8 CHAPTER VIIICEREBRAL CORTEX. AFFECTION AND IRRITATIVE SIGNS OF CEREBRAL CORTEX. CEREBROSPINAL FLUID. MENINGEAL SYNDROME.
Figure8.1Functional and Structural Areas of the Cerebral Cortex (L. Hendon, University of Alabama, 2005)
The cerebral cortex, the thin, gray covering of both hemispheres of the brain consists of two types: the neocortex (90%) and the allocortex (10%).
The neocortex consists in six layers.
I. Lamina zonalis, or the molecularlayer or plexiform layer, is the most superficial.
II. Lamina granularis externa, or the external granular layer of small pyramidal cells, consists of many small, closely packed pyramidal cells.
III. Lamina pyramidalis, or the external pyramidal layer. It gives rise to association and commissural fibers and is the major source of corticocortical fibers.
IV. Lamina granularis interna, or the internal granular layer is characterized by the presence of a large number of small multipolar cells with short axons, among which are some
small pyramidal cells.
V. Lamina ganglionaris, or the internal pyramidal layer of medium-sized and large pyramidal cells, contains the largest cells of the cortex. In the motor region these are known as the giant pyramidal cells of Betz, which gives rise to corticobulbar and corticospinal fibers.
VI. Lamina multiformis or the layer of polymorphic cells. It gives rise to projection, commissural, and association fibers.
Fissures – deep grooves – separate major regions of the brain. Transverse fissure – separates cerebrum and cerebellum. Longitudinal fissure – separates cerebral hemispheres. Sulci – grooves on the surface of the cerebral hemispheres. Gyri – twisted ridges between sulci. Prominent gyri and sulci are similar in all people. Deeper sulci divide cerebrum into lobes. Lobes are named for the skull bones overlying them.
The two hemispheres are connected by commissural fibers, which enable bihemispheric coordination of function. The most important commissural tract is the corpus callosum; because many tasks are performed primarily by one of the two hemispheres (cerebral dominance), interruption of the corpus callosum can produce various disconnection syndromes. Total callosal transection causes splitbrain syndrome, in which the patient cannot name an object felt by the left hand when the eyes are closed, or one seen in the left visual hemifield (tactile and optic anomia), and cannot read words projected into the left visual hemifield (left hemialexia), write with the left hand (left hemiagraphia), or make pantomimic movements with the left hand (left hemiapraxia). Anterior callosal lesions cause alien hand syndrome (diagonistic apraxia), in which the patient cannot coordinate the movements of the two hands. Disconnection syndromes are usually not seen in persons with congenital absence (agenesis) of the corpus callosum.
The meninges lie immediately deep to the inner surface of the skull and constitute the membranous covering of the brain. The pericranium of the inner surface of the skull and the dura mater are collectively termed the pachymeninges, while the pia mater and arachnoid membrane are the leptomeninges. Between the dura mater and the arachnoid is the (normally only virtual) subdural space; between the arachnoid and the pia mater is the subarachnoid space. The subarachnoid space contains the cerebrospinal fluid (CSF). The functions of CSF are physical (compensation for volume changes, buffering and equal distribution of intracranial pressure despite variation in venous andarterial blood pressure) and metabolic (transport of nutrients and hormones into the brain). The cerebrospinal fluid is formed in the choroid plexuses of the four cere- bral ventricles (right and left lateral ventricles, third ventricle, and fourth ventricle). It flows through the ventricular system, then enters the subarachnoid space surrounding the brain and spinal cord (external CSF space). It is resorbed in the arachnoid granulations of the superior sagittal sinus and in the perineural sheaths of the spinal cord.
Meningeal syndrome include in neck stiffness (passive flexion of the neck is restricted and painful), Kernig sign (resistance to passive extension of the knee while the hip is flexed), attempts at neck flexion, pressing the pubis may induce flexion of the hip or knee Brudzinski signs (upper, middle). Passive extension of the knee may induce flexion of the opposite hip or knee. and general cerebral signs (headache, vomiting, nausea, general hyperesthesia, seizures and changes of consciousness).
What is meningism? It’s a condition in which the symptoms simulate meningitis, but in which no actual inflammation of these membranes is present.
The functional organization of the cerebral cortex can be studied with various techniques: direct electrical stimulation of the cortex during neurosurgical procedures, measurement of cortical electrical cortical activity (electroencephalography and evoked potentials), and measurement of regional cerebral blood flow and metabolic activity. Highly specialized areas for particular functions are found in many different parts of the brain. A lesion in one such area may produce a severe functional deficit, though partial or total recovery often occurs because adjacent uninjured areas may take over some of the function of the lost brain tissue. (The extent to which actual brain regeneration may aid functional recovery is currently unclear.) The specific anatomic patterns of functional localization in the brain are the key to understanding much of clinical neurology.
