Potential-dependent potassium currents in the slowly adapting stretch receptor neuron of the crayfish

Mechanotransduction and the crayfish stretch receptor

Mechanotransduction or mechanosensitivity is found in almost every cell in all organisms from bacteria to vertebrates. Mechanosensitivity covers a wide spectrum of functions from osmosensing, cell attachment, classical sensory mechanisms like tactile senses in the skin, detection of sound in hair cells of the hearing apparatus, proprioceptive functions like recording of muscle length and tension in the muscle spindle and tendon organ, respectively, and pressure detection in the circulation etc. Since most development regarding the molecular aspects of the mechanosensitive channel has been made in nonsensory systems it is important to focus on mechanosensitivity of sensory organs where the functional importance is undisputed. The stretch receptor organ of the crustaceans is a suitable preparation for such studies. The receptor organ is experimentally accessible to mechanical manipulation and electrophysiological recordings from the sensory neuron using intracellular microelectrode or patch clamp techniques. It is also relatively easy to inject substances into the neuron, which also makes the neuron accessible to measurements with fluorescent techniques. The aim of the present paper is to give an up to date summary of observations made on the transducer properties of the crayfish stretch receptor (Astacus astacus and Pacifastacus leniusculus) including some recent unpublished findings. Finally some aspects on future line of research will be presented.

Ion channels for mechanotransduction in the crayfish stretch receptor

Mechanosensitivity is found in almost every cell in all organisms from bacteria to vertebrates and covers a wide spectrum of function from osmosensing to mechanical sensing in the specialized receptors, such as the hair cells of the cochlea. The molecular substrate for such mechanosensitivity is thought to be mechanosensitive ion channels (MSCs). Because most development regarding the molecular aspects of the MSC has been made in nonsensory or sensory systems, which have not been accessible to recordings from ion channels, it is important to focus on the mechanosensitivity of sensory organs where their functional importance is undisputed. The stretch receptor organ (SRO) of the crustaceans is a suitable preparation for such studies. Each organ contains two receptors: one slowly and one rapidly adapting receptor neurons. The primary mechanosensitivity is generated by two types of MSC of hitherto unknown molecular type located in the neuronal dendrites, which are inserted into a receptor muscle fiber. In addition to the MSCs, the neurons contain voltage-gated Na(+) channels, which seem to be differently located in the slowly and rapidly adapting neurons. At least three types of voltage-gated K(+) channels are present in the sensory neurons, the location of which is not known. The spatial distribution of ion channels and the kinetics of the channels, together with the viscoelastic properties of the receptor muscles, determine the overall transducer properties and impulse firing of the two receptor neurons, including their typical adaptive characteristics.

Characterization of a delayed rectifier potassium channel in the slowly adapting stretch receptor neuron of crayfish

Single channel recordings were performed on enzyme-cleaned slowly adapting sensory neurons of crayfish, in cell-attached configuration, with a physiological K(+) gradient across the neuronal membrane. An outward rectifying, voltage-gated K(+) channel with a slope conductance of 13 pS and a K(+) ion permeability of P(K)=6.5 x 10(-14) cm(3)/s was characterized. This 13 pS K(+) channel started to be activated at around 20 mV depolarization. Its open probability increased upon depolarization with V(0.5)= -25.3 mV and P(max)=0.83. The averaged currents showed a delay following the onset of depolarization. The activation time constant was voltage-dependent. The maximal value was 17.0 ms at -25 mV and at +35 mV the time constant was 1.7 ms. Little inactivation was observed throughout the 80- or 1500-ms long depolarization pulses. A sum of two exponentials provided the optimal fit for open time and closed time distribution. At 80-mV depolarization, the open time constants were 0.4 and 10.4 ms; the close time constants were 0.4 and 2.3 ms. The first-latency distribution suggested that at least two closed states preceded two open states. This 13 pS delayed rectifier plays a minor role in the maintenance of the resting membrane potential but contributes to the action potential repolarization. It may also modify the stretch-induced receptor potential and affect the adaptation behaviours in this neuron.

Temperature effects on the discharge frequency of primary and secondary endings of isolated cat muscle spindles recorded under a ramp-and-hold stretch

The effects of changes in temperature on primary and secondary endings of isolated cat muscle spindles were investigated under ramp-and-hold stretches and different degrees of pre-stretch. Temperature-induced alterations of the discharge frequency were compared over a temperature range of 25-35 degrees C. Both primary and secondary endings responded to warming with increasing discharge frequencies when the spindle was pre-stretched by 5-10% of its in situ length. The following differences between the temperature effects on primary and secondary endings were observed: (1) The temperature coefficients (Q(10)) obtained from the discharge frequencies during the dynamic and static phase of a stretch were similar for endings of the same type, but they were larger in primary endings (range of Q(10): 2.3-3.3; mean: 2.9) than in secondary endings (range of Q(10): 1.6-2.2; mean: 2.0); (2) With primary endings, but not with secondary endings, the temperature sensitivity (imp s(-1) degrees C(-1)) was larger during the dynamic phase than during the static phase of a stretch; (3) In primary endings, the fast and slow adaptive components occurring in the discharge frequency during the static phase of a stretch clearly increased with warming while in secondary endings, the slow decay was less affected, and the fast decay showed no change; (4) In relaxed spindles, the excitatory effect of warming was overlaid by a strong inhibitory effect as soon as the temperature exceeded about 30 degrees C, resulting in an abrupt cessation of the background activity in most secondary endings, but not usually in primary endings. In general, warming induced an enhanced stretch sensitivity in both types of ending, and additionally an inhibitory effect that is obvious only in secondary endings of relaxed spindles. The different effects of temperature on the discharge frequency of primary and secondary afferents are assumed to be caused by different properties of their sensory membranes.

Macrocurrents of voltage gated Na+ and K+ channels from the crayfish stretch receptor neuronal soma

Currents from the slowly adapting stretch receptor neuron of the crayfish (Pacifastacus leniusculus) were studied in a cell attached configuration using patch pipettes with an opening diameter of 2-10 microm. The neuronal membrane was enzymatically freed from the glial layer. The voltage gated Na+ and K+ channels seemed to be more concentrated in the lower part of soma close to the axon hillock. The Na+ and K+ currents could be analysed by fitting the currents to a fourth-order exponential function for Na+ current and a second-order exponential function for the K+ current. The macropatch recordings of enzymatically treated neurons are superior to two electrode voltage clamp recordings when analyzing voltage gated Na+ and K+ currents.

Different spatial distributions of sodium channels in the slowly and rapidly adapting stretch receptor neuron of the crayfish

Inward Na+ currents were studied, using a two-microelectrode intracellular voltage-clamp technique, in the slowly adapting (SA) and rapidly adapting (RA) stretch receptor neurons of the crayfish after the axons were cut at different distances from the soma. In the SA neuron, inward Na+ currents were recorded in the soma even when the axon was cut as close as 100 microm from the center of the soma, indicating the presence of Na+ channels in these parts. Also, two populations of Na+ channels seem to exist in the SA neuron. In the RA neuron, only minute Na+ currents were observed if the axon was shorter than 250 microm. The results strongly indicate that the voltage-gated Na+ channels in the SA and RA neurons have different distributions and that the difference in the spatial distribution of Na+ channel types may be important for the difference in firing properties in the two types of neurons.

Voltage-activated potassium outward currents in two types of spider mechanoreceptor neurons

We studied the properties of voltage-activated outward currents in two types of spider cuticular mechanoreceptor neurons to learn if these currents contribute to the differences in their adaptation properties. Both types of neurons adapt rapidly to sustained stimuli, but type A neurons usually only fire one or two action potentials, whereas type B neurons can fire bursts lasting several hundred milliseconds. We found that both neurons had two outward current components, 1) a transient current that activated rapidly when stimulated from resting potential and inactivated with maintained stimuli and 2) a noninactivating outward current. The transient outward current could be blocked by 5 mM tetraethylammonium chloride, 5 mM 4-aminopyridine, or 100 microM quinidine, but these blockers also reduced the amplitude of the noninactivating outward current. Charybdotoxin or apamin did not have any effect on the outward currents, indicating that Ca2+-activated K+ currents were not present or not inhibited by these toxins. The only significant differences between type A and type B neurons were found in the half-maximal activation (V50) values of both currents. The transient current had a V50 value of 9. 6 mV in type A neurons and -13.1 mV in type B neurons, whereas the V50 values of noninactivating outward currents were -48.9 mV for type A neurons and -56.7 mV for type B neurons. We conclude that, although differences in the activation kinetics of the voltage-activated K+ currents could contribute to the difference in the adaptation behavior of type A and type B neurons, they are not major factors.

The response of muscle spindle primary afferents to simultaneously presented sinusoidal and ramp-and-hold stretches

Fifteen primary (Ia) muscle spindle afferents from the tibial anterior muscle of the cat were subjected to a ramp-and-hold stretch (stretch rate 10 mm/s, stretch amplitude 8.5 or 7 mm) of the muscle, upon which was superimposed a sinusoidal stretch (10 Hz) of five different amplitudes (50, 100, 500, 1000 and 2000 microm peak to peak). The response of the Ia afferents to the sinusoidally overlaid ramp-and-hold stretch was subjected to computer analysis, by means of which the response to the superimposed sinusoids and the response to the underlying ramp-and-hold stretch were each obtained separately. Four basic discharge frequencies were determined from the response to the underlying ramp-and-hold stretch (obtained after elimination of the response to the concomitant sinusoidal stretch). The evaluation yielded the result that the concomitant sinusoidal stretch affected the response to the underlying ramp-and-hold stretch: the level of discharge of the Ia afferent was significantly increased and the dynamic and static indices were significantly diminished. These two effects intensified with increasing amplitude of the concomitant sinusoids. From among the oscillatory responses of the Ia afferent to the concomitant sinusoidal stretch (obtained after elimination of the response to the underlying ramp-and-hold stretch) the amplitudes of four selected responses were determined. These four oscillatory responses occurred during four time spans during which the four basic discharge frequencies were read. Evaluation showed that the response to the underlying ramp-and-hold stretch affected the modulation depth of the oscillatory responses, which decreased during the ramp and increased again during the plateau. The absolute amount of the change was independent of the amplitude of the sinusoids. The effect of the sinusoids on the response to the underlying ramp-and-hold stretch and that of the underlying ramp-and-hold stretch on the response to the sinusoidal stretches are interpreted in terms of ion currents occurring at the Ia sensory terminals. It is assumed that the probability of the stretch activated (SA) channels opening is increased by a stretching of the sensory terminals. A depolarizing Na+ and Ca++ inward current due to the activation of the SA channels in its turn activates a Ca++-activated and/or a potential-dependent K+ outward current which has a repolarizing effect. Ideas are put forward in the discussion as to how the inward and outward currents that are sinusoid-related and those that are ramp-and-hold-stretch related interact.

Action potential and sodium current in the slowly and rapidly adapting stretch receptor neurons of the crayfish (Astacus astacus)

Action potentials (APs) and sodium current from the slowly and the rapidly adapting stretch receptor neurons in the crayfish (Astacus astacus) were recorded with a two microelectrode voltage- and current-clamp technique. In the rapidly adapting neuron the APs had a duration of 3.2 +/- 0.2 ms (means +/- SE) and an amplitude of 55.2 +/- 1.5 mV. In the slowly adapting receptor neuron APs had a duration of 4.1 +/- 0.2 ms and an amplitude 79.9 +/- 2.0 mV. APs in the rapidly adapting neuron had a larger amplitude if they were recorded from the axon. In the rapidly adapting neuron adaptation of the impulse response was prolonged by hyperpolarization or by exposure to scorpion venom. Also, sinusoidal current stimulation added to the current steps prevented impulse adaptation. Block of the potassium currents in the slowly adapting neuron resulted in a rapid adaptation of the impulse response. The maximum sodium current amplitude was 313 +/- 15 nA in slowly adapting neuron and 267 +/- 11 nA in the rapidly adapting neuron. The current-voltage relationship showed a hump most marked in the slowly adapting neuron and abolished when a depolarizing prepulse was given. In the rapidly adapting neuron the inactivation starts at a more negative potential (Eh = -45 mV) and is faster compared with the slowly adapting neuron (Eh = -41 mV). The crude scorpion venom of Leiurus quinquestriatus (ScVLq) shifted hinfinity curve toward more positive potentials and slowed down the rate of inactivation. The results indicate the possible presence of more than one Na+ channel population and that the relative density and the spatial distribution is different in the slowly and rapidly adapting neuron. The difference contributes to the adaptive properties of the two receptor neurons.

Changes in primary muscle spindle ending excitability induced by a ramp-and-hold stretch

36 primary (Ia) muscle spindle afferents from the tibial anterior muscle of the cat were subjected to a ramp-and-hold stretch (stretch rate 10 mm/s, stretch amplitude 5 or 8.5 mm) of the muscle, on which a sinusoidal stretch (50 Hz) of four different amplitudes (25, 50, 250 and 500 microns) was superimposed. In 54 discharge patterns a Ia afferent subjected to a ramp-and-hold stretch with a sinusoidal stretch superimposed responded only to the superimposed sinusoidal stretch. In 25 of the cases the Ia afferent responded with an one-to-one driven action potential (AP) and in 29 of the cases with two APs per sinusoidal stretch. For these 54 discharge patterns the phase of the sinusoidal cycle was determined at which each AP occurred. Where the Ia afferent responded with one AP per cycle an accelerating phase advance was observed during the ramp stage of the underlying ramp-and-hold stretch and a decelerating phase advance during the plateau. This phase shift means that the excitability of the site generating the AP increased during the ramp stage and decreased during the plateau. If the Ia afferent responded with two APs per superimposed cycle, the second AP per cycle evinced a decelerating phase advance during the ramp and an accelerating phase advance during the plateau. The phase of the second AP per cycle showed a second, contrary change in excitability at the AP generating site. The excitability decreased during the ramp and increased during the plateau. The first kind of excitability change is interpreted as a consequence of an inward current at the AP generating site. The second, contrary type of excitability points to an interplay between an inward and an outward current. An increasing outward current lowers the excitability for the second AP per cycle during the ramp. A decreasing outward current raises the excitability during the plateau.

Transducer properties of the rapidly adapting stretch receptor neurone in the crayfish (Pacifastacus leniusculus)

1. The transducer properties of the rapidly adapting stretch receptor neurone of the crayfish (Pacifastacus leniusculus) were studied using a two-microelectrode voltage clamp technique. 2. The impulse response to ramp-and-hold extensions of the receptor muscle typically consisted of a high frequency burst followed by cessation of impulses within a relatively short time depending on the amplitude of extension. The type of adaptation was consistent with earlier studies. The stimulus-response relationship for the impulse frequency was non-linear and had a slope in a log-log plot of 2.9. 3. When impulse generation was blocked by tetrodotoxin (TTX), (block of Na+ channels) the receptor potential was extension dependent and similar to that found in the slowly adapting receptor. For small extensions there was an initial peak followed by a fall to a steady potential level. For large extensions the potential response during the ramp phase consisted of a peak followed by a constant potential level lasting to the end of the ramp. When the extension changed to the hold phase the potential fell towards a steady state. The relation between extension and amplitude of receptor potential was non-linear and saturated at -40 to -30 mV (extensions > 15% of zero length, lo). 4. When potassium channels were blocked by TEA (50 mM) and 4-aminopyridine (4-AP, 5 mM) (and Na+ channels blocked by TTX) the shape of the generator potential become less complex with an increased amplitude for large extensions. 5. When the receptor neurone was voltage clamped at the resting potential, extension of the receptor muscle produced an inwardly directed receptor current, the stretch-induced current (SIC). The response consisted of a fast transient phase which decayed towards a steady state. The SIC peak amplitude was dependent on extension in a sigmoidal fashion and saturated at 190 nA (extensions > 25% of lo). The slope of the steepest part of the stimulus-response relation (between 10 and 20% extension) was 4.7 +/- 0.25 (mean +/- S.E.M.) in a log-log plot. 6. The peak amplitude of the SIC increased with increasing extension speed (ramp steepness), the relation between the slope of the ramp and current amplitude being a first order (hyperbolic) function. The amplitude of the receptor current was voltage dependent and had a reversal potential of +16.2 +/- 1.8 mV (mean +/- S.E.M., 32 cells). From the reversal potential the permeability ratio, PNa/PK, of the transducer permeability system was calculated to be 1.5. The I-V curve of SIC was non-linear.(ABSTRACT TRUNCATED AT 400 WORDS)

A light emitting diode microspectrophotometer: intracellular Ca2+ measurements in isolated stretch receptor

A microspectrophotometer was designed to measure absorbance changes in single cells. The device utilizes sequentially activated light emitting diodes (LED) to provide different wave lengths of light. The instrument has the advantage of relative simplicity and less cost compared to other devices. The spectrophotometer was tested by measuring absorbance changes of the metallochromic Ca2+ indicator Arsenazo III (AIII) injected into the crayfish (Astacus astacus) stretch receptor. Under the conditions described the detection limit of the concentration of AIII was 0.05 mM and absorbance changes of 0.0005 can be reliably determined which correspond to a detection limit of 10-20 nM for free Ca2+ changes assuming a light path length of 0.003 cm and an apparent dissociation constant (KD) of 2 microM for the Ca(2+)-AIII complex. The upper frequency limit of the device is 3000 Hz. The absorbance measurements of AIII injected into the crayfish stretch receptor neurons revealed a Ca(i) of 375 +/- 177 nM (mean +/- SD: 14 cells). LiCl substituted for NaCl increased Ca(i) 45-100 nM in different cells, suggesting that a Na+ gradient is necessary for Ca2+ homeostasis in this receptor.

Block of potassium outward currents in the crayfish stretch receptor neurons by 4-aminopyridine, tetraethylammonium chloride and some other chemical substances

The effects of 4-aminopyridine (4-AP) and tetraethylammonium (TEA) on the outward potassium currents in the rapidly and slowly adapting stretch receptor neurons (SRNs) of the crayfish (Pacifastacus leniusculus) were studied using a two micro-electrode voltage-clamp technique. The leakage current was not affected by either 4-AP or TEA. External 4-AP blocked the peak outward current in a dose-dependent manner (1:1 stoichiometry) with an apparent dissociation constant (Kd) of 2.3 +/- 0.2 mM (mean +/- SEM) in the slowly and 1.4 +/- 0.2 mM in the rapidly adapting SRN, the block being voltage dependent. External application of TEA resulted in a block of the steady state current enhancing the transient characteristics of the current response. The block appeared to deviate from a 1:1 stoichiometry and the apparent Kd for TEA was 9.6 +/- 3.4 mM with a cooperativity factor n = 0.43 +/- 0.03 in the slowly adapting SRN and 34.5 +/- 9.2 mM and 0.37 +/- 0.03 respectively in the rapidly adapting SRN. Low Ca2+, apamin and charybdotoxin, which are known to block Ca(2+)-dependent K-currents, had no effects on the outward current as was also the case with catechol. It is concluded that the different effects of TEA and 4-AP on the outward current in the two types of SRNs can be explained by the presence of at least two, probably heteromultimeric, channel populations having similar sensitivity to 4-AP but different sensitivity to TEA. One channel has a high affinity (Kd = 0.8-1.6 mM) for TEA and the other a low affinity (Kd = 173-213 mM) for TEA. The low-affinity channel seems to dominate in the slowly adapting SRN while both channels are equally common in the rapidly adapting SRN. Further, the present results do not support the existence of a macroscopic Ca(2+)-dependent K+ current in the SRNs.

Inward current caused by sodium-dependent uptake of GABA in the crayfish stretch receptor neurone

A two-microelectrode current-voltage clamp and Cl(-)-selective microelectrodes were used to examine the effects of gamma-aminobutyric acid (GABA) on membrane potential, current and intracellular Cl- activity (aiCl) in the crayfish stretch receptor neurone. All experimental solutions were CO2-HCO3- free. 2. GABA (500 microM) produced a mono- or biphasic depolarization (amplitude < or = 10 mV), often with a prominent initial depolarizing component followed by a transient shift to a more negative level. In some neurones, an additional depolarizing phase was seen upon washout of GABA. Receptor desensitization, being absent, played no role in the multiphasic actions of GABA. 3. The pronounced increase in membrane conductance evoked by GABA (500 microM) was associated with an increase in aiCl which indicates that the depolarizing action was not due to a current carried by Cl- ions. 4. The currents activated by GABA under voltage clamp conditions were inwardly directed when recorded at the level of the resting membrane potential, and they often revealed a biphasic character. The reversal potential of peak currents activated by pulses of 500 microM-GABA (EGABA) was 9-12 mV more positive than the reversal potential of the simultaneously measured net Cl- flux (ECl). ECl was 2-7 mV more negative than the resting membrane potential. 5. EGABA (measured using pulses of 500 microM-GABA) was about 10 mV more positive than the reversal potential of the current activated by 500 microM-muscimol, a GABA agonist that is a poor substrate of the Na(+)-dependent GABA uptake system. 6. In the absence of Na+, the depolarization and inward current caused by 500 microM-GABA were converted to a hyperpolarization and to an outward current. Muscimol produced an immediate outward current both in the presence and absence of Na+. 7. Following block of the inhibitory channels by picrotoxin (100-200 microM), the depolarizing effect of 500 microM-GABA was enhanced and the transient hyperpolarizing shifts were abolished. 8. In the presence of picrotoxin, GABA (> or = 2 microM) produced a concentration-dependent monophasic inward current which had a reversal potential of +30 to +60 mV. This current was inhibited in the absence of Na+ and by the GABA uptake blocker, nipecotic acid. Unlike the channel-mediated current, the picrotoxin-insensitive current was activated without delay also at low (2-10 microM) concentrations of GABA.(ABSTRACT TRUNCATED AT 400 WORDS)

Block of receptor response in the stretch receptor neuron of the crayfish by gadolinium

The trivalent lanthanide gadolinium was found to block the mechanotransducer response in the stretch receptor neuron of the crayfish. At normal calcium concentration (13.5 mM) a 50 per cent block of the receptor current was found at 395 +/- 59 (mean +/- SD) microM gadolinium. At a calcium concentration of 1.35 mM a 50 per cent block of the receptor current was obtained at 103 +/- 14 (mean +/- SD) microM gadolinium. The potential activated potassium current was also affected by gadolinium. At 200 microM the amplitude of the peak outward current as a result of a 90 mV positive potential step was decreased by about 40 per cent. The fast inward sodium current was decreased less than 10 per cent by gadolinium. It is concluded that in the crayfish stretch receptor gadolinium blocks the receptor current, reflecting block of stretch-activated channels, but at higher concentrations than have been described for other stretch-activated channels. In addition the outward rectifier potassium current is also blocked reflecting a block of potassium channels.

Stimulus-response properties of the slowly adapting stretch receptor neuron of the crayfish

The receptor potential and receptor current in response to ramp-and-hold extensions were measured in the slowly adapting stretch receptor of the crayfish, using potential clamp technique. The stimulus-response relationship for the peak amplitude of the receptor current showed a linear behaviour for extensions less than 2% and a nonlinear behaviour for extensions larger than 5%. Using the Stevens power law, R = k(S--S0)n, where R is response, S is stimulus, S0 is threshold stimulus and n the power coefficient, n was found to be 3 for extensions between 5 and 15%. The receptor current saturated at extensions above 20-25% of the zero length of the muscle, resulting in a lower n value. However, the n value is difficult to define in this region due to the saturation. The stimulus-response relation for the receptor current can be explained by the properties of the stretch-activated channels for which the open probability is exponentially dependent on the square of the membrane tension, as suggested by recent findings. The receptor potential, using tetrodotoxin, in response to identical ramp-and-hold extensions as those used to record current responses showed a more complex time-course, indicating involvement of potential-dependent channels, potassium channels being the most probable candidate. This was supported by a mathematical model which takes into account the viscoelastic properties of the receptor muscle, the properties of the stretch-activated channels and a potential dependent K+ current.

Potential-dependent potassium currents in the rapidly adapting stretch receptor neuron of the crayfish

The outward current was analysed in the rapidly adapting stretch receptor neuron of the crayfish Pacifastacus leniusculus with a two-micropipette potential clamp technique and K(+)-selective microelectrodes in an attempt to establish if the properties of this current could explain the difference in adaptive behaviour compared to the slowly adapting receptor. A fast activating outward current carried by K+ was revealed. The time constant of activation(tau n) was dependent on potential and had a mean value of 0.5 ms at potential steps to 0 mV. Activation followed a second-order process according to the Hodgkin-Huxley model. The potential dependence of activation (n infinity) followed by a sigmoid curve n infinity = 1/(1 + exp/[(E - En)/a]) with a half maximal activation potential En = -44 mV and a = -13 mV. When long pulses were applied the outward potassium current decreased with two time constants, one that was potential independent (0.2 s) and one that was potential dependent (2-8 s). The latter could be explained by accumulation of K+ in the extracellular space of the neuron. The potential dependence of inactivation followed a sigmoid function infinity = 1/(1 + exp[(E - Ek)/+a]) with Ek = -36 mV and a = 13 mV. The inactivation properties are different from those of the classical fast transient (IA) current. The transport system for the outward potassium current during depolarizing potential steps in the rapidly adapting stretch receptor is similar to the current found in the slowly adapting receptor neuron. However, the activation is faster and seems to occur at potentials more negative than in the slowly adapting receptor. These differences can contribute to but not entirely explain the difference in adaptive behaviour between the slowly and rapidly adapting receptor.

Viscoelastic properties of the slowly adapting stretch receptor muscle of the crayfish

The viscoelastic properties of the muscle associated with the slowly adapting stretch receptor organ of the crayfish (Astacus astacus) were studied by recording the tension response to various length changes. When steady-state length changes were applied to the muscle, the tension developed in a non-linear way, increasing slowly for small extensions and rapidly when extension increased. Muscle tension responses to ramp-and-hold extensions were characterized by a transient peak followed by a gradual decline in tension. At the onset of the ramp the tension increased rapidly, similar to the response seen in resting skeletal muscle. The relation between peak dynamic tension and extension was non-linear. In a log-log plot the relation was linear with a mean slope of 1.4. At small extensions (less than 5%) the slope seemed to be lower. The experimental results have been analysed in relation to a viscoelastic model consisting of a Voigt element in series with a non-linear spring. The model could describe both the static length-tension relation and the dynamic response, but different parameters for the springs had to be used for the two cases. When the measured tension response was transformed by an exponential function of the squared tension, in accord with recent findings on stretch-activated channels, a good agreement was obtained with the time course of the receptor currents. Adaptation is thus likely to be caused by both the mechanical properties of the receptor muscle and the characteristics of stretch-activated channels of the neuron.