Orgasm is a complex, multimodal reflex induced typically by genital stimulation. Genitally stimulated orgasms (GSOs) activate excitatory neurochemical pathways in the brain and spinal cord that ultimately stimulate sympathetic outflow and the inhibition of parasympathetic spinal circuits in the lower lumbar cord. However, some women claim to have orgasms spontaneously without genital stimulation.
Source: Case of Female Orgasm Without Genital Stimulation | Sexual Medicine | Oxford Academic