  In Diffusive Molecular Communication (DMC), information is transmitted by
diffusing molecules. Synaptic signaling is a natural implementation of this
paradigm. It is responsible for relaying information from one neuron to
another, but also provides support for complex functionalities, such as
learning and memory. Many of its features are not yet understood, some are,
however, known to be critical for robust, reliable neural communication. In
particular, some synapses feature a re-uptake mechanism at the presynaptic
neuron, which provides a means for removing neurotransmitters from the synaptic
cleft and for recycling them for future reuse. In this paper, we develop a
comprehensive channel model for synaptic DMC encompassing a spatial model of
the synaptic cleft, molecule re-uptake at the presynaptic neuron, and
reversible binding to individual receptors at the postsynaptic neuron. Based on
this model, we derive an analytical time domain expression for the channel
impulse response (CIR) of the synaptic DMC system. Our model explicitly
incorporates macroscopic physical channel parameters and can be used to
evaluate the impact of re-uptake, receptor density, and channel width on the
CIR of the synaptic DMC system. Furthermore, we provide results from
particlebased computer simulation, which validate the analytical model. The
proposed comprehensive channel model for synaptic DMC systems can be exploited
for the investigation of challenging problems, like the quantification of the
inter-symbol interference between successive synaptic signals and the design of
synthetic neural communication systems.
