insular cortex

**Anatomy** - Sometimes called the "5th lobe" or island of Reil. It lies deep to the lateral (Sylvian) fissure and is overlain by the frontal, parietal, and temporal opercula. - A central insular sulcus divides it into a larger anterior portion (short gyri) and a posterior portion (long gyri). - Blood supply: the insular (M2) segment branches of the middle cerebral artery (MCA). - Bordered by the claustrum and extreme capsule medially, with the external capsule and putamen deeper.

**Functions** - Gustatory: primary taste cortex, in the anterior insula and frontal operculum. Taste fibers travel from the solitary nucleus to the VPM of the thalamus and then to the insula. - Interoception and visceral sensation: awareness of heartbeat, breathing, bladder or gut distension, and temperature. - Autonomic regulation: has sympathetic and parasympathetic influence. This is the basis for cardiac arrhythmias and sympathetic surges after insular stroke. - Pain: processes the affective and discriminative aspects of pain. The posterior insula receives nociceptive input from the VPI/spinothalamic pathways. - Emotion and social cognition: disgust, empathy, and subjective emotional awareness. It is a key node of the salience network (with the anterior cingulate cortex), which detects salient stimuli and switches between the default mode network and the central executive network. - Addiction: implicated in craving, and insular lesions have been associated with spontaneous cessation of smoking. - Vestibular and auditory integration, plus speech/motor planning in the left anterior insula.

**Clinical correlations** - MCA stroke: insular involvement is common. Possible effects include autonomic dysfunction (arrhythmias such as atrial fibrillation, QT prolongation, and neurogenic cardiac injury), dysphagia, and altered taste. - Left anterior insula lesions: apraxia of speech. - Insular lesions can also produce impaired pain perception (asymbolia for pain), loss of disgust recognition, and altered interoceptive awareness. - Insular epilepsy: seizures may mimic temporal lobe epilepsy. Auras can include throat constriction, paresthesias, viscerosensory sensations, and unpleasant tastes or smells. - Huntington disease is associated with impaired disgust recognition, and the insula is implicated. - Psychiatric: increased anterior insula activity is linked to anxiety disorders, panic (misinterpretation of bodily signals), and OCD. Altered insular function is also reported in anorexia nervosa, depression, and PTSD. - Pain syndromes: hyperactivity is seen in chronic pain and fibromyalgia. - Neurodegeneration: von Economo neurons are found in the anterior insula and anterior cingulate cortex. They are selectively lost in behavioral variant frontotemporal dementia. - Insular cortex lesions are also a cause of central post-stroke pain (Dejerine-Roussy-like presentations), though the thalamus is the classic site.

**High-yield pearls** - Insula = taste, visceral sensation, autonomic control, and disgust/interoception. - Think "MCA stroke with arrhythmia or dysphagia" and "von Economo neurons in bvFTD." - Salience network = anterior insula + dorsal anterior cingulate cortex. - The gustatory pathway is CN VII, IX, and X to the solitary nucleus, then the VPM of the thalamus, then the insula and frontal operculum.