Inhibition Helps Shape the Spatial and Temporal Profiles of the Local Bend Response of the Medicinal Leech
37th Annual Meeting of the Society for Neuroscience, San Diego, CA, 2007. Prog. no. 312.10
In response to local pressure applied anywhere on its body, the European medicinal leech contracts muscles ipsilateral to the pressure and relaxes contralateral muscles, thus bending its body away from the touch location. Previous work from our lab has shown that the relative strength of contraction and relaxation around the body circumference directs bending precisely away from the stimulus, despite the fact that only 4 sensory neurons per ganglion respond to pressure stimuli. Here we study the role of inhibition in shaping a well-controlled response.
We used a mechanical stimulator to deliver forces of 0.75–400 mN (corresponding to pressures of about 0.75–400 kPa) to a section of leech body wall innervated by a single ganglion, and recorded the consequent motion of the skin using a computer-attached CCD camera. These measurements were repeated after inhibition was blocked using bath application of bicuculline methiodide (BMI), a blocker of GABA-A receptors. This resulted in a roughly twofold increase in contraction ipsilateral to the stimulus, at all force levels. Contralateral to the stimulus, the (normally weak and brief) initial contractile phase was augmented in amplitude and duration, and the subsequent relaxation was delayed and reduced, or even abolished. This shows that the role of inhibition goes well beyond mediating contralateral relaxation; it strongly tempers the ipsilateral contraction as well. Conversely, contralateral relaxation is tempered by a contractile force which is revealed when inhibition is blocked. While four of the motor neurons involved in the response are inhibitory, applying BMI locally to the muscles had no effect. Thus, the observed effects must be central.
In conclusion, inhibition regulates the dynamic balance between contraction and relaxation in response to contralateral touch, and it controls the amplitude of the contraction in response to ipsilateral touch as well as its spatial extent.