Structure and synaptic activation of the fast coxal depressor motoneurone of the cockroach, Periplaneta americana.

Now that behavioural descriptions of walking in the cockroach are available (Delcomyn, 1971) attempts are being made to elucidate the neuronal mechanisms involved in leg movements. Innervation of some of the muscles utilized in walking has been studied and the patterns of activity of motoneurones to both coxal levator and depressor muscles have been described in the intact, free-walking animal (Pearson, 1972) and in dissected preparations in which sensory input from the legs has been interrupted (Pearson & lies, 1970). The basic features of the normal pattern are retained in the de-afferented preparation leading to the conclusion that walking movements are to some extent centrally programmed. Further study of this central system requires intracellular recording from motoneurones within the thoracic ganglia and examination of their synaptic inputs. The general organization of cells and fibres in the cockroach metathoracic ganglion has been described (Guthrie & Tindall, 1968; Pipa, Cook & Richards, 1959) but few details are yet available for single identified cells. A method of injecting individual crustacean motoneurones with the fluorescent dye Procion Yellow was introduced by Stretton & Kravitz (1968). This technique allows detailed reconstruction of cell structure to be made and has since been applied to insect neurones (Bentley, 1970; Hoyle & Burrows, 1970), leech motoneurones (Stuart, 1970) and various mammalian spinal neurones (Jankowska & Lindstrom, 1970a, b, 1971). In this paper a modified technique which avoids the necessity of locating and penetrating the cell with a microelectrode (lies & Mulloney, 1971) is used to fill the largest coxal depressor motoneurone of the cockroach with Procion Yellow. This cell is active during leg depression when a cockroach jumps or is running rapidly (Pearson, 1972). The histological detail obtained from dye injection has facilitated an analysis of the jumping response. A stimulus to the anal cerci of a cockroach induces a jump followed by running (Roeder, 1959; Pearson & lies, 1970). Afferents from receptors in the cerci are known to excite giant axons which originate in the last abdominal ganglion (Hess, 1958; Harris & Smyth, 1971; Milburn & Bentley, 1971). The giant axons ascend in the abdominal cord to the thoracic ganglia and have been assumed to synapse with leg motoneurones completing an 'evasion reflex* (Roeder, 1948, 1962). However, it has recently been suggested that axons smaller than the giant fibres (but also excited by