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R Fettiplace

Publications and source records attributed to R Fettiplace.

51 records · Page 3Linked to original sources
PubMed1980

The origin of tuning in turtle cochlear hair cells.

Intracellular recordings were made from single hair cells in an in vitro preparation of the cochlea of the turtle. The response of each hair cell to sound stimuli was sharply tuned with a characteristic frequency which was related to the position of the cell along the basilar membrane. When small current steps were injected through the recording electrode, the hair cell potential exhibited damped oscillations at the cell's characteristic frequency. We suggest that the property of the hair cells which is responsible for this oscillatory behaviour could make a major contribution to their acoustic tuning.

Acoustic Stimulation↗
PubMed1980

The frequency selectivity of auditory nerve fibres and hair cells in the cochlea of the turtle.

1. The electrical responses of single auditory nerve fibres or cochlear hair cells were recorded in the isolated half-head of the turtle Pseudemys scripta elegans. Responses to sound stimuli presented to the tympanum could be recorded for at least 4 hr after isolation.2. Impulses were recorded extracellularly from single auditory nerve fibres. For tones of suprathreshold intensity the impulses occurred with a preferred phase relation (i.e. they were phase-locked) to the cycles of the sound stimulus. Nerve fibres had sharp tuning curves (Q(10 db) = 0.5-7.5) with single characteristic frequencies (c.f.) ranging from about 30 to 700 Hz. Best threshold sensitivities of fibres at their c.f. were in the region of 30-40 db sound pressure level with respect to 20 muPa.3. Intracellular recordings were made from hair cells in the basilar papilla. Following injection of a fluorescent dye into a cell through the recording electrode, the dye was localized in a single hair cell in a transverse section of the cochlea.4. Hair cells had resting potentials of about -50 mV, and, to low frequency tones, gave periodic responses graded with the intensity and frequency of the stimulus. Recordings were obtained from cells with characteristic frequencies between 70 and 670 Hz.5. The voltage response to a pure tone at low sound pressure was sinusoidal for all frequencies of stimulation; at higher sound pressures a number of non-linearities were apparent in the response wave form. One of these was a steady depolarizing component, which, relative to the periodic component of the response, was most prominent at high frequencies.6. The amplitude of the response evoked in a hair cell by a low intensity tone was linearly related to the sound pressure; for loud sounds, the response eventually reached a saturating amplitude, which in some cells was as great as 30-45 mV peak-to-peak.7. The linear sensitivity of a hair cell is defined as the r.m.s. voltage for a linear response of the cell at its c.f. divided by the sound pressure at the tympanum. In the most sensitive cells this value was 30-90 mV/Pa.8. If the frequency selectivity of a hair cell was expressed in terms of the sound pressure needed to produce a constant amplitude of response, the sharpness of this frequency selectivity was found to be virtually independent of the response criterion for responses between 1 and 10 mV; in the cells which gave the largest responses, the frequency selectivity expressed in this way was comparable to that of the nerve fibres. Cells with smaller maximum responses often had broader tuning curves.9. Responses of hair cells to short low intensity tone bursts at the c.f. built up approximately exponentially during the tone, and decayed away exponentially when the tone was terminated. The terminal oscillations were at the c.f. of the cell, and independent of the frequency of stimulation.10. From the time constant of the build up and decay of the linear response to a tone burst at the c.f. the sharpness of tuning of the cell was estimated and found to agree with that obtained from the responses of the cell to continuous tones. The most highly tuned cells had quality factors (Q(3 db)) in the range 5-10.11. The c.f. of a hair cell was correlated with its position along the basilar membrane. Low frequency hair cells were located towards the apical or lagenar end and high frequency cells were found towards the basal or saccular end. On the assumption of an exponential distribution of c.f. with distance, each octave occupied about 94 mum along the membrane.12. A hair cell's response to a click was a decaying oscillation at the characteristic frequency of the cell. From the initial polarity of the responses to condensation and rarefaction clicks it was concluded that the hair cell depolarized as a result of movements of the basilar membrane towards the scala vestibuli, and hyperpolarized for motion towards the scala tympani.13. In the absence of deliberate sound stimulation, the hair cell voltage fluctuated continuously about its mean level. The principal frequency components in the noise were concentrated around the c.f. of the cell. The voltage noise in the hair cells showed no significant cross-correlation with sound pressure fluctuations at the tympanum.

Acoustic Stimulation↗
PubMed1979

Synaptic drive and impulse generation in ganglion cells of turtle retina.

1. Light reponses and electrical constants of ganglion cells in the retina of the turtle were examined by intracellular recording in eyecup preparations. 2. In 'on', 'off', and 'on/off' cells, the impulses produced by illumination of the centre of the receptive field arose from slow synaptic depolarizations. The ganglion cells also exhibited inhibitory synaptic potentials. 3. The synaptic depolarization evoked by a step change in light intensity rose more slowly than the response of the cones in which the excitation originated, and the depolarization then declined in spite of a well maintained cone response. This behaviour is consistent with the notion advanced previously that, during transmission to ganglion cells, receptor signals are relayed through the equivalent of a bandpass filter. 4. The e.p.s.p.s evoked by light grew when the membrane was hyperpolarized by injected current and decreased when the membrane was depolarized. The i.p.s.p.s reversed at a level slightly negative to the resting potential in darkness. 5. In neither 'on' nor 'off' ganglion cells did the synaptic potentials evoked by step changes in illumination show the hyperpolarizing phases expected of a linear filter. The absence of hyperpolarizations is consistent with a rectification which permits transmission of depolarizations but not hyperpolarizations from bipolar to ganglion cells. 6. In darkness the membrane potential of some ganglion cells showed random depolarizations which brought the potential near the threshold for impulse generation. 7. With very small spots in the receptive field centre the 'on' responses of ganglion cells to flashes and steps of light grew approximately linearly with stimulus intensity. The step reponse was not, however, related to the flash response by superposition. Larger spots in the field centre gave responses which grew non-linearly with the intensity of even dim stimuli. 8 Depolarizing current passed through the recording electrode elicited a repetitive discharge of impulses. The frequency of firing increased linearly with current strength above a rheobase value of about 10(-10) A. Accommodation occurred during steady currents, the main decline taking place with a time constant of about 15 msec. 9. Strength-latency measurements and bridge records of ganglion cell charging by constant currents gave time constants of 10--20 msec and input resistances of 100--150 M omega.

Action Potentials↗
PubMed1978

The coding of sound pressure and frequency in cochlear hair cells of the terrapin.

Intracellular recordings have been made from single hair cells in the cochlea of the terrapin, and the site of recording has been verified by injection of a fluorescent dye through the recording electrode. A hair cell gives periodic voltage responses graded with the intensity and frequency of the sound stimulus, and produces the largest response at its characteristic frequency. When small current steps are injected through the recording electrode, the voltage response of the cell exhibits damped oscillations at its characteristic frequency. The results are consistent with the idea that the cochlear frequency selectivity arises in two stages and it is suggested that the second stage resides within the hair cell itself.

Action Potentials↗
PubMed1978

The influence of the lipid on the water permeability of artificial membranes.

The water permeabilities of artificial membranes formed formed from various monoglycerides or phospholipids in alkane solvents have been measured using an osmotic method, and it has been shown that the permeability depends upon the type of lipid used. For monoglycerides, the permeability was found to increase with the unsaturation and decrease with the length of the acyl chain. Membranes formed from either egg phosphatidylcholine or dioleyl phosphatidylcholine had an osmotic permeability coefficient of approx. 35--40 micrometer/s at 25 degrees C; with sphingomyelin as the membrane lipid, the permeability was an order of magnitude lower than that for phosphatidylcholine. It is suggested that the water permeabilities of biological membranes might be partly controlled by the types of lipid present.

Glycerides↗
PubMed1977

Transmission from photoreceptors to ganglion cells in turtle retina.

1. Synaptic transfer between photoreceptors and impulse-generating cells was studied in isolated eyecups from turtles. Single red-sensitive cones or rods were stimulated by current passed through an intracellular electrode, and impulses generated by the resulting synaptic action were recorded with an external micro-electrode. This technique permits study of retinal transmission without the operation of the visual transduction mechanism. Antidromic stimulation of the optic nerve indicated that most of the impulse-generating cells were ganglion cells.2. Individual ganglion cells responded transiently to changes in the membrane potential of a receptor and could be classified into three groups on the basis of the direction of the effective change in potential. Off centre ganglion cells responded selectively to depolarizations of a receptor, while on centre ganglion cells responded selectively to hyperpolarizations. On-off ganglion cells responded to both depolarizations and hyperpolarizations of a receptor.3. Ganglion cells gave the same pattern of response to electrical hyperpolarization of a receptor and to light in the centre of their receptive fields. Subthreshold depolarizing currents passed in a receptor antagonized the ganglion cell's response to light, and subthreshold hyperpolarizing currents reinforced the response. These observations are consistent with the view that the hyperpolarization generated by visual transduction is responsible for regulating the release of transmitter at the first retinal synapse.4. When a receptor was stimulated with weak current pulses of fixed intensity the number and latency of the ganglion cell impulses fluctuated randomly in successive trials. The relation between the fraction of trials yielding a response and the stimulus intensity was broad. These results indicate that the link between retinal input and output is noisy.5. In the most sensitive pairs of cells, a response of one or more impulses could be obtained in half the trials with a current of about 2 x 10(-11) A, which changed the potential of the receptor by 1-2 mV. A current of similar magnitude would be developed by about 130 photoisomerizations in a red-sensitive cone or 50 photoisomerizations in a rod.6. Dim background light producing a steady hyperpolarization of a few millivolts in the rods raised the threshold for electrically-evoked transmission from a rod to a ganglion cell. In experiments on red-sensitive cones, background light raised the threshold in the off pathway, in which depolarization was the effective stimulus, and lowered the threshold in the on pathway, in which hyperpolarization was the effective stimulus. These changes in sensitivity were not accompanied by obvious changes in the input resistance of the stimulated receptor. Regulation of retinal sensitivity in background light thus involves changes in synaptic transfer as well as changes in the sensitivity of the visual transduction mechanism.

Animals↗
PubMed1977

Kinetics of synaptic transfer from receptors to ganglion cells in turtle retina.

1. Synaptic transfer between the retinal input and output was studied in turtle eyecups by injecting rectangular current pulses into a single cone or rod while recording externally from a ganglion cell.2. When a receptor was activated with weak steps of polarizing current, the probability of obtaining a ganglion cell impulse rose after an S-shaped delay to a peak at about 0.1 sec and then declined. This suggests that the transmission chain behaves like an electrical band-pass filter containing delay and differentiating elements.3. To further characterize the kinetics of excitation in the subthreshold region, the duration and polarity of the polarizing current pulses were varied while determining the magnitude of the threshold current and the delay to the ganglion cell impulses. The results of these experiments were described with linear models which assume that synaptic transfer occurs over a cascade of first-order delay stages and a single differentiating stage.4. The pathways which relay off responses to light from rods and red-sensitive cones were formally similar, but the time scale in the rod path was several times slower. The path carrying off responses from the red-sensitive cones was faster than the on path. These kinetic differences indicate that independent pathways mediate each of the three categories of response and suggest that the kinetics of each path are ;matched' to the input signals generated by light.5. The strength-latency relations for the responses of on-centre ganglion cells to flashes and steps of light were approximately predicted from the description of synaptic transfer developed here and the description of visual transduction in red-sensitive cones from a previous study.6. It is suggested that the retinal paths have kinetics which might be useful in transmitting light-evoked signals whilst attenuating noise present near the input.

Action Potentials↗
PubMed1976

The adsorption of lysine vasopressin at ionized interfaces.

It has been shown by surface potential measurements that lysine vasopressin and oxytocin may be bound by ionic surfaces to very varied extents. To dodecyl sulphate and phosphatidylserine monolayers the binding is very strong and is comparable to that for biological receptors such as those in toad bladder. For dioleyl phosphate and the carboxyl group of the polypeptide alamethicin, the binding is rather weaker while, for the zwitterionic lipids phosphatidylcholine and phosphatidylethanolamine, and for the erythrocyte surface, which contains two varieties of carboxylic acid group, no interaction seems to take place. In no system does the lysyl amino group of the vasopressin appear necessary for adsorption and, in the dodecyl sulphate monolayers, the interaction is strong even when the ionization of the terminal alpha-amino group is suppressed.

Animals↗
PubMed1976

Transmission of signals from photoreceptors to ganglion cells in the eye of the turtle.

Synaptic transfer between receptors and ganglion cells was studied in the retina of the turtle. In the normal operation of this pathway, signals are relayed across two or more chemical synapses. Previous work indicates that the receptors give graded hyperpolarization to light, and that the bipolar cells also respond with graded potential changes; the ganglion cells give impulses. Injection of weak electrical currents into a single receptor provides an alternative means of activating the pathway. We have used this technique to examine (1) the role of the receptors' hyperpolarization in signal transmission and (2) the sensitivity and temporal properties of the pathway. The response of a ganglion cell evoked by light on the retina can be duplicated by electrically hyperpolarizing a receptor and antagonized by electrical depolarization. This indicates that the hyperpolarization is responsible for regulating the flow of information from the receptors to the second-order retinal cells. The sensitivity of the synaptic path from cones to ganglion cells, determined from electrical stimulation of single receptors, was found to be sufficient to permit detection of about 100 photoisomerizations. Using the same technique, the kinetics of transfer were studied and found to exhibit a delay or integrating process and also a slow differentiation that blocks steady-state transmission. For the path from red-sensitive cones to ganglion cells, the apparent time constant of the delay process was of the order of 75 ms and that of the differentiation, about 100 ms. In the path from rods to ganglion cells, the differentiation was several times slower.

Animals↗
PubMed1975

Light path and photon capture in turtle photoreceptors.

1. The directional selectivity of individual cones was examined by intracellular recording in the eye of the turtle. Sensitivites were determined from linear responses to dim flashes of monochromatic light incident on a cell over a range of angles to its long axis. 2. With light near the optimum wave-length, some red- and green-sensitive cones showed a high sensitivity for light entering axially and lower sensitivities for light entering obliquely. In contrast, other cells had lower peak sensitivities and less pronounced directional selectivities. The highest axial sensitivities observed in red receptors were about 320 muV photon(-1) mu2; in these cells, the sensitivity declined to half for rays 6-9 degrees off the axis as measured in the retina. Green receptors had lower axial sensitivities and broader angular profiles. 3. On the assumption that rays at all angles contribute independently to the over-all sensitivity, the sensitivity of a cell to large cones of rays was successfully predicted from the angular selectivity determined with a narrow pencil of rays. The shape of small responses to dim stimuli delivered on and off the axis of the cell was invariant, implying that a cone signals the number of photons absorbed but not their angle of incidence. 4. Short wave-lengths have previously been shown to be filtered out by the oil droplets present in turtle cones. At short wave-lengths, the angular profiles showed a depression in axial sensitivity consistent with this filtering action. 5. Diameters of inner segments, oil droplets, and outer segments were measured in red-, green-, and blue-sensitive cones, since these dimensions are expected to influence the cones' angular acceptances and ability to collect light. The diameters of the structure were in approximately the same proportions for each type of receptor, but the absolute values of the diameters were found to be scaled in relation to the wave-length of maximum sensitivity. 6. Optical determinations of the efficiency with which axial rays are concentrated by red receptors gave a mean value of 55%. 7. Receptors in histological sections of the whole eye were found to be oriented with their long axes directed approximately toward the pupil. 8. The observed directional selectivities and collecting efficiencies agree well with the behaviour of a model retinal cone developed by Winston & Enoch (1971) on a geometrical optical treatment. 9. Effective collecting areas are derived for red-, green- and blue-sensitive cones; these permit conversion of observed flash sensitivities into the mean peak hyperpolarization produced by isomerization of a visual pigment molecule. The figure obtained is about 25 muV for red-sensitive cones and 21muV for green-sensitive cones.

Animals↗