Kaplan + Sadock's Synopsis of Psychiatry, 11e

10

Chapter 1: Neural Sciences

The skillful use of the hands is called praxis, and deficits in skilled movements are termed apraxias. The three levels of apraxia are limb- kinetic, ideomotor, and ideational. Limb-kinetic apraxia is the inability to use the contralateral hand in the presence of preserved strength; it results from isolated lesions in the supplementary motor area, which contains neurons that stimulate functional sequences of neurons in the motor strip. Ideomotor apraxia is the inability to perform an isolated motor act on command, despite preserved comprehension, strength, and sponta- neous performance of the same act. Ideomotor apraxia simultaneously affects both limbs and involves functions so specialized that they are localized to only one hemisphere. Conditions in two separate areas can produce this apraxia. Disconnection of the language comprehension area, Wernicke’s area, from the motor regions causes an inability to fol- low spoken commands, and lesions to the left premotor area may impair the actual motor program as it is generated by the higher-order motor neurons. This program is transmitted across the corpus callosum to the right premotor area, which directs the movements of the left hand. A lesion in this callosal projection can also cause an isolated ideomotor apraxia in the left hand. This syndrome implies the representation of specific motor acts within discrete sections of the left premotor cortex. Thus just as some cells respond selectively to specific environmental features in the higher sensory cortices, some cells in the premotor cortex direct specific complex motor tasks. Ideational apraxia occurs when the individual components of a sequence of skilled acts can be performed in isolation, but the entire series cannot be organized and executed as a whole. For example, the sequence of opening an envelope, removing the letter, unfolding it, and placing it on the table cannot be performed in order, even though the individual acts can be performed in isolation. The representation of the concept of a motor sequence may involve several areas, specifically the left parietal cortex, but it likely also relies on the sequencing and executive functions of the prefrontal cortex. This apraxia is a typical finding of diffuse cortical degeneration, such as Alzheimer’s disease. Autonomic Motor System The autonomic system is divided into a sensory component (described earlier) and a motor component. The autonomic motor system is divided into two branches: the sympathetic and the parasympathetic. As a rule, organs are innervated by both types of fibers, which often serve antagonistic roles. The para- sympathetic system slows the heart rate and begins the process of digestion. In contrast, the sympathetic system mediates the fight or flight response, with increased heart rate, shunting of blood away from the viscera, and increased respiration. The sympathetic system is highly activated by sympathomimetic drugs, such as amphetamine and cocaine, and may also be activated by withdrawal from sedating drugs such as alcohol, benzodiazepines, and opioids. Investigators who have found an increased risk of heart attacks in persons with high levels of hostility have suggested that chronic activation of the sympa- thetic fight or flight response, with elevated secretion of adrena- line, may underlie this association. The brain center that drives the autonomic motor system is the hypo- thalamus, which houses a set of paired nuclei that appear to control appetite, rage, temperature, blood pressure, perspiration, and sexual drive. For example, lesions to the ventromedial nucleus, the satiety cen- ter, produce a voracious appetite and rage. In contrast, lesions to the upper region of the lateral nucleus, the hunger center, produce a pro- found loss of appetite. Numerous research groups are making intense efforts to define the biochemical regulation of appetite and obesity and frequently target the role of the hypothalamus.

In the regulation of sexual attraction, the role of the hypothala- mus has also become an area of active research. In the 1990s, three groups independently reported neuroanatomical differences between certain of the hypothalamic nuclei of heterosexual and homosexual men. Researchers interpreted this finding to suggest that human sexual orientation has a neuroanatomical basis, and this result has stimulated several follow-up studies of the biological basis of sexual orientation. At present, however, these controversial findings are not accepted without question, and no clear consensus has emerged about whether the struc- ture of the hypothalamus consistently correlates with sexual orientation. In animal studies, early nurturing and sexual experiences consistently alter the size of specific hypothalamic nuclei. Primitive Reflex Circuit Sensory pathways function as extractors of specific features from the overwhelming multitude of environmental stimuli, whereas motor pathways carry out the wishes of the organ- ism. These pathways may be linked directly, for example, in the spinal cord, where a primitive reflex arc may mediate the brisk withdrawal of a limb from a painful stimulus, without immedi- ate conscious awareness. In this loop, the peripheral stimulus activates the sensory nerve, the sensory neuron synapses on and directly activates the motor neuron, and the motor neuron drives the muscle to contract. This response is strictly local and all-or-none. Such primitive reflex arcs, however, rarely gener- ate an organism’s behaviors. In most behaviors, sensory systems project to association areas, where sensory information is inter- preted in terms of internally determined memories, motivations, and drives. The exhibited behavior results from a plan of action determined by the association components and carried out by the motor systems. Localization of Brain Functions Many theorists have subdivided the brain into functional sys- tems. Brodmann defined 47 areas on the basis of cytoarchitec- tonic distinctions, a cataloging that has been remarkably durable as the functional anatomy of the brain has been elucidated. A separate function, based on data from lesion studies and from functional neuroimaging, has been assigned to nearly all Brodmann’s areas. At the other extreme, certain experts have dis- tinguished only three processing blocks: The brainstem and the thalamic reticular activating system provide arousal and set up attention; the posterior cortex integrates perceptions and gener- ates language; and, at the highest level, the frontal cortex gener- ates programs and executes plans like an orchestra conductor. Hemispheric lateralization of function is a key feature of higher cortical processing. The primary sensory cortices for touch, vision, hearing, smell, and taste are represented bilat- erally, and the first level of abstraction for these modalities is also usually represented bilaterally. The highest levels of feature extraction, however, are generally unified in one brain hemi- sphere only. For example, recognition of familiar and unfamiliar faces seems localized to the left inferior temporal cortex, and cortical processing of olfaction occurs in the right frontal lobe. Hypotheses about the flow of thought in the brain are based on few experimental data, although this scarcity of findings has not impeded numerous theoreticians from speculating about functional neuroanat- omy. Several roles have been tentatively assigned to specific lobes of the brain, on the basis of the functional deficits resulting from localized