>wouldn’t there technically be some other function that initials the spark in reaction to the stimulus?

Our sensory organs (retina, skin, ...) are what converts the different stimuli into electrical messages. These travel up the sensory nerves into the brain where they induce an endless train of activity, including the reactivation of memories, the complex neural process of decision making and so on. Even something as simple as a feeling of anxiety/hunger/feeling your heartbeats/... is considered sensory input, so it would be extremely hard to completely isolate the brain from it. These serve as cues to spark a certain though or memory, which in turn sparks other memories and so on in a continuous chain.

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>And what about a thought not stimulated by external factors?

There are many types of neurons, some of which will periodically fire spontaneously due to certain chemical processes (e.g., pacemaker neurons). Even you were to perfectly isolate the brain, it would still have a baseline activity, and would still think. Though in this case I'd imagine you'd be drawing blanks most of the time, the activity would translate to random flashes of thoughts/memories here and there until one of them induces a new chain of coherent thoughts.

You could look at it as if the very initial spark was the first neuron that fired in your developing brain when you were a fetus, and your brain has been following an endless causal chain of neural activity, altered by incoming messages from the sensory nerves and noise from the spontaneously firing neurons.

Consciousness, whatever it is, seems to be "just along for the ride". Whatever activity is taking place in the brain, that is what you are conscious of, yet it has no influence on that activity. Hopefully science reaches the real answer soon, brain simulations is what I'm really excited about.