The capacitance C'(e), presumed to be located across the walls of the transverse tubules of twitch fibers, was identified in earlier impedance measurements by virtue of having a resistance in series with it. When the voltage V(m) across the surface membrane is made to vary, the voltage V(c) across C'(e) will be delayed with respect to V(m), the extent of the delay depending on the location of the series resistance. Model 1 assumes that the resistivity of the lumen of the tubules is negligible; model 2 assumes that the series resistance arises entirely in the tubular lumen; model 3 assumes that the resistivity of the tubular lumen is small, but not negligible and that the bulk of the resistance arises in a structure directly in series with C'(e) and having a similar geometric distribution. If V(m) varies sinusoidally, the relative value of V(c(max)) will fall with increasingly higher powers of the frequency at the center of the fiber if model 2 is applicable, whereas models 1 and 3 predict that V(c(max)) will fall at high frequency only in proportion to the frequency everywhere in the cross-section of the fiber. Equations have been derived for the voltage change V(c) in response to a step change of V(m) and during an action potential. On the assumption that contraction is initiated when V(c) reaches mechanical threshold, the delay between the activation of myofibrils on the axis of the fiber and at the surface would amount to 2.6 msec in model 2 and 0.25 msec in model 3 for frog fibers of about 100 mum diameter during a twitch.

Action Potentials↗

PubMed1968

Passive electrical properties of rod outer segments.

1. Measurements on a packed suspension of randomly oriented, dark-adapted frog rods at frequencies of 15 c/s-0.5 Mc/s indicate a behaviour similar to that of other biological materials.2. Results are analysed on the assumption that the low-frequency limiting resistance is determined by current flowing in the suspending medium and that, of the rods, two thirds are oriented perpendicular to the applied field and one third parallel to it. Those parallel to the field are treated as non-conductors.3. From the high-frequency limiting resistance the conductivity of the rod interior is calculated to vary linearly with the conductivity of the medium. The slope of the relation of internal to external conductivity is 0.50 with a limiting internal conductivity (at zero external) of 280 mumho/cm.4. On the assumption that the suspension can be represented as a single-capacitance network, the characteristic frequency of impedance is used to calculate a capacitance for the rod surface of 1.54 muF/cm(2). On the assumption of a distribution in properties of the suspension according to the theory of Bruggeman, the capacitance is calculated to have a value of about one half this.5. At frequencies below 5 kc/s the impedance locus deviates from the curve describing the behaviour at higher frequencies. It is suggested that this may involve conduction in a thin layer extending along the surface of the rod.

Animals↗

PubMed1968

Conductance changes produced by light in rod outer segments.

1. Changes in the admittance of rod outer segments produced by illumination with brief flashes were studied by two methods: one, in which maintained changes in real and imaginary parts of admittance were observed in the frequency range 15 c/s-60 kc/s; the other, in which the time course of change in absolute value of admittance (Delta|Y|) was observed at frequencies of 100 kc/s-1.0 Mc/s.2. The response to light absorbed by rhodopsin was resolved into components. One of these components was a transient increase in conductance which arose from a rapid degradation into heat of the light energy. Another component, prominent at high frequencies where the conductivity of the rod interior was accessible to measurement, was produced by the uptake of H(+) by visual pigment in its conversion from metarhodopsin I to metarhodopsin II, causing a change in ionization of buffer.3. Two other components, designated I and II, appeared as maintained changes of admittance involving the organized structure of the rod. Component I appeared as a frequency-independent increase in the real part of admittance (DeltaG), the amplitude of which varied in proportion to the conductivity of the medium, without specificity as to ion species. Component II appeared as a DeltaG which rose linearly with log frequency over the range 1-60 kc/s, while the imaginary part of admittance change (DeltaB) rose to a plateau which was maintained for more than a tenfold frequency range. This component was unaffected by variations in conductivity in the region of low conductivities.4. When rods were suspended in a solution containing 100 mM hydroxylamine, component II no longer appeared as a maintained admittance change while component I was unaffected. Examination of the time course of response showed component II to appear transiently, decaying over the course of 2 sec following a flash.5. Measurements of Delta|Y| for rods in solutions of widely different conductivities showed component II to have a more rapid time course of development than component I and to be only slightly delayed in its early part relative to the buffer component.6. The amplitude of component I varied with temperature to the extent of 4.1%/ degrees C (Q(10) of 1.5) over the range -2-25 degrees C. The amplitude of component II was nearly constant over the range 15-27 degrees C, but fell steeply at temperatures below 10 degrees C, the Q(10) at low temperatures being about 2.4. The effect of temperature on amplitude and time course of component II is consistent with its dependence on the formation and continued presence of metarhodopsin II. The failure of component I to decrease steeply at temperatures below 10 degrees C indicates a dependence on an earlier stage in the thermal conversion of rhodopsin photoproducts.7. With light flashes each bleaching less than 1% of the rhodopsin content of the rod, all components of response were proportional to the amount of rhodopsin bleached (which would be proportional to the light absorbed). For brighter flashes components I and II failed to increase in proportion to the amount of rhodopsin bleached, the deviation from proportionality being greater for component I than for component II. The failure of summation of response extended to successive responses separated by up to 5 min.8. It is suggested that component I arises from a non-selective increase in ionic permeability of the surface membrane of the rod or from a change in rod volume, while component II arises from a change in conduction along the surface membrane.

Animals↗

PubMed1968