INNERVATION PATTERN OF A POOL OF 9 EXCITATORY MOTOR-NEURONS IN THE FLEXOR TIBIAE MUSCLE OF A LOCUST HIND LEG

The flexor tibiae muscle of a locust hind leg consists of 10-11 pairs of fibre bundles in the main body of the muscle and a distal pair of b undles that form the accessory flexor muscle, all of which insert onto a common tendon. It is much smaller than the antagonistic extensor ti biae muscle and yet it is innervated by nine excitatory motor neurons, compared with only two for the extensor. To determine the pattern of innervation within the muscle by individual motor neurons, branches of the nerve (N5B2) that supplies the different muscle bundles were back filled to reveal somata in the metathoracic ganglion. This showed that different muscle bundles are innervated by different numbers of excit atory motor neurons. Physiological mapping of the innervation was then carried out by intracellular recordings from the somata of flexor mot or neurons in the metathoracic ganglion using microelectrodes. Spikes were evoked in these neurons by the injection of current, and matching junctional potentials were sought in fibres throughout the muscle usi ng a second intracellular electrode. Each motor neuron innervates only a restricted array of muscle fibres and, although some innervate a la rger array than others, none innervates fibres throughout the muscle, Some motor neurons innervate only proximal fibres and others only more distal fibres, so that the most proximal and most distal bundles of m uscle fibres are innervated by non-overlapping sets of motor neurons. More motor neurons innervate proximal bundles than distal ones, and th ere are some asymmetries in the number of motor neurons innervating co rresponding bundles on either side of the tendon. Individual motor neu rons cause slow, fast or intermediate movements of the tibia, but thei r patterns of innervation overlap in the different muscle bundles, Fur thermore, individual muscle fibres may also be innervated by motor neu rons with different properties.