Organisation of neurons that control reflex movements

A summary of the main patterns of interconnections found among neurons controlling local reflexes of a locust's leg is drawn in Fig. 8.8. Some non-spiking interneurons make excitatory output connections and others make inhibitory connections, but none has been found to excite some of its targets and inhibit others. As with the local spiking interneurons, the pattern of output connections from non-spiking interneurons is diffuse in that a non-spiking interneuron can control several different motor neurons and each motor neuron is controlled by several non-spiking interneurons (Burrows, 1980). Some non-spiking interneurons connect laterally with others, always making inhibitory connection, which might help to ensure that incompatible movements involving antagonistic muscles do not occur.

The most intriguing aspect of the organisation of the neurons that control local reflexes of a locust's leg is that one type of interneuron operates with spikes and the other without them. The situation contrasts with the way information is processed in visual systems, where the first layers of neurons often operate without spikes but later neurons transmit trains of spikes along axons. It is interesting that all the interneurons that have been found to drive flight motor neurons in locust thoracic ganglia

Figure 8.8 A circuit diagram indicating the kinds of connections that have been discovered between neurons involved in local reflexes of a locust's hind leg. Excitatory connections are indicated by triangles, and inhibitory connections by circles.

produce spikes (see section 7.6). It might be that the graded way in which non-spiking interneurons control motor neurons is useful for the kinds of small postural adjustments that legs can make. Spiking rather than non-spiking communication is not simply related to neuronal size, because both the extent and the complexity of the branching pattern are similar in non-spiking and spiking local interneurons (compare Fig. 8.6a with Fig. 8.7a). It is possible that signals other than spikes cannot travel along the narrow, constricted neurite that connects the dorsal and ventral fields of a spiking local interneuron. Spikes may also provide a mechanism for synchrony by depolarising all of a neuron's presynaptic terminals together and by a similar amount. In contrast, the use of smoothly graded membrane potentials in a non-spiking interneuron probably allows different regions of the neuron to operate independently to some extent. In support of this idea, electron microscopy has shown that input and output synapses are intermingled along all the processes of non-spiking interneurons and no separate regions of input or output can be distinguished (Watson & Burrows, 1988), and signals do seem to reduce in amplitude as they travel from one part of a non-spiking interneuron to another (Burrows & Siegler, 1978).

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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