The Limbic System

The hippocampus, amygdala, cingulate cortex, hypothalamus and their connections: what the 'emotional brain' idea got right, and why many neuroscientists now treat the term with caution.

The word limbic comes from the Latin limbus, "border". In 1878 the French anatomist Paul Broca used it for the ring of cortex on the inner border of each hemisphere, wrapping around the brainstem.

From a border to an emotional system

In 1937 the American neuroanatomist James Papez proposed that a loop of connected structures — the hippocampus, the mammillary bodies of the hypothalamus, the anterior thalamus and the cingulate cortex — formed the circuitry of emotion: the Papez circuit. In the 1950s Paul MacLean expanded the idea, added the amygdala, and named the whole set the limbic system, the brain's emotional core.

What is usually included

Definitions vary, but typical members are the hippocampus (memory), the amygdala (emotional significance, fear), the cingulate cortex (emotion, attention and pain), the hypothalamus (drives and hormones), parts of the thalamus, the septal nuclei, the mammillary bodies, and often the orbitofrontal cortex, insula and nucleus accumbens.

A useful label, a misleading theory

The limbic system idea captured something real: these structures are heavily interconnected and important for emotion, motivation and memory. But it has problems. The Papez circuit turned out to be more important for memory than emotion. Emotion also depends heavily on the prefrontal cortex, brainstem and body. And MacLean's related "triune brain" — the idea of a reptilian brain, wrapped in an old mammalian (limbic) brain, wrapped in a new mammalian neocortex — is rejected by comparative neuroanatomists: evolution modified existing brain parts in all vertebrates rather than stacking new layers on top.

Many neuroscientists still use "limbic" as convenient shorthand, while studying specific circuits rather than one emotional system.