Modeling of spontaneous activity in developing spinal cord using activity-dependent depression in an excitatory network

Spontaneous episodic activity is a general feature of developing neural net works. In the chick spinal cord, the activity comprises episodes of rhythmi c discharge (duration 5-90 sec; cycle rate 0.1-2 Hz) that recur every 2-30 min. The activity does not depend on specialized connectivity or intrinsic bursting neurons and is generated by a network of functionally excitatory c onnections. Here, we develop an idealized, qualitative model of a homogeneo us, excitatory recurrent network that could account for the multiple time-s cale spontaneous activity in the embryonic chick spinal cord. We show that cycling can arise from the interplay between excitatory connectivity and fa st synaptic depression. The slow episodic behavior is attributable to a slo w activity-dependent network depression that is modeled either as a modulat ion of cellular excitability or as synaptic depression. Although the two de scriptions share many features, the model with a slow synaptic depression a ccounts better for the experimental observations during blockade of excitat ory synapses.