Toward a more precise, clinically--informed pathophysiology of pathological laughing and crying.

Involuntary emotional expression disorder (IEED) includes the syndromes of pathological laughing and crying (PLC) and emotional lability (EL). Review of the lesion, epilepsy, and brain stimulation literature leads to an updated pathophysiology of IEED. A volitional system involving frontoparietal (primary motor, premotor, supplementary motor, posterior insular, dorsal anterior cingulate gyrus (ACG), primary sensory and related parietal) corticopontine projections inhibits an emotionally-controlled system involving frontotemporal (orbitofrontal, ventral ACG, anterior insular, inferior temporal, and parahippocampal) projections targeting the amygdala-hypothalamus-periaqueductal gray (PAG)-dorsal tegmentum (dTg) complex that regulates emotional displays. PAG activity is regulated by glutamatergic NMDA, muscarinic M1-3, GABA-A, dopamine D2, norepinephrine alpha-1,2, serotonin 5HT1a, 5HT1b/d, and sigma-1 receptors, with an acetylcholine/GABA balance mediating volitional inhibition of the PAG. Lesions of the volitional corticopontine projections (or of their feedback or processing circuits) can produce PLC. Direct activation of the emotional pathway can result in EL and the laughing or crying of gelastic and dacrystic epilepsy. A criterion-based nosology of PLC and EL subtypes is offered.