[Model of a neuron-regulator of the effectiveness of synaptic transmission]

In diad and triad synapses of vertebrate retina the transmitter released from one and the same presynaptic membrane can act simultaneously on two (or even three) postsynaptic neurones. The model of such a diad synapse is proposed, in which the positive electrical feed-back is caried out between one of postsynaptic and presynaptic neurones. The feed-back appears because of potential drop across the longitudinal resistance of intercellular gap (RIII) near the activated synapse (see [1]). It is shown that if the input resistance of this postsynaptic neurone (N3) is low (low R3 in fig. 1,), it can operate as a regulator of effectiveness of synaptic transmission between the presynaptic (N1) and the second postsynaptic (N2) neurones. The effectiveness of synpatic transmission (e. g. the steepness of transfer function) is the more, the higher the membrane potential in the neurone acting as a regulator (fig. 2), and the higher the resistance of intercellular gap (RIII) (fig. 2). The model reproduces the effect of polarization of horizontal cells (by means of light or current) on the membrane potential in bipolars of the turtle and fish retina (fig. 3). The analysis of the model also shows that one of the functions of horizontal cells, as regulators of synaptic transmission between photoreceptors and bipolars, may be the detection of small objects and borders of image against uniform background.