Maintenance of the CNS Environment Flashcards
does CSF recirculate?
NO, so we make about 550ml/day
How much CSF is there at any given moment
150 ml
Where is most of the CSF made?
The specialized tissue of the choroid plexus (50-70%)
Where is the remainder of the CSF made
the tissue that lines the ventricles and blood vessels
What is the two step process of CSF formation
- passive filtration of serum
- modification of CSF (HCO3-, Cl- and K+ concentrations controlled by channels on epithelial cells, aquaporins also exist)
Important note regarding CSF production and ICP
production of CSF is constant over a wide range of ICPs
things that ed up in equal concentrations in plasma and CSF (3)
Na
Cl
HCO3
things that are present in higher concentration in the CSF than plasma (2)
Mg2+
CO2
Things that are present in lower concentrations in CSF than plasma
(4)
K
Ca
protein
glucose
CSF flows through the _______. Flow is _______ and CSF must be ________.
ventricular system
one way
reabsorbed at the end
Absorption of CSF is through
arachnoid villi
arachnoid villi - how is CSF reabsorbed
endothelium of sinus and the membrane on the villi have fused.
bulk flow of CSF into venous sinus is primary
pinocytosis is also documented
Absorption of CSF related to ICP
absorption of CSF is proportional to ICP
at pressures below 68mm CSF, no absoption
normal pressure =112 mm CSF
increased pressure causes damage to neurons
Role of CSF
to protect the brain
weight in air= 1400g
weight in CSF= 50g
BBB - two components that limit exchange in cpillaries in brain
tight junctions between endothelial cells
glial endfeet come in close contact with blood vessel
things that can passively diffuse across the BBB
H2O
CO2
O2
FREE steroid hormones (most are bound to protein)
major energy source for neurons? Does it cross BBB
glucose, does NOT cross BBB readily
How does glucose cross the blood brain barrier
GLUT1 transporter
Is the GLUT 1 transporter insulin dependent?
no
Two forms of GLUT1 transporters
45K= astroglia 55K= capillaries
Importance of NaK2Cl transporter at BBB
moves all those ions from CSF to blood
expression tied to endothelin 1 and 3
ET production tied to a signal from the astrocytes, may be related to [K] in CSF
Many drugs do cross the BBB, but are moved back to the blood via?
P-glycoprotein
Function of BBB
to protect the chemical composition of the CSF from blood-borne agents, maintain electrolyte composition, protect from toxins, prevent escape of NTs
BBB in particular maintains electrolyte composition related to _____. Why?
K+ to keep Vm maintained
Circumventricular organs
places in teh brain where we want the brain exposed to the blood.
Circumventricular organs are different in that they dont have a BBB. What is different
the capillaries do not have tight junctions between endothelial cells.
4 circumventricular organs
area postrema - vomitting
posterior pituitary - releases hormones into the blood
OVLT - control of body water/thirst/ blood volume, angiotensin control
subfornical organ - control of body water/thirst/blood volume, angiotensin control
Cerebral circulation receives _______ml (___% of CO)
750
14
Circle of Willis is formed by
2 internal carodtids and the basilar artery
Does blood from different sources mix in the Circle of willis? What is the impact?
NO
disruption of one of the inputs prduces localized (discrete) areas of ischemia.
Sympathetic Innervation of the vasculature
NT
R
effect
NE, NPY
alpha adrenergics
leads to constriction when systemic cardiac output/BP increases
Parasympathetic innervation of cerebral circulation
where
NT
effect
larger blood vessels
Ach, VIP, PHM-27 (derived from pre pro VIP)
cause vasodilation
Sensory innervation
NT
effect
impact
substance P, neurokinin A, CGRP
DILATION
these sensory fibers render the blood vessls of the brain extremely sensitive to torsion/manipulation - lead to pain
Role of Sensory innervation
in reduced CSF volume, the brain is effectively heavier and simple motion can cause bain bc of torsion of the blood vessels
the activation of those afferents will also cause vasodilation and increase blood flow (returns total intracranial volume closer to normal?)
Cerebral blood flow is under what kind of control
local control
oxygen consumption dictates where in the brain blood will go
What happens to cerebral blood flow as ICP increases
venous outflow is obstructed, leading to reduced arterial flow
cerebral bloodflow is strongly autoregulated, meaning
it is held constant over a wide range of systemic (mean arterial) blood pressures
In the face of high BP (which could damage the BBB), sympathetics in the brain will _______ the vasculature
vasoconstrict
although this will increase the systemic vascular resistance, it protects the capillaries in the brain from damage - sacrifice the heart for the brain
a decrease in brain perfusion will activate the vasomotor centers and _______systemic BP
increase, sometimes drastically. –> the brain will drive the body’s bp up in hopes of sending enough blood to the brain to get adequate perfusion
what might incrrease the ICP
hydrocephalus of any variety
edema
bleeding within the skull
