Abstract
Touch triggers highly precise behavioural responses in the leech. The underlying network of this so-called local bend reflex consists of three layers of individually characterised neurons. While the population of mechanosensory cells provide multiplexed information about the stimulus, not much is known about how interneurons process this information. Here, we analyse the responses of two local bend interneurons (cell 157 and 159) to a mechanical stimulation of the skin and show their response characteristics to naturalistic stimuli. Intracellular dye-fills combined with structural imaging revealed that these interneurons are synaptically coupled to all three types of mechanosensory cells (T, P, and N cells). Since tactile stimulation of the skin evokes spikes in one to two cells of each of the latter types, interneurons combine inputs from up to six mechanosensory cells. We find that properties of touch location and intensity can be estimated reliably and accurately based on the graded interneuron responses. Connections to several mechanosensory cell types and specific response characteristics of the interneuron types indicate specialised filter and integration properties within this small neuronal network, thus providing evidence for more complex signal processing than previously thought.
Highlights
The medicinal leech possesses a relatively simple and accessible neuronal system[11,12] with individually identifiable, monopolar neurons[13], and accurate behavioural patterns
Most of the local bend INs receive input from all four P cells in one ganglion indicating that these INs are not specialised for eliciting only one local bend direction but are rather activated by a wider range of touch locations mediated by the corresponding mechanosensory cells[5]
Little is known about the response patterns of local bend INs to naturalistic stimulation or about their connections to mechanosensory cells other than P cells
Summary
The medicinal leech possesses a relatively simple and accessible neuronal system[11,12] with individually identifiable, monopolar neurons[13], and accurate behavioural patterns. We showed in preceding studies[21,22] that T cells encode touch locations very precisely These studies suggest that T cells might play a substantial role for the local bend response. At the network level, at least nine types of INs are known to be involved in the local bend response[5] These neurons have synaptic connections on MNs, which elicit the muscle contraction or elongation during the local bend[4,5]. At least some of the local bend INs receive input from T cells[22], but the relative contributions of the different types of mechanosensory cell inputs are not known yet. We use two complementary maximum-likelihood approaches for stimulus estimation: a pairwise discrimination of stimulus differences and a classification of all possible stimulus conditions[21]
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