Limbic system

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It was Papez (1937) who first postulated that there were a collection of structures within the brain, whose interdependence was such that they might legitimately, be conceptualized as an interrelated system. Their function he proposed was in the regulation of motivation and emotion. It was MacLean (1949) however, who contributed the term ‘limbic system’ and extended its scope to encompass further brain structures, thereby extending its complexity of function.

The objective of the present work is essentially of a bipartite nature. Firstly, one shall begin by identifying those brain structures and circuits presently implicated in the concept of ‘limbic system;’ and secondly, one shall briefly examine the functions of the most salient of these major structures, in the hope of elucidating the general function of the limbic system.

Prior to an examination of its major structures and circuits, it is important to note that the term ‘limbic system’ is used merely to conceptualise a hypothesized interaction between different brain structures and as such, does not constitute a separate and distinct sub-system of the brain. Indeed, Pinel (2000) notes that it shares with the basal ganglia motor system, the fact that , “It is not entirely clear exactly which structures should be included in them, or even whether it is appropriate to view them as unitary systems” [p74].

That said however, continued research in this area, involving increasingly sophisticated techniques and ever more ingenious experimental paradigms, has resulted in a growing sophistication of our understanding of the structures and circuits implicated. As noted previously, it was Papez was the first to suggest the existence of such a system. His conceptualization implicated the four basic structures of the hypothalamus (and mamillary bodies), the anterior thalamic nucleus, the cingulate gyrus and the hippocampus.

Extending this conceptualization, MacLean incorporated inter alia, the structures of the orbito-frontal cortex, the para-hippocampal gyrus, the amygdala, the medial thalamic nucleus, the septum, the olfactory cortex, the insula, and the fornix. MacLean (1985) conceptualized the brain in terms of a tripartite, evolutionary hierarchical structure, comprising the ‘primitive reptilian’ brain, the more elaborate ‘old mammalian’ and the most recent and advanced ‘new mammalian’ brain. It was the ‘old mammalian,'(or ‘Paleomammalian’) brain he stated, which comprised the structures of the limbic system.

As such, the limbic system is hypothesized to contribute to the survival of the species (on both an ontogenic and phylogenic level) as it provides a circuit which forms the bases of various motivational and emotional processes with important survival value; namely, feeding, fighting, fleeing, and mating behaviour. Furthermore, as the evolutionary ‘intermediate brain,’ the limbic system serves an important function in acting as a mediating circuit between the oldest and newest layers of the brain. How then are such processes achieved?

In order to gain a thorough appreciation of the overall function of the limbic system, it is necessary to examine the functions of its constituent parts. Obviously however, space restriction shall not allow for an examination of function of all limbic system structures. Thus, this discussion shall be limited to a consideration of four main structures; namely, the hypothalamus, the amygdala, the hippocampus and the cingulate cortex. One shall examine each in turn. The hypothalamus is , “A relatively small… ut extremely complex structure at the base of the brain… that is intimately involved in the control of… a variety of functions… including… temperature regulation, heartbeat, blood pressure, feeding behaviour, water intake, emotional behaviour and sexual behaviour” [Reber & Reber; 2001; p333].

The hypothalamus achieves control of autonomic processes by projecting to three important brain regions – the nucleus of the solitary tract; the rostral ventral medulla regions of the brainstem, and the autonomic outflow of the spinal cord [Kandel et al. 000]; and by via the release of hormones through its action upon the endocrine system, which in turn concentrate on the ANS. Traditionally, the hypothalamus was seen as comprising two main regions, the lateral hypothalamus (LH), and the ventromedial hypothalamus (VMH), which were hypothesized to control feeding and satiety respectively. The VMH was implicated as a ‘satiety center’ when a series of studies (e. g. Hetherington & Ranson; 1940) showed that lesions to the VMH resulted in hyperphagia and obesity in rats.

Similarly, lesions to the LH resulted in aphagia, thus suggesting the LH to be the ‘feeding centre’ of the brain. (Anand & Brobeck, 1951). Such theories have been largely displaced of late, owing to two main experimental findings. Firstly, the hypothalamus has been found to regulate energy metabolisation rather than eating per se; and secondly, effects previously attributed to LH & VMH lesions, have been shown to actually result from damage to surrounding brain structures such as the dopaminergic axons of the nigrostriatal (Stricker & Zigmond; 1976), the noradrenergic bundle (Gold et al. 1977) or the para-ventricular nuclei (Leibowitz, et al 1981). Present research in this area has focused upon the role of two neurotransmitters secreted by neurons, within the lateral hypothalamus and their activation (i. e. hunger signals) via neuropeptide Y (NPY) which also has at least two sites within the thalamus (Carlson; 2001).

The hypothalamus has also been implicated in sexual behaviour. For example, it has been noted that males have larger preoptic (Swaab & Fliers; 1985), anterior (Allen et al. , 1989) and suprachiasmatic (Swaab et al. 1994) hypothalamic regions than females. Furthermore, male sexual behaviour is regulated by a tract connecting the medial preoptic are and the lateral tagmental field, whilst in females the corresponding tract runs from the ventromedial nucleus to the periaqueductal gray (Pinel; 2000; p307). Another highly important structure of the limbic system is the amygdala, “… an almond shaped nucleus that is located in the medial temporal lobe, just anterior to the hippocampus [and which]… links feelings of fear and anxiety to appropriate stimuli and defensive responses. (Pinel & Edwards 1998; p190).

Kluver-Bucy’s-syndrome (e. g. Kluver & Bucy;1937) was the first indication that this structure was implicated in emotional perception and expression. Subsequent research has demonstrated the amygdala’s importance in the acquisition of conditioned emotional responses (e. g. LeDoux; 1995; Bechara et al. ; 1995 ), and modified startle responses (Davis el al. ; 1994); and in the recognition of emotional facial expressions (Adolphs et al. ; 1994) . Such functions are further supported by functional imaging studies involving both PET scans (Isenberg et al. 1999) and MRI (Benkelfat et al. 1995).

Of course the importance of such functions would be diminished greatly if one were unable to retain such information. Thankfully, however, another limbic system structure, namely the hippocampus, facilitates in this acquisition (Maren & Baudry; 1995). The hippocampus, located at the medial-temporal lobes, extends from the anterior amygdala to the posterior of the fornix. The most significant suggested function of the hippocampus arose from O’Keefe & Dostrovsky’s (1971) discovery of hippocampal pyramidal cells.

These so-called ‘place cells’ have been found to fire in relation to where the animal ‘thinks’ it is (Wilson & McNaughton; 1993) and thus are important spatial locators. There are three main contemporary explanations of such functions. O’Keefe & Nadel (1978) proposed ‘cognitive map theory’ stating that the hippocampus constructs and stores allocentric maps of the external environment, based upon sensory input. Whilst Rudy & Sutherland (1992) suggest that the hippocampus facilitates long-term retention of the behavioural significance of stimuli as opposed to the stimuli per se.

Whatever its exact function, the hippocampus can nonetheless be noted as being pertinent to both spatial location and the process of memory. A further structure of the limbic system, namely, the cingulate cortex, has been implicated in the perception of pain. More specifically, recent research suggests that it is important in the detection of aversiveness of stimulus.

For example, Rainville et al. (1997) found that hypnotized subjects (who had reported less aversion to a painful stimulus than experimental controls) correspondingly, showed less activation of the cingulate cortex as measured via PET scans. An important point of note, is that whilst all of the above structures and functions were presented individually for ease of conceptualization, the greatest understanding of their significance is to be grained when they are considered collectively. To borrow the old Gestalt idiom – ‘the whole is greater that the sum of its parts. But how then, might all of the above information, be integrated into a tenable set of principles capable of explaining the collective functions of the limbic system? Firstly, as has been noted previously, the limbic system is thought to equip us with survival value, and indeed from the evidence outlined above this appears to be true. For example in the case of the, the cingulate cortex, it helps us to interpret the severity, or aversiveness of any threatening stimuli and thus can prepare us for the flight or flight response.

This is of particular importance to the hypothalamus, which can exert control on the ANS (fundamental to the fight or flight response) and is involved in the regulation of sexual behaviour (and thus continuation of ones gene-pool) as well as eating and drinking behaviour (necessary to remain healthy). Similarly, the amygdala facilitates the recognition of emotional facial expression in others, affects our ability to perceive threatening situations, and is fundamental to the acquisition of conditioned emotional responses.

Whilst the hippocampus helps us to form memories of that information (all of which are behaviours which are of obvious value to the survival of the individual, and thus the species as a whole). Secondly, it is important to remember also, that those structures outlined above, are but only some of the components of which the limbic system comprises. That is, it constitutes a circuit which is flexible enough to receive information from several different sources at once, relay this information to all other relevant structures, (re)interpret the feedback received from those structures and still, somehow integrate all this information into a coherent course of action for use by one or more of its structures.

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