Many networks in the mammalian nervous system remain active in the absence
of stimuli. This activity falls into two main patterns: steady firing at lo
w rates and rhythmic bursting. How are these firing patterns generated? Spe
cifically, how do dynamic interactions between excitatory and inhibitory ne
urons produce these firing patterns, and how do networks switch from one fi
ring pattern to the other? We investigated these questions theoretically by
examining the intrinsic dynamics of large networks of neurons. Using both
a semianalytic model based on mean firing rate dynamics and simulations wit
h large neuronal networks, we found that the dynamics, and thus the firing
patterns, are controlled largely by one parameter, the fraction of endogeno
usly active cells. When no endogenously active cells are present, networks
are either silent or fire at a high rate; as the number of endogenously act
ive cells increases, there is a transition to bursting; and, with a further
increase, there is a second transition to steady firing at a low rate. A s
econdary role is played by network connectivity, which determines whether a
ctivity occurs at a constant mean firing rate or oscillates around that mea
n. These conclusions require only conventional assumptions: excitatory inpu
t to a neuron increases its firing rate, inhibitory input decreases it, and
neurons exhibit spike-frequency adaptation. These conclusions also lead to
two experimentally testable predictions: 1) isolated networks that fire at
low rates must contain endogenously active cells and 2) a reduction in the
fraction of endogenously active cells in such networks must lead to bursti
ng.