Electrical coupling between supernumerary motor neurones in the locust.

1. While recording from a slow tarsal levator motor neurone of the locust metathoracic ganglion, a second neurone of this type was discovered in the same half of the ganglion. In thirty animals from which recordings were subsequently made, the additional neurone was found in only oae. 2. Since two motor neurones could not be found routinely, other tests were made to determine how many motor neurones normally innervated the levator muscle. The tibial nerve containing the motor axons was stimulated whilst intracellular recordings were made from the levator muscle fibres; recordings were made simultaneously from the cell body of a levator motor neurone, from the tibial nerve and from the levator muscle fibres; and the composition of fine nerve branches that terminate on the levator muscle was examined by electron microscopy. 3. It is concluded that normally the levator muscle is innervated by one excitatory motor neurone and two inhibitory ones. The single excitatory motor neurone has the synaptic inputs ascribed in earlier studies to the 'slow' motor neurone. It can, however, evoke movements of the tarsus ranging from slow smooth ones, to rapid twitchlike ones. No evidence was found for a second, 'fast' motor neurone previously reported (Hoyle & Burrows, 1973). 4. In the two locusts found to possess a supernumerary levator motor neurone, the two motor neurones were electrically coupled by non-rectifying junctions that were strong enough to ensure that the pair invariably spiked in 1 :1 fashion. 5. The coupled levator motor neurones received different synaptic inputs. 6. One pair of coupled motor neurones was stained intracellularly with cobalt-silver. The cell bodies were some 50 /im apart, but their neurites converged as they entered the neuropil. The neurites and major side branches followed similar courses in the neuropil, and the two neurones shared the major anatomical features of the single levator motor neurones stained in other animals.