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'The Limbic System' by Jeremy Lander

The Limbic System- with thanks to the Kahn Academy

The limbic system is a set of small anatomical structures that sit on top of the brain stem and is thought to be the source of all those reactive value judgments that we make, often unconsciously, which exert such a strong influence on our behaviour. It also may act as a gatekeeper to our deep memory. Limbic comes from the Latin limbus, for "border" or "edge" because it borders the main part of the brain, the neo cortex or cerebral hemispheres. Its importance in controlling emotion was only identified in the 1930s and its relationship with the rest of our brain is still being debated.

It is believed that the limbic system first appeared in small mammals about 150 million years ago. If the Earth is a year old and we are at 23.59 on New Year’s Eve this equates to about 15 days ago—16th December.

The reptilian brain, now part of our brain stem and cerebellum, first appeared in fish, nearly 500 million years ago, 50 days ago, or early November, to give an idea of how relatively old it is. It continued to develop in amphibians and reached its most advanced stage in reptiles, roughly 250 million years ago (early December).

The largest part of our brain is the neo-cortex and it began its spectacular expansion in primates, scarcely 2 or 3 million years ago, as the genus Homo sapiens emerged. About teatime.

The reptile (or lizard) part of the brain controls the body's vital functions such as heart rate, breathing, body temperature, balance, and also possibly the more ritualistic behaviours as shown in birds. It is reliable but tends to be somewhat rigid and compulsive.

The limbic brain is responsible for producing emotions and regulating senses- it also records and processes memories of behaviours that produce agreeable and disagreeable experiences.

The main structures of the limbic brain are:

Hippocampus – this forms new memories and converts short term memories into long term memories (STM to LTM)

Amygdala – these two collections of cells help to regulate behaviour. If they are stimulated in humans they produce aggression and anxiety, even violence, if they are suppressed we become more mellow and, in extremis, this can lead to hypersexuality and disinhibited behaviour (known as Kluver-Bucy syndrome).

Thalamus – a twin structure that serves as a sensory relay station processing four of our five senses and sending them onward to our cortex (interestingly our sense of smell bypasses it, which is probably why a smell can be so powerfully evocative).

Hypothalamus – a tiny structure, about the size of a kidney bean, which produces the hormones (including adrenaline) which control our opposing modes of fight-flight-freeze or rest & digest depending on whether we are about to be attacked by a sabre-toothed tiger or whether we are tucking into a hearty meal of bison around our camp fire.

The rest of the brain, the neocortex, is huge by comparison. Consisting of two hemispheres it is responsible for the development of language, abstract thought, imagination, and consciousness. It has learning abilities which appear to be almost infinite.

These three parts of the brain, the retile, limbic and cortex, operate together and they influence one another. The theory, known as the Triune model, is that these different parts have been sort of thrown together by an evolutionary scrap heap challenge and they don’t always work together in harmony.

But we always have to be careful when we talk of ‘The Science’, because it too is evolving. The Triune model is perhaps a little too beguiling in its simplicity. Recent studies do not support the view of reptile behaviour as being wholly stereotyped and ritualistic. Birds have been shown to possess highly sophisticated cognitive and linguistic abilities, as demonstrated by the recent story of the five parrots banned from public areas at a zoo after they started swearing at visitors. Some parts of the limbic system, which according to the model are peculiar to mammals, have now been shown to exist across a range of present-day vertebrates, and the "paleomammalian" trait of parental care of offspring is widespread in birds of course, and occurs in some fish species as well.

So – surprise, surprise – it’s actually quite complicated. Who’d have thought it?

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