Limbic system · Grey Matter

The limbic system is a traditional name for the set of structures on the inner edge of each hemisphere that join emotion, motivation and memory to the body's responses: what is felt, what is wanted and what is remembered because it mattered.

Limbic system. The limbic system is a traditional name for the set of structures on the inner edge of each hemisphere that join emotion, motivation and memory to the body's responses: what is felt, what is wanted and what is remembered because it mattered.

Its membership varies by author. The core always includes the hippocampus, the amygdala, the cingulate cortex and the septal area, with the hypothalamus as its output to the body and the anterior nuclei of the thalamus as a relay. In 1937 James Papez proposed a loop through hippocampus, hypothalamus, anterior thalamus and cingulate as the circuit of emotion; that loop turned out to matter more for memory, while the amygdala, which Papez left out, became central to emotion.

It is a teaching convenience more than a unit. Its structures do different things, many of them also serve cognition, and emotional processing extends to the insula and the ventromedial prefrontal cortex, which the classic list does not include.

The amygdala and the prefrontal cortex form a two-way loop. The amygdala signals that something is dangerous, the prefrontal cortex can signal that it is only a film, and which side wins decides whether thought amplifies or calms the emotion.

The idea of a limbic layer wrapped around a reptilian core belongs to the triune brain model, an outdated map that the question on this card takes apart.

Questions: Is the brain really a reptilian core wrapped in a limbic layer and a rational neocortex? That picture is the triune brain model, proposed by Paul MacLean from the 1960s and popularised by his 1990 book, and it remains a common teaching map. As an account of evolution it is wrong: reptiles, birds and mammals all have homologues of the cortex, the basal ganglia and the limbic structures, which evolved together and were modified rather than stacked as new layers. As a functional map it misleads as well, since emotion and reason are carried by interacting circuits across all levels (the prefrontal cortex is central to emotion, the amygdala to decisions). It can still serve as a mnemonic for where the vital, the emotional and the deliberative tend to be concentrated, provided it is not taken as the brain's history. Was the Papez circuit the circuit of emotion, as first proposed? In 1937 James Papez proposed a loop from the hippocampus through the fornix to the mammillary bodies of the hypothalamus, on to the anterior thalamus, to the cingulate cortex and back to the hippocampus, as the anatomical basis of emotion. Later work showed that the loop matters above all for memory: damage at several of its stations (the hippocampus, the mammillary bodies in Korsakoff syndrome, the anterior thalamus) causes amnesia rather than a loss of emotion. The amygdala, which Papez left out, turned out to be the structure most closely tied to emotional learning.