Nieuwenhuys, Voogd, Huijzen. The Human Central Nervous System. Springer-Verlag, 2007
The limbic system is the set of brain structures that forms the inner border of the cortex. The components of the limbic system located in the cerebral cortex generally have fewer layers than the classical 6-layered neocortex, and are usually classified as allocortex or archicortex.
The limbic system is made up of the limbic lobe and other deep-lying structures. The limbic lobe consists of the parahippocampal, cingulate, and subcallosal gyrus, hippocampus, and dentate gyrus. The deep-lying structures include, the hippocampus, amygdala, mammillary body, habenula, anterior thalamic nuclei, and olfactory bulb. The limbic system has four major components that form two C-shaped structures. The first C-shape is made up of the hippocampus and the fornix, while the cingulate and parahippocampal gyri form the second C-shape.
The limbic system includes many structures in the cerebral pre-cortex and sub-cortex of the brain. The term has been used within psychiatry and neurology, although its exact role and definition have been revised considerably since the term was introduced. The following structures are, or have been considered to be, part of the limbic system:
Hippocampus and associated structures:
- Hippocampus (see below):
Required for the formation of long-term memories and implicated in maintenance of cognitive maps for navigation. The hippocampus consists of two “horns” that curve back from the amygdalae. It appears to be very important in converting things that are “on one’s mind” at the moment (in short-term memory) into things that one will remember for the long run (long-term memory). If the hippocampus is damaged, a person cannot build new memories and lives instead in a strange world where everything he or she experiences just fades away, even while older memories from the time before the damage are untouched (a condition depicted in the films Memento and 50 First Dates).
- Amygdala (see below):
Involved in signaling the cortex of motivationally significant stimuli such as those related to reward and fear in addition to social functions such as mating. Furthermore, the anatomy of amygdalae are two almond-shaped masses of neurons on either side of the thalamus at the lower end of the hippocampus. The amygdalae stimulate the hippocampus to remember many details surrounding the situation, as well.
- Fornix (see below):
is a C-shape bundle of axon that carries signals from the hippocampus to the mammillary bodies and septal nuclei.
The fornix is mainly a projection tract connecting the hippocampus with the mammillary body, the anterior thalamic nuclei, and the hypothalamus; it also has a small commissural component known as the hippocampal commissure (Crosby et al., 1962; Aggleton, 2008; Nieuwenhuys et al., 2008). Figure 11.2 shows two views of the fornix reconstructed with in vivo diffusion tensor tractography (Catani et al., 2002). Fibres arise from the hippocampus (subiculum and entorhinal cortex) of each side, run through the fimbria, and join beneath the splenium of the corpus callosum to form the body of the fornix. Other ﬁmbrial fibres continue medially, cross the midline, and project to the contralateral hippocampus. Most of the fibres within the body of the fornix run anteriorly beneath the body of the corpus callosum towards the anterior commissure. Above the interventricular foramen, the anterior body of the fornix divides into right and left columns. As each column approaches the anterior commissure it diverges again into two components. One of these, the posterior columns of the fornix, cun/e ventrally in front of the inter-ventricular foramen of Monroe and posterior to the anterior commissure to enter the mammillary body (post-commissural fornix), adjacent areas of the hypothalamus, and anterior thalamic nucleus. The second component, the anterior columns of the fornix, enter the hypothalamus and project to the septal region and nucleus accumbens (Aggleton, 2008). The fornix also contains some afferent fibres to the hippocampus from septal and hypothalamic nuclei (Niewenhuys et al., 2008).
- Mammillary body (see in Nieuwenhuys Collection):
locates at the ends of the anterior arches of the fornix. It is involved with the process of recognition memory.
The ﬁbres of the mammillo-thalamic tract (bundle of Vicq d’Azyr) originate from the mammillary bodies and after a very short course terminate in the anterior and dorsal nuclei of the thalamus (Figure 11.1). A ventrally directed branch projects from the mammillary bodies to the tegmental nuclei (mammillo-tegmental tract). According to Nauta (1958), the mammillo-tegmental tract together with other fibres of the medial forebrain bundle, forms an important circuit between medial limbic structures of the midbrain and hypothalamus to relate visceral perception to emotion and behaviour.
- Septal nuclei (see in Nieuwenhuys Collection):
Located anterior to the interventricular septum, the septal nuclei provide critical interconnections. The septal area isn’t related to the sense of smell, but is the pleasure zone in animals.
- Parahippocampal gyrus (see below):
Plays a role in the formation of spatial memory.
- Cingulate gyrus (see below):
Autonomic functions regulating heart rate, blood pressure and cognitive and attentional processing.
Figure 11.3A shows a lateral view of the trajectory of the cingulum as reconstructed with tractography. The cingulum contains fibres of different lengths, the longest running from the amygdala, uncus, and parahippocampal gyrus to sub-genual areas of the frontal lobe (Crosby et al., 1962; Nieuwenhuys et al., 2008). From the medial temporal lobe, these fibres reach the occipital lobe and arch almost 180 degrees around the splenium to continue anteriorly within the white matter of the cingulate gyrus. The dorsal and anterior fibres of the cingulum follow the shape of the superior aspect of the corpus callosum. After curving around the genu of the corpus callosum, the fibres terminate in the subcallosal gyrus and the paraolfactory area (Crosby et al., 1962). Shorter fibres that join and leave the cingulum along its length, connect adjacent areas of the medial frontal gyrus, paracentral lobule, precuneus, cuneus, cingulate, lingual, and fusiform gyri (Déjérine, 1895; Nieuwenhuys et al., 2008). The cingulum can be divided into an anterior-dorsal component, which constitutes most of the white matter of the cingulate gyrus, and a posterior-ventral component running within the parahippocampal gyrus, retrosplenial cingulate gyrus, and posterior precuneus. Preliminary data suggest that these subcomponents of the cingulum may have different anatomical features. For example, a higher fractional anisotropy has been found in the left anterior-dorsal segment of the cingulum compared to right, but reduced fractional anisotropy has been reported in the left posterior-ventral component compared to the right (Park et al., 2004; Gong et al., 2005; Wakana et al., 2007). Notwithstanding this, the volume of the cingulum is bilateral and symmetrical in most subjects (Thiebaut de Schotten et al., 2011).
- Dentate gyrus (see in Nieuwenhuys Collection):
thought to contribute to new memories.
In addition, these structures are sometimes also considered to be part of the limbic system:
- Entorhinal cortex (see in Nieuwenhuys Collection):
Important memory and associative components.
- Piriform cortex (see below):
The function of which relates to the olfactory system.
- Fornicate gyrus (see in Nieuwenhuys Collection):
Region encompassing the cingulate, hippocampus, and parahippocampal gyrus.
- Nucleus accumbens (see in Nieuwenhuys Collection):
Involved in reward, pleasure, and addiction.
- Orbitofrontal cortex and Gyrus Rectus (see below):
Required for decision making.
Anterior thalamic projections (see below):
The anterior thalamic nuclei receive projections from the fornix and mammillo-thalamic tract and connect through the anterior thalamic projections to the orbitofrontal and anterior cingulate cortex (Figure 11.1). The anterior thalamic projections run in the anterior limb of the internal capsule.
Uncinate fasciculus (see below):
The uncinate fasciculus connects the anterior part of the temporal lobe with the orbital and polar frontal cortex. In the tractography reconstruction the fibres of the uncinate fasciculus originate from the temporal pole, uncus, parahippocampal gyrus, and amygdala, then after a U-turn, enter the floor of the external/extreme capsule. Between the insula and the putamen, the uncinate fasciculus runs inferior to the fronto-occipital fasciculus before entering the orbital region of the frontal lobe. Here, the uncinate splits into a ventro-lateral branch, which terminates in the anterior insula and lateral orbitofrontal cortex, and an antero-medial branch that continues towards the cingulate gyrus and the frontal pole (Déjérine, 1895; Klingler and Gloor, 1960; Crosby et al., 1962). Whether the uncinate fasciculus is a lateralized bundle is still debated. An asymmetry of the volume and density of fibres of this fasciculus has been reported in one post-mortem neurohistological human study in which the uncinate fasciculus was found to be asymmetric in 80% of subjects, containing on average 30% more fibres in the right hemisphere compared to the left (Highley et al., 2002). However, diffusion measurements have also shown higher fractional anisotropy in the left uncinate compared to the right in children and adolescents (Eluvathingal et al., 2007) but not in adults (Thiebaut de Schotten etaL,201ll)
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