1 CEREBROSPINAL FLUID (CSF) AND THE BLOOD-BRAIN BARRIER (BBB)

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CEREBROSPINAL FLUID (CSF) AND THE BLOOD-BRAIN

BARRIER (BBB)

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Student Preparation

Textbook of Medical Physiology, Guyton and Hall, Ch. 61

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Cerebrospinal Fluid

• Cerebrospinal fluid (CSF) is a clear fluid present in the ventricles of the brain, the central canal of the spinal cord, and the subarachnoid space.

• Normal: almost no blood cells, little protein.

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CSF Cont.

• Its rate of formation, flow, and absorption is sufficiently high to cause its replacement several times daily. Measuring its pressure, performing cell counts, and analyzing levels of various biochemical constituents is accomplished with a spinal tap.

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CSF Cont.

• Injecting radiopaque dyes into the subarachnoid space is the basis of a common neuroradiologic technique called a myelogram. Obstruction of flow of CSF is a common cause of hydrocephalus.

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CSF Cont.

• An understanding of the formation, flow, and absorption of CSF is essential to an understanding of these diagnostic procedures and the pathophysiology of hydrocephalus.

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Cerebrospinal Fluid• A clear, colorless fluid that surrounds and

permeates the CNS. Offers support, protection and nourishment.

• Functions:– Protection of cranial contents

– Modulates pressure changes (same specific gravity as brain)

– Serves as a chemical buffer to maintain constant ionic environment

– Serves as a transport medium for nutrients and metabolites, endocrine substances and even neurotransmitters

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Location of CSF

• Two lateral ventricles• Third ventricle• Fourth ventricle• Spinal cord central

canal• Subarachnoid space• Continuous with

extracellular fluid of brain parenchyma

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Formation of CSF

• Choroid plexuses of lateral, third and fourth ventricles

• Ependymal lining of ventricular system

• Pia-glial membrane• Blood vessels

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Formation of CSF

• Cells are believed to actively secrete Na+ into the ventricular system in exchange for K+. Sodium ions electrically attract Cl- and osmotically draw water from the blood vascular system to constitute the CSF.

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Composition of CSF

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Circulation of CSF

• Due to pulsation of blood in choroid plexus• Due to pulsation of ependymal cells• Lateral ventricles interventricular

foramen of Monroe third ventricle

mesencephalic aqueduct (aqueduct of Sylvius) fourth ventricle spinal cord central canal; also, out the lateral apertures to the subarachnoid space to the venous system

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Absorption of CSF

1. Through the arachnoid villi, a protrusion of arachnoid membrane into the central venous sinus and other sinuses

• A valve opens when CSF pressure exceeds venous pressure

2. Absorption by veins and capillaries of CNS

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CSF Pressure

• Dog - 170 mm H2O• Cow - 200 mm H2O• Horse - 400 mm H2O

• Subarachnoid hemorrhage blocks absorption, raises pressure

• Congenital excessive formation or blockage to flow raises pressure

• Regulated by rate of absorption and rate of formation.

• Brain tumors lower absorption & raise pressure

• Inflamed meninges can cause excessive formation & raise pressure

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Jugular compression maneuver (Queckenstedt maneuver)

• Compress jugular veins in neck - increases CSF pressure at the cerebellomedullary cistern and the lumbar cistern (normal)

• Decreased pressure at lumbar cistern indicates blockage of spinal central canal

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Cerebrospinal fluid collection

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Cerebrospinal fluid collection

• Small animals:atlanto-occipital(cerebellomedullary) cistern

neck

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Cerebrospinal fluid collection

• Large animals– cerebellomedullary

cistern

– Lumbar cistern (L5-L6)

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CSF collection

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Dog Lumbar Myelogram

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Hydrocephalus(water on the brain)

• An abnormal increase in the volume of CSF

• Symptoms: depression, sleep, changes in behavior, spastic paresis, papilledema, bulging of skull in young, behavioral change, pacing, head pressing, seizures, cranial nerve deficits, vestibular signs

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Hydrocephalus

• Communicating –due to excessive formation of CSF or lack of absorption

• Non-communicating – due to obstruction of flow through ventricle system

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Ventriculoperitoneal Shunt

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Coup – Coup-ContrecoupClosed Head Injuries

Coup Coup-contrecoup

With a blow to theback of thehead, the forehead strikes the surface(coup), then thebrain bouncesback againstrear skull (contrecoup)

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Blood-Brain Barrier (BBB)

• Direct injection of large molecules (e.g., a dye) into the venous system will often penetrate all tissues except the brain. This observation led to the notion of a physiological barrier to flow of substances from blood to brain tissue – the blood brain barrier.

• The BBB functions to preserve a stable environment for neurons and glia of the CNS

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Differences between brain and general capillaries

• General capillary– small solutes can

diffuse through intercellular clefts

– pinocytosis independent of molecular size (pass large molecules)

– fenestrated

• Brain capillary– tight junctions– reduced

pinocytosis– no fenestra– astrocyte foot

processes

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General vs. Brain Capillaries

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Brain capillary

• Tight junctions• Surrounded by

astrocytes• Few vesicles• Carrier mediated

transport of glucose and amino acids

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Blood-Brain Barrier• Lipid soluble molecules will cross• Negatively charged molecules cross more

freely than positively charged ones

• High CO2/low O2 produce vasodilation and decrease resistance of BBB

• Injury or inflammation decreases resistance of BBB (allows some antibiotics to be used for treatment)

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Blood-Brain Barrier (BBB)

• Selected brain areas are not protected by the BBB – the circumventricular organs (CVO) bordering on the 3rd and 4th ventricles– parts of the hypothalamus - median eminence– neurohypophysis– pineal gland– area postrema– subfornical organ– subcommissural organ

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Blood – CSF Barrier

• Barrier from blood to the CSF

• Similar structural mechanism to the BBB