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Biomedical subjects

M V Bennett

Publications and source records attributed to M V Bennett.

At least 145 records · Page 8Linked to original sources
PubMed1979

Voltage dependence of junctional conductance in early amphibian embryos.

Isolated pairs of blastomeres from early amphibian embryos (Ambystoma, Rana, Xenopus) are electrontonically coupled. Junctional conductance and permeability to the dye Lucifer Yellow are markeldy and reversibly decreased by moderate transjunctional polarization in either direction. The relationship between junctional conductance and transjunctional voltage is sufficiently steep that a physiological role in regulation of intercellular communication is plausible.

Ambystoma↗
PubMed1979

Plasticity of feeding behavior in the opisthobranch mollusc Navanax.

Several forms of plasticity of feeding behavior were investigated in Navanax. Navanax is a gastropod mollusc which lacks a radula, and ingests prey with suction caused by rapid pharyngeal expansion. Feeding is little affected by handling or posture and is resistant to the noxious stimulus of cutting through the body wall. Feeding is affected by arousal, as shown by a decrease in latency following initial exposure to food. The feeding response also habituates, as shown by an increase in latency when feeding responses are elicited without allowing animals to engulf food. The latency increase is not likely to be due to motor fatigue, since it can be partially reversed by dishabituation with an alternate prey species. Continued feeding causes satiation, as shown by increased feeding latencies and eventual cessation of feeding after a mean weight gain of 42.0% of initial animal weight (N = 12). Another form of plasticity occurs when Navanax are presented with food too large to swallow whole. Navanax either cease to respond after a few presentations, perhaps due to habituation, or maintain suction on the partially swallowed prey for an extended period of time. During extended sucks, the more deeply ingested portions are digested.

Animals↗
PubMed1979

The oscillatory responses of skate electroreceptors to small voltage stimuli.

Tonic nerve activity in skate electroreceptors is thought to result from spontaneous activity of the lumenal membranes of the receptor cells which is modulated by applied stimuli. When physiological conditions are simulated in vitro, the receptor epithelium produces a current which flows inward across the lumenal surface. This epithelial current exhibits small spontaneous sinusoidal fluctuations about the mean that are associated with corresponding but delayed fluctuations in postsynaptic response. Small voltage stimuli produce damped oscillations in the epithelial current similar in time-course to the spontaneous fluctuations. For lumen-negative, excitatory stimuli, these responses are predominantly an increase over the mean inward current. For inhibitory stimuli they are predominantly a decrease. Increased inward current across the lumenal membranes of the receptor cells increases depolarization of the presynaptic membranes in the basal faces leading to increased release of transmitter and an excitatory postsynaptic response. Decreased inward current decreases depolarization of the presynaptic membranes leading to a reduction in transmitter release and an inhibitory postsynaptic response. Clear changes in postsynaptic response are detectable during stimuli as small as 5 microV with saturation occurring at +/- 400 microV. The evoked oscillations in epithelial current are damped and the postsynaptic responses decline during maintained stimuli with large off-responses occurring at stimulus termination. The initial peak of the off-response is similar to the response produced by onset of an oppositely directed stimulus. These observations substantiate the role of receptor cell excitability in the detection of small voltage changes.

Animals↗
PubMed1979

The ionic basis of oscillatory responses of skate electroreceptors.

When physiological conditions are simulated, skate electroreceptors produce small maintained oscillatory currents. Larger damped oscillations of similar time-course are observed in voltage clamp. Subtraction of leakage in voltage clamp data shows that the oscillations involve no net outward current across the lumenal surface of the epithelium. The oscillations are much faster than the late outward current generated by the lumenal membranes of the receptor cells. Treatment of the basal surface of the epithelium with tetraethyl ammonium (TEA), high K, Co, or EGTA reversibly blocks the oscillations in voltage clamp, but has little or no effect on the epithelial action potential in current clamp or on the current-voltage relation. The TEA sensitivity of the oscillations indicates that they involve a potassium conductance in the basal membranes of the receptor cells. Treatment of the basal membranes with TEA and high calcium, with strontium, or with barium causes these membranes to produce large regenerative responses. Direct stimulation of the basal membranes then elicits a lumen-positive action potential whereas stimulation of the lumenal membranes elicits a diphasic action potential. Excitability of the basal membranes is abolished by extracellular Co, Mn, or La. Modulation of the lumenal membrane calcium conductance by the basal membrane conductances probably gives rise to the oscillatory receptor currents evoked by small voltage stimuli. The slower calcium-activated late conductance in the lumenal membranes may be involved in sensory accommodation.

Animals↗
PubMed1979

Transduction at electroreceptors: origins of sensitivity.

Electroreceptors of four different classes were analyzed in terms of adaptations leading to high sensitivity. The large receptor of mormyrids is a sensitive phasic receptor. Voltage amplification is provided by an impulse-generating mechanism in the receptor cells. The cells are specialized in that their threshold is very close to the resting potential. Inward current may be mediated by Ca channels, which show little inactivation. Transmission to the afferent nerve is electrotonic and is apparently not otherwise specialized. Other phasic receptors appear to use regenerative responsiveness more for active filtering than for voltage gain. Tonic electroreceptors of freshwater fishes lack regenerative responses in their receptor cells. Stimuli act directly on the presynaptic membrane to alter Ca permeability and modulate the release of transmitter. The absence of regeneration is ascribable to shunting by fixed conductances. The relation between transmitter release and depolarization is much steeper than the corresponding relation at the squid giant synapse. Indirect arguments suggest that the great voltage sensitivity does not reside in any properties of the Ca channels, but in subsequent processes leading to transmitter release. The ampulla of Lorenzini is the most sensitive electroreceptor. Its sensitivity apparently resides in the regenerative oscillatory activity of the receptor cells. The receptor is kept in its operating range by accommodative processes, probably involving Ca-activated outward current. A number of mechanisms of electroreception appear relevant to mechanosensitive acoustico-lateralis receptors. The specialized accommodative processes and the adaptations increasing sensitivity are likely to be relevant to many other systems as well.

Action Potentials↗
PubMed1978

Junctional complexes and variations in gap junctions between spinal cord ependymal cells of a teleost. Sternarchus albifrons (Gymnotoidei).

Junctions between ependymal cells lining the central canal in the spinal cord of the weakly electric teleost, Sternarchus albifrons, were investigated by means of thin-section and freeze-fracture electron microscopy. Junctional complexes representing the 'terminal bar' consist of a zonula occludens, followed by intermediate junctions and desmosomes. This pattern is common in many types of epithelia but uncommon in ependyma. Three types of gap junctions were found below the level of the terminal bar and intermixed with desmosomes: (1) gap junctions at least partially enclosed by a single strand of apparent tight junction, (2) ordinary gap junctions, partially surrounded by a particle-free halo and (3) segmented gap junctions, in which linear arrays several particles wide are separated by fairly regular linear particle-free regions. Some of the observations may have implications for the mechanism of formation of gap junctions.

Animals↗
PubMed1977

The Ranvier nodes in the neurogenic electric organ of the knifefish Sternarchus: a freeze-etching study on the distribution of membrane-associated particles.

The two types of Ranvier nodes (type I with narrow gap, type II with giant gap) and internodes in nerve fibers composing the Sternarchus electric organ have been studied by means of freeze-etching electron microscopy. Numerical analysis of the distribution of membrane-associated particles revealed the following features: (1) the P-faces of both types of nodes and of the internodal axon bear a similarly high density of particles (1000-1200 particles/sq. micron on the average). (2) particle density is differential in E-faces: the histogram for type I nodes has a wider range of particle concentrations (114-1522 particles/sq. micron) than that for type II nodes (45-576 particles/sq. micron) whose density values are in the same range as those of the internodal axon. At least some type I nodes (narrow gaps) generate spikes and probably have a low resistivity; these nodes may be those with high particle density on E-faces. The low particle density on E-faces of type II nodes may be associated with high resistivity and absence of excitability. Similarly, the low particle density in internodes may reflect inexcitability. There is evidence that the transition from one nodal type to the next is gradual: as the gap width of type I nodes increases, there is an occurrence of surface elaborations and the density of E-face particles tends to drop towards the range of type II nodes.

Animals↗
PubMed1977

Calcium-activated conductance in skate electroreceptors: current clamp experiments.

When current clamped, skate electroreceptor epithelium produces large action potentials in response to stimuli that depolarize the lumenal faces of the receptor cells. With increasing stimulus strength these action potentials become prolonged. When the peak voltage exceeds about 140 mV the repolarizing phase is blocked until the end of the stimulus. Perfusion experiments show that the rising phase of the action potential results from an increase in calcium permeability in the lumenal membranes. Perfusion of the lumen with cobalt or with a zero calcium solution containing EGTA blocks the action potential. Perfusion of the lumen with a solution containing 10 mM Ca and 20 mM EGTA initially slows the repolarizing process at all voltages and lowers the potential at which it is blocked. With prolonged perfusion, repolarization is blocked at all voltages. When excitability is abolished by perfusion with cobalt, or with a zero calcium solution containing EGTA, no delayed rectification occurs. We suggest that repolarization during the action potential depends on an influx of calcium into the cytoplasm, and that the rate of repolarization depends on the magnitude of the inward calcium current. Increasingly large stimuli reduce the rate of repolarization by reducing the driving force for calcium, and then block repolarization by causing the lumenal membrane potential to exceed ECa. Changes in extracellular calcium affect repolarization in a manner consistent with the resulting change in ECa.

Action Potentials↗
PubMed1977

Calcium-activated conductance in skate electroreceptors: voltage clamp experiments.

Voltage clamp experiments allow further characterization of the calcium-dependent repolarizing process in skate electroreceptor epithelium. Four current components are described: a prolonged capacity current, a leakage current, an early active current which flows inward across the lumenal membranes of the receptor cells, and a late current which flows outward. The leakage and capacity currents are linear and may be substracted from the total current, giving net active currents. The early active current is carried by calcium and does not undergo inactivation for at least several seconds. When large stimuli exceed the reversal potential for the early calcium current, the late current is suppressed. Reduction of the ionized calcium concentration in the lumen lowers the reversal potential for the early current and the suppression potential for the late current by the same amount. We conclude that the late current is initiated by a calcium influx into the cytoplasm. During pulses of moderate duration, activation of the late current does not begin until a fixed amount of calcium has entered the receptor cells. The required amount of calcium is reduced if a recent calcium influx has occurred. We suggest that the calcium-activated outward current is mediated by a distinct macromolecule that is insensitive to voltage. Such macromolecules are likely to have an important role in the regulation of electrical activity in excitable cells.

Action Potentials↗
PubMed1976

Electrotonic coupling: effective sign reversal by inhibitory neurons.

Neurons in the buccal ganglia of Navanax inermis which control circumferential muscles of the pharynx showed typical electrotonic coupling when there was little synaptic activity in them. When there was much inhibitory activity, the effective sign of coupling was reversed; that is, hyperpolarization and depolarization of one cell caused depolarization and hyperpolarization of the others. A neural circuit explaining these results involes inhibitory neurons electronically coupled to and also inhibitory to the circumferential neurons that are themselves coupled. This circuit offers considerable flexibility for mediation of different activity patterns in this simple neuronal system.

Action Potentials↗
PubMed1976

Stimulation-induced depletion of vesicles, fatigue of transmission and recovery processes at a vertebrate central synapse.

The Mauthner fiber giant fiber synapses of the hatchetfish are chemically transmitting axo-axionic synapses in the medulla. Tetanic stimulation at room temperature depletes the presynaptic Mauthner terminal of vesicles and leads to the appearance of large numbers of irregular membraneous compartments in the terminal. Stimulation during cooling to 12 degrees C depletes the terminal of vesicles and greatly increases the external surface, which forms large whorls of invaginating double membranes. Many coated vesicles are attached to the surface and the invaginating whorls. It is concluded that vesicles are discharged by exocytosis and fusion of their membrane with the external surface, and that at room temperature, membrane is reinternalized by coated vesicles and formed into irregular compartments. In completion of the cycle, these compartments disappear, and the vesicle population recovers over an hour or two of rest. When the Mauthner fibers are stimulated at low rates, the p.s.p.'s in the giant fibers are large and suprathreshold. Minature p.s.p.'s are generated spontaneously or can be evoked by subthreshold depolarization or tetanic stimulation of the Mauthner fiber. Stimulation of the Mauthner fibers at gradually increasing frequencies depresses p.s.p. amplitude to or below the level of miniature p.s.p.'s, but no failures are observed. Small p.s.p.'s without failures suggest that the quantum number remains high but that quantal size is greatly reduced, either by partial filling, as is supported by the morphological observation of vesicle depletion, or by desensitization. When stimulation is stopped, recovery of p.s.p. amplitude occurs in 1 or 2 seconds, but if tetanic stimulation is resumed immediately, p.s.p. amplitude decreases again and much more rapidly than in the initial rundown. This result suggests that depression of p.s.p. amplitude is not due to desensitization and leaves partial filling as the most likely explanation of small quanta. Calculated quantal size following a tetanus recovers in 200-500 ms, which probably largely reflects the time for filling since enough vesicles can be supplied to prevent failures with much shorter intervals between stimuli. Because quantal size appears to decrease gradually as stimulation frequency increases, it appears that release of vesicles can interrupt filling, leading to the conclusion that filling and release sites are very close together. This conlusion is consistent with other data in the literature obtained by different techniques.

Animals↗
PubMed1976

Responses of cells of posterior lateral line lobe to activation of electroreceptors in a mormyrid fish.

Activity of neurons in the lateral line lobe was studied by intracellular recording of responses to stimulation of the lateral line nerves and of electroreceptors on the skin surface. Two modes of activation occur for cells responding to inputs from medium receptors. There is a direct monosynaptic input mediated by a single fiber. Short latency of response and antidromic spread from cell to afferent fiber indicate that the mediating synapse is electrotonic. The second input is from a number of additional fibers and is relayed, presumably by the granule cells. At shortest latency this input is disynaptic, probably involving at least one electrotonic synapse. A relay is indicated by heterosynaptic facilitation of the PSP and by pronounced depression with repetitive stimulation. The monosynaptic input may be on the axon. Disynaptic inputs are distributed over the dendrites, and impulses can arise in the dendrites. What appear to be spikes restricted to dendritic regions are often recorded as small brief potentials in the cell body. There is a somatotopic projection of the electroreceptors to the lateral line lobe. The monosynaptic input comes from a specific receptor in the periphery. Strong disynaptic inputs come from a group of receptors generally found anterior, but less commonly posterior or lateral, to the receptor giving rise to the monosynaptic input. Additional inputs that are inhibitory come from surrounding receptors. The inhibition only affects responses to the disynaptic input. The different inputs and multiple sites of impulse initiation must modify the cell's response as compared with the input-output relations that would be obtained with inputs acting on a single summation point. Cells responding to activation of large receptors are infrequent. They are characterized by low threshold, little latency change near threshold, and ability to follow high frequencies of stimulation.

Animals↗
PubMed1976

Interaction of electrosensory and electromotor signals in lateral line lobe of a mormyrid fish.

A signal associated with the neural command to discharge the electric organ is recorded in cells of the lateral line lobe. Responses of cells activated by medium receptor inputs are facilitated or less frequently inhibited during this command-associated signal. Only responses to disynaptic inputs are affected, the monosynaptic response is not altered. The periods of facilitation and inhibition occur at times at which electroreceptor activity evoked by organ discharge reaches the lateral line lobe. Presumably the command-associated signal is important in electrolocation. Cells responding to large receptor inputs are inhibited by the command-associated signal. Activity evoked by large receptors is transmitted in a mesencephalic fiber tract. The tract response is also inhibited by the command-associated signal. Since each organ discharge would excite all the large receptors at short latency, there would be little information contained in their responses. Inhibiting discharge-evoked activity may allow the system to return to maximum sensitivity most rapidly.

Animals↗