The Limbic System

The limbic system (also known as the paleomammalian brain) is a collection of brain structures located in the middle of the brain. It was first defined by Paul Broca in the nineteenth century as the structures between the cerebral hemisphere and the brainstem (i.e., the limbus, or border of the brain). The limbic system is not a discrete system itself but rather a collection of structures—anatomically related but varying greatly in function. The term has been in use for about 70 years and does suggest a functionally unified system, but as this is not the case some neuroscientists believe it should be abandoned. Regardless of terminology, collectively we can think of the limbic system as the centre for emotional responsiveness, motivation, memory formation and integration, olfaction, and the mechanisms to keep ourselves safe. These are broad strokes to be sure, which is not to suggest that the neo-cortex is not involved in these functions, but these are the focal activities of the limbic system. The amygdala, hippocampus, and hypothalamus are considered the main limbic structures of clinical relevance to the practising psychotherapist. There is also the very important hub of information transfer, the thalamus, which feeds the limbic system with sensory input.

The Limbic System in 60 Seconds from The Neuropsychotherapist on Vimeo.

The basal ganglia, a set of subcortical structures located near the thalamus and hypothalamus, are also included in the limbic system and are involved in intentional movements. It is important to note that inadequate dopamine supply to the basal ganglia may affect posture and movement, leading to the symptoms of Parkinson’s disease. The limbic system is closely connected to the prefrontal cortex, and it is this prefrontal–limbic connection that is strengthened when practising mindfulness. The functional relevance of the limbic system to psychotherapy is obvious—as affect, memory, sensory processing, time perception, attention, consciousness, autonomic control, motor behaviour, and more are all mediated in and through this collection of structures.

This diagram below shows some of the complexity of the limbic system. Note how the olfactory bulbs feed directly into the amygdala, giving smell a particularly important role in emotional memory and evaluation of circumstances—we can smell danger faster than our “smart brain” will recognise a problem, and we can automatically recall the emotions of a past love at the smell of the perfume they used to wear. It is also striking that the size of these structures has little correlation to their power and importance in this amazing system.

The_Limbic_System_and_Nearby_Structures_smallAC Anterior commissure
AN Anterior nucleus of thalamus
DG Dentate gyrus
FR Fasciculus retroflexus
IN Interpeduncular nucleus
LT Lamina terminalis
MB Mammillary body
MD Mediodorsal thalamic nucleus
MF Medial forebrain bundle
MT Mammillothalmic tract
NA Nucleus accumbens
OB Olfactory bulbs
OC Optic chiasm
OL Olfactory striae lateral
OS Olfactory striae medial
OT Olfactory tract
PG Pituitary gland
PT Paraterminal gyrus
SA Subcallosal area
SM Stria medullaris
SN Septal nuclei
SP Septum pellucidum
ST Stria terminallis

 

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