The juncture between two neurons is called the synapse.  Each of the perhaps 100 billion neurons in the brain is connected to about 1,000 other neurons.  At the synapse, a firing neuron either passes a neurochemical signal to the next neuron, or it does not pass a signal, with the passing or not passing depending on the complex neurochemistry of the synapse.  If, within a millisecond, a certain number of signals are passed on to a neuron, then that neuron will fire.  Otherwise it will not fire.  Thus what happens at the various synapses—signal passed on or not passed on—is the sole determinant of the firing pattern of the neurons in the brain.  The synapses are the control points for our flow of thoughts.