Local reflex movements of a locusts leg

Jointed limbs, such those of a mammal or an insect, are used in a wide variety of ways. Besides being involved in locomotion, they are capable of finely controlled movements such as grooming, in which the end of the limb is brought precisely to a particular spot on the animal's surface. Since the 1970s, Malcolm Burrows and colleagues have investigated in detail the neurons responsible for controlling local movements of a locust's hind leg, providing a very complete description of the links between sensory analysis and motor control (see Burrows, 1996). Compared with the local bending movements of the leech, local reflexes of a locust's leg involve many more neurons, arranged in two layers between the sensory and motor neurons. Each of these layers consists of local interneurons, which have their branches restricted to the third thoracic ganglion, often to one part of it. The first layer mostly receives inputs from sensory neurons and consists of spiking interneurons. The second layer makes connections with motor neurons, and consists of non-spiking interneurons. The major route for the flow of information is from sensory neurons to spiking interneurons, then to non-spiking interneurons and finally to motor neurons. Within this route there are no feedback pathways, so motor neurons do not synapse back onto either type of interneuron, and non-spiking interneurons do not synapse back onto local spiking interneurons. Feedback is provided by sense organs that monitor movements of the limbs, and these mainly connect with interneurons rather than directly with motor neurons.

If a locust's leg is touched anywhere on its surface, it will be moved away from the touch (Fig. 8.5a). Usually, the movement involves several joints so that, for example, when the ventral surface of the tibia is touched, the tibia is raised and during this response the angle between the tibia and the foot changes to keep the foot parallel with the ground. These movements are controlled by neurons contained within the third thoracic ganglion, including about 70 motor neurons and a few hundred local interneurons. About 10 000 sensory neurons project into each half of the third thoracic ganglion, and many of these originate on the surface of the leg, so there is a great deal of convergence as information flows from sensory neurons to motor neurons.

Many of the sensory neurons belong to basiconic sensilla, each of which consists of a short tactile hair with one mechanosensory neuron and four chemosensory neurons. Trichoid sensilla are another type of sense organ

Figure 8.5 Local reflex movements of a hind leg of a locust, Schistocerca. (a) Movements evoked by touching the tibia on its anterior or posterior (large arrows). The leg is shown as an outline in its initial position and in black in its final position. (b) The projection of a trichoid sensillum on the right foot into the third thoracic ganglion, viewed from below. The dashed line indicates the extent of the neuropile into which sensilla on different parts of the right leg project. Only the anterior section of the right half of the ganglion is drawn. (a redrawn after Siegler & Burrows, 1986; b from Newland, 1991; reprinted by permission ofWiley-Liss, Inc., a subsidiary of John Wiley & Sons Inc.)

Figure 8.5 Local reflex movements of a hind leg of a locust, Schistocerca. (a) Movements evoked by touching the tibia on its anterior or posterior (large arrows). The leg is shown as an outline in its initial position and in black in its final position. (b) The projection of a trichoid sensillum on the right foot into the third thoracic ganglion, viewed from below. The dashed line indicates the extent of the neuropile into which sensilla on different parts of the right leg project. Only the anterior section of the right half of the ganglion is drawn. (a redrawn after Siegler & Burrows, 1986; b from Newland, 1991; reprinted by permission ofWiley-Liss, Inc., a subsidiary of John Wiley & Sons Inc.)

which consist of a sensory cell that responds phasically when its hair (which can be up to 0.8 mm long) is deflected towards the centre of the body. Other types of sense organs, such as campaniform sensilla (see Chapter 4), monitor strains in the cuticle or span joints, responding whenever the leg is moved. Sensory neurons project in an orderly manner into the ganglion, so that their terminal branches form a somatotopic map of the surface of the leg (Newland, 1991). All the hair-like sensilla project to a particular region of neuropile in the ventral part of the third thoracic ganglion. A sensillum near the foot projects to a relatively posterior region of this neuropile (Fig. 8.5b), whereas a sensillum close to the joint of the femur with the base of the leg projects more anteriorly.

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Essentials of Human Physiology

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