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PubMed1983

Generation and coordination of heartbeat timing oscillation in the medicinal leech. I. Oscillation in isolated ganglia.

1. The interactions among the four pairs of interneurons (HN(1)-HN(4)) of the heartbeat timing oscillator are confined to the third and fourth ganglia (G3 and G4). In isolation, G3 and G4 each produces a rhythm essentially the same as that shown when the two ganglia are linked together. 2. The local circuits in both ganglia have the same general form. In both the oscillation centers on a bilateral pair of HN cells that are linked by reciprocal inhibition (the HN(3) pair in G3 and the HN(4) pair in G4). In addition, there is reciprocal inhibition between an HN(3) or HN(4) cell and the intersegmental processes of the ipsilateral HN(1) and HN(2) cells. 3. These connections account for the phase relationships in an isolated G3 or G4, since cells linked by reciprocal inhibition produce bursts in alternation. 4. In isolated ganglia, reciprocal inhibition not only coordinates the activity of the HN cells but also appears to help generate their bursts. 5. Yet reciprocal inhibition alone cannot account for the activity of the network. An endogenous property of the HN(3) and HN(4) cells appears to create the instability necessary for oscillation.

Animals↗
PubMed1998

The leech receptor protein tyrosine phosphatase HmLAR2 is concentrated in growth cones and is involved in process outgrowth.

Developing neurons extend long processes to specific distal targets using extracellular molecules as guidance cues to navigate through the embryo. Growth cones, specialized structures at the tip of the extending processes, are thought to accomplish this navigation through receptors that recognize guidance cues and modulate growth accordingly. In Drosophila, several receptor tyrosine phosphatases (rPTPs), including DLAR, have been shown to participate in directing neurite outgrowth. As yet, however, it is not known how rPTPs act to affect navigation. To gain insight into the mechanisms of rPTP-mediated outgrowth guidance, we have investigated the role of HmLAR2, a Hirudo medicinalis homologue of DLAR, in process outgrowth. HmLAR2 is expressed by, among other cells, a transient neuron-like template cell, the Comb cell. Here we present evidence that HmLAR2 protein becomes concentrated within their growth cones at a stage when C cell processes undergo rapid outgrowth. When antibodies raised against the extracellular domain of HmLAR2 were injected into intact embryos, they bound specifically to the C cell surface at growth cones and along processes and caused the partial internalization of HmLAR2 receptors. Moreover, the C cell processes were found to project aberrantly, to deviate from their normally highly regular trajectories and to extend shorter distances in the presence of the antibodies. We propose that HmLAR2 is required by the C cell for guidance and extension and suggest that it functions via its ectodomain to transduce extracellular guidance cues.

Animals↗
PubMed1989

New growth elicited in adult leech mechanosensory neurones by peripheral axon damage.

1. New growth in cutaneous mechanosensory neurones elicited by axotomy or axon crush was studied using intracellular injection of horseradish peroxidase at different times after the lesion, ranging from a few days to over a year. 2. Cutting or crushing major, large-calibre axon branches of mechanosensory neurones elicits sprouting of new processes, either centrally within the ganglion neuropile or at the site of the lesion in the peripheral nerve. In contrast, cutting or crushing fine-calibre axon branches supplying accessory parts of the receptive field does not elicit sprouting of the main arbor or main axon branches. 3. Different modalities of mechanosensory neurone respond differently to lesions of their axons. Cutting the axons of high-threshold units responding to noxious stimulation of the skin elicits sprouting of additional processes from the axon hillock region within the central nervous system (CNS), whereas cutting or crushing the axons of low-threshold cells responding to light touch of the skin elicits sprouting at the site of the lesion only, and not within the CNS. 4. In addition to the new growth directed into the peripheral nerve, damaged nociceptive neurones also form new processes that wrap the somata of particular cells within the ganglion. 5. Sprouted processes of axotomized neurones are retained for long periods after the lesion (up to 425 days). 6. The electrical properties of touch and nociceptive cells were studied between 1 and 60 days after axotomy, by intracellular recording from the centrally located cell bodies. The amplitude, width and maximum dV/dt of the action potential and after-hyperpolarization, as well as the resting potential and input resistance, did not change significantly after axotomy, despite the considerable process sprouting known to occur during this time.

Action Potentials↗
PubMed1978

Neurones in the leech that facilitate an avoidance behaviour following nearfield water disturbances.

1. A multimodel, multisegmental interneurone (Röhde's fibre, RF) and previously identified mechanoreceptors (T-cells) are shown to respond to nearfield disturbances. Both the T-cells and RF can fire for hundreds of milliseconds following a brief stimulus, and both have subthreshold excitatory synapses onto motor neurones that cause longitudinal contraction of the body wall, an avoidance response. 2. Natural stimulation or electrical stimulation of T-cells in one hemiganglion causes synaptic excitation of T-cells in adjacent ipsilateral hemiganglia and re-excitation of T-cells in the hemiganglion stimulated. A model of repetitive T-cell activity that incorporates previously described synapses among T-cells is presented: the T-cells in adjacent ipsilateral hemiganglia form a reverberatory circuit, re-exciting one another via electrical synapses; repetitive firing is terminated by synaptic inhibition onto T-cells provided by an interneurone excited by the T-cells. With repeated stimulation (0.1--0.2 Hz, 0.2 ms pulses) of a segmental root (directly exciting all the T-cells of a hemiganglion), the number of T-cell impulses per stimulus decreases. Facilitation of inhibition may contribute to the response decrement. 3. The T-cell-RF pathway is investigated. T-cell stimulation can elicit RF impulses in the same and in adjacent ganglia. The long delay between mechanoreceptor stimulation and a response in the interneurone suggests that spatial and temporal summation of T-cell inputs may be required to reach firing threshold in the interneurone. 4. The impulse frequency of the RF response was compared for a travelling surface wave that is approaching a segment v. one that is moving away from the segment. It was found that the frequency was greater as the stimulus approaches; this should allow more effective temporal summation of the subthreshold synaptic potentials which RF evokes in motor neurones that cause longitudinal contraction of the body wall. Therefore, the probability of contraction is greater in segments toward which a stimulus is moving.

Animals↗