Kaplan + Sadock's Synopsis of Psychiatry, 11e

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Chapter 1: Neural Sciences

Limbic System Function The limbic system was delineated by James Papez in 1937. The Papez circuit consists of the hippocampus, the fornix, the mam- millary bodies, the anterior nucleus of the thalamus, and the cin- gulate gyrus (Fig. 1.2-4). The boundaries of the limbic system were subsequently expanded to include the amygdala, septum, basal forebrain, nucleus accumbens, and orbitofrontal cortex. Although this schema creates an anatomical loop for emo- tional processing, the specific contributions of the individual components other than the hippocampus or even whether a given train of neural impulses actually travels along the entire pathway is unknown. The amygdala appears to be a critically important gate through which internal and external stimuli are integrated. Information from the primary senses is interwoven with inter- nal drives, such as hunger and thirst, to assign emotional sig- nificance to sensory experiences. The amygdala may mediate learned fear responses, such as anxiety and panic, and may direct the expression of certain emotions by producing a par- ticular affect. Neuroanatomical data suggest that the amygdala exerts a more powerful influence on the cortex, to stimulate or suppress cortical activity, than the cortex exerts on the amyg- dala. Pathways from the sensory thalamic relay stations sepa- rately send sensory data to the amygdala and the cortex, but the subsequent effect of the amygdala on the cortex is the more potent of the two reciprocal connections. In contrast, damage to the amygdala has been reported to ablate the ability to distin- guish fear and anger in other persons’ voices and facial expres- sions. Persons with such injuries may have a preserved ability to recognize happiness, sadness, or disgust. The limbic system appears to house the emotional association areas, which direct the hypothalamus to express the motor and endocrine compo- nents of the emotional state.

in psychiatry because patients with temporal lobe seizures often mani- fest bizarre behavior without the classic grand mal shaking movements caused by seizures in the motor cortex. A proposed TLE personality is characterized by hyposexuality, emotional intensity, and a perseverative approach to interactions, termed viscosity. Patients with left TLE may generate references to personal destiny and philosophical themes and display a humorless approach to life. In contrast, patients with right TLE may display excessive emotionality, ranging from elation to sadness. Although patients with TLE may display excessive aggression between seizures, the seizure itself may evoke fear. The inverse of a TLE personality appears in persons with bilat- eral injury to the temporal lobes after head trauma, cardiac arrest, herpes simplex encephalitis, or Pick’s disease. This lesion resem- bles the one described in the Klüver-Bucy syndrome, an experimen- tal model of temporal lobe ablation in monkeys. Behavior in this syndrome is characterized by hypersexuality, placidity, a tendency to explore the environment with the mouth, inability to recognize the emotional significance of visual stimuli, and constantly shifting attention, called hypermetamorphosis. In contrast to the aggres- sion–fear spectrum sometimes seen in patients with TLE, complete experimental ablation of the temporal lobes appears to produce a uniform, bland reaction to the environment, possibly because of an inability to access memories. The prefrontal cortices influence mood in a complementary way. Whereas activation of the left prefrontal cortex appears to lift the mood, activation of the right prefrontal cortex causes depression. A lesion to the left prefrontal area, at either the cortical or the subcortical level, abolishes the normal mood-elevating influences and produces depres- sion and uncontrollable crying. In contrast, a comparable lesion to the right prefrontal area may produce laughter, euphoria, and witzelsucht, a tendency to joke and make puns. Effects opposite to those caused by lesions appear during seizures, in which occurs abnormal, excessive activation of either prefrontal cortex. A seizure focus within the left prefrontal cortex can cause gelastic seizures, for example, in which the ictal event is laughter. Functional neuroimaging has documented left prefrontal hypoperfusion during depressive states, which normalized after the depression was treated successfully.

Figure 1.2-4 Schematic drawing of the major anatomic structures of the limbic system. The cingulate and parahippocampal gyri form the “limbic lobe,” a rim of tissue located along the junction of the diencephalon and the cerebral hemispheres. (Adapted from Hendelman WJ. Student’s Atlas of Neuroanatomy . Philadelphia: WB Saunders; 1994:179.)