A mechanosensory system that controls feeding in adult Mnemiopsis

We describe here the food groove complex and mechanism of prey capture used by adult Mnemiopsis spp. ctenophores to obtain prey swept into the auricular grooves by feeding currents. Tentilla that emerge from the tentacular groove of the food groove complex extend into the auricular grooves and capture prey upon their sticky surfaces. The prey-laden tentilla contract and drag the prey to the edge of the transport groove, which is also part of the food groove complex. The transport groove undergoes a focal eversion to capture and transport prey orally. Focal eversion exposes the inner ciliated surface of the transport groove as it extends toward the prey. Focal eversion can be evoked by mechanical stimuli from a probe, but only if it is positioned directly over the tentacular groove. We propose that g-cilia located within the tentacular groove are mechanoreceptors whose output triggers a sensory-motor pathway that in turn everts the transport groove. The mechanosensory-motor pathway is ectodermal and sensitive to Mg²⁺ anesthesia, which defocuses and amplifies eversion. Tentilla are not strictly necessary for eversion to occur, because preparations lacking tentilla can still display eversion; however, they may amplify the sensory signal by interacting with g-cilia as they contract.     

Two Different Conductances Contribute to the Anion Currents in Coffea arabica Protoplasts

The anion conductance of the plasma membrane of Coffea arabica protoplasts was isolated and characterized using the whole-cell patch clamp technique. Voltage pulse protocols revealed two components: a voltage-gated conductance (G _s ) and a voltage-independent one (G _l ). G _s is activated upon depolarization (e-fold activation every +36 mV) with time constants of 1 sec and 5 sec at all potentials. G _l and G _s also differ by their kinetic and biophysical properties. In bi-ionic conditions the current associated with G _s shows strong outward rectification and its permeability sequence is F⁻ > NO₃ ⁻ > Cl⁻. In the same conditions the current associated with G _l does not rectify and its permeability sequence is F⁻≫ NO₃ ⁻= Cl⁻. Furthermore, at potentials over +50 mV G _s , but not G _l , increases with a time constant of several minutes. Finally the gating of G _s is affected by stretch of the membrane, which leads to an increased activation and a reduced voltage sensitivity. Anion conductances similar to the ones described here have been found in many plant preparations but G _l -type components have been generally interpreted as the background activation of the slow voltage-gated channels (corresponding to G _s ). We show that in coffee protoplasts G _l and G _s are kinetically and biophysically distinct, suggesting that they correspond to two different molecular entities. Received: 25 November 1996/Revised: 9 April 1997     

Subthreshold activation of spinal motoneurones in the stretch reflex: experimental data and modeling

Responses of gastrocnemius–soleus motoneurones to stretches of the homonymous muscles were recorded intrasomatically in decerebrate cats; changes of membrane potential (MP) were evoked by smoothed trapezoid stretches of the muscles. Amplitudes of separate excitatory postsynaptic potentials (EPSPs) were defined via differences between values of MP at the end and beginning of the positive derivative waves, which were also used as basic elements in the model of the excitatory postsynaptic currents (EPSCs). EPSCs were assumed to be transformed into EPSPs by low-pass filtering properties of the somatic membrane; parameters of the filtering were firstly defined from analysis of Ia EPSP in the same cell and then were applied in model P _m0. The model showed unsatisfactory quality in tracking slow components of MP; to overcome the disadvantage there was proposed model P _m1 based on addition to P _m0 the difference between two low-pass filtered signals MP and P _m0 (the cutoff frequency 10 or 20 Hz). An overestimation of EPSPs’ amplitudes was corrected in model P _m2. The mismatch in tracking slow changes of MP was assumed to be connected with summation of a great number of low-amplitude EPSPs generated at distal dendrites; information about waveform of separate EPSPs could disappear in this process. One can speculate that slow components of membrane depolarization at least partly are linked with the persistent inward currents in dendrites; variable and, sometimes, too fast decays in EPSPs seem to reflect inhibitory synaptic influences.     

The abdominal motor system of the crayfish, Cherax destructor. II. Morphology and physiology of the deep extensor motor neurons

Two opposing muscle systems underlie abdominal contractions during escape swimming in crayfish. In this study we used extracellular and intracellular stimulation, recording and dye-filling to systematically identify each of the five deep extensor excitors and single inhibitor of the crayfish, Cherax destructor. Functional associations of each neuron were characterised by recording its responses to sensory and abdominal cord inputs, its extensor muscle innervation pattern, and its relationships with other neurons. Each excitor receives excitatory input from the tonic abdominal stretch receptors and the largest neuron also receives input from the phasic stretch receptor. The two largest excitors innervate the muscle bundle containing the fastest fibres and may be electronically coupled. The smaller neurons may also be electronically coupled and innervate the remaining deep extensor fibres which display dynamic characteristics from fast to medium-fast. The inhibitor does not receive input from the stretch receptors, but is strongly excited by tactile afferents. The implications of these findings for the current models of the control of abdominal tailflips and swimming are discussed. Accepted: 21 June 1998