The locus coeruleus is rendered here as a procedural icosphere at the left-side neuromelanin-imaging centroid (MNI -6, -37, -22). The contralateral right-side LC sits at (+6, -37, -22); a separate mesh exists for the right side but does not yet have its own region page. The nucleus is anatomically tiny: ~2 mm in diameter, ~15 mm long along its rostro-caudal axis, with the cells arranged in a thin column. The sphere here is a generic placeholder; the actual nucleus is elongated.

Aston-Jones and Cohen’s 2005 adaptive-gain theory of LC function frames the LC as continuously modulating cortical signal-to-noise based on the expected utility of attending to the current task versus exploring alternatives. Tonic LC firing signals general arousal; phasic LC bursts signal that something task-relevant has just happened. The balance between tonic and phasic firing predicts task performance across a range of attentional paradigms.

The LC has emerged as a target of intense interest in the synucleinopathy and tauopathy literature. Braak’s staging of Parkinson’s disease places LC degeneration at Stage 2 (before midbrain dopaminergic involvement), explaining why prodromal REM-sleep behaviour disorder and autonomic dysregulation often precede classic motor symptoms by years. Tau pathology in LC is now believed to be the earliest detectable site of Alzheimer’s-type neurofibrillary degeneration on autopsy.