Pathological personality traits in negative affectivity: Emotional liability – Emotions that are easily aroused, intense, and/or out of proportion to events and circumstances.
What we know
Lucas and others (1989), using computerized tomography, studied brain structures of people with BPD and controls. They found no significant differences between the two groups on any of measures except for a narrower third ventricle in borderline patients, which could be accounted for by the narrower third ventricle observed in female subjects overall. They concluded that while borderline patients may show signs of subtle neurological dysfunction, they do not show evidence of structural brain pathology.
In another study, Koenigsberg and others  (2009), used fMRI’s on 18 BPD patients and 16 controls to study responses to pictures involving social interactions. BPD patients showed a different pattern of activation compared with HC subjects when looking at negative versus neutral pictures. The BPD group showed less signal change in dorsal anterior cingulate cortex and IPS, less deactivation in the amygdala, and greater activation in the superior temporal sulcus and superior frontal gyrus. They concluded that BPD patients do not engage the cognitive control regions to the extent that HCs do when employing a distancing strategy.
So what does this mean in plain English? When it comes to personality traits, the study by Lucas and others suggest that we do not have any biological structures in our brain that account for our BPD. However, other studies have shown that BPD does run in families but there is no clear indication of how much of that is genetic and how much is environmental. Even studies of identical twins that show similarities in emotional and cognitive functioning tend to have the same overall environmental experiences. What seems to be agreed is that we may have similar genetic predispositions that may involve ability to cope with anxieties and some deficiencies in Serotonin regulation. Thus a predisposition or trait.
From the Koenigberg study we see that people with full blown BPD have dysfunctions in self-regulating, and self-soothing with irregularities in the prefrontal cortex functions and limbic/amygdala emotion regulation. In other words it is highly likely that our problems may involve genetic based personality traits coupled with problems during childhood that result in the creation of dysfunctional neural patterns. However, this study was done on BPD patients and may not be evident in people who are still in control of their traits. We do know that the brain has enormous powers of plasticity and is very good at creating new neural circuits and the white brain matter that goes with it. So a trait does not have to develop into pathological neural circuitry and dysfunctional thought and behavioral patterns.
In conclusion, these traits are only a combination of genetic tendencies and the creation of neural circuitry during a life time of existence. We created them and we can change them.
 Lucas, Peter B.; Gardner, David L,; Cowdry, Rex W. and Pikar, David. Cerebral Structure in borderline personality disorder. Elsevier Psychiatry Research. 1989.
 Koenigsberg, Harold, W.; JinFan, kevin; Ochsner, N.; XunLiu, Kevin; Guise, G.; Pizzarello, Scott; Dorantes, Christine; Guerreri, Stephanie; Tecuta, Lucia; Goodman, Marianne, New, Antonio and Siever, Larry J.. Neural Correlates of the Use of Psychological Distancing to Regulate Responses to Negative Social Cues: A Study of Patients with Borderline Disorder. Elsevier, Bilogical Pschiatry. 2009.