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PubMed1976

The connective tissue coverings of leech peripheral nerves: anatomical evidence for the absence of cerebrospinal fluid in the leech.

The central nervous system of Hirudo medicinalis is contained within a blood vessel, the ventral longitudinal sinus, but the nervous system is separated from the blood by the visceral endothelium. The visceral endothelium possesses many pinocytotic vesicles and basal infoldings and thus appears active whereas the parietal endothelium appears inactive. The junction between the visceral and parietal endothelia is abrupt. Peripheral nerves in this animal, as in vertebrates, are covered by endoneurium, perineurium, and epineurium. The endoneurium is continous with the fibrous tissue of the segmental ganglia. The perineurium, consisting of a single layer of flattened cells that surrounds the peripheral nerve like a sleeve, is not continuous with the endothelium of the ventral sinus, but is separated from it by 5-10 microns. Therefore, at the point where the peripheral nerve joins the segmental ganglion, the extracellular spaces of the central nervous system, the peripheral nervous system, and the body wall are all confluent. Thus, there are only two compartments of the extracellular space in the leech: the blood, which is enclosed by the endothelia of the coelomic sinuses, and the extracellular fluid of the body, which includes the extracellular fluid of the nervous system. There seems to be no equivalent of cerebrospinal fluid in the gnathobdellid leech.

Animals↗

PubMed1978

Modulation of transmission at an inhibitory synapse in the central nervous system of the leech.

The synaptic interactions among a group of cells in the leech C.N.S. that regulate the animal's heartbeat exhibit several remarkable features (Thompson & Stent, 1976 a, b, c). We have examined in detail the properties of the inhibitory synapse between two of these cells, the heart interneurone (HN cell) and the heart excitor motoneurone (HE cell). 1. Impulses in the presynaptic HN cell gave rise to monosynaptic i.p.s.p.s in the HE cell that were blocked by high concentrations of Mg and were reversed when the membrane potential of the post-synaptic motoneurone was hyperpolarized beyond--75 m V or when Cl was injected into the cell body. These i.p.s.p.s were chemically mediated, and involved an increase in chloride conductance. 2. In contrast to chemical synapses between sensory and motor cells in the leech C.N.S., little facilitation or depression of transmission occurred when the HN cell was stimulated at frequencies of 0.1--50 Hz. 3. Steady subthreshold depolarization of the presynaptic HN interneurone evoked a maintained hyperpolarization of the post-synaptic HE cell, indicating that currents injected into the HN cell body could spread to the terminals and cause continuous release of transmitter. 4. The size of the i.p.s.p. evoked in the HE motoneurone by an action potential in the HN interneurone varied with the resting membrane potential of the presynaptic cell. An impulse superimposed on a prolonged, subthreshold, depolarizing pulse produced a larger i.p.s.p.; conversely, prolonged hyperpolarization of the HN interneurone reduced the i.p.s.p. amplitude recorded in the HE cell. This effect was most obvious when the natural, rhythmical bursts of activity in the HN interneurone were interrupted by bathing the preparation in leech Ringer fluid containing elevated concentrations of Mg. Under these conditions a 10 mV depolarization of the HN cell increased the size of the i.p.s.p. in the HE cell approximately sixfold. Significant changes in i.p.s.p. amplitude occurred without any noticeable change in the amplitude and duration of the presynaptic action potential. With large presynaptic depolarizations, which produced the biggest i.p.s.p.s, there was some reduction in the amplitude and increase in the duration of the action potential. 5. Following a step depolarization of the presynaptic cell, the size of successive i.p.s.p.s increased with a time constant of about 1 sec. Upon repolarization the i.p.s.p.s decreased in amplitude to the original level. 6. stimulation of one HN cell also gives rise to an i.p.s.p. in its contralateral homologue (Thompson & Stent, 1976c). Trains of i.p.s.p.s produced in this way hyperpolarized at HN cell to such an extent that the size of the synaptic potential it evoked in an HE cell was reduced. 7. Thus, an HN interneurone inhibitis transmission between the contralateral HN and HE cells presynapitcally in addition to inhibiting directly the ipsilateral HE motoneurone.

Animals↗

PubMed1976

Current excitation threshold in sensory neurons of leech central nervous system.

1. Sensory cells in segmental ganglia of the leech (Hirudo medicinalis L.), N (noxious), P (pressure), and T (touch) cells, were stimulated by linearly rising currents. The electrical response to intracellular stimulation with prolonged square-wave currents, the shape of the action potential, and the degree of repetitive activity were also examined. 2. The thresholds for first production of an action potential by linearly rising currents (expressed as multiples of rheobase, I/IO), plotted as a function of the action-potential latency, provide a measure of accomodation. These T-L curves show that the P and T cells accommodate more rapidly than the N cells. 3. An index of accommodation, the accommodation coefficient (I/Io)s/2, was defined as the current required to elicit a first action potential 0.5 s after the beginning of a linearly rising stimulus current. These accommodation coefficients also reflect the property that P and T cells accommodate more rapidly, on the average, than N cells. 4. Threshold depolarization is higher with linearly rising currents than with square-wave currents. 5. Comparison of the accommodation rates of these sensory somata with the published data for other somata and axons, on the basis of accommodation coefficient, minimal current gradient, and/or threshold-latency curve, shows that a) N cells, at the slow end of the leech-soma accommodation range, accommodate at about the same rate as most vertebrate neurons; and b) some vertebrate neurons have accomodation rates as high as or higher than those of the T and P cells. The accommodation rates of most vertebrate neurons are related in a characteristic way to undershoot shape, repetitive-firing properties, and adaptation. Comparable relationships are found in the leech if the N and T cells alone are considered, but the P cells (with extremely rapid accomodation) depart notably from this pattern--they adapt slowly, fire repetitively in response to a maintained stimulus, and have an undershoot of intermediate amplitude and duration.

Adaptation, Physiological↗

PubMed1978

Quantitative mapping of cutaneous receptive fields in normal and operated leeches, Limnobdella.

1. The receptive fields and physiological properties of the sensitive cutaneous mechanoreceptive neurones in the leech Limnobdella australis were found to be very similar to those previously described in Hirudo medicinalis. 2. Following separation from the central nervous system (C.N.S.), the distal dendrite stump and cutaneous receptive field remained unchanged for at least 160 days. 3. There was little spreading of receptive fields into regions of skin isolated from the C.N.S. for at least 184 days. 4. Cutting one dendrite of a mechanoreceptive neurone which has two major dendrites produced little change in the receptive field of the intact dendrite. 5. Abnormalities were found in most of the receptive fields of operated leeches, irrespective of the site of operation. These abnormalities were not seen in normal leeches.

Animals↗

PubMed1975

[Leeches in the respiratory system (author's transl)].

The leech as a foreign body and parasite in the human respiratory tract occurs principally in the Mediterranean countries, in Africa and Asia. It reaches the respiratory tract when water is drunk directly from rivers, lakes, etc. Ignorance of this fact may cause diagnostic difficulties leading to errors in treatment. Anaemia and respiratory obstruction due to leeches cause danger to health and life, especially in children. Fatalities have been known. Treatment consists of endoscopic removal of the parasite, which may be technically difficult, especially when the leech is in the region of the larynx and thus presents dramatic conditions for the procedure. A series of 120 cases treated in Algeria between 1962 and 1971 is presented.

Age Factors↗

PubMed1976

A central inhibitory action of 5-hydroxytryptamine in the leech.

Previous studies indicated that 5-HT reduced the number of spontaneous excitatory junctional potentials (ejp's) that occurred in leech body wall muscle cells. The present study confirms these findings and shows that the ejp's arise from impulses in motoneuron L. This study further shows that 5-HT acts by hyperpolarizing and reducing the membrane resistance of neuron L, thus inhibiting the motoneuron and reducing the frequency of spontaneous ejp's on body wall muscle cells. These effects of 5-HT are not seen when the ganglion is bathed in a high magnesium solution, a finding that suggests that 5-HT does not act directly on the membrane of motoneuron L. This study demonstrates that 5-HT can have a central inhibitory effect on body wall muscle contractions. Previous studies provide evidence that 5-HT may act as a direct neuromuscular inhibitory transmitter and may also take part in peripheral presynaptic inhibition. Thus, if further studies confirm these suggestions, the well-known inhibitory effect of 5-HT on leech body wall muscle is a more complex process than was previously thought.

Action Potentials↗

PubMed1978

The caudal ganglion of the leech, with particular reference to homologues of segmental touch receptors.

The caudal ganglion of the leech, which provides sensory and motor innervation to the posterior sucker, represents the fusion of seven embryonic segmental ganglia. Although fused, each of the seven contributing ganglia ("subganglia") of the caudal ganglion can be distinguished morphologically and functionally. The roots from each subganglion carry the axons of mechanoreceptors homologous to "touch" cells found in the segmental ganglia and the subesophageal compound ganglion. The receptive fields supplied by the touch cells of the caudal ganglion are uniquely arranged and reveal the modified segmentation of the circular posterior sucker. Extensive overlap of sensory innervation occurs between adjacent segments of the sucker, beyond the overlap characteristic of the homologous cells of body segments. It thus appears that the touch receptors of the caudal ganglion are less restricted than receptors of the segmental ganglia with regard to their territories of innervation. The caudal ganglion has additional unique properties that establish it as a distinct integrative center of the leech CNS.

Animals↗

PubMed1979

Identified neurones isolated from leech CNS make selective connections in culture.

Neurones cultured in vitro offer distinct advantages for studying how processes grow towards their targets and form synaptic connections. In contrast to the complex events occurring during the development of the nervous system, synapse formation in culture can be analysed in a few neurones at a time and under controlled conditions. We have now dissected out and cultured single identified neurones from the central nervous system (CNS) of the adult leech. Various types of sensory cells, motor cells, and interneurones can be identified in leech ganglia--each with a stereotyped set of properties, including: (1) the electrical characteristics of its membrane, (2) the arborisation of its branches and the morphology of its terminals and (3) the pattern of connections it makes with other identified neurones, skin or muscle. Thus, cultured cells can be compared in detail with their counterparts in situ. We have found that isolated cells survive for several weeks, maintain their membrane properties, sprout and form selective connections.

Animals↗

PubMed1976

Modification and regeneration of synaptic connections in cultured leech ganglia.

Segmental ganglia of the central nervous system of the leech were maintained in culture medium outside the animal for several weeks in order to study the properties of synapses and regeneration by identified sensory and motor nerve cells. A variety of preparations were used, including single ganglia, chains of ganglia and ganglia connected to the areas of skin and muscle that they normally innervate in the animal. (1) For up to 10 weeks after removal from the animal, resting and action potentials recorded from sensory and motor neurons resembled those seen in normal ganglia. The same individual cell in a cultured ganglion could be recorded from with intracellular electrodes on a second occasion after an interval of a few days. (2) Sensory cells, identified as touch, pressure or nociceptive according to their morphology and electrical properties, continued to respond selectively to stimuli of the appropriate modality applied to their receptive fields in the skin; action potentials in motor cells caused contractions in the appropriate muscles. Culture of ganglia for more than 3 weeks caused the disappearance of synaptic potentials and a loss of transparency in ganglia. (3) Certain chemically mediated synaptic interactions between sensory and motor nerve cells became markedly changed in cultured ganglia. These changes appeared over the first 3 weeks and consisted of abnormally large excitatory and inhibitory synaptic potentials. The changes in synaptic transmission observed in culture were in many respects similar to those occurring in ganglia maintained within an animal after lesions have been made in the nervous system (Jansen et al. 1974). (4) The morphological appearances of sensory cells were compared in cultured and normal ganglia after injection of horseradish peroxidase. In cultured ganglia, the branching pattern appeared normal, but varicosities became more conspicuous. (5) When connectives linking cultured ganglia were crushed or cut, regeneration occurred. By 7 days, impulses propagated through the regenerated fibers and evoked synaptic potentials on cells within the next ganglion. The course taken by regenerating axons was observed in cells injected with horseradish peroxidase. The results again resembled those seen in animals with similar lesions. (6) The cultured ganglia provide preparations in which it is possible to analyze the mechanisms that underlie long-term changes similar to those seen in the leech central nervous system in situ.

Animals↗

PubMed1978

Quantal analysis of transmitter release at an inhibitory synapse in the central nervous system of the leech.

The quantal nature of transmitter release has been analysed at central inhibitory synapses in the leech nervous system between an interneurone (HN) and a motoneurone (HE) that regulate the heartbeat. 1. Ganglia were bathed in leech Ringer fluid containing 20 mM-Mg and 1.8 mM-Ca and the membrane of the presynaptic HN interneurone was hyperpolarized by current injection. Under these conditions successive inhibitory potentials in the HE motoneurone, evoked by impulses in the HN interneurone, showed striking fluctuations in amplitude. 2. Assuming a Poisson distribution of the i.p.s.p.s and estimating the number of failures from the amplitude histograms of the observed responses, the mean size of the quantal unit was estimated as 0.25 +/- 0.015 mV (S.E. of mean, n = 26). When m, the mean number of quanta released per trial, was varied by changing the membrane potential of the presynaptic HN cell (Nicholls & Wallace, 1978), the experimentally observed amplitude distributions could be predicted by the Poisson theory. 3. An independent estimate of the unit size was obtained by noise analysis. A long subthreshold depolarizing pulse applied to the presynaptic HN interneurone evoked a sustained hyperpolarization of the HE motoneurone, apparently caused by an increase in the rate of on-going release of quanta by the HN cell terminals. From the mean change in membrane potential and the increase in variance, the size of the unit was calculated as 0.21 +/- 0.039 mV (S.E. of mean, n = 11). For ten pairs of cells an estimate of unit amplitude was made both from the Poisson analysis and the analysis of variance, again with good agreement. For these cells the estimated unit sizes were 0.24 +/- 0.023 mV (S.E. of mean, n = 10) from the failures and 0.21 +/- 0.043 m V (S.E. of mean, n = 10) from the noise. 4. A similar analysis was made of the inhibitory synaptic potentials evoked in one HN interneurone by stimulation of its contralateral homologue. Transmission again appeared to be qualtal; the mean unit amplitude from Poisson analysis was 0.31 +/- 0.022 mV (S.E. of mean, n = 19) and from the noise 0.29 +/- 0.027 mV (S.E. of mean, n = 3). 5. We conclude that transmitter is released from the terminals of the HN interneurone in quantal units that evoke miniature i.p.s.p.s of about 0.25 mV in the post-synaptic cells. Furthermore, modulation of transmission proudced by variation in the presynaptic resting potential and during presynaptic inhibition results from changes in the mean number of quanta released by each impulse.

Animals↗

PubMed1976

Sensory neurons in leech central nervous system: changes in potassium conductance an excitation threshold.

1. The sensory neurons in the leech central nervous system differ in their accommodation to linearly rising currents. Advantage was taken of these differences to study the ionic mechanism of accommodation in single pairs of N (noxious), P (pressure), and T (touch) cells. 2. Nonlinearities in membrane-potential changes and current-voltage relationships with square-wave and ramp currents are more pronounced in P and T cells than in N cells. The accommodation coefficients increase in conditions that reflect this delayed rectification. When rectification is absent, the accommodation coefficients depart from unity only slightly or not at all. 3. Accommodation coefficients remain unchanged when half of the chloride in the bathing medium is replaced by sulfate. Accommodation coefficients become greater when the extracellular potassium concentration is reduced from 4 to 0 mM, and decrease when the concentration is raised to 8 mM. The membrane potential changes by only a few millivolts. 4. As extracellular potassium concentration is increased, the action potential is lengthened and the maximal rate of fall of the action potential is reduced. With concentrations greater than 4 mM these relationships are linear, but depart from linearity at lower concentrations. The amplitude of the undershoot decreases linearly as the extracellular potassium concentration increases from 4 to 16 mM, and increases non-linearly at concentrations below 4 mM. 5. The rapid accommodation of leech neurons is based primarily on an increased potassium conductance. The possibility is considered that concentration changes like those produced experimentally may occur naturally, affecting integrative processes in the central nervous system.

Action Potentials↗

PubMed1977

Extrasynaptic receptors on cell bodies of neurons in central nervous system of the leech.

1. A systematic study has been made of the sensitivity of identified sensory and motoneurons in the leech central nervous system to chemical transmitter substances. 2. The following substances elicited responses from the cell bodies of individual neurons: acetylcholine, 5-hydroxytryptamine, gamma-aminobutyric acid, glutamic acid, glycine, dopamine, and norepinephrine. Since the cell bodies of leech neurons are free of synapses, the receptors that give rise to these responses are extrasynaptic. 3. Sensory and motoneurons of different function had characteristic complements of extrasynaptic receptors. For example, mechanosensory cells responding to light touch, to pressure, and to noxious stimuli could be distinguished by their responses to iontophoretically applied compounds. For one of these modalities (nociceptive), neurons with different receptive fields but otherwise similar properties had markedly distinct extrasynaptic receptors. The possible significance of extrasynaptic receptors is discussed.

Animals↗

PubMed1977

Synthesis of acetylcholine by excitatory motoneurons in central nervous system of the leech.

1. A study was made of the synthesis of acetylcholine (ACh) and other transmitters by the cell bodies of functionally identified neurons in leech segmental ganglia. 2. Choline acetyltransferase, the synthetic enzyme for ACh, was detected in excitatory motoneurons but not in mechanosensory cells or Retzius cells. The ability of motoneurons to synthesize ACh was also demonstrated by their accumulation of [3H]ACh following incubation of segmental ganglia with [3H]choline. [3H]ACh was not detected in the other cell types. When eserine was included in [3H]choline incubations, the amount of [3H]ACh in motoneurons increased severalfold and small amounts of [3H]ACh (1% that in motor cells) appeared in extracts of sensory and Retzius cells. 3. In addition to [3H]ACh segmental ganglia synthesized [3H]5-HT, [3H]gamma-aminobutyric acid, [3H]dopamine, and [3H]octopamine from exogenous, labeled precursors. None of these labeled transmitters was detected in identified neurons except [3H]5-HT, which was found in Retzius cells. 4. These results provide biochemical evidence that excitatory motoneurons in the leech are cholinergic, but leave open the identity of the sensory transmitter(s).

Acetylcholine↗

PubMed1979

Identifiable neurons controlling penile eversion in the leech.

1. This paper describes the neuroanatomy and electrophysiology of motor neurons causing penile eversion in the leech. 2. The male organ is innervated by ganglia 5 and 6 of the 34 ganglia in the leech brain through special sex nerves deriving from anterior roots. These sex ganglia have at least 200 more neurons than the other midbody ganglia. Many of the extra neurons are involved in reproductive behavior. 3. Two pairs of motor neurons on the ventral side of ganglion 6, named rostral and lateral neurons, are the only ones that elicit full penile eversion. Evidence that the lateral and rostral neurons are, in fact, motor neurons comes from HRP and electrophysiological studies. HRP injections reveal that each neuron's single primary axon grows into the sex nerve. Electrophysiological evidence is twofold: a) action potentials of lateral and rostral cells can still contract the genitalia after the neurons are deafferented from chemical synaptic input in the ganglion by high Mg2+, b) their action potentials are followed by junction potentials in male organ muscle fibers.

Animals↗

PubMed1979

Neural control of heartbeat in the leech and in some other invertebrates.

The heartbeat of the leech Hirudo consists of the contractile rhythm of the circular muscles in the wall of a bilateral pair of celomic sinuses, the heart tubes, that run the length of the leech body. The constriction cycles of the segmental heart-tube sections are coordinated so that on one body side they constrict in a rear-to-front progression (peristalsis), while on the other side they constrict nearly in concert (nonperistalsis). Spontaneous right-left reciprocal transitions between peristaltic and nonperistaltic coordination modes occur every few dozen heartbeat cycles. The constriction of each segmental heart-tube section is controlled via excitatory synapses by a rhythmically active heart motor neuron, or HE cell, of which 17 bilateral pairs are iterated in segmental ganglia of the ventral nerve cord. The activity rhythm of the HE cell ensemble is in turn controlled via inhibitory synapses by a rhythmically active heart interneuron, the HN cell, of which seven bilateral pairs are iterated in the rostral segmental ganglia. The HN heart interneuron owes its activity rhythm to an endogenous polarization cycle, and the cycles of all members of the HN cell ensemble are locked into an appropriate phase relation thanks to their mutual interconnection via excitatory and inhibitory synaptic connections. The observed activity pattern and identified synaptic connections of HE cells and HN cells can account not only for the generation of the two bilaterally asymmetric heartbeat coordination modes but also for the right-left coordination mode transitions. In contrast to the heartbeat of Hirudo, the beat of the single-chambered heart of the lobsters Panulirus and Homarus is controlled by a set of nine rhythmically active neurons that make up the cardiac ganglion. Of these, five larger cells are heart motor neurons that innervate the heart muscle fibers via excitatory synapses. The remaining four smaller neurons of the cardiac ganglion are interneurons that provide excitatory input to each other and to the heart motor neurons. Although all the neurons of the cardiac ganglion appear capable of producing their own endogenous polarization rhythm, it is currently believed that one of the interneurons acts as a pacemaker for the whole ensemble of interneurons and motor neurons. The beat of the two-chambered heart of the marine snail Aplysia is generated by yet an entirely different mechanism. Here, the basic contractile rhythm of the heart is due to an endogenous polarization cycle of the heart muscle fibers. That myogenic rhythm is controlled and modulated by a set of cardiovascular motor neurons located in the abdominal ganglion, some of which make excitatory and others of which make inhibitory connections with the heart muscle fibers. The activity of these cardiovascular motor neurons is controlled by three types of heart interneurons via both inhibitory and excitatory connections. The interneurons are in turn interconnected in a manner that prevents the simultaneous activation of antagonistic cardiac motor acts...

Action Potentials↗

PubMed1977

[Morphologic changes in the axo-dendritic synapses of the leech following excitatory conduction block in a medium with decreased ionic concentration].

Reactive changes in the synaptic apparatus of the leech neuropile were studied in low ionic strength media using supravital methylene blue staining. Special preparation developed in the laboratory permitted obtaining clearly identified axodendritic synapses in the leech. It was shown that the reactive changes were pronounced during a decrease in the electrical conductance and block in the abdominal chain. Certain correlations between the structural alterations in axodendritic synapses and phasic reactive changes in axosomatic synapses were observed.

Animals↗

PubMedDate not supplied

[Neural control of somatic muscle function in the earthworm, Allobophora longa, and in the leech, Hirudo medicinalis].

Studies have been made on the electrical activity of the segmentary nerves and connectives of the abdominal nervous chain in the earthworm and leech. It was shown that the electrical activity of the isolated piece of the abdominal chain of the leech is manifested of periodic outbursts of impulsation. Presumably this central periodicity accounts for the discharge-like pattern of muscle rhythmic activity which was revealed in our earlier investigations. The electrical activity in the central nervous system of the earthworm depends on afferent influences which pass to the ganglia from the peripheral sensory nervous cells. Stimulation of the abdominal nervous chain did not result in extra discharges of muscle activity, but only affected some of the parameters of the latter.

Afferent Pathways↗

PubMed1975

A multisomatic axon in the central nervous system of the leech.

There is one particularly large axon in the medial bundle of the nerve cord of the leech. It extends along the entire length of the cord and is connected to a single cell body in each ganglion. The cell bodies in adjacent ganglia are tightly electrically coupled, and dye injected into one cell body can diffuse along the axon and into the cell body of the next ganglion. If the nerve cord is cut between two ganglia, neither end of the axon degenerates. Repair of the axon appears to occur by end-to-end fusion.

Action Potentials↗