#05 Flashcards
Lack of significant anaerobic
metabolism by the brain means that
- Neuronal glycogen storage is very slight
- Brain oxygen stores are very slight
Cerebral blood
flow rate is approximately a ml/100g of brain tissues per min
Cerebral blood flow less than b/100 results in irreversible brain damage
a: 50
b:15
4 mechanisms to regulate cerebral blood flow (CBF)
Cerebral pressure autoregulation (homeostatic)
Metabolic factors (CO2)
Neurogenic factors
Blood viscosioty
Under what conditions may autoregulation become impaired?
- Old age – mean CBF decreased
- Epilepsy – mean CBF increased 2-3×
- Arteriosclerosis – if severe, mean CBF decreased
Under what circumstances does hypertension result in decreased blood flow?
Damage to vessels
Edema (odem)
Raised intracranial pressure
When arterial CO2 ↑ brain arterioles
dilate (CBF ↑).
Steal Syndrome (Reverse Robin Hood
Syndrome):
After prolonged ischemia, normal
CO2 response fails, and blood flows away from the region where CO2 is elevated, i.e. from the region where it is most needed.
PET measures
glucose utilization
SPECT measures
blood flow
Cerebral vessels are innervated by a
(from superior cervical ganglion), but, due to b, this innervation doesn’t
usually affect blood flow
a: noradrenergic postganglionic sympathetic fibers
b: autoregulation
CBF is a to blood viscosity
a: inversely proportional
How are the ventricles interconnected?
lateral ventricles -> interventricular foramina (of Monro) -> IIIrd ventricle -> cerebral aqueduct (of Sylvius) -> IVth ventricle
Core functions of CSF
- Maintains a constant external environment for neurons and glia (communicates with brain interstitial fluid)
- Primarily one-way flow of CSF serves as a functional waste clearance pathway (glymphatic system) for removal of potentially harmful brain metabolites
- Provides a mechanical cushion to protect the brain from impact with the bony calvaria when the head moves
- By its buoyant action, CSF allows the brain to float, thereby reducing its effective weight in situ to less than 50g
- May serve as a conduit for polypeptide hormones (secreted by hypothalamic neurons) acting at remote sites in the brain
- Homeostatic role (pH of CSF affects both pulmonary ventilation and CBF)
Ependymocytes
These cells create and secrete CSF and beat their cilia to help circulate CSF
through the system
CSF is mostly produced in modified ependymal cells called
choroid plexus.
all four ventricles (L and R lateral ventricles, IIIrd ventricle, and IVth
ventricle) have choroid plexus, but the vast majority (~70%) is located in
lateral ventricles
Foramina of Luschka (location?) empty ventrally into a
loc: lateral apertures of ventricle IV
a: pontine cistern
Foramen of Magendie (location?) empties dorsally into the a
loc: median aperture of ventricle IV
a: cisterna magna
CSF production
Choroid plexus (%50-70)
other ependymal cells
around blood vessels in subarachnoid space
produced at a rate of 500 ml/day
CSF absorption
Arachnoid granulations (majority)
along crnial and spinal nerves into lymph system
Brain can contain only 135-150 ml, so CSF must turn over 3.7 times daily
Monro-Kellie doctrine
An increase in the volume of any one of the contents of the calvaria—brain tissue, blood, CSF, or other brain fluids—will produce increased intracranial pressure because the bony calvaria rigidly fixes the total cranial volume.
noncommunicating hydrocephalus vs communicating hydrocephalus
- Interventricular obstructions (e.g., stenosis of the cerebral aqueduct) cause noncommunicating hydrocephalus (the CSF does not “communicate” with the
subarachnoid space). - Extraventricular obstructions (e.g., obstruction at the tentorial notch, blockage of the arachnoid granulations) cause communicating hydrocephalus. Tracer die injected
into the lateral ventricle will ccumulate in the lumbar cistern in the case of
communicating hydrocephalus, as CSF is able to evacuate the ventricular system and enter the subarachnoid space.
Entry into the blood/CSF is achieved primarily in three ways
- by diffusion of lipid-soluble substances – substances with high lipid solubility (e.g., ethanol, nicotine) enter the brain rapidly, those with low/no lipid solubility (e.g., sodium, dopamine) enter the brain slowly or not at all
- by facilitative (e.g., Glutl) and energy-dependent (e.g., Na+-K+-ATPase)
receptor-mediated transport of specific water-soluble substances - by ion channels (e.g., Na+ channels)
advantages of BBB
Maintains a precisely regulated microenvironment for reliable neuronal signaling
Keeps out bacteria
disadvantages of BBB
Keeps out white blood cells
Keeps out drugs
