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PubMed1998

Acid/base transport across the leech giant glial cell membrane at low external bicarbonate concentration.

1. We have studied acid/base transport across the cell membrane of the giant neuropile glial cell in the leech (Hirudo medicinalis) central nervous system induced by changing the external pH (pHo), using double-barrelled, pH-sensitive microelectrodes. In the presence of 5 % CO2 and 24 mM HCO3-, the intracellular pH (pHi) rapidly changes due to a potent, reversible Na+-HCO3- cotransport across the glial membrane. We have now investigated the transport mechanism which leads to pHi changes in the nominal absence of CO2/HCO3-, where the HCO3- concentration is expected to be below 1 mM. 2. The intracellular pH increased and then decreased when pHo was altered from 7.4 to 7.8 and then 7.0 with a rate of increase of +0.026 +/- 0.008 and a rate of decrease of -0.028 +/- 0.009 pH units min-1 (+/- s.d., n = 49), indicating an acid/base flux rate of 0.64 and 0.71 mM min-1 across the glial membrane, respectively. 3. In the absence of external sodium (Na+replaced by N-methyl-D-glucamine), pHi slowly decreased, and the rate of alkali and acid loading was reduced to 19 and 28 %, respectively, (n = 12). Amiloride (2 mM), which inhibits Na+-H+ exchange, had no effect on the alkali/acid loading (n = 6). 4. The alkali and acid loading were not impaired after the removal of external chloride (Cl-o, replaced by gluconate; n = 11), but were significantly reduced by the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS, 0.5 mM) to 23 and 16 %, respectively, of the control (P < 0.001; n = 5). 5. Alkali and acid loading were affected differently by manipulating the availability of residual HCO3-. After adding the membrane-permeable carbonic anhydrase inhibitor ethoxyzolamide (EZA, 2 microM) to the saline, the acid loading, but not the alkali loading, was significantly reduced (by 25 %, P < 0.01), while lowering the residual CO2/HCO3- concentration in the saline by O2 bubbling significantly reduced the alkali loading (by 59 %, P < 0. 02), but not the acid loading. 6. Changing the membrane holding potential in voltage-clamped glial cells or raising the external K+ concentration to 30 mM had no significant effect on acid/base loading. 7. It is concluded that a residual HCO3- concentration of less than 1 mM in nominally CO2/HCO3--free salines and HCO3- produced endogenously in the glial cells support alkali and acid loading across the glial cell membrane, presumably by activation of the reversible Na+-HCO3- cotransporter. The results suggest a very high selectivity and affinity of this cotransporter for HCO3-; they imply that HCO3--dependent processes may not be negligible even in the nominal absence of CO2/HCO3-, when the HCO3- concentration is expected to be in the submillimolar range.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed2000

Coding and adaptation during mechanical stimulation in the leech nervous system.

The experiments described here were designed to characterise sensory coding and adaptation during mechanical stimulation in the leech (Hirudo medicinalis). A chain of three ganglia and a segment of the body wall connected to the central ganglion were used. Eight extracellular suction pipettes and one or two intracellular electrodes were used to record action potentials from all mechanosensory neurones of the three ganglia. When the skin of the body wall was briefly touched with a filament exerting a force of about 2 mN, touch (T) cells in the central ganglion, but also those in adjacent ganglia (i.e. anterior and posterior), fired one or two action potentials. However, the threshold for action potential initiation was lower for T cells in the central ganglion than for those in adjacent ganglia. The timing of the first evoked action potential in a T cell was very reproducible with a jitter often lower than 100 us. Action potentials in T cells were not significantly correlated. When the force exerted by the filament was increased above 20 mN, pressure (P) cells in the central and neighbouring ganglia fired action potentials. Action potentials in P cells usually followed those evoked in T cells with a delay of about 20 ms and had a larger jitter of 0.5-10 ms. With stronger stimulations exceeding 50 mN, noxious (N) cells also fired action potentials. With such stimulations the majority of mechanosensory neurones in the three ganglia fired action potentials. The spatial properties of the whole receptive field of the mechanosensory neurones were explored by touching different parts of the skin. When the mechanical stimulation was applied for a longer time, i.e. 1 s, only P cells in the central ganglion continued to fire action potentials. P cells in neighbouring ganglia fully adapted after firing two or three action potentials.P cells in adjacent ganglia, having fully adapted to a steady mechanical stimulation of one part of the skin, fired action potentials following stimulation of a different region of the skin. These results indicate that a brief and localised stimulation of the skin can activate more than a dozen different mechanosensory neurones in the three ganglia and after 100 ms of steady stimulation many of these mechanosensory neurones stop firing action potentials and fully adapt. Adaptation occurs primarily at the nerve endings and mechanosensory neurones can quickly respond to mechanical stimulation at a different location on the skin.

Action Potentials↗
PubMed1983

Chromatographic and histochemical identification of dopamine within an identified neuron in the leech nervous system.

Each segmental ganglion of the leech nervous system has two pairs of lateral roots extending to the body wall and viscera. A cluster of about eight neuron cell bodies is located proximal to the first major branch of each anterior root and is termed the anterior root ganglion (ARG). Only one of these eight cells is vitally-stained by Neutral Red dye and fluoresces an intense blue-green following the glyoxylic acid histochemical condensation. The emission spectrum of this anterior root cell (AR) is stable under continuous UV illumination and is bimodal, with peaks at about 480 and 515 nm. This spectrum is indistinguishable from that of millimolar solutions of dopamine (DA) in gelatin droplets following glyoxylic acid histochemistry. We utilized high performance liquid chromatography and an amperometric detector to measure DA within the AR neurosomata at 1.01 pmol/cell. The AR cells in this study had an average diameter of 23 micron and therefore, the minimum intrasomatal concentration of DA is 160 mM, an unusually high level for any neurotransmitter. We measured DA in anterior axons at 0.83 pmol, in segmental ganglia at 1.07 pmol, and in longitudinal connectives at 0.16 pmol. Control neurosomata (Retzius cells) and axonal tracts which lack blue-green fluorescence (posterior and distal anterior roots), had no detectable DA (less than 0.06 pmol/sample). These data establish that the catecholamine DA is responsible for the fluorescence of the AR cell.

Animals↗
PubMed1985

Elemental composition and water content of neuron and glial cells in the central nervous system of the North American medicinal leech (Macrobdella decora).

Elemental (Na, P, S, Cl, K, Ca, Mg) composition and water content of neurons and glial cells of the leech (Macrobdella decora) were determined by x-ray microanalysis of frozen hydrated and dried section techniques. Results are reported as elemental mass fractions (mass/mass) and water content as percent mass. Specific cell compartments and cell types had distinct elemental patterns and water content which suggests that chemical composition of specific cell types is unique and may represent an expression of cell differentiation analogous to morphological specialization. Water content of cells was also cell specific and ranged from 55% (neurons) to 90% (vacuolated zone of glial cells). K and Na were present in concentrations greater than predicted by ion-selective microelectrode measurements, indicating that not all the K and Na were simultaneously accessible to such electrodes.

Animals↗
PubMed1988

Effects of increased extracellular K on the elemental composition and water content of neuron and glial cells in leech CNS.

Elemental (Na, Cl, K) and water contents of leech (Macrobdella decora) neurons and glial cells were determined under steady-state exposure to 4, 10, and 20 mM KCl concentrations (bathing media) using x-ray microanalysis for quantitative digital imaging of frozen hydrated and dried cryosections. Effects of furosemide, 5-hydroxytryptamine (5-HT), and ouabain on elemental distribution changes, induced by exposure to 20 mM K, were also determined. Results demonstrated that packet glial cells and neurons accumulated substantial amounts of K that appeared evenly distributed throughout the cytoplasm. Cell water content also increased as a function of increased cytoplasmic K so that the net effect was an unchanged wet-weight K concentration (expressed as millimoles per kilogram wet weight). Dry-weight Na and Cl concentration (expressed as millimoles per kilogram dry weight) increased slightly in glial cells; however, because cell water increased, both Na and Cl (wet-weight) concentrations decreased. Neurons, in contrast, had no significant change in either Na or K on a wet-weight basis, so a relatively constant Na/K ratio was maintained despite a small, but significant, increase in K (dry weight) and cell water. These increases, like those in packet glia, were a function of exposure to different concentrations of extracellular space K. These changes were completely abolished by 10(-4) M ouabain. Neither furosemide nor 5-HT appeared to affect neuronal or glial K wet-weight concentrations. These data show that both glial cells and neurons can act as substantial reservoirs for K while maintaining stable K concentrations (by altering cell water content and elemental composition). This process appears to depend on a functioning Na+, K+-ATPase system.

Animals↗
PubMed1990

The specificity of 130-kDa leech sensory afferent proteins is encoded by their carbohydrate epitopes.

From early development through adulthood in the leech, sensory afferents, glial cells, and connective tissue express different epitopes located on a group of 130-kDa glycoproteins. The sensory epitope [reactive with monoclonal antibody (mAb) Lan3-2] is shared by the peripheral sensory afferents of different sensory modalities. In contrast, three other immunocytochemically distinct epitopes (reactive with mAbs Laz2-369, Laz7-79, and Laz6-212) differentiate these sensory afferents according to their sensory modalities. The glial epitope (mAb Laz6-297) is expressed on all macroglial processes, and the connective tissue epitope (mAb Laz9-84) is located on connective tissue surrounding the CNS, as well as in the peripheral tissues. The hydrophilic-hydrophobic nature of the 130-kDa sensory afferent and glial proteins was determined by phase separation with Triton X-114 and hypoosmotic extraction. They behave as peripheral membrane proteins. Deglycosylation of 130-kDa glycoproteins with N-Glycanase or preincubation of their respective mAbs with alpha-methylmannoside showed that the sensory epitope contains mannose, whereas the modality epitopes are of an undefined carbohydrate character. Immunoprecipitation and a peptide mapping experiment confirmed the existence of four distinct sensory afferent epitopes. Previous studies provided evidence that the mannose-containing Lan3-2 epitope mediates normal sensory afferent growth in the synaptic neuropile. We, therefore, postulate that the carbohydrate epitopes on sensory afferent glycoproteins participate in synapse formation.

Animals↗
PubMed1999

Destabilase complexes--natural liposome produced by medicinal leeches Hirudo medicinalis.

Electrophoretic analysis of destabilase preparation demonstrates the presence of protein combinations with MW 12.3, 25 and 50 kD. Fraction (MW 12.3 D) is a monomer of destabilase aggregation having properties of micellar proteins and represents a stable lipid-protein complex, where the role of lipid component is played by the stable analogue of prostacyclin (MW 391 D). The synthesis of a low molecular fraction of destabilase is fulfilled with bacteria--symbiont of leeches Aeromonas hydrophila. When the destabilase (MW 12.3 kD) contacts with blood a process of complexe formation is triggered with hirudin and blood plasma kallikrein inhibitor, forming a stable 'destabilase complex' (DC; MW 25 kD), possessing also a high aggregation capacity. Polymer forms of the destabilase complex form a liposome changing its spatial orientation depending on the nature of the solvent. Such structural organization provides a high stability of DC components and a rapid penetration through cellular membranes (transmembrane transfer) and it also provides prophylactic antithrombotic action in the case of peroral application to animals, due to the blockade of vascular platelets (inhibition of platelet aggregation by prostacyclin analogue) and plasmic (inhibition of thrombin activity and blood plasma kallikrein) links of the hemostasis process. Destabilase fraction with MW 50 kD is a dimer of the destabilase complex. As a result of DC destruction (liposome), hirudin, prostacycline analogue and blood plasma kallikrein inhibitor are released.

6-Ketoprostaglandin F1 alpha↗
PubMed1974

The action of 5-hydroxthryptamine and related compounds on the activity of Retzius cells of the leech Hirudo medicinalis.

1 The equipotent molar ratios of a range of tryptamine analogues, as compared with 5-hydroxytryptamine (5-HT), have been determined on the basis of their ability to hyperpolarize the membrane potential of the Retzius cell of the leech, Hirudo medicinalis.2 The substitution of methyl, fluoro, chloro, methoxy or acetyl groups onto the 5-HT molecule progressively reduced the potency.3 5-Methoxylation or terminal N-methylation of tryptamine considerably increased the potency of tryptamine but these compounds tended to depolarize cells rather than cause hyperpolarization. In some experiments they were ineffective on preparations pretreated with 5-HT.4 It is suggested that these compounds may act by a different mechanism from the 5-hydroxylated indoles, perhaps involving a different receptor.

Action Potentials↗
PubMed1980

Structure-activity studies on an excitatory receptor for glutamate on leech Retzius neurones.

1 Intracellular recordings were made from Retzius cells from the segmental ganglia of Hirudo medicinalis and Haemopis sanguisuga. Glutamate had a direct excitatory effect on the leech Retzius cells. 2 L-Glutamate was 25 times more potent than D-glutamate. 3 L-Glutamate was approximately equipotent with ibotenic acid and 11.2 times more potent than L-aspartic acid. 4 Quisqualic acid and kainic acid were both approximately 100 times more potent than L-glutamate. DL-1-Amino-cis-1-3-dicarboxyclyclopentane was approximately 5 times more potent than L-glutamate, while the trans isomer was 105 times less potent. 5 alpha-NH2-pimelic acid and beta-CH3-glutamic acid reduced the response to L-glutamate. 6 It is suggested that glutamic acid may interact with the Retzius cell glutamate receptor in an extended conformation.

Animals↗
PubMed1995

Differential modulation of voltage-activated conductances by intracellular and extracellular cyclic nucleotides in leech salivary glands.

1. Two-electrode voltage clamp was used to study the effects of adenosine 3':5'-cyclic monophosphate (cyclic AMP) and guanosine 3':5'-cyclic monophosphate (cyclic GMP) on voltage-dependent ion channels in salivary gland cells of the leech, Haementeria ghilianii. 2. Intracellular cyclic AMP specifically blocked delayed rectifier K+ channels. This was shown by use of 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor), forskolin (an activator of adenylyl cyclase) and intracellular injection of cyclic AMP and its dibutyryl and 8-bromo analogues. Cyclic AMP appeared to be the second messenger for the putative neuroglandular transmitter, 5-hydroxytryptamine. 3. Intracellular injection of cyclic GMP specifically potentiated high-voltage-activated (HVA) Ca2+ current and the effect was mimicked by zaprinast, an inhibitor of cyclic GMP-dependent phosphodiesterase. 4. Extracellularly, cyclic GMP and cyclic AMP specifically decreased the amplitude and increased the rate of inactivation of HVA Ca2+ current. These effects of the cyclic nucleotides are identical to those known for extracellular ATP, which activates a presumed purinoceptor. The pyrimidine nucleotide, UTP, was almost equipotent to ATP (threshold dose < 10(-6) M), indicative of a vertebrate-type nucleotide receptor. However, suramin (5 x 10(-5) M), a non-specific P2-receptor antagonist, failed to block the effects of 5 x 10(-6) M ATP (higher suramin doses could not be reliably tested because of the depolarization and increase in membrane conductance produced by the drug). 5. Activation of the putative purinoceptor by ATP did not affect inward rectifier Na+/K+ current which is known to be potentiated by intracellular cyclic AMP and reduced by intracellular cyclic GMP. 6. The preparation may provide a useful model for study of nucleotide actions, and interactions, in channel modulation. It has technical advantages such as large cells (1200 microns in diameter) which lack intercellular coupling and may be individually dissected for biochemical studies.

Action Potentials↗
PubMedDate not supplied

Lessons from leeches: a call for DNA barcoding in the lab.

Many evolution of development labs study organisms that must be periodically collected from the wild. Whenever this is the case, there is the risk that different field collections will recover genetically different strains or cryptic species. Ignoring this potential for genetic variation may introduce an uncontrolled source of experimental variability, leading to confusion or misinterpretation of the results. Leeches in the genus Helobdella have been a workhorse of annelid developmental biology for 30 years. Nearly all early Helobdella research was based on a single isolate, but in recent years isolates from multiple field collections and multiple sites across the country have been used. To assess the genetic distinctness of different isolates, we obtained specimens from most Helobdella laboratory cultures currently or recently in use and from some of their source field sites. From these samples, we sequenced part of the mitochondrial gene cytochrome oxidase I (COI). Sequence divergences and phylogenetic analyses reveal that, collectively, the Helobdella development community has worked on five distinct species from two major clades. Morphologically similar isolates that were thought to represent the same species (H. robusta) actually represent three species, two of which coexist at the same locality. Another isolate represents part of a species complex (the "H. triserialis" complex), and yet another is an invasive species (H. europaea). We caution researchers similarly working on multiple wild-collected isolates to preserve voucher specimens and to obtain from these a molecular "barcode," such as a COI gene sequence, to reveal genetic variation in animals used for research.

Animals↗
PubMed1989

Growth and synapse formation by identified leech neurones in culture: a review.

Within hours after identified neurones have been isolated from the CNS of the leech, they begin to sprout and to form synapses. Electrical recordings made by loose-patch clamp show that the tip of the isolated neurone has distinct properties with a high density of sodium channels. Neurites grow out from this tip after about 30 min and continue to grow for the next few days. The extent of growth, the branching pattern and the distribution of calcium channels all depend critically upon the molecular composition of the substrate. The tip of the neurone also represents a preferred region for synapse formation. For example when the tips of two serotonin-containing neuromodulatory neurones, the Retzius cells, are placed in contact, chemical synapses develop within about 6 h. These chemical synapses are bidirectional and become stronger over the next 2 days. Electrical synapses between the two Retzius cells develop more slowly and appear only after about 20 h. When the tip of one Retzius cell is apposed to the soma of another, chemical transmission develops more slowly. When other regions of these same cells are placed in contact, electrical transmission can appear before chemical. Together these results show that specialized areas of neuronal membrane are involved in neurite extension and in the formation of specific synaptic connections.

Animals↗
PubMed1969

After-effects of nerve impulses on signalling in the central nervous system of the leech.

A long-lasting hyperpolarization has been observed in sensory neurones of the C.N.S. of the leech following natural stimulation applied to their receptive fields. The underlying mechanism of this hyperpolarization and its effects on signalling were investigated by intracellular recording.1. The amplitude and duration of the hyperpolarization depended on the number of action potentials. Whereas a brief burst of impulses had little effect, a prolonged sensory discharge was followed by a hyperpolarization of up to 30 mV that gradually declined over a period of several minutes.2. The hyperpolarization was abolished by ouabain, reversibly inhibited by strophanthidin or cooling, unaffected by Mg and accompanied by an increase in membrane resistance. These observations suggest that the hyperpolarization is the result of membrane current generated by an electrogenic pump.3. A hyperpolarization similar to that recorded in the cell body also occurred in the neuronal processes within the neuropile, where synaptic contacts are made. This led to significant changes in integrative activity, such as an increase in the amplitude of excitatory synaptic potentials, a reversal of inhibitory synaptic potentials or a conduction block in parts of the neurones. All of these effects could be mimicked by injecting hyperpolarizing currents into resting cells, or abolished by injecting a depolarizing current into a cell hyperpolarized by previous impulses.4. During neuronal hyperpolarization the sensitivity of the membrane potential to small increments of external K was enhanced by a factor of about three. This effect varied with the magnitude of the hyperpolarization produced by preceding impulses and could not be mimicked by applying K to a neurone hyperpolarized by the injection of current into its cell body.5. These findings are discussed in relation to possible effects that changes in the intracellular and extracellular concentrations of ions produced by activity might have on integration in the C.N.S.

Action Potentials↗
PubMed1969

Chemical and electrical synaptic connexions between cutaneous mechanoreceptor neurones in the central nervous system of the leech.

Experiments have been made to study the synaptic connexions between sensory cells in the C.N.S. of the leech. Each segmental ganglion contains six neurones that respond specifically to light touch applied to the skin; each of these ;touch cells' innervates a discrete area on the surface of the body and has a characteristic set of properties by which it can be recognized. Using intracellular electrodes it has been shown that these sensory cells interact with one another through chemical and electrical synapses by way of a stereotyped set of pathways.1. Action potentials occurring in one touch cell gave rise to synaptic potentials in the five other touch cells in the same ganglion and also in the three ipsilateral touch cells in the adjacent ganglia. Thus, synaptic interactions took place between sensory cells whose receptive fields lay within the same segment and on the same side of adjacent segments.2. The post-synaptic potentials consisted of a short-latency coupling potential, followed by an excitatory potential and a number of inhibitory potentials. These delayed synaptic potentials occurred inconsistently and with a variable latency; they could also be recorded in the cell which had been stimulated. All of the touch cells appeared to be equally effective in initiating synaptic potentials.3. The short-latency coupling potential was shown to be mediated through an electrical synapse by observing a voltage change in one touch cell when current was injected into its neighbour. It was not abolished by high concentrations of Mg in the bathing fluid, which blocked chemical synapses in this ganglion. This electrical synapse displayed remarkable rectification; a depolarization could spread from cell to cell in both directions, while a hyperpolarization could spread in neither.4. The inhibitory potentials were reversed by injecting Cl into the cell. In Cl-free Ringer solution this effect was so marked that the reversed IPSPs caused long trains of impulses in touch cells, which tended to excite each other by a process of positive feed-back.5. Synaptic potentials evoked by activation of a touch cell did not usually reach threshold since excitation and inhibition tended to cancel. The connexions between touch cells that mediated the delayed excitatory and inhibitory potentials are polysynaptic; the interneurones have not yet been found but some of their connexions could be inferred from electrical recordings.6. Action potentials in sensory cells of a different modality (responding to pressure) also initiated synaptic potentials in the same family of touch cells.7. The possible significance for integration of these synaptic interactions between sensory cells is discussed.

Action Potentials↗
PubMed1972

A comparison of chemical and electrical synaptic transmission between single sensory cells and a motoneurone in the central nervous system of the leech.

In leech ganglia, three sensory cells of different modality converge on a motoneurone, where they form chemical and electrical synapses. Each of these synapses behaves in a characteristic manner and the nature of the transmission mechanism has significant functional consequences for the operation of the reflexes. An analysis has been made of the effects of trains of impulses on synaptic transmission through these pathways, using frequencies that correspond to natural firing.1. At the chemical synapse between the nociceptive sensory cell and the motoneurone, two opposing events occur: facilitation and depression. Thus, with trains of impulses, the synaptic potentials first increase in amplitude and then decrease. The two processes could be separated by altering the Mg and Ca content of the bathing fluid. In concentrations of Mg that reduced the amplitude of a single control chemical synaptic potential, pure facilitation occurred during a train. Depression predominated during brief trains in raised concentrations of Ca, although synaptic potentials were initially larger. These results suggest that changes in the amount of transmitter released by each presynaptic action potential can account for the changes observed in chemical synaptic transmission.2. In contrast, electrical transmission between the sensory cell responding to touch and the same motoneurone did not show facilitation or depression. The electrical coupling potential in the motoneurone was relatively constant when the touch cell fired at high or low frequencies in normal Ringer fluid, high Mg, or high Ca fluid.3. Further differences between chemical and electrical synapses were apparent when the preparation was cooled to 4 degrees C. In the cold the latency of chemically evoked synaptic potentials in the motoneurone increased and their amplitude declined drastically with repetitive stimulation, while electrical coupling potentials were unaffected.4. A brief hyperpolarization of the presynaptic cell by injected current produced a marked and prolonged increase in chemically evoked synaptic potentials, but did not influence electrical synaptic transmission.5. The synapses of the sensory cell responding to pressure, which are both chemical and electrical, behaved as expected: the chemical synaptic potentials showed facilitation and depression while electrical transmission remained relatively constant.6. These experiments emphasize the different functional consequences of electrical or chemical synapses in reflex pathways for the transmission of signals that arise as a result of natural sensory stimuli.

Action Potentials↗
PubMed1972

Analysis of responses in visual cells of the leech.

1. Potentials were recorded from the cytoplasm and from the vacuole of leech photoreceptors. Since the vacuole is lined with microvilli and is connected to the outside by narrow channels, the potential drops between vacuole and outside measure the current through the microvillar membrane.2. In darkness, the potential of the cytoplasm with respect to the outside is about - 45 mV while the potential of the vacuole is approximately zero.3. Following illumination the negativity of the cytoplasm decreases and the vacuole becomes negative relative to the outside.4. For dim intensities, the response to a flash of light may grow proportionately more than the intensity of the flash. This is probably due to development of a depolarizing local response.5. The resistance from the cytoplasm to the outside was about 150 MOmega in darkness and decreased to approximately 40 MOmega at the peak of the response to a bright flash (on average). Corresponding measurements from the vacuole gave 50 MOmega in darkness and 35 MOmega at the peak of the response.6. Charging curves produced by steps of constant currents applied to the cytoplasm or to the vacuole include two time constants (about 5 and 50 msec on average). The longer time constant decreases greatly with bright illumination.7. The results are consistent with the interpretation that the response to light is brought about by an increase of conductance of the microvillar membrane.

Action Potentials↗
PubMed1973

The contribution of membrane hyperpolarization to adaptation and conduction block in sensory neurones of the leech.

The factors underlying sensory adaptation and conduction block have been studied in cutaneous mechanoreceptor neurones of the leech. A touch-sensitive cell was activated by applying mechanical or electrical stimuli to its receptive field on the skin. Impulses were recorded extracellularly from its axons and intracellularly from its cell body, which is situated within the C.N.S.1. Activation of the touch cell by mechanical stimuli revealed two distinct types of adaptation with characteristically different time courses. Sustained pressure on the skin caused a brief burst of impulses at the onset of the stimulus. This rapid adaptation to pressure was restricted to the part of the receptive field that had been stimulated mechanically. A second type of adaptation developed more slowly during the course of repetitive mechanical stimulation. It persisted for many seconds after the end of a train of impulses and appeared as an increase in the threshold to mechanical stimuli not only in the region of skin that had been rubbed but throughout the receptive field of the cell.2. Impulses initiated in the cell body propagated antidromically towards the skin and also raised the threshold to touch, indicating that after-effects of impulse activity were responsible for the long-lasting threshold increase.3. Repetitive mechanical stimulation could also produce a reversible conduction block in branches of the touch cell. The block occurred in discrete regions of low safety factor such as axonal branch points both within the ganglion and in the periphery. In some experiments impulses intermittently failed to reach one axonal branch yet continued to invade a separate branch of the same cell.4. Several lines of evidence indicate that both conduction block and the slow component of adaptation are linked to a prolonged hyperpolarization that follows repetitive stimulation of the touch cell. Strophanthidin, which blocks the after-hyperpolarization in touch cells, reduced the adaptation following trains of impulses and also relieved a conduction block previously established by repetitive stimulation. Furthermore, a comparison of the effects of hyperpolarizations produced by current injection and by repetitive firing showed that most of the threshold increase in the cell body after a train of impulses could be attributed directly to the membrane hyperpolarization.5. These experiments suggest several ways in which repetitive activity can have pronounced and long-lasting effects on the performance of a highly branched sensory cell. Thus a relatively small number of impulses in a touch cell can markedly decrease its sensitivity to touch. The functional role of the conduction block observed during vigorous stimulation is not as clear because activity for many seconds or minutes is usually needed to establish a block in the larger branches of the cell.

Action Potentials↗