Collision warning neurons in the locust

Optomotor responses help to stabilise an animal when it is stationary or proceeding along a straight course, and they do this by referring to movements that occur in the background. The animal also needs to be able to respond to individual objects, such as potential predators, mates or perching sites. Like many animals, locusts and grasshoppers will escape from rapidly approaching objects. This is apparent to anyone who has tried to catch one of these insects; they respond to approach by a powerful jump, caused by rapid extension of the hind legs. Often, the wings are opened during the jump, and the animal extends the range of its jump by flying or gliding.

A particular neuron in the hindbrain of the locust has been found to respond vigorously to the images of approaching objects. The axon of the neuron travels to the thorax, where it excites motor neurons and interneu-rons that are concerned with the control of the jumping and flying. Probably, therefore, it plays a vital role in channelling sensory information about an approaching object to the motor control circuits responsible for executing escape movements.

It is relatively easy to record responses from this neuron by using extracellular electrodes placed around a nerve cord because it produces spikes that are usually much larger than those from the axons of other neurons. Because the axon of the neuron crosses the brain, and responds to movements detected by the opposite (contralateral) eye, the neuron is called the descending contralateral movement detector (DCMD). The DCMD receives its input from a single, large, fan-shaped neuron in the lobula, called the lobula giant movement detector (LGMD; Fig. 5.10). Spikes in the DCMD follow those in the LGMD one for one. The two neurons are extremely sensitive to small movements anywhere within the visual field of their eye, and respond with a brisk, but brief, burst of spikes. As is to be expected from neurons that receive their input through large monopolar cells in the lamina, the response to a sustained stimulus rapidly disappears and they respond to changes in contrast over a wide range of background light intensities.

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Figure 5.10 Two feature-detecting neurons in the visual system of a locust (Locusta), the lobula giant motion detector (LGMD) and the descending contralateral motion detector (DCMD). The drawings were made from neurons that had been stained by injecting them with cobalt ions. The LGMD receives excitatory input from lower-order interneurons on the fanlike array of dendrites on the left. (Modified from Rind, 1984.)

Figure 5.10 Two feature-detecting neurons in the visual system of a locust (Locusta), the lobula giant motion detector (LGMD) and the descending contralateral motion detector (DCMD). The drawings were made from neurons that had been stained by injecting them with cobalt ions. The LGMD receives excitatory input from lower-order interneurons on the fanlike array of dendrites on the left. (Modified from Rind, 1984.)

The LGMD generates a vigorous and prolonged train of spikes in response to an approaching object. The frequency of spikes increases throughout the approach movement, as if the neuron has locked on to the approach movement (Fig. 5.11a). Images of receding objects, or of objects that are moving around the locust, generate only brief responses. Deviations as small as 2-3° from a direct collision course result in a reduction in response in the LGMD by a half, so it is remarkably tightly tuned to respond to objects that will collide with the animal (Judge & Rind, 1997). The intensity of response depends on the speed with which the object is approaching, and the neuron would respond very well to a predatory bird swooping towards a locust. Wide field movements, like those that cause optomotor responses, inhibit responses by the LGMD.

To determine the types of movement that are most effective at exciting the LGMD, Claire Rind made recordings while a locust viewed a video of a space movie (Rind & Simmons, 1992). This approach was an effective way of providing a wide variety of visual stimuli and rapidly indicated that the

Motor Neurons Locust

Figure 5.11 Detection of approaching objects by the locust LGMD neuron. (a) Response by the LGMD to the image of an object approaching and then receding at 3.5 m/s. The top trace is an intracellular recording from the LGMD, and the lower trace is a monitor of the size of the object's image, which was displayed on a screen. (b) Stimulation of one part of the visual field inhibits the response to stimulation of another part. The drawing shows the stimulus screen, with two vertically oriented bars that could move from left to right across the screen. Movement of either bar alone across the screen caused EPSPs and spikes in the LGMD, but when the left bar moved just before the right one, the response to movement of the right bar was greatly reduced. (c) Drawing of an electron micrograph of a dendrite of the LGMD with input synapses onto it from two adjacent presynaptic elements, pre-1 and pre-2. Each presynaptic element contains a densely staining bar-like structure that is believed to direct vesicles of neurotransmitter to their site of release at an insect synapse. The LGMD receives synapses from each presynaptic element, and the two elements also synapse with each other as indicated by the arrows. (a recordings from Rind, 1996; reprinted with permission of the American Physiological Society; b from Rind & Simmons, 1998.)

LGMD responded more vigorously to images of approaching objects than to other types of movement. Subsequently, responses were analysed in more detail by generating carefully controlled moving images with a computer, and then determining exactly which features in the image of an approaching object are the important cues (Simmons & Rind, 1992). Although many invertebrates show avoidance reactions when a shadow is cast over them, the LGMD only responds weakly to overall decreases in light intensity. To excite it, an image of the edges of an approaching object must move over the surface of the eye. Two different features of the image are important as cues that an object is moving nearer to the eye: first, the edges grow in length; and, second, the edges accelerate as they move across the retina.

As with the lobula plate neurons in the fly, selectivity by the LGMD for a particular kind of moving stimulus is established by circuits among neurons in the medulla. The way that these circuits work has been established by using a variety of experimental approaches. Electrophysiological recordings demonstrated that lateral inhibition operates among the columns of neurons in the medulla (O'Shea & Rowell, 1976). When one area of the retina is stimulated by a moving image, the response to stimulation of other parts of the retina is depressed (Fig. 5.11b). Although the first movement clearly decreases the response by the LGMD to the second movement, no inhibitory postsynaptic potentials are recorded from the LGMD. This means that the inhibition must occur at an earlier stage, pre-synaptically to the LGMD.

Using electron microscopy, it has been shown that the dendrites of the LGMD are covered with input synapses, probably from neurons that originate in the medulla. These synapses are arranged in a remarkable manner (Fig. 5.11c). Each neuron that synapses with the LGMD also synapses with its neighbours; in other words, the neurons that drive the LGMD are reciprocally coupled to each other. These micro anatomical circuits provide a route for local lateral inhibition among the elements that excite the LGMD, which is a key feature of the input circuitry to the LGMD, allowing it to filter out approaching objects. In the lamina, the lateral inhibition between cartridges serves a different function - to sharpen the detection of edges in the image.

The way in which the LGMD filters out the image of an approaching object can be envisaged as a kind of race between excitation and inhibition

Figure 5.12 A diagram of the circuits thought to drive the LGMD. An array of units drives the LGMD through excitatory inputs (—and inhibits their neighbours. Each unit is excited by a small number of photoreceptors fl>—) and inhibits its neighbours (—•). When an object approaches the eye, its edges spread outwards (arrows), and this creates a race between excitation of units by edge movement, and inhibition between neighbours. The LGMD is excited strongly when excitation is winning the race. A second input to the LGMD causes inhibition in response to wide-field movements.

Figure 5.12 A diagram of the circuits thought to drive the LGMD. An array of units drives the LGMD through excitatory inputs (—and inhibits their neighbours. Each unit is excited by a small number of photoreceptors fl>—) and inhibits its neighbours (—•). When an object approaches the eye, its edges spread outwards (arrows), and this creates a race between excitation of units by edge movement, and inhibition between neighbours. The LGMD is excited strongly when excitation is winning the race. A second input to the LGMD causes inhibition in response to wide-field movements.

of the units in the medulla that synapse with the LGMD (Fig. 5.12). The excitation comes from photoreceptors that are stimulated sequentially by the edges of the image as it travels over the eye; and the inhibition travels laterally between the medulla units. As the number of excited units increases, the strength of the lateral inhibition also increases. Consequently, in order for the excitation to outstrip the inhibition and be passed on to the LGMD, a large number of new medulla units must next be stimulated. For this to occur, the extent of the edges in the image or the speed with which they move must be increasing. A network like this has been modelled in a computer, and it responds as predicted, being excited most strongly by the images of approaching objects (Rind & Bramwell, 1996). The network incorporates a second kind of inhibition that also occurs in the LGMD system, and this acts directly on the LGMD itself, causing IPSPs in it. This inhibition acts to reduce responses by the LGMD when the whole background moves, or when overall light intensity changes.

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