Материал: Kaplan USMLE-1 (2013) - Anatomy

Clinical Correlate

Lesion of the Frontal Eye Field

The frontal eye field lies in front of the motor cortex in Brodmann area 8. This cortical area is the center for contralateral horizontal gaze. A lesion here results in an inability to make voluntary eye movements toward the contralateral side. Because the activity ofthe intact frontal eye field in the opposite cortexwould also be unopposed after such a lesion, the

result is conjugate slow deviation ofthe eyes toward the side ofthe lesion.

If motor cortex is involved in the lesion, the patient may have a contralateral spastic paresis. The intact frontal

eye field in the opposite hemisphere deviates the eyes away from the paralyzed limbs.

Prefrontal cortex

The prefrontal cortex is located in front ofthe premotor area and represents about a quarter ofthe entire cerebral cortex in the human brain. This area is involved in organizing and planning the intellectual and emotional aspects ofbehavior, much as the adjacent premotor cortex is involved in planning its motor aspects.

Clinical Correlate

Lesions in the Prefrontal Area

Lesions in the prefrontal area produce what is called the frontal lobe syndrome. The patient cannot concentrate and is easily distracted; there is a general lack of initiative, foresight, and perspective. Another common aspect is apathy (i.e., severe emotional indifference). Apathy is usually associated with abulia, a slowing of intellectual faculties, slow speech, and decreased participation in social interactions. Prefrontal lesions also result in the emergence of infantile suckling or grasp reflexes that are suppressed in adults. In the suckling reflex, touching the cheek causes the head to turn toward the side ofthe stimulus as the mouth searches for a nipple to suckle. In the grasp reflex, touching the palm ofthe hand results in a reflex closing ofthe fingers, which allows an infant to grasp anything that touches the hand.

Clinical Correlate

Expressive Aphasia

Broca area is just anterior to the motor cortex region that provides upper motoneuron innervation of cranial nerve motor nuclei. This area in the left or dominant hemisphere is the center for motor speech and corresponds to

Brodmann areas 44 and 45. Damage to Broca area produces a motor, nonfluent, or expressive aphasia that reflects a difficulty in piecing together words to produce expressive speech. Patients with this lesion can understand written and spoken language but normally say almost nothing. When pressed on a question such as "What did you do today?" they might reply "Went town." The ability to write is usually also affected in a similar way (agraphia) in all aphasias, although the hand used for writing can be used normally in all other tasks. Patients are keenly aware of and frustrated by an expressive aphasia, because of their lack ofthe ability to verbalize their thoughts orally or in writing.

Broca area damage often extends posteriorly into the primary motor cortex and might be combined with a contralateral paralysis ofthe muscles ofthe lower face, resulting in a drooping of the corner ofthe mouth. Ifthe lesion is larger, the patient might have a spastic hemiparesis of the contralateral upper limb.

Clinical Correlate

Lesions, usually in the dominant hemisphere and which include areas 5 and 7 ofthe posterior parietal association areas, often result in apraxia (also seen with lesions to the premotor cortex). Apraxia is a disruption ofthe patterning and execution of learned motor movements. This deficit seems to reflect a lack of understanding how to organize the performance of a pattern of movements (i.e., what should be done first, then next, etc.). The patient may be unable, for example, to draw a simple diagram (constructional apraxia) or describe how to get from his home to his work.

Another deficit, with lesions of areas 5 and 7 is astereognosia (inability to recognize objects by touch). There is no loss oftactile or proprioceptive

sensation; rather, it is the integration ofvisual and somatosensory information that is impaired. Both apraxia and astereognosia are more common after left hemisphere damage than in right hemisphere damage. The astereognosia is usually confined to the contralateral side ofthe body; in contrast, apraxia is usually bilateral. Apraxia is probably a result ofthe loss of input to the premotor cortex (area 6), which is involved in the actual organization of motor movements into a goal-directed pattern.

Wernicke area

The inferior part of the parietal lobe and adjacent part of the temporal lobe in the dominant (left) hemisphere, known as Wernicke area, are cortical regions that function in language comprehension. At a minimum, Wernicke area consists of area 22 in the temporal lobe but may also include areas 39 and 40 in the parietal lobe. Areas 39 (the angulargyrus) and 40 (the supramarginal gyrus) are regions of convergence ofvisual, auditory, and somatosensory information.

Note

Any blockage of the left middle cerebral artery that results in an aphasia (Broca Wernicke, conduction) or Gerstmann syndrome will also result in agraphia.