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This difference makes the CSF sensitive tochanges in PCO2 of the blood, an important factor in regulation of respiration by the brain. Substances within the CSF arefreely exchanged with the interstitial fluid of the brain, incontrast to substances within blood plasma. Because CSF issecreted at a rate of approximately 500 mL/day and the totalvolume of CSF is approximately 150 mL, it turns over at a rateof three to four times per day.TelencephalonTHE CENTRAL NERVOUS SYSTEMThe general structure of the central nervous system and brainis illustrated in Figure 4.1.BrainThe brain can be subdivided into the telencephalon (alsoknown as the cerebrum or cerebral hemispheres), diencephalon (thalamus, hypothalamus), cerebellum, and brainstem(midbrain, pons, and medulla).Blood-Brain BarrierThe environment of neurons within the CNS is maintained inpart by the blood-brain barrier.
Endothelial cells of capillarieswithin the CNS are joined by tight junctions, preventing themovement of water-soluble substances, highly charged molecules, and cells between blood and brain. Astrocytes (nonneuronal cells of the CNS) are also involved in maintainingthe integrity of the blood-brain barrier.In the telencephalon, the right and left cerebral hemispheresconsist of the outer cerebral cortex (gray matter) and innerwhite matter. (Gray matter contains unmyelinated axons, andwhite matter contains myelinated axons). The hemispheresare linked anatomically and functionally by bundles ofnerves known as commissures: the large corpus callosum andsmaller anterior, posterior, and hippocampal commissures(Fig.
4.4).The cerebral cortex consists of five major areas, based on grossanatomical features: the frontal, parietal, temporal, and occipital lobes and the insula (Fig. 4.5). It has afferent and efferentconnections to the thalamus and basal ganglia (two sets ofnuclei deep in the cerebral hemispheres, connecting with thecerebrum, thalamus, and brainstem) and many other regionsof the brain. Regions within the cerebral cortex receive andintegrate sensory information, integrate motor function,and perform other high-level functions such as learning andreasoning (see Fig. 4.4). Much of the sensory information isreceived indirectly, passing through the thalamus, except inCerebrospinal FluidThe formation, circulation, and regulation of the compositionof cerebrospinal fluid (CSF) compose a second importantfactor in the homeostasis of the environment within the CNS.CSF, which is secreted by ependymal (epithelial) cells of thechoroid plexus, differs from blood plasma somewhat in composition and circulates through the ventricles of the brain andthe subarachnoid space around the brain and spinal cord(Table 4.1 and Figs.
4.2 and 4.3). Specifically, the CSF has alower concentration of bicarbonate anion than plasma, result-The term neocortex refers to the outermost layer of thecerebral hemispheres. This area is the most recentlyevolved part of the cerebral cortex and is only a few millimetersthick. The neocortex is considered the highest center in thebrain, where the most complex functions ascribed to thecerebral cortex are performed, including sensory perception,initiation of motor function, language skills, and consciousthought.Precentral sulcusCingulate gyrusCentral (rolandic) sulcusCingulate sulcusLeft lateral viewAtlas (C1)Axis (C2)CervicalcurvatureCorpus callosumMedial frontal gyrusPrecuneusSulcus of corpus callosumC7T1CuneusThalamusCalcarine cortexHabenular commissureHypothalamic sulcusCalcarine sulcusParaterminal gyrusThoraciccurvatureLingual gyrusGyrus rectusCalcarine cortexAPLamina terminalisPineal glandT12Straight sinusOptic recessL1Great cerebral vein(of Galen)Optic chiasmCerebellumMammillary bodyLumbarcurvatureSuperior medullary velumPituitary gland (anterior and posterior)Midbrain4th ventricle and choroid plexusPonsMedullaoblongataInferior medullary velumL5Cerebral aqueduct(of Sylvius)Sacrum(S1–S5)SacralcurvatureCoccyxFigure 4.1 Major Parts of the Central Nervous System and the Vertebral Column The brain is illustrated as a midsagittal section,with the frontal, parietal, and occipital lobes of the cerebral cortex (outer layer of the cerebral hemispheres) labeled (the temporal lobes are not seenin this section), as well as other major divisions of the brain.
The vertebral column consists of cervical, thoracic, lumbar, and sacral vertebrae. Ithouses the spinal cord, which extends from the medulla oblongata to lumbar level L1–L2.Left lateral phantom viewRight lateral ventricleFrontal (anterior) hornCentral partTemporal (inferior) hornLeftlateralventricleOccipital (posterior) hornCerebral aqueduct (Sylvius)Fourth ventricleLeft lateral aperture(foramen of Luschka)Left lateral recessLeft interventricularforamen (Monro)Median aperture(foramen of Magendie)Third ventricleCentral canal of spinal cordFigure 4.2 Brain Ventricles and CSF Composition Circulation of CSF through the four ventricles of the brain and in the subarachnoidspace is essential for maintaining the environment of the brain and spinal cord. The composition of CSF, which is secreted by the choroid plexus,differs from that of plasma (see Table 4.1).Organization and General Functions of the Nervous Systemthe case of olfactory signals.
Generally, the right and left cerebral hemispheres receive input from the contralateral side ofthe body.The basal ganglia consist of two sets of nuclei (in the rightand left hemispheres) located deep within the cerebralhemispheres. With connections to the various lobes of thecerebral cortex, thalamus, and brainstem, the basal gangliaare involved in regulation of movement, among otherfunctions. Two additional deep formations, the hippocampusand amygdala, are located within the medial temporallobes. Both are part of the broader limbic system, involvedin emotion and long-term memory and affecting theendocrine and autonomic nervous systems (Fig.
4.6).The hippocampus has an important role in memoryand spatial navigation; the amygdala is involved inemotion. Through its connections to the hypothalamus, thehippocampus also affects sympathetic nervous systemfunction.Choroid plexus of lateralventricle (phantom)Comparison of CSF and BloodCompositionTable 4.1ParameterCSFBlood PlasmaNa+ (mEq/L)140-145135-147K+ (mEq/L)33.5-5.0Cl− (mEq/L)115-12095-1052022-2850-7570-1100.05-0.076.0-7.87.37.35-7.45HCO3− (mEq/L)Glucose (mg/dL)Protein (g/dL)pH(Reprinted with permission from Hansen J: Netter’s Atlas of HumanPhysiology, Philadelphia, Elsevier, 2002.)Superior sagittal sinusSubarachnoid spaceCistern of corpus callosumArachnoid granulationsDura materArachnoidInterventricularforamen (Monro)Choroid plexus of third ventricleCerebral aqueduct (Sylvius)Lateral aperture (foramen of Luschka)Choroid plexus of fourth ventricleDura mater51Median aperture(foramen of Magendie)ArachnoidSubarachnoid spaceCentral canal of spinal cordFigure 4.3 Circulation of CSF CSF, produced by the choroid plexus, circulates through the two lateral ventricles and third andfourth ventricles, leaving the fourth ventricle through the lateral and medial apertures and entering the subarachnoid space.
Much of thefluid is reabsorbed at the arachnoid granulations, into the venous system, as well as into the capillaries of the CNS and pia mater.52The Nervous System and MuscleMotorPremotor; orientation; eyeand head movementsMs IMs IISm ISensorySm IISensoryanalysisPrefrontal; inhibitorycontrol of behavior;higher intelligenceVisual IIIVisual IIVisual ILanguage; reading; speechMotor control of speechAuditory IAuditory IIMotorPremotorMs IMs IISm ISensorySm IIIPrefrontal; inhibitorycontrol of behavior;higher intelligenceVisual IIIVisual IIVisual ICingulate gyrus (emotionalbehavior) and cingulumOlfactoryCorpus callosumHippocampal commissureAnterior commissureFigure 4.4 Cerebral Cortex: Association Pathways and Localization of Function The two cerebral hemispheres are linked functionally and anatomically by the corpus callosum and anterior, posterior, and hippocampal commissures.
In the cerebral cortex, functional regionsinvolved in sensory and motor activity as well as integration of information from various sources have been identified. The cortex is essential forhigh-level intellectual functions including learning, reasoning, memory and language skills, and conscious thought and is involved in planning andexecution of fine motor activity.Table 4.2Major Functions of the HypothalamusHypothalamic AreaMajor Functions*Preoptic and anteriorHeat loss center: cutaneous vasodilation and sweatingPosteriorHeat conservation center: cutaneous vasoconstriction and shiveringLateralFeeding center: eating behaviorVentromedialSatiety center: inhibits eating behaviorSupraoptic (subfornical organ and organum vasculosum)ADH and oxytocin secretion (sensation of thirst)ParaventricularADH and oxytocin secretionPeriventricularReleasing hormones for the anterior pituitary*Stimulation of the center causes the responses listed.(Reprinted with permission from Hansen J: Netter’s Atlas of Human Physiology, Philadelphia, Elsevier, 2002.)Organization and General Functions of the Nervous SystemCentral sulcus (Rolando)53Postcentral gyrusPostcentral sulcusPrecentral gyrusSuperior parietal lobulePrecentral sulcusInferior parietal lobuleSupramarginal gyrusAngular gyrusParietooccipital sulcusOccipital poleFrontal poleCalcarine sulcusLateral sulcus (Sylvius)Temporal poleSuperior temporal gyrusInferior temporal gyrusMiddle temporal gyrusFrontal lobeParietal lobeTemporal lobeOccipital lobeInsula (island of Reil)Figure 4.5 Organization of the Brain: Surface Anatomy of the Cerebral Cortex The outerlayers of the cerebrum (telencephalon) are the gray matter, containing neurons with unmyelinated fibers.The cerebral cortex is only 2 to 4 mm thick, but is highly convoluted in primates, such that most of thecortex is actually buried in sulci (grooves).
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