Local bending reflexes in the leech

If you touch a small area of the skin of a leech, it will bend its body away from the touch, a movement caused by contraction of longitudinal muscles in a few segments close to the site of the touch. The body of the leech locally assumes a U shape. Touching the top of the body causes a downward bend, whereas touching the left side causes a bend towards the right, and so on (Fig. 8.2a). The main sensory neurons involved in triggering these local reflex movements are pressure-sensitive P neurons. Each segmental ganglion contains four P neurons, each of which has a receptive field that covers one quadrant of the skin of its segment - upper or lower right, and upper or lower left. The muscles that produce the bending movements run longitudinally along the body wall, and in each segment there are six bilateral pairs of dorsal, ventral and lateral longitudinal muscles. Each dorsal or ventral longitudinal muscle is controlled by two motor neurons, an excitor and an inhibitor, and each lateral muscle is controlled only by an excitatory motor neuron.

Excitation of a particular set of P neurons reliably activates motor neurons in a particular way (Fig. 8.2b). No direct connections between P neurons and motor neurons have been found, so interneurons must be involved in the local bending reflexes. A straightforward way of arranging the circuitry of a ganglion to control these local avoidance reflexes would be to connect sensory neurons with motor neurons through interneurons that are responsible for particular reflex behaviours, in the manner indicated in Fig. 8.2c. Shawn Lockery and Bill Kristan characterised these interneurons by using intracellular recording and staining (Lockery & Kristan, 1990a, 1990b). They studied interneurons that are involved in dorsal bending, the production of a downward U shape when the top surface of a leech is touched. In a survey of 73 ganglia, they found nine types of interneuron

Figure 8.2 Local bending reflex movements of the leech (Hirudo). (a) Drawings of a leech viewed from its right side: resting (top), and responding to touches to the top (middle) or side (bottom), as indicated by the arrows. (b) Intracellular recordings from the excitor motor neuron of the left dorsal muscle in response to stimulation of P neurons. In the upper and middle recordings, the four P neurons were stimulated individually. In the lower recordings, lateral touches were mimicked by stimulating the dorsal and ventral P neurons of either side simultaneously. (c) A circuit that incorporates interneurons which are dedicated to particular bending movements. The triangles indicate excitatory synapses and the circles indicate inhibitory synapses. (a redrawn after Lockery et al., 1989; b and c redrawn from Lockery & Kristan, 1990a.)

Figure 8.2 Local bending reflex movements of the leech (Hirudo). (a) Drawings of a leech viewed from its right side: resting (top), and responding to touches to the top (middle) or side (bottom), as indicated by the arrows. (b) Intracellular recordings from the excitor motor neuron of the left dorsal muscle in response to stimulation of P neurons. In the upper and middle recordings, the four P neurons were stimulated individually. In the lower recordings, lateral touches were mimicked by stimulating the dorsal and ventral P neurons of either side simultaneously. (c) A circuit that incorporates interneurons which are dedicated to particular bending movements. The triangles indicate excitatory synapses and the circles indicate inhibitory synapses. (a redrawn after Lockery et al., 1989; b and c redrawn from Lockery & Kristan, 1990a.)

that were both activated by dorsal P neurons and made connections onward to dorsal bend motor neurons. Each type had a distinct morphology, allowing it to be distinguished from others and to have its cell body position identified on a map of a segmental ganglion. One, neuron 125, is illustrated in Fig. 8.3a.

Each interneuron responded to touch anywhere on the skin of its

Figure 8.3 Interneuron 125, which is involved in local reflex bending movements in the leech. (a) A diagram of a ganglion in which the neuron was stained by intracellular injection of the fluorescent stain lucifer yellow. (b) Amplitudes of EPSPs in a left 125 caused by stimulating the four different P neurons (dorsal and ventral left and right) of the same segment. Each bar indicates, from three experiments, the mean amplitude and standard error of the EPSP evoked by stimulating a particular P neuron. (c) Amplitudes of postsynaptic potentials evoked by spikes in interneuron 125 in the excitor (ex.) and inhibitor (inh.) motor neurons of dorsal and ventral longitudinal muscles. Upward-directed bars indicate EPSPs and downward-directed bars indicate IPSPs. Means and standard errors are indicated, as in (b). (a redrawn after Lockery & Kristan, 1990b.)

Figure 8.3 Interneuron 125, which is involved in local reflex bending movements in the leech. (a) A diagram of a ganglion in which the neuron was stained by intracellular injection of the fluorescent stain lucifer yellow. (b) Amplitudes of EPSPs in a left 125 caused by stimulating the four different P neurons (dorsal and ventral left and right) of the same segment. Each bar indicates, from three experiments, the mean amplitude and standard error of the EPSP evoked by stimulating a particular P neuron. (c) Amplitudes of postsynaptic potentials evoked by spikes in interneuron 125 in the excitor (ex.) and inhibitor (inh.) motor neurons of dorsal and ventral longitudinal muscles. Upward-directed bars indicate EPSPs and downward-directed bars indicate IPSPs. Means and standard errors are indicated, as in (b). (a redrawn after Lockery & Kristan, 1990b.)

segment (Fig. 8.3b), and made connections with most of the longitudinal motor neurons in its ganglion (Fig. 8.3c). None of the interneurons appeared to be dedicated to just the dorsal reflex bending behaviour because each was also active during ventral or lateral bending. Therefore, the networks responsible for analysing touch stimuli and triggering bending movements seem to be arranged in a distributed manner in which particular interneurons are not assigned to specific local bending responses. However, is it possible that other interneurons, not found by Lockery and Kristan, are the ones that are primarily responsible for specifying particular bending responses?

One test for the involvement of a neuron in a behaviour is to remove that neuron and observe any change in behaviour. When hyperpolarising currents were injected into single interneurons to reduce their excitability, responses by motor neurons to stimulation of P neurons were reduced in amplitude. But these experiments were inconclusive because the reductions in response by motor neurons were small, which is not

Sensory Movements neurons

Interneurons

Sensory Movements neurons

Interneurons

Figure 8.4 A diagram indicating the kind of organisation of the circuit that controls local bending movements in the leech. The diagram is simplified in many ways so that it does not indicate connections between left and right neurons or the relative strengths of different connections, and includes only three of the 17 interneurons known to exist in each ganglion.

Figure 8.4 A diagram indicating the kind of organisation of the circuit that controls local bending movements in the leech. The diagram is simplified in many ways so that it does not indicate connections between left and right neurons or the relative strengths of different connections, and includes only three of the 17 interneurons known to exist in each ganglion.

surprising because each motor neuron is driven by several different interneurons.

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