The Identification of Motor and Unpaired Median Neurones Innervating the Locust Oviduct

Abstract

The different classes of neurones supplying the locust oviduct were individually identified by intracellular recording and staining. We could thus show that different regions of the oviduct are innervated by different sets of neurones. Three motor neurones (oviductal neurones 1–3, OVN1-3) supply the oviduct via nerve N2B of the seventh abdominal ganglion. Whereas all three motor neurones innervate the junctional area of the lateral and the common oviduct (OVN1, 2 and 3), the lateral oviduct is innervated by only one motor neurone (OVN2) and the common oviduct by two motor neurones (OVN1 and 2). The cell bodies of all three motor neurones lie ventrally, near the origin of the sternal root, and their neuropilar branches are confined to the seventh abdominal ganglion. The neuropilar branches of OVN1 and 2 extend mainly in the ipsilateral half of the ganglion; those of OVN3 reside exclusively in the contralateral half. The oviductal motor neurones, produce a phasic motor pattern, the oviductal rhythm, which causes neurogenic contractions of the junctional area and the common oviduct. These contractions serve to retain eggs in the lateral oviduct. The oviduct is also supplied by a large number (16–20) of median neurones with bilateral axons. All of these appear to innervate the lateral oviduct, but only two project to the junctional area and the common oviduct. The cell bodies of the median neurones are situated in the seventh abdominal ganglion and are arranged in two groups: a posterior group made up of 10–12 cells and an anterior group with 6–8 cells. Their primary neurites run towards the centre of the ganglion in the dorsal plane, where they bifurcate, sending a secondary neurite through each oviductal nerve. Their neuropilar branches are confined to the seventh abdominal ganglion, but some also possess thin axon collaterals projecting to the terminal abdominal ganglion. The anterior and posterior median neurones show considerable differences in their branching pattern within the ganglion. The posterior cells are all likely to be neurones of the well-known DUM cell group, but the anterior median cells probably represent a different class of neurone. Posterior median neurones support overshooting soma action potentials of 60–80 mV amplitude, with a characteristic undershoot of 6–15 mV. Orthodromic stimulation of these neurones results in a reduction of the amplitude and frequency of the oviductal contractions, suggesting that they have a modulatory role. Note: To whom reprint requests should be sent.