neurologic disorders
Overview of Cerebral Function
Frontal lobes
The frontal lobes are anterior to the central sulcus. They are essential for planning and executing learned and purposeful behaviors; they are also the site of many inhibitory functions. There are several functionally distinct areas in the frontal lobes: Theprimary motor cortexis the most posterior part of the precentral gyrus. The primary motor cortex on one side controls all moving parts on the contralateral side of the body (shown on a spatial map called a homunculus—see figure); 90% of motor fibers from each hemisphere cross the midline in the lower brain stem and upper cervical spinal cord. Thus, damage to the motor cortex of one hemisphere causes weakness or paralysis mainly on the contralateral side of the body. Themedial frontal cortex(sometimes called the medial prefrontal area) is important in arousal and motivation. If lesions in this area are large and extend to the most anterior part of the cortex (frontal pole), patients sometimes become abulic (apathetic, inattentive, and markedly slow to respond). Theorbital frontal cortex(sometimes called the orbital prefrontal area—see figure) helps modulate social behaviors. Patients with orbital frontal lesions can become emotionally labile, indifferent to the implications of their actions, or both. They may be alternately euphoric, facetious, vulgar, and indifferent to social nuances. Bilateral acute trauma to this area may make patients boisterously talkative, restless, and socially intrusive. The disinhibition and abnormal behaviors that can occur with aging and in many types of dementia probably result from degeneration of the frontal lobe, particularly the orbital frontal cortex. ) controls expressive language function. Lesions in this area cause expressive Aphasia (impaired expression of words). Thedorsolateral frontal cortex(sometimes called the dorsolateral prefrontal area) manipulates very recently acquired information—a function called working memory. Lesions in this area can impair the ability to retain information and process it in real time (eg, to spell words backwards or to alternate between letters and numbers sequentially). Specific parts of the cortex control specific motor and sensory functions on the contralateral side of the body. The amount of cortical space given to a body part varies; eg, the area of the cortex that controls the hand is larger than the area that controls the shoulder. The map of these parts is called the homunculus (“little person”). 