Материал: Kaplan USMLE-1 (2013) - Anatomy

Section IV • Neuroscience

*In the dominant hemisphere. Eighty percent of people are left-hemisphere dominant.

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Chapter 10 • Cerebral Cortex

ChapterSummary

•The external layer ofthe gray matter covering the surface ofthe cortex is characterized by numerous convolutions called gyri, separated by grooves called sulci. The cortex is divided into the frontal, parietal, occipital, and temporal lobes by several prominent sulci. Different areas ofthe cortex are concerned with sensory and motor functions. The frontal lobe contains the primary motor and premotor cortex, frontal eye field, and Broca speech area. The primary somatosensory and association cortex is found in the parietal

lobe. The temporal lobe contains the primary auditory cortex and Wernicke area. The primary visual cortex is at the posterior pole ofthe occipital lobe.

•The blood supply ofthe cortex is supplied by branches ofthe 2 internal carotid arteries and 2 vertebral arteries. On the ventral surface ofthe brain, the anterior cerebral and middle cerebral branches ofthe internal carotid arteries connect with the posterior cerebral artery, derived from the basilar arteryform the circle ofWillis. This circle ofvessels is completed by the anterior and posterior communicating arteries. The middle carotid artery mainly supplies the lateral surface ofthe frontal, parietal, and upper aspect ofthe temporal lobe. Deep branches also supply part ofthe basal ganglia and internal capsule. The anterior cerebral artery supplies the medial aspect ofthe frontal and parietal lobes. The entire occipital lobe, lower aspect oftemporal lobe, and the midbrain are supplied by the posterior cerebral artery.

•The homunculus of the motor and sensory cortex indicates that the upper limb and head are demonstrated on the lateral surface of the cortex. The pelvis and the lower limb are represented on the medial surface of the hemispheres. Therefore, the motor and sensory functions of the lower limb are supplied by the anterior cerebral artery while the motor and sensory functions ofthe upper limb and head are supplied by the middle cerebral artery.

•The primary language centers (Broca and Wernicke areas) are functionally located only in the dominant hemisphere, usually the left hemisphere. Both ofthese are supplied by the middle cerebral artery. Lesions ofthe Broca area result in motor or expressive aphasia (intact comprehension). Lesions of the Wernicke area produce receptive aphasia (lack of comprehension). Conduction aphasia results from a lesion ofthe arcuate fasciculus that connects the Broca and Wernicke areas.

•The internal capsule is a large mass ofwhite matter that conducts almost all tracts to and from the cerebral cortex. It is divided into an anterior limb, genu, and posterior limb. The anterior limb is supplied by the anterior cerebral artery, and the genu and posterior limb are supplied by the middle cerebral artery. The primary motor and sensory systems course through the posterior limb and genu.

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Limbic System 11

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Section IV • Neuroscience

Clinical Correlate

Olfactory deficits may be incomplete (hyposmia), distorted (dysosmia), or complete (anosmia). Olfactory deficits are caused bytransport problems or by damage to the primary olfactory neurons or to neurons in the olfactory pathway to the central nervous system (CNS). Head injuries that fracture the cribriform plate can tear the central processes of olfactory nerve fibers as they pass through the plate to terminate in the olfactory bulb, orthey may injure the bulb itself. Because the olfactory bulb is an outgrowth ofthe CNS covered by meninges, separation ofthe bulb from the plate may tearthe meninges, resulting in cerebrospinal fluid (CSF) leaking through the cribriform plate into the nasal cavity.

LIMBIC SYSTEM

The limbic system is involved in emotion, memory, attention, feeding, and mat­ ing behaviors. It consists of a core of cortical and diencephalic structures found on the medial aspect of the hemisphere. The limbic system modulates feelings, such as fear, anxiety, sadness, happiness, sexual pleasure, and familiarity.

The Papez Circuit

A summofarythe simplified connections of the limbic system is expressed by the Papez circuit (Figure IV-11-1). The Papez circuit oversimplifies the role ofthe lim­ bic system in modulating feelings, such as fear, anxiety, sadness, happiness, sexual pleasure, and familiarity; yet, it provides a useful starting point for understanding the system. Arbitrarily, the Papez circuit begins and ends in the hippocampus. Ax­ ons of hippocampal pyramidal cells converge to form the funbria and, finally, the fornix. The fomix projects mainly to the mammillarybodies in the hypothalamus. The mammillary bodies, in turn, project to the anterior nucleus of the thalamus by way ofthe mammillothalamic tract. The anterior nuclei project to the cingulate gyrus through the anterior limb of the internal capsule, and the cingulate gyrus communicates with the hippocampus through the cingulum and entorhinal cortex.

The amygdala functions to attach an emotional significance to a stimulus and helps imprint the emotional response in memory.

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