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R3b1 is a Leech Neuron

Basic Information

Figure 1: The soma of R3b1 filled with neurobiotin and then labeled with Cy3 streptavidin. Subpackets of the subesophageal neuromeres are labeled.

R3b1 is a bilaterally paired interneuron found in the head ganglion of the Leech. R3b1 is a gating neuron of the leech crawl (i.e. it must be depolarized for the animal to crawl). R3b1 is also a "decision" neuron involved in the initiation of the swim and crawl. A tactile stimulus to the tail of the leech triggers a depolarization of R3b1. This depolarization can initiate a swim or a crawl, depending on the environment. When the leech is fully immersed in water, it will swim to R3b1 depolarization. Conversely, when the animal is in shallow or no water, R3b1 depolarization will elicit a crawl. It appears that much of the water depth sensory information is transmitted through head ganglion nerves.



R3b1 is a bilaterally paired neuron in the posterior subpacket of R3 (R3b) (Figure 1).

Figure 2: Tactile stimulus elicits depolarization in R3b1. A) In shallow water the stimulus elicits a crawl. B) In deep water, stimulus elicits a swim


R3b1 identification can be verified electrophysiogically. The neuron depolarizes in response to a tactile stimulus to the tail of the leech (Figure 2). In deep water, the tactile stimulus will elicit a crawl. In shallow water the same stimulus will elicit a swim. Depolarizing R3b1 can also elicit a swim or crawl motor pattern, again, depending on the level of saline in the experimental chamber (Figure 3).

Figure 3: Depolarizing R3b1 can elicit a swim or a crawl. A) R3b1 elicits a crawl when in low saline levels. B) R3b1 elicits a swim in high saline levels.


  1. Esch T, Mesce KA, Kristan WB (2002) Evidence for sequential decision making in the medicinal leech. J Neurosci 22:11045-11054.
  2. Esch T, Kristan WB (2002) Decision-making in the leech nervous system. Integ and Comp Biol 42:716-724.