Following the description of some typical variables of escape running in the cricket Gryllus bimaculatus in a companion paper, this study gives an account of the physiological characteristics of identified interganglionic cell types recorded during normal and wind-evoked walking.
Intracellular recording and staining of axons in the prothoracic ganglion revealed a group of intersegmental wind-sensitive neurones with large axons in the laterodorsal tract and somata in the pro- or mesothoracic ganglion. These interneurones rapidly conduct signals to their projections in the thoracic and cephalic ganglia. Wind pulses evoke strong, non-habituating spike reactions, which tend to summate during repeated stimulation.
During walking, the sensory response to wind stimulation is suppressed in a velocity-dependent manner in all ascending interneurones tested (N=40). During slow walking, the sensory responsiveness is merely reduced, whereas it is completely blocked during fast escape running bouts. Conversely, during pauses occurring during wind-evoked escape behaviour, the sensory responsiveness in ascending cells is significantly enhanced.
One type of interneurone that descends from the suboesophageal ganglion and projects to the thorax and abdominal connectives has been identified. In the resting animal, this neurone fires in the rhythm of abdominal ventilatory contractions. During walking, the rhythmic spike discharges disappear and, as in ascending interneurones, velocity-dependent spike suppression is observed.
In contrast to all other types of interneurones, which uniformly showed reduced spike activity during walking, cells descending from the brain were tonically excited during walking. Brain cells (N=21) have been classified according to whether their spike activity during walking was correlated with forward speed or with the intended walking direction.
Mechanisms underlying the observed gating of sensory responsiveness are discussed in terms of their possible functional significance. Modulated spike activity in ascending cells during walking suggests a role in tuning the thoracic motor centres for a central walking command. It is proposed that descending interneurones from the suboesophageal ganglion coordinate different behavioural rhythms. Possible functions of different types of brain neurones in the control of specific variables of walking behaviour are discussed.