Morphological and electrophysiological features of the GABAergic substantia gelatinosa (SG) neurons in the caudal spinal trigeminal nucleus (the medullary dorsal horn; MDH) were examined in the GAD67-GFP knock-in mouse.Visually guided whole-cell patch-clamp recording and biocytin-injection combined with histochemical staining were preformatted in horizontal slices.The results showed that 96% GABAergic SG neurons were intrinsic neurons that sent axons only to the MDH,and almost all of these intrinsic neurons had sparse axonal arborization.Only one GABAergic SG neuron was projection neuron that sent axons to regions outside the MDH.The input resistance of GABAergic SG neurons averaged (564.51 +45.98) M Ω,and the mean resting membrane potential was-62.99±+7.97 mV.When stimulated by rectangular current injections,80% of GABAergic SG neurons showed tonic action potential generation pattern,20% showed initial bursting pattern.