Fig. 2. Spontaneous synaptic activity in Retzius neurones. (A) Simultaneous somatic recordings of Retzius neurones in leech Ringer’s solution showing spontaneous and synchronous excitatory postsynaptic potentials. V₁ is the recording from one of the neurones and V₂ is the recording from the other. Six consecutive series are shown. (B) Partial substitution of Ca²⁺ by Mg²⁺ in the Ringer’s solution abolished the spontaneous activity in both neurones. (C) Synaptic activity was restored in normal Ringer’s solution.