2 cns protection

8/14/2019 2 CNS Protection

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The Nervous System

SMS 1084

Dr. Mohanad R. Alwan

IMS-MSU.MY


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Protection & Nutrition of CNS


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Protection

Bone

Cranial bones & vertebral arches

MeningesDura mater

Arachnoid mater

Pia mater

Cerebrospinal fluid (CSF)


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Definition

Meninges = membranes covering the brain

and spinal cordThey are

1. Duramater

2. Arachnoid mater3. Pia mater

Meninges


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Meninges

Dura mater Arachnoid mater

Pia mater


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Duramater

Two layers of dense fibrous tissueIn the skull outer layer takes the place of periosteum

on the inner surface of the skull

Inner layer covers the brain

Forms a partition between the two cerebralhemispheres called the falx cerebri

Between the cerebellar hemispheres falx cerebelli

Between the cerebrum and cerebellum tentoriumcerebelli


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Spinal Dura mater

Foms a loose sheath around the spinal cord

Extends from the foramen magnum to the S2vertebra Then it invests the filum terminale

Fuses with the periosteum of the coccyx

It is an extension of the cerebral dura It is separated from the periosteum of the vertebraeand ligaments within the neural canal by the epiduralspace.

Applied anatomy : epidural space epiduralanaesthesia

Subarachnoid space of the spinal canal spinalanaesthesia


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Arachnoid mater

Middle layer of the three membranes covering the

brain and spinal cord

Separated from the dura mater by the subdural space

and from the pia mater by the subarachnoid space Subarachnoid space contains the cerebrospinal fluid

Covers the spinal cord also and ends by merging with

the dura mater at the level of S2


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Pia mater

Inner most layer of the meninges

Fine connective tissue

Contains minute blood vessels

Closely invests the brain

Completely covers the convolutions Dips into fissures

Invests the spinal cord

Beyond the end of the spinal cord continues as thefilum terminale

Pierces arachnoid mater and with dura mater fuseswith periosteum of the coccyx


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Meninges - Arachnoid and Pia

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Cerebrospinal Fluid

Slide 7.46

Similar to blood plasma

composition Formed by the choroid

plexus

Forms a watery

cushion to protect thebrain

Circulated in arachnoidspace, ventricles, and

central canal of thespinal cord


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Production: floor of the lateral ventricle

and third ventricle, by choroid plexus.

Circulation: L.V. -> III.V. -> IV.V. -> exitventricular system into various basalcisterns and then to subarachroid space

Circulates in through ventricles, canals, &between meninges

Drain back to blood via arachnoidgranulation to superior sagittal sinus, or via

spinal nerve roots, or via olfactory tracts

From Johanson CE


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CSF volume in human

Total: 164.5 47.8 ml

Ventricles: 31.9 17.8 ml

IIIrd ventricle: 0.95 0.62 ml Extraventricular: 132.6 43.2 ml

Total cranium: 1051.7 86.9 ml


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Function of CSF

Maintenance of a constant external

environment for neurons and glia

Mechanical cushion to protect the brainand buoyant to the heavy brain (1400 g)

Serve as a lymphatic system and a conduit

for neuropeptides pH of CSF regulates pulmonary ventilation

and CBF


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Supports (buoys) mass Cushions CNS - like

waterbed

Nourishes brain tissue Contains proper

electrolytes for CNS


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Filtered from blood

plasma in choroid

plexuses

Circulates in throughventricles, canals, &

between meninges

Returned to blood at

superior sagittal sinus


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Circulation of cerebrospinal fluid

CSF drains from lateral ventricle interventricular foramina third ventricle

mesencephalic aqueductfourth ventricle

median and two lateral apertures

subarachnoid space arachnoid granulations superior sagittal sinus vein


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Ventricular System

Within the brain is a communicating system of

cavities that are lined with ependyma cells and filled

with cerebrospinal fluid (CSF)

There are: two lateral ventricles,

the third ventricle,

the cerebral aqueduct,

and the fourth ventricle within the brain stem.


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Ventricles of the Brain

2 Lateral ventricles (1, 2) in cerebral hemispheres Third ventricle (3) between hemispheres

Fourth ventricle (4) in midbrain and medulla


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Ventricles and Location of the CerebrospinalFluid

Slide

Figure 7.17a


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What is the Blood Brain Barrier?

Structural and functional barrier whichimpedes and regulates the influx of

most compounds from blood to brain.

Formed by brain microvascularendothelial cells (BMEC), astrocyte endfeet and pericytes

Essential for normal function of CNS.

Regulates passage of molecules in andout of brain to maintain neuralenvironment.

Responsible for metabolic activitiessuch as the metabolism of L-dopa toregulate its concentration in the brain.


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Blood-brain barrier CNS is sensitive to chemicals

Capillaries have thicker, non-leaky walls.

Protects brain from drugs, metabolites, toxins


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Functions and Properties of the BBB

The BBB has several important functions:

1. Protects the brain from "foreign substances" in the blood that may injure the brain.

2. Protects the brain from hormones and neurotransmitters in the rest of the body.

3. Maintains a constant environment for the brain.


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Functions and Properties of the BBB

General Properties of the BBB

1. Large molecules do not pass through the BBB easily.

2. Low lipid (fat) soluble molecules do not penetrate into the brain.

However, lipid soluble molecules rapidly cross the BBB into the

brain.3. Molecules that have a high electrical charge to them are slowed.

Therefore:

The BBB is selectively permeable to :Oxygen, Carbon dioxide and

glucose The BBB is not permeable to

hydrogen ions


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Structure of Blood Brain Barrier


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Transport of substances across the BBB

Ions

Amino Acids and organic acids

Glucose and other carbohydrates

Biogenic amines

Nucleotide precursors

Peptides, proteins and lipoproteins

Steroid and thyroid hormones Vitamins, trace metals

Chemotherapy agents, antibiotics


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Transport at the BBB

There are five basic mechanisms by which solute

molecules move across membranes:

1. simple diffusion

2. facilitated diffusion3. simple diffusion through an aqueous channel

4. active transport through a protein carrier

5. Endocytosis


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Blood supply & Nutrition

Brain is highly vascular

blood delivers O2and glucose supply

Can't be deprived of O2. No capacity foranaerobic respiration.

Brain cells die w/in 5-6 minutes when deprived

Requires glucose (or ketone bodies) as fuelCan't use fats, amino acids.


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