Anatomy Cerebrospinal Fluid



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Anatomy
Cerebrospinal Fluid

  • Osm 295, Na same as plasma, Cl/Mg higher than plasma, K/Cl/uric acid/glucose lower than plasma (glucose 2/3 plasma level, usu 45-80; protein nl<45)

  • 70% made by choroid plexus, 18% from capillaries, 12% from metabolic water production

  • volume is 150 cc (25 cc in ventricles); make 450 cc/day

  • arachnoid granulations absorb if pressure is 3-6 cmH2O above venous pressure

  • normal pressure is 10-15 cmH2O; 3-6 cmH2O in children; 20-30 cmH2O when sitting

  • secretion increased by CO2 and volatile anesthetics; secretion decreased by NE and carbonic anhydrase inhibitors

  • 150 ml at any given time

  • Produce 450 ml/day (turnover CSF volume 3 times/day)

  • Active secretion requiring NA/K pump

  • Composition

    • Has similar Na+ to serum

    • Increased Cl- and Mg++

    • Decreased K+ and glucose

  • Blood Brain Barrier

    • Formed by capillary endothelial tight junctions

    • Permeable to H2O, CO2, O2, and lipid-soluble molecules

    • Not permeable to glucose and protein-bound molecules

  • Circumventricular Organs

    • Pineal – secretes melanin for circadian rhythm

    • Subforniceal organ – between intraventricular foramen; connected to choroid plexus; involved with body fluid regulation

    • Subcommisural organ – unknown function; only one with intact BBB

    • Organum vasculosum of the lamina terminalis – outflow of hypothalamic peptides; monitors peptides, amino acids, and proteins in blood

    • Median eminance of hypothalamus – site of secretion of hypothalamic releasing factors

    • Neurohypophysis (post pit)

    • Area postrema – only paired structure; in inferior fourth ventricle; involved with emesis


Meninges

  • Dura consists of two layers

  • Arachnoid

  • Pia – 2 layers

    • Intimal layer

    • Epipia – absent over convexities; filum is epipia

  • Somatic innervation

    • CN V for anterior/middle fossa

    • CN IX and X for posterior fossa



Anatomy of Cerebral Cortex

  • Area of 2.5 square feet

  • About 14 billion neurons

  • Development

    • from telencephalic vesicle

    • neurons originate in germinal zone, migrate peripherally to form cortical mantle

    • at 6 months fetal age neurons begin to form 6 layers of neopallium (neocortex)

    • paleopallium (olfactory cortex) and archipallium (hippocampal formation and dentate gyrus) consist of 3 layers

  • Cell Types

    • granular cells (stellate) – short axons, intracortical, glutamate/GABA, more common in sensory/association areas

    • fusiform cells – for output

    • pyramidal cells (like Betz) – for output to spinal cord and subcortical association fibers; large cells with large axons

  • Cell Layers – 1-3 intracortical association fibers, 4=input, 5=output to brainstem/spinal cord, 6=output to thalamus

    • I – molecular layer; cells with horizontal axons, Golgi type II cells; receive input from cells of Marinotti

    • II – external granular layer – ipsilateral cortico-cortical association fibers

    • III – external pyramidal layer – commissural fibers (to contralateral hemisphere); some ipsilateral cortico-cortical association fibers. Pyramidal cells (acetylcholine, cells of Betz)

    • IV – internal granular layer – myelinated fibers of external band of Baillanger – receive thalamocortical fibers

    • V – internal pyramidal layer – project to subcortical structures – mostly corticostriate, corticospinal, corticotectal; cells of Betz here (pyramidal)

    • VI – multiform layer – project to subcortical structures – mostly corticothalamic

  • Summary of Cell Layers

    • Layers II and III – output to different cortical areas (SI, SII, motor cortex)

    • Layers V and VI – output to subcortical areas, thalamus

    • Layer IV – receives input from thalamus

  • Cytochemistry

    • Glutamate and Aspartate – excitatory; most projection neurons

    • GABA – inhibitory; nonpyramidal cortical neurons that inhibit pyramidal

    • Peptides – include CCK, VIP (a vasodilator), and Neuropeptide Y (a vasoconstrictor)

    • Acetylcholine – include neurons from substantia inominata (in subcommisural region ventral to corpus striatum, includes basal nucleus of Meynert)

    • Norepinephrine – from locus ceruleus

    • Serotonin – from median and dorsal raphe nuclei of raphe

  • Cortical Areas (Brodmann)

    • Broca’s – area 44

    • Wernicke’s – area 22

    • Auditory (Heschl) – areas 41-42

    • Gustatory – area 43

    • Angular gyrus – areas 39-40

    • Visual (calcarine) cortex – area 17

    • Secondary visual areas – areas 18-19

    • Sensory areas – areas 3, 1, 2

  • Sensory Cortical Areas

    • Thick layer 4

    • Area 3a (most anterior) – Muscle stretch

    • Area 3 – slow and fast cutaneous

    • Area 1 – fast cutaneous

    • Area 2 (most posterior)

    • Primary Sensory Area (SI) – areas 1-3

      • Area 3a – info from deep stimuli, muscle spindle (muscle stretch, group Ia)

      • Area 3b – from skin; slow and rapid

      • Area 1 – from skin; rapid

      • Area 2 – from deep stimulation; pressure and joint position

    • Secondary Sensory Area (SII) – lies along superior bank of lateral sulcus; extends laterally into parietal lobe, has bilateral body representation

      • Afferents from thalamus (VPLc and VPM) and ipsilateral and contralateral SI

      • Efferents to SI and motor cortex

    • Primary Visual Area (striate cortex)

      • Area 17; on walls and floor of calcarine sulcus

      • Lesion of left visual cortex causes right homonymous hemianopsia

      • Retinal cells have circular receptive rields with either “on” or “off” center

      • In the cortex, receptive fields tend to be oriented in discrete columns

        • Simple cells respond best to light in one axis of orientation with basic “on” or “off” response

          • Located in layer IV

          • Receive impulses directly from lateral geniculate nucleus

          • Receive monocular input

        • Complex cells get input from multiple simple cells; respond to motion of light slits in sustained manner depending on orientation and direction of movement

          • Located in layers above or below IV

          • Receive binocular input

      • Two regions of striate cortex are monocular and do not contain ocular dominance columns:

        • (1) region representing blind spot of retina – area of optic disc (in nasal half of each retina) transmits no visual impulses to contralateral striate cortex; this region of straite cortex receives sole input from temporal half of ipsilateral retina

        • (2) region representing monocular temporal crescent –optic disc in each retina extends farther nasally than temporally, so extreme nasal part of each retina has no corresponding temporal part in the contralateral eye

    • Secondary Visual Area (area 18) and Tertiary Visual Area (area 19)

      • Input from pulvinar

    • Primary Auditory Area (areas 41-42)

      • On two transverse gyri (of Heschl) on dorsomedial surface of superior temporal convolution and in floor of lateral sulcus

      • Input from medial geniculate body

      • Unilateral cortical lesions produce partial deafness greatest contralaterally

    • Gustatory Area (area 43)

      • Parietal operculum

      • Receives information from nucleus of solitary tract

      • Projects to VPM

  • Motor Cortical Areas

    • Premotor area (area 6)

      • Contains the supplementary motor cortex and the premotor cortex

      • Supplementary motor cortex is superior/medial

      • Premotor cortex is on the lateral surface of the hemisphere

    • Architecture

      • No layer 4 (little input from thalamus)

      • Giant pyramidal cells of Betz in layer 5

    • Primary Motor Area (MI, area 4)

      • Precentral gyrus

      • Contain giant pyramidal cells of Betz (in layers III and V); otherwise cortex is agranular (minimal layer IV; most thalamocortical fibers go to layer III)

      • Corticospinal fibers – Betz cells give rise to 3% of these

        • 90% between 1-4 µm

        • 40% poorly myelinated

        • Origin

          • 31% from area 4

          • 29% from area 6

          • 40% from parietal lobe

      • Organization

        • Pharynx (swallowing functions) – most inferior opercular portion of precentral gyrus, tongue, jaw, lips, larynx, eyelid, fingers, hand, wrist, elbow, shoulder, hip, knee, ankle, toes (extending into paracentral lobule), anal sphincter

    • Premotor Area (area 6aα on lateral convexity of hemisphere)

      • Voluntary motor function dependent on sensory inputs

      • Activated in response to visual, auditory, somatosensory stimuli

      • Higher threshold for activation than MI; no giant cells of Betz

      • Lesions do not cause paresis, pathologic reflexes (e.g., grasp reflex), or hypotonia (as MI lesions)

    • Supplementary Motor Area (MII, area 6aα on medial convexity of hemisphere)

      • Programming and planning of motor activities

      • Higher threshold for activation than MI

      • Unilateral ablations produce no permanent deficit in maintenance of posture or movement

      • Reciprocal connections to areas 4, 6, 5, 7, and contralateral MII

      • 5% of neurons project to spinal cord (bilaterally)

      • Subcortical projections to caudate, putamen, VA, VLo, and MD thalamus

      • Bilateral influences on both proximal and distal musculatures

      • Lesions associated with:

        • reduction of voluntary motor activity (akinesis)

        • poverty of spontaneous speech

    • Input to motor cortical areas

      • VA and VLo (mostly VLo)

        • From GP and SNpr

        • To premotor and MII

      • VLc and VPLo

        • From deep cerebellar nuclei

        • To motor MI (minor premotor/MII)

  • Cortical Eye Fields

    • Frontal Eye Fields – caudal middle frontal gyrus (area 8)

      • Stimulation produces conjugate deviation to opposite side

    • Occipital Eye Center – subserve eye movements induced by visual stimuli, also involuntary smooth pursuit

      • Stimulation produces conjugate deviation to opposite side

      • Compared to frontal eye field: threshold for excitation higher, latency of response longer, eye movements smoother

  • Corticothalamic Projections

    • Frontal cortex to/from MD thalamus

    • Area 6 to/from VA, VLc, VPLo, PF (parafascicular) nucleus

    • Area 4 to/from CM (centromedian) nucleus






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