Background: We obtained microelectrode recordings from four patients with intractable aggressivity who underwent surgery at posteromedial hypothalamus under general anaesthesia. We described two general types of extracellular action potentials (EAPs): typical/canonical and atypical. Methods: We analysed 337 units and 67 traces, which were characterized by the mean action potential (mAP). For the first phase, depolarization and repolarization, we computed amplitudes (V{}_{\text{FP}}, V{}_{\text{Dep}} and V{}_{\text{Rep}}) and durations (d{}_{\text{FP}}, d{}_{\text{Dep}} and d{}_{\text{Rep}}), maximum and minimum values of the first derivative (dV{}_{\text{max}}, dV{}_{\text{min}}), and amplitude and duration ratios. Results: Most of the canonical mAPs were positive (81.1%). EAPs with atypical mean action potentials (amAPs) were recorded in 42/337 cases. Only 35.6% of mAPs showed 2 phases. We identified the following types: N1P1N2 (38.3%), P1N1 (35.9%), amAP (12.5%), P1P2N1 (12.2%), N1P1 (4.7%), P1N1P2 (4.1%) and N1N2P1 (3.2%). We can define the properties of canonical forms as those units with (i) at least two opposite phases; (ii) V_{\text{Dep}}\in [1.2,2.7]\times |V_{\text{Rep}}| and strongly related by this function V_{\text{Rep}}=-0.56\left(\pm 0.01\right)V_{\text{Dep}}-1.83(\pm 0.79); (iii) a very strong relationship between dV{}_{\text{max}} and dV{}_{\text{min}}, given by the equation d{V}_{\text{min}}=-0.91\left(\pm 0.03\right)d{V}_{\text{max}}-0.37(\pm 0.12), both of which were included in the depolarization phase; (iv) related with V{}_{\text{Dep}} by the equation d{V}_{\text{max}}=0.08\left(\pm 0.001\right)V_{\text{Dep}}-0.28(\pm 0.14); and (v) d_{\text{Dep}}\mathrm{~}0.38d_{\text{Rep}}. However, the first phase does not pertain to the same dynamic process responsible for depolarization and repolarization. Conclusions: Atypical units are described here for the first time and are true EAPs that differ strikingly from canonical forms. To date, they have been observed only in the hypothalamus, but future research is needed to assess their existence in other brain structures.