General Neurophysiology Flashcards
is Cerebrospinal fluid recirculated?
no
always about 150 ml at any given moment
make about 550 a day
What are the 4 ventricles in the brain?
2 lateral ventricles
third ventricle
fourth ventricle (over medulla, and under the cerebellum)
What are the 4 connections of the ventricles
Intraventricular foramen: lateral ventricles to the third ventricle
Cerebral aqueduct: 3rd ventricle to the 4th ventricle
Median Aperture: 4th ventricle to Cisterna magna
Lateral apertures: 4th ventricle to arachnoid space
What produces 50-70 percent of the CSF
Choroid plexus
what makes 30 percent of the CSF
made by the tissue that lines the ventricles and blood vessels
What is the first step of the process of making CSF, and how is this possible
Have passive filtration of plasma from the blood vessels into the ventricles
Hydrostatic pressure
- large capillary “bloodpressure”
- small tissue hydrostatic pressure
- – push fluid into capillary
Oncotic pressure or Osmotic pressure
- this is the pull pressure
- Identical at the choroid plexus
What is the second step in the formation of the CSF
modification of Ion composition
What ion concentrations are controlled by channels of epithelial cells
HCO3
Cl
K
aquaporin 1 allows H2O
if intracranial pressures change what happens to the production of CSF
Production of CSF is constant over a wide range of intracranial pressures, even high or low pressures
What ion concentrations stay equal between both the plasma and the CSF
Na+
and HCO3
What ions are concentrated more in CSF than in plasma
Mg++
Cl-
CO2 (brain produces lots of CO2
What ions are more concentrated in plasma than CSF
K+ (therefore CSF can tolerate the efflux of K+ from action potentials)
Ca++
Protein (affects pressure and pH)
Glucose
where is the CSF absorbed
Arachnoid villi
flows into the venous sinus
some pinocytosis occurs
What plays a factor in the amount of CSF absorbed
absorption of CSF is proportional to intracranial pressure
at pressures below 68mm CSF = no absorption
normal pressure is 112 mm CSF
increased pressure can damage neurons
what is the role of the CSF
protect the brain, in CSF the brain only weighs 50 g
what are the two components that limit exchange in the blood brain barrier
tight junctions between endothelial cells
glial endfeet come in close contact with blood vessel
what substances can cross the BBB via passive diffusion
H20 via AQP -4
CO2
O2
free steroid hormones
how does glucose cross the blood brain barrier, and what are the two forms of this protein
Glut 1 transporter
not insulin dependant
55k for capillaries
45k on astrogila
Neurons use Glut -3 to move glucose in
what plays an important role in keeping the lower concentration of K+ in the CSF
Na/K/2Cl transporter
moves these ions from the CSF to the blood
these transporters are expressed via the endothelin 1 and 3 from the endothelial cells of the blood vessels
some drugs are able to cross the BBB but how are they pumped back to the blood
P-glycoprotein (pump glycoprotein)
glycoprotein binds to variety of substances and then will be pumped back to the blood from the CSF
what is the function of the BBB
protects the chemical composition of the CSF from blood borne agents
particularly the K+
and the maintenance of Vm
what are the four circumventricular organs and what does this mean
several places in the brain where the brain is exposed to blood, and their are no tight junctions
Posterior pituitary : for releasing hormones into the blood
Area postrema: chemotactic trigger zone to induce vomitting
Organum casculosum of the lamina terminalis (OVLT)
Subfornical organ
(both are involved in control of the body water/thirst/blood volume control)
Despite the apperance of having an anstamoses of the circle of willis what is the clinical relevence of this anastamoses
disruption of one of the inputs produces localized (discrete) ares of ischemia because the brain would rather lose one part of the brain than divert blood from other areas to help that area
What is the sympathetic innervation of the Cerebral circulation
Neurotransmitter: Norepi and Neuropeptide Y (NPY)
Receptors: Alpha adrenergics
leads to constriction when systematic cardiac output/blood pressure increases
what is the parasympathetic innervation of the Cerebral cirulation
innervation of larger vessels
Ach
Vasoactive intestinal polypeptide
PHM-27
all three of these cause vasodilation
what are the three neurotransmitters released and act on the blood vessels from its own sensory innervation
Substance P (dilation)
Neurokinin A (dilation)
CGRP (dilation)
when do the sensory fibers render the blood vessels of the brain
extremely sensitive to torsion and manipulation (lead to pain)
in the presence of decreased CSF the brain is heavier and a simple motion can cause pain
this ill then activate the afferents that will cause the vasodilation and increase blood flow (to try and return total intracranial volume closer to normal)
how is cerebral blood flow controlled
local control
oxygen consumption dictates where in the brain the blood will go
how is the cerebral blood flow regulated
it is autoregulated and is held at a constant over a wide range of systematic (mean arterial) blood pressures
therefore even at high and low blood pressures the brain will get the same amount of blood flow
What happens in the face of high systematic blood pressure
the sympathetics will take over and VASOCONSTRICT the vasculature
even though this will increase the systematic vascular resistance it protects the capillaries in the brain and the blood brain barrier from damage
how does intracranial pressure influence the cerebral blood flow
as intracranial pressure goes up, venous outflow is obstructed leading to reduced arterial blood flow
a decrease in brain perfusion will activate the cardiovascular control centers in the medulla and increase systematic BP, sometimes dramatically
What are some common causes that increase intracranial pressure
hydrocephalus of any variety
Cerebral edema
Intracranial bleeding
