Ventricular system, CSF & BBB Flashcards
What is the homeostasis of fluid compartments in the brain regulated by?
blood brain barrier and blood CSF barriers
How are fluids seperates in the brain?
the interstitial fluid in the brain and the CSF in the intraventricular subarachnoid spaces are seperated compartmentally
What is the ventricular system in the brain?
the cerebral ventricles are a series of interconnected cavities in the core of the forebrain and brainstem filled with CSF that is prduced by a modifiec vascular structure called the choroid plexus
What is the choroid plexus?
specialized capillary netoworks that secrete CSF
present in all ventricles
What is the blood-CSF barrier?
barrier imposed by the choroidal eptihemlium prevents the transport of materials form blood into the CSF
This is the blood-CSF barrier analagous to the blood-brain barrier
What is the structure of the choroid plexus?
the cellular constituents are blood vessels and pia, which form the core of the choroid plexus, and the choroid epithelium which is specialized to secret CSF
What is the total CSF production in a day?
~500 mL/day
entire volume of CSF (~150 mL) turns over about three times each day
What are the choroid epithelial cells?
specialized ependymal cells contiguous with the ependymal lining of the ventricles at the margins of the choroid plexus
How are choroid epithelial cells bound to eachother?
by tight gap junctions that insulates the ECF around the capillaries from the CSF
What type of capillaries are present in each choroid plexus?
high density of fenestrated, leaky choroidal capillares (outside the blood brain barrier)
What are the 2 stages of CSF formation?
- ultrafiltration of plasma occurs across the fenestrated capillary wall into the ECF beneath the basolateral membrane of the choroid epithelial cell
- choroid epithelial cells secrete fluid inot the ventricle
What drives water movement during CSF production?
a net transfer of NaCl and NaHCO3 that drives water movement isosmotically
What are the 2 steps in the net secretion of Na+ from plasma to CSF?
- Na-K pump in choroid plexus epithelia apical membrane moves Na+ outs of the cell into CSF
- active movement of Na+ out of the cell generates inward Na+ gradient across basolateral membrane, energizing basolateral Na+ entry through Na-H exchange and Na+-coupled HCO3-transport
What is the path of CSF circulation?
1 - lateral ventricles
2- interventricular foramina of Monro
3 - thrid ventricle
4 - cerebral aquaduct of Sylvius
5 - fourth ventricle
6 - foramino of Magendie and Luschka
7 - subarachnoid space, over SC, over convexity of the brain (and down spinla cord)
How is CSF absorbed?
by the dural venous sinuses (particulary the superior sagittal sinus) through arachnoid granulations a diverticululm of arachnoid space
How do the arachnoid granulations function in CSF absorpotion?
as pressure sensitive, one-way valves that allow flow of CSF from subarachnoid spaces into venous blood
CSF can cross inot venous blood, but venous blood cannot enter CSF
what is the net CSF movement into the venous blood promoted by?
the pressure of the CSF, which is higher than that of the venous blood
What is the rate of CSF formation insensitive to?
changes in the pressure of the CSF, while the absorption of CSF increases steeply as CSF pressures are about ~70 mm H2O
What does CSF absorption selectively increasing so that absorption exceeds formation achieve?
a lower CSF volume and a tendency to counteract and stabilize the increased intracranial pressure
What produces and increase in intercranial pressure?
an increase in volume of brain tissues, blood, or CSF because the cranium is a fixed volume and none of the intercranial residents are compressible
ICP is increased with an increase in volume of the brain produced by edema, tumor formation, a cerebral abscess, or hematoma
How can small intracranial masses be compensated for?
by reductions in intracranial CSF and blood volume without causing much rise in ICP
What is the result of large intracranial masses?
overcome compensatory mechanisms and lead to a steep rise in intracranial pressure, causing reduced cererbal perfudion and, ultimatley, herniation and death
What can severely elevated intracranial pressure cause?
decreased cerebral blood flow and brain ischemia
- cerebral blood flow depends on cerebral perfusion pressure, which is defined as the MAP - ICP
How does an increase in ICP affect cerebral perfusion pressure?
as ICP increases, cerebral perfusion pressure decreases
autoregulation of cerebral vessel caliber can compensate for modest reductions in cerebral perfusion pressure, leading to relatively stable cerebral blood flow
What does large increases in ICP lead too?
can exceed the capacity for autoregulation, leading to reduced cerebral blood flow and brain ischemia, irreversible brain damage and death
What are symptons of elevated ICP?
headache, altered mental status, especially irritability and depressed level of alertness and attention, nausea and vomiting, papilledema, visual loss, double vision (Diplopia), cushings triad: hypertension, bradycardia, irregular respirations
What is a lumbar puncture (spinal tap)?
a need is inserted between the fourth and fifth lumbar vertebrae and into the lumbar subarachnoid space
when the CSF flows freely through the needle, the hub of the needs is attaches to a manometer, and the fluid is allowed to rise
Why is there no risk of spinal cord injury during a lumbar puncture (spinla tap)?
the spinal cord only extends to L1
What is normal ICP?
65 to 195 mm CSF
or 5 to 15 mm Hg
What is papilledema?
Optic disk swelling caused by increased ICP
How does increased ICP cause papilledema?
elevated ICP is transmitted through the subarachnoid space to the optic nerve sheath, obstructing axonal transport and venous return to the optic nerve
pressure of the potic nerve head forces it inward
Can papilledema be seen during an eye exam?
yes
through an opthalmoscope
see the bulgin optic disk
What is hydrocephalus caused by?
caused by excess in the CSF intracranial cavity
What are the 3 ways hydrocephalus results from?
- excess CSF production (rare)
- obstruction of flow at any point in the ventricles but especially narrow points or subarachnoid space
- decrease in reabsorption via the arachnoid granulations when damaged or clogged
What are the two categories of hydrocephalus?
- communicating hydrocephalus: caused by impaired CSF reabsorption in the arachnoid granulations, obstruction of flow in the subarachnoid space, or (rarely) excess CSF production
- noncommunicating hydrocephalus: (common) caused by an obstruction of flow within the ventricular system
What are the symptoms of hydrocephalus?
mainly similar to elevated ICP
can be acute or chronic
headache, nausea, vomiting, cognitive impairment, decreased level of consciousness, papilledema, decreased vision
What is the treatment for hydrocephalus?
Usually involves a procedure that allows CSF to bypass the obstruction and drain from the ventricles
an external ventricular drain works by draining fluid from the lateral ventricles into a bag outside the head
a more permanent treatment is a ventriculoperitoneal shunt, in which the shunt tubing passes from the lateral ventricle out of the skull and is then tunnelled under the skin to drain into the peritoneal cavity of the abdomen. A valve prevents the flow of fluid in the reverse direction
What are the three locations of the barrier sites in the CNS?
- the brain endothelium forming the blood-brain barrier
- the arachnoid epithelium forming the middle layer of the meninges
- the choroid plexus epithelium which secretes CSF
What is the physical barrier caused by at each site?
tight junctions that reduce the permeability of the paracellular pathway
What is the blood-brain barrier?
diffusion barrier that impedes influx of most compounds from blood to brain
essential for maintaining a constant internal environment
What is the role of endothelial cell tight junctions in the BBB?
prevent water-soluble ions and molecules from passing from blood into the brain through the paracellular route
What is the role of astrocyte endfeet in the BBB?
provide a nearly continuous covering of the capillaries and facilitate the transport of substances between cells and blood
What can readily cross the BBB?
gases such as CO2, O2
drugs such as ethanol, caffeine, nicotine, opioids
What are the three principal transmembranous proteins that compose the tight junction of the BBB?
claudins
occludins
junctional adhesion molecules
as well as numerous cytoplasmic accessory proteins
What links brain capillary endothelial cells?
continuous tight junctions
What are the 3 primary ways entry into the brain is achieved?
- diffusion of lipid-soluble substances
- facilitative and energy-dependent receptor-mediated transport
- ion channels and exchangers
