Limbic system structure

The limbic system is a set of brain structures located on top of the brainstem and buried under the cortex. Limbic system structures are involved in many of our emotions and motivations, particularly those that are related to survival. Such emotions include fear, anger, and emotions related to sexual behaviour. The limbic system is also involved in feelings of pleasure that are related to our survival, such as those experienced from eating and sex. 

Neuropathological studies in autistic brains have also demonstrated smaller neuronal size and increased cell packing density in the hippocampus, another limbic system structure. The morphology of hippocampal neurons in two cases of infantile autism was studied and investigators found smaller cells with reduced perikaryon area and dendritic branching in comparison with neurons of age-matched control children [63], This result is consonant with MRI results evidencing a smaller area dentata of the hippocampus in autistic children from 2 to 4 years [64]. However, the density and distribution of some 5-HT receptors in the hippocampus were not statistically different between autistic children and controls [65]. 

Limbic system A network of brain structures involved in the regulation of a number of behavioural functions. 

Note The limbic system is made up of several brain structures, including the hippocampus, amygdala, and basal forebrain (Figure 1.3b). 

Figure 1.3b This cross-section of the human brain shows additional major structures. Note that the limbic system, which controls emotions, is made up of structures including the amygdala and hippocampus at the core of the brain.

From this brief description, it is apparent that the brain is really an assembly of organs all of which are structurally and functionally interconnected. Undoubtedly one of the most important areas for the psychopharmacologist is the so-called limbic system which is concerned with emotion. This region consists of the hippocampus (concerned with memory... 

The hippocampus has innumerable afferent and efferent connections to other brain structures both within the limbic system and beyond. There are receptors for many different chemical signals ranging from the "classical neurotransmitters such as acetylcholine to steroid hormones and neurotrophic factors. Some of these receptors are located in the synapses that form the intrinsic hippocampal circuits and others are the targets of specific projection pathways from other brain areas. A comprehensive review of all neurotransmitter interactions relevant to function is not within the scope of this chapter. There are detailed reviews of modulation of neurochemical systems on place learning in the watermaze (McNamara and Skelton, 1993) or other limbic-system dependent tasks (Izquierdo and Medina, 1995) in animals. The effects of key neurochemical, other than NMDA channel-mediated, and environmental influences are discussed below. 

NK3 is also abundantly expressed in many limbic structures (Rothman et al. 1984 Shughrue et al. 1996 Spitznagel et al. 2001), while NKB is only produced in discrete areas, including the bed nucleus of the stria terminalis, the septal nuclei, and the central gray (Merchenthaler et al. 1992). It is conceivable that NKB is not the only ligand at NK3 receptors in the limbic system. 

Acetylcholine neuromodulatory system. The neurons that synthesize acetylcholine (molecular formula in box) are located in the pontine brainstem and basal forebrain. The brainstem nuclei (called Ch.5 and 6 in Mesulam s nomenclature) project locally and forward into the thalamus, subthalamus, basal forebrain, and limbic system. The basal forebrain nuclei (Ch.1-4) project to the cerebral cortex and limbic system. Compare with figure 2.1 to identify structures shown. In this and the following three drawings, the very extensive and complex projections to the cerebral cortex are not shown. 

Dopaminergic neuromodulatory system. The neurons that synthesize dopamine (structural formula in box) are found in the midbrain, from which they project to the limbic system (the mesolimbic pathway), the cerebral cortex (the mesocortical pathway), as well as to the extrapyramidal motor system (the nigrostriatal pathway). 

All of these drugs, benzodiazepine and non-benzodiazepine alike, exert their effects only in presence of GABA, enhancing the action of GABA at its receptor. The highest concentration of these receptors is found in the neocortex, hippocampus, cerebellum, and throughout the limbic system (which is involved in producing both pleasant and unpleasant emotional responses). The presence of these receptors within the hippocampus may explain why benzodiazepines can produce amnesia. They may inactivate the neural circuits in this structure that are critical for the consolidation of memories. 

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