Mod4-Obj7: Meninges, Cerebrospinal fluid and the blood brain barrier Flashcards
The brain is protected by
- The cranium
- Meninges
- Cerebrospinal fluid (CSF)
- The blood-brain barrier
The meninges
Three connective tissue membranes that lie external to the central nervous system which:
- ) Cover and protect the CNS
- ) Protect blood vessels and enclose venous sinuses
- ) Contain CSF
- ) Form partitions in the skull
The meninges: membranes (superficial to deep)
- Dura mater=Top layer
- Arachnoid mater=middle layer
- Pia mater=Inner most layer
The meninges: The dura mater
Tough mother=strongest meninx
- Two layered sheet of fibrous connective tissue
- Dural periosteal (endosteal) layer: superficial, attached to the inner surface of the skull
- Dural meningeal layer: deep, covers the brain and spinal tissue
- Two layers are fused, except in certain areas where they form venous sinuses=collect and drain venous blood
- Dura mater folds form divisions within the cranium
The meninges: The arachnoid mater
Spider mother-Middle meninx
- Separated from the dura mater by the subdural space which contains blood vessels (site of subdural haemorrhage)
- Loose covering with web-like extensions that extend to the subarachnoid space
- The subarachnoid space is filled with CSF and contain large blood vessels (subarachnoid haemorrhage)
- Arachnoid villi=projections of the meninx through the dura mater into the dural venous sinus=allows CSF to drain away
The meninges: The pia mater
Gentle mother-Inner most layer
- Delicate connective tissue that clings tightly to the brain and closely follows its contours
- Contains mainly small blood vessels
The meninges sequence
- Bone (skull or vertebral column)
- Epidural space (spinal cord only)
- Dura mater (3 components)
- Subdural space (serous fluid and blood vessels)
- Arachnoid mater
- Subarachnoid space (CSF & blood)
- Pia mater
- Nervous tissue
Meningitis
Inflammation of the meninges
-Generally caused by a bacterial or viral infection (bacterial generally more serious)
Typical route of infection-Meningitis
Microbes infect the nasal mucosal surface=blood=subarachnoid blood vessels=large-scale inflammation=cerebral oedema=increased ICP
Symptoms of meningitis
Severe headache, neck stiffness, fever
Diagnosis of meningitis
CSF sample examined for WBC and microbes
Treatment of meningitis
Antibiotics, corticosteroids
Cerebrospinal fluid
Clear fluid that circulates within and around the brain and spinal cord
- Capillary filtrate derived from plasma but has very little protein and limited ca2+ and K+ ions
- Total volume=150ml, replaced every 8 hours
Functions of CSF
- ) Buoyancy-reduces brain weight by 97% and thus prevents crushing of inferior brain tissues
- ) Shock absorption (prevent physical trauma)
- ) Helps nourish the brain and removes wastes
CSF formation-ventricles
Site of CSF production
- 4 chambers within the brain
- Continuous with each other
- 4th connects to the subarachnoid space and central canal of the spinal cord
CSF formation
CSF is produced by a choroid plexus (network of capillaries) found within each ventricle
- Capillaries are surrounded by ependymal cells, which control the composition of the capillary filtrate
- Ependymal cells bear cilia, which keeps the CSF in motion
CSF
- ) CSF is produced by the choroid plexus of each ventricle
- ) CSF flows through the ventricles and into the subarachnoid space via the median and lateral apertures. Some CSF flows through the central canal of the spinal cord
- ) CSF flows through the subarachnoid space
- ) CSF is absorbed into the dural venous sinuses via the arachnoid villia
CSF exit
The CSF flows from the subarachnoid space into dural venous sinuses (e.g. superior sagittal sinus) through arachnoid villi-and drains away
Cerebrospinal fluid production and drainage
CSF is produced at a constant rate and must be drained at the same rate
Obstruction in CSF
Leads to fluid accumulation -Increase ICP Condition=HYDROCEPHALUS symptoms include: -Headache -Irritability -Nausea -vomiting -sleeplessness -coma May be caused by: -Blockage to flow e.g.) tumor, haemorrhage -Inflammation e.g.) meningitis -Birth injury
Hydrocephalus in young children
Enlargement of the head as cranial bones are not yet fused
-Minor symptoms, minimal long-term effects if treated properly
Hydrocephalus in adults
Cranial bones are fused
- Increased ICP
- Impaired neuron function and damage
- Impaired blood flow=ischaemic damage, CVA
- Midline shift and herniation (coning)
Treatment of hydrocephalus
Shunt fluid to other body cavities
The blood brain barrier
A protective mechanism that helps provide a stable environment for the brain=controls ISF composition
Formation of the blood brain barrier
Formed by astrocytes (glial cells) that encircle capillaries and stimulate the endothelial cells to form tight junctions=least permeable capillaries in the body
The blood-brain barrier: Selective barrier-water soluble substance
Must pass through endothelial cells to enter the ISF
- Nutrients (e.g. glucose, essential amino acids) and some electrolytes move into ISF by facilitated diffusion
- Metabolic wastes, proteins, certain toxins and most drugs are denied entry
- Non-essential amino acids and K+ ions are excluded and/or actively pumped out
The blood-brain barrier: Selective barrier-Lipid soluble substance
Readily pass through endothelial cells and enter the ISF
- O2, CO2, lipid hormones
- Alcohol, nicotine anaesthetics
The blood-brain barrier is incomplete in the adult in the:
- Vomiting centre of medulla oblongata
- Hypothalamus
- Monitors chemical composition of the blood e.g.) glucose levels=regulates appetite
- Monitors blood temperature
The blood-brain barrier is incomplete in the newborn brain:
Sever jaundice can interfere with normal neuronal development (kernicterus)
Blood supply to the brain
Right and left internal carotid arteries and the right and left vertebral arteries take blood to the cerebrum
Each internal carotid artery branched to form an:
-Anterior cerebral artery Supplies anterior/superior cerebrum -Middle cerebral artery Supplies lateral cerebrum -Posterior communicating artery
The vertebral arteries…
Fuse to form the basilar artery which gives rise to the R. and L. posterior cerebral arteries=supplies posterior cerebrum
The cerebral arterial circle (circle of Willis)
- Unites the anterior and posterior blood supplies
- Equalises blood pressure
- Provides alternate routes for blood to reach the cerebrum if a vessel is occluded (supplying the circle, within the circle)
- Provides a constant blood supply to the cerebrum
Venous drainage
Most venous blood and CSF drains from the brain into the dural venous sinuses
- Dural venous sinuses converge at the back of the skull to form the sigmoid sinus
- The sigmoid sinus gives rise to the internal jugular veins
Meninges summary
Cover and protect the CNS
- Dura mater-tough membrane, includes venous sinuses
- Arachnoid mater-space includes blood vessels and CSF
- Pia mater-covers neural tissue
CSF summary
Buoyancy, shock absorption, nourishment
-Produces by choroid plexus in ventricles, composition controlled by ependymal cells, circulates in subarachnoid space, exits via dural venous sinuses
Blood brain barrier summary
Controls the composition of the ISF to maintain stable environment
-Incomplete in the medulla and hypothalamus
Blood supply summary
Internal carotid and vertebral arteries supply much of the brain via the
- Cerebral arterial circle: blood reservoir that supplies the cerebrum, equalises pressure and compensates for blockages
- Venous blood exits via the dural venous sinuses
