B

Puff

Cercus

Figure 3.12 Startle response of the cockroach. (a) Video recording, made from above the cockroach, of the turning response to a wind puff delivered from the front left of the animal. The outlines of the body and head are traced from every second frame. (b) Leg movements during an escape turn caused by a puff of wind coming from the right and slightly to the front. The cockroach was held so that its body could not move, but its legs slipped over a lightly oiled glass plate. The initial positions of the legs are indicated with dotted lines; the final positions of the middle and hind legs are drawn as solid lines. The arrow above the cockroach indicates the direction in which the animal would have faced if its body had been free to rotate. (a redrawn after Comer & Dowd, 1993; b redrawn after Ritzmann, 1993.)

Puff

Cercus

Figure 3.12 Startle response of the cockroach. (a) Video recording, made from above the cockroach, of the turning response to a wind puff delivered from the front left of the animal. The outlines of the body and head are traced from every second frame. (b) Leg movements during an escape turn caused by a puff of wind coming from the right and slightly to the front. The cockroach was held so that its body could not move, but its legs slipped over a lightly oiled glass plate. The initial positions of the legs are indicated with dotted lines; the final positions of the middle and hind legs are drawn as solid lines. The arrow above the cockroach indicates the direction in which the animal would have faced if its body had been free to rotate. (a redrawn after Comer & Dowd, 1993; b redrawn after Ritzmann, 1993.)

their natural habitat, which for the American cockroach Periplaneta americana is leaf litter on the forest floor, cockroaches have been shown to be able to evade the strikes of one of their predators, the toad, by detecting the movement of air caused by movement of the toad's tongue (Camhi & Tom, 1978). Cockroaches are particularly sensitive to sudden increases in the velocity of air currents, which is how they can distinguish an attack by a toad from meteorological wind (Plummer & Camhi, 1981).

Unlike the startle response in crayfish, cockroach startle behaviour is normally controlled by activity in several giants, and involves many spikes in these neurons rather than the single spike that marks the decisionmaking process in the crayfish lateral giant or Mauthner neuron. Undoubtedly associated with this, the cockroach startle response is initiated more slowly than that of the crayfish or fish, but is nevertheless a rapid behaviour, with the first movement occurring within 50 ms of the start of a wind stimulus. It is a rapid turn away from the direction of attack (Fig. 3.12a) that, unlike walking or running, involves the co-ordinated move ment of all six legs at the same time (Fig. 3.12b). Each leg pushes or pulls, so that the cockroach swivels to face away from the direction from which the air current is coming. The cockroach then runs forward, using the usual tripod gait mode of locomotion in which the legs are moved in two sets of three.

The sense organs that excite the giants are called filiform sensilla; each is a slender, whip-like structure inserted in a socket on the ventral surface of a cercus, the sensory structure that projects from each side of the last abdominal segment. The sensillum articulates in its socket, and can move in one particular plane. It is attached to a single bipolar sensory cell that is excited when its sensillum is deflected in one direction. The axon of the sensory cell enters the last abdominal ganglion, where it terminates in a series of fine branches that are restricted to one side of that ganglion and which make chemical synapses on to dendrites of the giant interneurons. The filiform sensilla are arranged in 14 columns, most of which run the length of the cercus. Sensilla of a particular column all share the same direction for deflection, and so all respond best to air currents coming from the same direction. A cercus of an adult cockroach has about 20 segments, and each column is usually represented by one filiform sensillum on each segment. The arrangement for two adjacent segments is shown in Fig. 3.13a.

Seven pairs of interneurons that originate in the last abdominal ganglion and have axons that run in the ventral nerve cord have been labelled as giants. The axons range from 20 to 60 ^m across, so they are considerably smaller than the giants of crayfish or Mauthner neurons, but are appreciably larger than other axons in the cockroach nerve cord. Anatomically, two separate groups of giants can be distinguished, one with axons located dorsally and the other with axons located ventralfy in each connective nerve. The giants in the ventral group are larger than those in the dorsal group and trigger escape running. The structure of one ventral giant, number 1, is shown in Fig. 3.13b.

Carefully conducted experiments have revealed that each giant responds most vigorously to air currents from a particular direction (Kolton & Camhi, 1995). To determine the directional selectivity of a giant, spikes were recorded from its axon using a microelectrode that contained a stain so that the neuron could later be identified from its anatomy. Carefully controlled puffs of air, always with a peak velocity of 0.85 m/s, were directed at the

Figure 3.13 Giant interneurons and sensory analysis of wind-puff direction in the cockroach. (a) Best directions of deflection for exciting sensory neurons of the filiform sensilla of the seventh segment of the right cercus, indicated by arrows. Each circle shows the location of one filiform sensillum on the ventral surface of segment 7 or 8; the diameter of the circle depends on the length of the hair. (b) Drawing of right giant 1 in the last abdominal ganglion. (c) Polar plot of the directional sensitivities of the three ventral giants (GI 1-3) that have their axons in the right connectives, constructed as explained in the text. (a redrawn after Dagan & Camhi, 1979; b modified after Harrow et al., 1980; c redrawn after Kolton & Camhi, 1995.)

Figure 3.13 Giant interneurons and sensory analysis of wind-puff direction in the cockroach. (a) Best directions of deflection for exciting sensory neurons of the filiform sensilla of the seventh segment of the right cercus, indicated by arrows. Each circle shows the location of one filiform sensillum on the ventral surface of segment 7 or 8; the diameter of the circle depends on the length of the hair. (b) Drawing of right giant 1 in the last abdominal ganglion. (c) Polar plot of the directional sensitivities of the three ventral giants (GI 1-3) that have their axons in the right connectives, constructed as explained in the text. (a redrawn after Dagan & Camhi, 1979; b modified after Harrow et al., 1980; c redrawn after Kolton & Camhi, 1995.)

right cercus from a nozzle that could be rotated to stimulate it from different directions. Air currents were applied from an angle of 45° above the cockroach, which is the type of angle from which an attack might be made. Spikes were counted for the first 50 ms following the start of each stimulus; this is roughly the time over which a giant neuron would respond before a cockroach starts to move in response to an air current. The results are expressed as a polar plot (Fig. 3.13c). The origin of the plot represents the cercus, and each response is plotted as a point whose direction from the origin represents the direction of the stimulus, and whose distance from the origin represents the number of spikes.

In Fig. 3.13c, each point is the mean of several stimuli in experiments on five different cockroaches. For each interneuron, the different points are joined by a line, and the resulting shape gives an immediate impression of the directional sensitivity of that giant. The shape and location of the plot for each giant are called its receptive field, a term that is commonly used to refer to the region around an animal from which a neuron receives sensory input. All three interneurons respond preferentially, but not exclusively, to stimuli coming from the same side as their axons (the right side in this case), and interneuron 1 responds well to stimuli from the front or from the rear. There is a little overlap between the receptive fields of interneurons 2 and 3, but if we look at the receptive field to determine which stimulus direction elicits the best responses in these two neurons, we find that the directions for the two giants are almost exactly at right angles to each other: interneuron 2 prefers stimuli from behind and to the side of the animal, whereas interneuron 3 responds best to stimuli from the front and the side. In fact, the best direction for interneuron 3 corresponds to the direction in which the cercus normally points, and the best direction for interneuron 2 is at right angles to this. Thus, a cockroach can determine whether an air current is coming from the left or from the right by comparing the responses of its left and right giants; and it can distinguish stimuli coming from the front from those coming from the rear by the relative excitation of interneurons 2 and 3. If a current of air is coming directly from the side, these two giants will respond with almost equal vigour.

A cockroach giant interneuron can generate 25 spikes or more between the times when it starts to respond to wind and when the cockroach starts to move. By stimulating single giants to generate extra spikes during responses to wind, Liebenthal, Uhlman & Camhi (1994) showed that the most critical parameter in the response is the number of spikes in left compared with right giants. It is, therefore, not possible to equate a single spike, or any other single event, with a distinct decision to initiate an escape turn in the same way as a spike in a Mauthner neuron or crayfish lateral giant marks the decision to initiate the startle responses. No single giant interneuron responds exclusively to air currents from the left or from the right, yet the cockroach always makes quite accurately oriented turns that take it away from a source of danger. Control of the activity in leg motor neurons must, therefore, involve quite sophisticated mechanisms to combine signals that originate in different giant interneurons (Levi & Camhi, 1996).

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