Isolated midbody ganglia of the leech Hirudo medicinalis were surveyed for interneurons contributing to the dorsal component of the local bending reflex, i.e., to the excitation of dorsal excitatory motor neurons that follows stimulation of dorsal mechanoreceptors responsive to pressure (P cells). Nine types of local bending interneuron could be distinguished on physiological and morphological grounds--8 paired and 1 unpaired cell per ganglion. Synaptic latencies from sensory neurons to interneurons were consistent with a direct or possibly disynaptic pathway. Connections between interneurons appeared to be rare and hyperpolarization of individual interneurons during local bending produced small but reliable decrements in motor neuron response, suggesting that multiple parallel pathways contribute to the behavior. Paradoxically, most interneurons received substantial inputs from ventral as well as dorsal mechanoreceptors, indicating that interneurons that were distinguished by their contribution to dorsal local bending were, in fact, active in ventral and lateral bends as well. The capacity to detect a particular stimulus and produce the appropriate response cannot be localized to particular types of interneuron; rather, it appears to be a distributed property of the entire local bending network.

Segmental variation in identified neurons may provide an opportunity to examine extrinsic influences on neuronal phenotype, since segmentally homologous neurons must contain much the same intrinsic information, having arisen from very similar or identical precursors. Two large serotonergic Retzius (Rz) cells are found in each segmental ganglion of the leech Hirudo medicinalis. While most Rz cells innervate the body wall in their own segment and, by way of axons in the interganglionic connectives, the body wall of adjacent segments, the Rz cells in ganglia 5 and 6 [Rz(5,6)] lack interganglionic axons and innervate only the reproductive tissue (Glover and Mason, 1986). Here we describe and quantify the development of differences between Rz(5,6) and other Rz cells in peripheral innervation, neuropilar arborization, and soma size. We filled individual Rz cells with Lucifer yellow or HRP in adults and in staged embryos. During the first 72 hr of outgrowth of Rz cell processes, the morphology of Rz(5,6) was indistinguishable from that of other Rz cells. Only after the processes of Rz(5,6) reached the reproductive tissue did they begin to differ from their segmental homologs. This temporal correlation suggests that these morphological differences arise because of some interaction between Rz(5,6) and their target tissue.

The leech whole-body shortening reflex consist of a rapid contraction of the body elicited by a mechanical stimulus to the anterior of the animal. We used a variety of reduced preparations - semi-intact, body wall, and isolated nerve cord - to begin to elucidate the neural basis of this reflex in the medicinal leech Hirudo medicinalis. The motor pattern of the reflex involved an activation of excitatory motor neurons innervating dorsal and ventral longitudinal muscles (dorsal excitors and ventral excitors respectively), as well as the L cell, a motor neuron innervating both dorsal and ventral longitudinal muscles. The sensory input for the reflex was provided primarily by the T (touch) and P (pressure) types of identified mechanosensory neuron. The S cell network, a set of electrically-coupled interneurons which makes up a 'fast conducting pathway' in the leech nerve cord, was active during shortening and accounted for the shortest-latency excitation of the L cells. Other, parallel, interneuronal pathways contributed to shortening as well. The whole-body shortening reflex was shown to be distinct from the previously described local shortening behavior of the leech in its sensory threshold, motor pattern, and (at least partially) in its interneuronal basis.

The swimming movement of the leech is produced by an ensemble of bilaterally symmetric, rhythmically active pairs of motor neurons present in each segmental ganglion of the ventral nerve cord. These motor neurons innervate the longitudinal muscles in dorsal or ventral sectors of the segmental body wall. Their duty cycles are phase-locked in a manner such that the dorsal and ventral body wall sectors of any given segment undergo an antiphasic contractile rhythm and that the contractile rhythms of different segments form a rostrocaudal phase progression. This activity rhythm is imposed on the motor neurons by a central swim oscillator, of which four bilaterally symmetric pairs of interneurons present in each segmental ganglion appear to constitute the major component. These interneurons are linked intra- and intersegmentally via inhibitory connections to form a segmentally iterated and inter-segmentally concatenated cyclic neuronal network. The network appears to owe its oscillatory activity pattern to the mechanism of recurrent cyclic inhibition.

In the leech Hirudo medicinalis inhibitory motor neurons to longitudinal muscles make central inhibitory connections with excitatory motor neurons to the same muscles. We have used a variety of physiological and morphological methods to characterize these inhibitory connections. The efficacy of the transmission between the inhibitors and the excitors was measured by using three intracellular electrodes, two in the inhibitor (one for injecting current and one for measuring voltage) and a third electrode in the excitor for measuring the resultant voltage changes. We have determined that delta Vpre/delta Vpost, or what we have called the transmission coefficient, is X = 0.51, as measured in the somata of the two cells. Evidence which we have obtained leads us to propose that these inhibitory connections between motor neurons are probably monosynaptic. The synaptic latency is consistent with a monosynaptic connection. In addition, a double-labeling technique, whereby one neuron was filled with Lucifer Yellow and the other with horseradish peroxidase (HRP), was used to determine the anatomical relationship between inhibitors and excitors in whole mounts. This revealed varicosities on the processes of inhibitor motor neurons which appear to make contact with processes of excitor motor neurons. A second double-labeling technique, whereby one neuron was filled with HRP and the other with an electron-dense particulate marker, revealed adjacent processes between an inhibitor and an excitor in electron microscopic thin sections which could be the sites of synaptic contact between the neurons. The connections appear to be mediated largely by graded transmitter release from the inhibitory motor neurons. Three different methods were used to demonstrate that synaptic transmission remained in the absence of impulses in the inhibitory motor neurons. These included eliminating the impulse-supporting portion of the motor neuron by pinching off its axon, abolishing impulses by replacing Na+ with Tris in the medium bathing the nerve cord, and increasing the threshold for impulse production by raising the Mg2+ and Ca2+ concentrations in the medium bathing the nerve cord.