and inhibitory postsynaptic potentials (e.p.s.p.'s and i.p.s.p.'s) of lobster muscle fibers were studied using intracellular microelectrodes Acetylcholine, d-tubocurarine, strychnine, and other drugs of respectively related actions on vertebrate synapses were without effects even in 1 per cent solutions (10(-)w/v). Gamma-aminobutyric acid (GABA) acted powerfully and nearly maximally at 10(-7) to 10(-6)w/v. Membrane resistance fell two- to tenfold, the resting potential usually increasing slightly. This combination of effects, which indicates activation of inhibitory synaptic membrane, was also produced by other short chain omega-amino acids and related compounds that inactivate depolarizing axodendritic synapses of cat. The conductance change, involving increased permeability to Cl(-), by its clamping action on membrane potential shortened as well as decreased individual e.p.s.p.'s. Picrotoxin in low concentration (ca. 10(-7)w/v) and guanidine in higher (ca. 10(-3)w/v) specifically inactivate inhibitory synapses. GABA and picrotoxin are competitive antagonists. The longer chain omega-amino acids which inactivate hyperpolarizing axodendritic synapses of cat are without effect on lobster neuromuscular synapse. However, one member of this group, carnitine (beta-OH-GABA betaine), activated the excitatory synapses, a decreased membrane resistance being associated with depolarzation. The pharmacological properties of lobster neuromuscular synapses and probably also of other crustacean inhibitory synapses appear to stand in a doubly inverted relation to axodendritic synapses of cat.

Acetylcholine↗
PubMed1959

Synaptic components of cerebellar electrocortical activity evoked by various afferent pathways.

Electrical responses evoked in different regions of the cerebellar cortex of cat by stimulating various cerebello-petal pathways have been analyzed for their component postsynaptic potentials (p.s.p.'s). The principal analytical tools of the present work were pharmacological agents; the selective inactivator of depolarizing (excitatory) axodendritic synapses, gamma-aminobutyric acid (GABA, or C(4)); the homologous C(6) and C(8) omega-amino acids, which inactivate selectively the hyperpolarizing (inhibitory) axodendritic synapses; and the general inactivator of inhibitory synapses, strychnine. Some experiments employed the analytical possibilities of activity cycles. The potentials evoked in one cerebellar region by different exciting pathways may differ markedly in their responses to drugs or may show different types of activity cycle. Also, the potentials evoked in various cortical regions by one cerebello-petal pathway are acted upon differently by the testing drugs. These differences are believed to be due to involvement of different proportions of excitatory and inhibitory, axosomatic and axodendritic p.s.p.'s. The analyses of a number of different responses confirm an earlier conclusion, that the cerebellar cortex is relatively lacking in inhibitory axodendritic p.s.p.'s in comparison with the cerebral cortex. Only the cortex of the paramedian lobule appears to be endowed with a considerable proportion of inhibitory p.s.p.'s, a finding which correlates with other data.

Afferent Pathways↗
PubMed1959

Electrophysiology of electric organ in Gymnotus carapo.

The electric organ of G. carapo is formed by linearly arrayed electroplaques which lie in four tubes on each side of the fish. In one tube the electroplaques are innervated on their rostral surfaces, in the others on the caudal. Both surfaces of each electroplaque produce spikes, and either can be excited alone by a suitably oriented externally applied stimulating current. The innervated surface, however, has a lower threshold, and in the normal organ activity, which is a continuous discharge at 35 to 60/sec., it is always fired first by the large neurally evoked postsynaptic potential. The spike of the innervated face then fires the opposite face. The potential recorded external to the innervated face is initially negative and becomes positive when the other face fires. The potential outside the other face is inverted. The p.s.p.'s are electrically inexcitable, have short duration, and are augmented by hyperpolarization. A single electroplaque is innervated by several nerve fibers, which produce summative p.s.p.'s. Homosynaptic facilitation of p.s.p.'s is common. The synapses are cholinoceptive. The organ discharge begins with synchronized activity in the rostrally innervated electroplaques. After a brief interval, the electroplaques in the other three tubes fire. The organ discharge therefore is triphasic, resulting from the summation of the two diphasic components that are oppositely directed and asynchronous. Observations on the sensory role of the organ are included.

Animals↗
PubMed1959

Anesthetic and calcium action in the voltage-clamped squid giant axon.

Changes in spike configuration and in the inward and outward currents of voltage-clamped axons agree in indicating that the increases in permeability to sodium and potassium ions during activity are depressed by procaine and cocaine and augmented by calcium. At low levels of depolarization, the effect of the multivalent ion is similar to that of the local anesthetics, in keeping with their similar effects on the threshold of excitability. The reduction of membrane conductance at rest requires a higher concentration of the drugs than that needed to affect the increase in permeability with activity.

Anesthetics, Local↗
PubMed1959

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.

Action Potentials↗
PubMed1959

The mechanism of dual responsiveness in muscle fibers of the grasshopper Romalea microptera.

Dually innervated Romalea muscle fibers which respond differently to stimulation of their fast and slow axons are excited by intracellularly applied depolarizing stimuli. The responses, though spike-like in appearance, are graded in amplitude depending upon the strength of the stimuli and do not exceed about 30 mv. in height. In other respects, however, these graded responses possess properties that are characteristic of electrically excitable activity: vanishingly brief latency; refractoriness; a post-spike undershoot. They are blocked by hyperpolarizing the fiber membrane; respond repetitively to prolonged depolarization, and are subject to depolarizing inactivation. As graded activity, these responses propagate decrementally. The fast and slow axons of the dually responsive muscle fibers initiate respectively large and small postsynaptic potentials (p.s.p.'s) in the muscle fiber. These responses possess properties that characterize electrically inexcitable depolarizing activity. They are augmented by hyperpolarization and diminished by depolarization. Their latency is independent of the membrane potential. They have no refractory period, thus being capable of summation. The fast p.s.p. evokes a considerable or maximal electrically excitable response. The combination, which resembles a spike, leads to a twitch-like contraction of the muscle fiber. The individual slow p.s.p.'s elicit no or only little electrically excitable responses, and they evoke slower smaller contractile responses. The functional aspects of dual responsiveness and the several aspects of the theoretical importance of the gradedly responsive, electrically excitable component are discussed.

Animals↗