  In this paper we present a simple microscopic stochastic model describing
short term plasticity within a large homogeneous network of interacting
neurons. Each neuron is represented by its membrane potential and by the
residual calcium concentration within the cell at a given time. Neurons spike
at a rate depending on their membrane potential. When spiking, the residual
calcium concentration of the spiking neuron increases by one unit. Moreover, an
additional amount of potential is given to all other neurons in the system.
This amount depends linearly on the current residual calcium concentration
within the cell of the spiking neuron. In between successive spikes, the
potentials and the residual calcium concentrations of each neuron decrease at a
constant rate.
  We show that in this framework, short time memory can be described as the
tendency of the system to keep track of an initial stimulus by staying within a
certain region of the space of configurations during a short but macroscopic
amount of time before finally being kicked out of this region and relaxing to
equilibrium. The main technical tool is a rigorous justification of the passage
to a large population limit system and a thorough study of the limit equation.
