Cerebral Anatomy and Physiology Part Iids08

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NEUROLOGICAL SYSTEM

PART II

CEREBRAL ANATOMY AND PHYSIOLOGY

DENNIS STEVENS CRNA, MSN, ARNP

SEPTEMBER 2008

FLORIDA INTERNATIONAL UNIVERSITY

ADVANCED BIOSCIENCE IN ANESTHESIOLOGY II

NGR 6145


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ANATOMY AND PHYSIOLOGY

OBJECTIVES

Identify gross anatomical features of the brain.

Discuss functions associated with membranes of the

cranial meninges. Describe significant aspects of arterial cerebral

vascularization.

Explain cerebral perfusion pressure and autoregulation

associated with cerebral blood flow. List divisions of the brain and specific functions related to

each division.

Discuss significant differences between gray and whitematter.


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INTRODUCTION

Brain weighs ~ 1300 Gms

Divided into four principle parts: Brain stem

Diencephalon

Cerebrum

Cerebellum Brain is protected by cranial

bones, cranial meninges, andCSF


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INTRODUCTION

Brain stem consists of the medulla oblongata, pons, and

midbrain. Lower end is a continuation of the spinal cord. Diencephalon consists primarily of the thalamus and

hypothalamus

Cerebrum spreads over the diencephalon and occupies most

of the cranium Inferior to the cerebrum and posterior to the brain stem is

the cerebellum

Cranial meninges surround the brain and are continuouswith the spinal meninges


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CRANIAL BONES

Frontal bone forms the forehead,roofs of the orbits, and most ofthe anterior portion of thecranial floor

Parietal bones form greaterportion of the sides and roof of

the cranial cavity Temporal bones form inferior

sides of cranium and part of thecranial floor


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CRANIAL BONES

Occipital bone forms posterior part and significant

portion of the base of the cranium Sphenoid bone is situated at the middle part of the

base of the skull and articulates with all other cranialbones

Ethmoid bone is principle supporting structure of thenasal cavities. Forms part of the anterior portion ofthe cranial floor, medial wall of the orbits, superiorportions of the nasal septum, and most of thesidewalls of the nasal roof


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CRANIAL BONES

Four prominent skull sutures are immovable jointsfound only between skull bones and contain verylittle connective tissue:

Coronal suture

Sagittal suture

Lambdoidal suture

Squamosal suture


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CRANIAL BONES

At birth fontanels, membrane-filled spaces found

between cranial bones, will eventually be replaced bybone:

Anterior (frontal) fontanel

Posterior (occipital) fontanel

Anterolateral (sphenoidal) fontanel

Posterolateral (mastoid) fontanel


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CRANIAL MENINGES

Three membranes envelopethe brain:

Dura (outermost layer)

Arachnoid

Pia (innermost layer)


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CRANIAL MENINGES

Dura:

Tough fibrous structure containing an inner (meningeal)layer and outer (periosteal) layer

Most of the duras venous sinuses lie between the durallayers

Dural layers are generally fused, except where theyseparate to provide space for the venous sinuses andwhere the inner layer forms septa between the brainportions

Outer layer firmly attached to inner surface of cranial

bones; inner layer continuous with spinal dura


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CRANIAL MENINGES

Arachnoid:

Delicate avascular membrane covers the subarachnoidspace

Between the arachnoid and dura mater lies thesubdural space

Arachnoid granulations project into the superior

sagittal sinus Subarachnoid space between the arachnoid and the

pia is relatively narrow over the surface of the cerebralhemisphere and is much wider at areas at the base ofthe brain


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CRANIAL MENINGES

Pia:

Thin connective tissue membrane that covers thebrain surface and extends into sulci and fissuresand around blood vessels throughout the brain

Invaginations of the pia form choroid plexuses ofthe ventricles

Clinical considerations: Various types of lesions, malformations, or

pathology may present in one or more intracranialcompartments


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CEREBRAL VASCULARIZATION

~ 18% of total blood volume circulates in the brain

Brain is responsible for 20% of total body oxygenconsumption

Constant flow of oxygen must be maintained:

Loss of consciousness occurs in less than 15 seconds

Irreparable damage occurs within 5 minutes

Cerebrovascular disease occurs as a result of vascularcompromise or hemorrhage in the central nervoussystem


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ARTERIAL SUPPLY OF THE BRAIN

Extra cerebral vessels; R carotid artery

arises from R subclavian, L carotid arteryarises from aortic arch

Intracranial cerebral vessels; internalcarotid artery divides into anteriorcerebral and middle cerebral arteries

Two vertebral arteries (arising from thesubclavian arteries) join to form thebasilar artery which gives rise to theposterior cerebral artery; supplyingoccipital lobes and brain stem


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ANATOMY AND PHYSIOLOGYARTERIAL SUPPLY OF THE BRAIN

Circle of Willis is a confluence of vesselsthat gives rise to all major cerebral

arteries It is fed by the paired internal carotid

arteries and the basilar artery

When the circle is complete, it contains aposterior communicating artery on each

side and an anterior communicatingartery

Each major artery supplies a certainterritory

Sudden occlusion affects its territoryimmediately, sometimes irreversibly


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REGULATION OF CEREBRAL BLOOD FLOW

Cerebral perfusion pressure is the difference between

mean arterial pressure and intracranial pressure CPP = MAP ICP

CPP is normally 80-100 mm Hg

CPP values less than 50 mm Hg often show slowing onEEG

CPP values between 25-40 mm Hg typically flat EEG Sustained CPP less than 25 mm Hg results in

irreversible damage


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REGULATION OF CEREBRAL BLOOD FLOW

Autoregulation:

CBF remains nearly constant between MAP of 60-160mm Hg

Pressures greater than 150-160 mm Hg can disrupt theblood brain barrier

Extrinsic mechanisms influencing cerebral blood flow:

Respiratory gas tensions

Temperature

Viscosity

Autonomic influences


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VENOUS DRAINAGE

Venous drainage of the brain and

coverings includes veins of thebrain itself, dural venous sinuses,duras meningeal veins, anddiploic veins

Eventual cerebral venousdrainage is the internal jugularvein

Cerebral veins contain no valves


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DIVISIONS OF THE BRAIN

Develop from embryonic brain

vesicles that form from thecranial end of the neural tube

Consists of:

Brain stem

Diencephalon Cerebrum

Cerebellum


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ANATOMY AND PHYSIOLOGYBRAIN STEM

Medulla:

Relays motor and sensory impulses between other parts

of the brain and spinal cord (some tracts decussate)

Reticular formation functions in consciousness andarousal

Contains vital reflex centers (heartbeat, breathing, and

blood vessel diameter) Nonvital reflex centers coordinate swallowing, vomiting,

coughing, sneezing, and hiccupping

Contains nuclei of origin for CNs VIII, IX, X, XI, and XII

Vestibular nuclear complex helps maintain equilibrium


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ANATOMY AND PHYSIOLOGYBRAIN STEM

Pons:

Relays impulses within the brain and between parts of

the brain and the spinal cord

Contains nuclei of origin for CNs V, VI, VII, and VIII

Pneumotaxic and apneustic areas help regulate breathing

Midbrain:

Relays motor impulses from cerebral cortex to pons andspinal cord and relays sensory impulses from spinal cordto thalamus

Coordinates movement of eyeballs and head and trunk

Contains nuclei of origin for CNs III and IV


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ANATOMY AND PHYSIOLOGYDIENCEPHALON

Thalamus:

Serves as relay station for all sensory impulses, except

smell, to cerebral cortex Relays motor impulses from cerebral cortex to spinal cord

Interprets pain, temperature, light touch, and pressuresensations

Functions in emotions and memory

Hypothalamus:

Controls and integrates the ANS, articulates with thepituitary gland, center for mind-over-body phenomena,rage and aggression, controls normal body temperature,

food intake and thirst, maintains waking state and sleep


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CEREBRUM

Functional areas of cerebral cortex divided intosensory, motor, and association areas

Sensory areas interpret sensory impulses, motorareas control muscular movement, and associationareas function in emotional and intellectual processes

Basal ganglia control gross muscle movements andregulate muscle tone

Limbic system functions in emotional aspects ofbehavior related to survival

Language; contained in the left hemisphere in 90%of the population, located in frontal (Brocas area),

parietal, and temporal lobes


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CEREBELLUM

Second-largest portion of the brain, occupies inferior

and posterior aspects of the cranial cavity Separated from the cerebrum by the transverse

fissure and the tentorium cerebelli

Controls subconscious skeletal muscle contractions

required for coordination, posture, and balance Assumes a role in emotional development,

modulating sensations of anger and pleasure


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REFERENCES

Morgan, G.E., Mikhail, M.S., and Murray, M.J. (2006).

Clinical Anesthesiology. (4th Ed.) New York, NY:McGraw-Hill.

Nagelhout, J.J. and Zaglaniczny, K.L. (2005). Nurse

Anesthesia. (3rd Ed.) St. Louis, MO: Elsevier-

Saunders.

Waxman, S.G. (2000). Correlative Neuroanatomy (24th

ed.). New York, NY:McGraw-Hill.

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Cerebral Anatomy and Physiology Part Iids08