This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Neuroscience. Please check back later for the full article.
The medicinal leech (Hirudo verbana) is an annelid (segmented worm) that has been one of the classic model systems in neuroscience, in use for over 50 years, and it was one of the first animals in which intracellular recordings of mechanosensory neurons were carried out. Remarkably, the leech has three main classes of mechanosensory afferents that exhibit many of the same properties found in vertebrates. The most sensitive of these are the touch cells, which are rapidly-adapting neurons that detect low intensity mechanical stimuli. Next are the pressure cells, which slow adapting sensory neurons that respond to higher intensity, sustained mechanostimulation. Finally, there are nociceptive neurons, which have the highest threshold and respond to potentially damaging mechanostimuli, such as a pinch. As observed in mammals, the leech actually has separate mechano-sensitive and polymodal nociceptors, the latter responding to mechanical, thermal, and chemical stimuli. The cell bodies for all three types of mechanosensitive neurons are found in the central nervous system, where they are arranged as bilateral pairs. Each neuron extends processes to the skin, where they form discrete receptive fields. In the touch and pressure cells, these receptive fields are arranged along the dorsal-ventral axis. For the mechano-only and polymodal nociceptive neurons, the peripheral receptive fields overlap with the mechano-only nociceptor, also innervating the gut. The leech also has a type of mechanosensitive cell located in the periphery that responds to water movement and is used, in part, to detect potential prey nearby.
In the central nervous system, the touch, pressure, and nociceptive cells all form synaptic connections with a variety of motor neurons, interneurons, and even each other using glutamate as the neurotransmitter. Synaptic transmission by these cells can be modulated by a variety of activity-dependent processes and due to the influence of neuromodulators, such as serotonin. The output of these afferents can also be modulated by a process known as conduction block, in which action potentials fail to propagate to all the synaptic release sites, decreasing synaptic output. Activity in these sensory neurons leads to the initiation of a number of different motor behaviors involved in locomotion, such as swimming and crawling, as well as behaviors, such as local bending and shortening, designed to recoil from aversive or noxious stimuli. In the case of local bending, the leech is able to bend in the appropriate direction away from the offending stimuli due to a combination of which pressure cell receptive fields have been activated and the relative activation of multiple pressure cells that are decoded by a layer of downstream interneurons.