ABSTRACT
A behavioural sequence of Spisula is described that involves local reflexes, siphon retraction and valve adduction. The response to low-intensity tactile stimulation is locally mediated reflex activity of the siphon wall musculature. In response to medium-intensity stimulation, the siphon retractor muscles are activated along with local reflex activity.
A neural correlate of this behavioural discrimination between low and medium stimulus intensities was obtained by simultaneous intracellular recordings from pairs of efferent neurones, one neurone type involved in local reflexes and one efferent tai the siphon retractor muscles. The peripherally located neurones of the siphon wall musculature (cluster cells) were found to have a lower spike threshold than the efferents to the siphon retractor muscles (pallial neurones).
Several lines of evidence are presented that support the assumption that intra-somatic recordings from these neurones accurately reflect the activity of the synaptic region.
The membrane electrical properties (i.e. input resistance, rheobasic current, threshold potential) of the two cell types were measured by passing current through single intrasomatic micro-electrodes. From these measurements it was found that the ‘critical firing level ’ of the cluster cells is significantly lower than that of the pallial neurones. By way of their differential sensitivities to synaptic input, the stimulusintensity discrimination could be mediated by cluster cells and pallial neurones directly.
