which = excitatory neurons ?

Context:

The cells in question are the class of neurons that release the neurotransmitter called GABA (gamma aminobutyric acid). GABA neurons function to inhibit or dial down the intensity of nerve signals propagated by excitatory neurons, which are triggered by neurotransmitters such as glutamate.

Excitatory neurons account for about 80% of all the neurons in the mammalian cortex. But without the modulatory intervention of the much rarer GABA neurons within the circuits they form, normal brain function would be impossible. Uninhibited neuronal excitation would lead to a constant state of seizure something like what is seen, episodically, in epilepsy.

More:

http://www.sciencedaily.com/releases/2011/09/110921132342.htm