bdivisions of the autonomic nervous system? It is divided into the sympathetic and the parasympathetic nervous systems. What is the major neurotransmitter of the parasympathetic autonomic nervous system? Acetylcholine (ACh). ACh is released in- to the synaptic clefts from the pre-and the postsynaptic neurons of the parasympathetic nervous system. In the sympathetic nervous system, what neurotransmitter is released from the pregan- glionic neuron into the synaptic cleft? -