Leech swimming: the neural story.

Leech swimming: The neural story

Contrary to modern popular understanding, leeches give as well as take. They can suck up a blood meal nine times their weight, but they offer a variety of research services as well. Using leeches in the lab, scientists at the University of Virginia in Charlottesville have discovered a cellular link in a neural chain that enables them to explain the animal's rhythmic swimming movements in terms of neural mechanisms. Because such movements have features that are common to all rhythmic motor behaviors, including chewing, walking and breathing, the scientists say their findings extend far beyond the leech.

Whereas the human nervous system is woven from billions of cells, the nervous system of the leech species Hirudo medicinalis has about 13,000 cells, all of which scientists suspect are hard-wired to the point where any neuron can be labeled and identified from leech to leech. The leech has two ganglia, or discrete collections of nerve cells, in the head region and one in the tail region. In addition, there are 21 nearly identical segmental ganglia along the animal's axis. It is the exceptional regularity in its neural architecture that makes the leech especially suitable for detailed neuroethological studies, says W. Otto Friesen, who reports on the work in the Nov. 21 SCIENCE with Peter D. Brodfuehrer, now at Cornell University. Neuroethology is the study of animal behavior in terms of the underlying neural mechanisms.

By removing nearly the entire nervous system of the leech, the scientists were able to eavesdrop on individual neurons in different parts of the nervous system. Using two microelectrodes, they stimulated either of two brain neurons called Tr1 cells and observed electrical activity downstream in the segmental ganglia, in neurons already known to be involved in swimming. To determine that Tr1 cells were linked also to cells on the body wall that initiate swimming and that sense touch, pressure and pain, the scientists stimulated these sensory cells while monitoring the Tr1 cells. They found that stimulation of the sensory cells was followed immediately by bursts of activity in the Tr1 cells.

Finally, to make sure their observations in isolated nervous systems actually corresponded to the leech's swimming behavior and were not just artifacts from their experimental procedures, the researchers recorded firing patterns from Tr1 cells of leeches that were nearly entirely intact. In this way, they could monitor what specific cells were doing during whole-animal behaviors such as swimming. They observed the same electrical pattenrs in Tr1 cells regardless of whether the cells were in intact swimming leeches or in their isolated nervous systems.

Before Friesen and Brodfuehrer's discovery of the Tr1 cells, scientists did not know hoe leech sensory cells were hooked up to the neurons that generate the oscillating swim signals. Several labs have been looking for the connection, notes neurobiologist Bill Kristan of the University of California at San Diego, but they were searching in the segmental ganglia. The missing links, report Friesen and Brodfuehrer, are the Tr1 cells, whose somas or bodies are located in the relatively large subesophageal ganglion in the head region and whose axons extend down the length of the animal. Says invertebrate biologist Ronald Calabrese of Emory University in Atlanta, "Finding these critical interneurons [such as the Tr1 cells] is the most difficult thing."

With the final link in the neural chain fastened into place, biologists can now tell a neural story of leech swimming. The two Tr1 cells receive input converging from roughly 150 sensory cells along the body and from other cells not yet identified. The Tr1 cells then trigger at least 92 neurons distributed among the segmental ganglia. These cells in turn set off another category of neurons that generate oscillating signals, which are sent to still other neurons that control the muscles. The result of such a chain of events is the undulatory movement that leeches use to swim. And since similar mechanisms are used in some rhythmic movements of most animals, Friesen suggests that more neural stories, analogous to the one for leech swimming, could be constructed and tested.