Cerebrospinal Fluid & Blood-brain Barrier
Describe CSF formation.
- An ultrafiltrate of plasma - Modified ependymal cells perform this "ultrafiltration function" & then secrete this ultrafiltrate into the ventricles - CO2 & O2 move freely between blood & CSF
What is the rate of formation of CSF?
- Dog: 3 ml/hr - Cat: 1 ml/hr - Human: 20 ml/hr
What are some characteristics of molecules that may be able to cross the blood-brain barrier?
- Small, uncharged - Lipid-soluble (nicotine, ethanol) - Non-protein bound - Gases (O2, CO2) - Glucose, some amino acid - mediated transport
What do the modified ependymal cells secrete?
- Sodium - H2O, Cl-, & bicarbonate follow
What pumps molecules back out of the capillaries into the blood?
- The MDR pump - MDR genetic deficient dogs can have Ivermectin toxicity
What makes the cerebral spinal fluid (CSF)?
Primarily by the choroid plexus of the lateral ventricles, 3rd ventricle, & 4th ventricle
How is CSF reabsorbed?
- Arachnoid villi act as 1-way valves for CSF to enter the venous circulation (CSF pressure dependent) - CSF also resorbed by epidural veins around nerve roots - At steady rate, rate of formation = rate of exit - rate of formation unaffected by CSF pressure
How is the blood-brain barrier formed?
- Capillaries in the brain & spinal cord have tight junctions between them that prevent many molecules from passing from the blood into the brain - Consists of capillaries w/ tight junctions, pericytes, continuous basement membrane
What is the function of the cerebrospinal fluid?
- Cushions the brain & CNS from impact damage - Supplies nutrients to the CNS - Helps maintain a stable microenvironment for neurons/glia since CSF in equilibrium w/ extracellular fluid around brain cells - chemical buffer - Waste excretion & toxin removal - Some nutritional properties - pH of CSF by respiratory center near surface of medulla oblongata
What are circumventricular organs?
- Located in several regions of the brain - near brain or ventricular surfaces - Small, discrete regions - Lack a blood-brain barrier
What is the cerebrospinal fluid?
- Low protein fluid that exists in the brain ventricles, the central canal of spinal cord, & in the subarachnoid space around the brain & spinal cord - Created in the 3rd ventricle
Describe the choroid plexus.
- Many fenestrated capillaries - Modified ependymal (epithelial) cells around these capillaries that tight junctions between them - Constitute the blood-CSF barrier
What are the functions of circumventricular organs?
- Monitor serum solutes - osmolality of the blood, glucose levels, hormones, other chemicals - In communication w/ other brain structures that respond to these solute changes
Describe the flow of CSF.
- Produced in lateral ventricle - Travels to third ventricle - Enters cerebral aqueduct of Sylvius - Travels to fourth ventricle & lateral aperture of 4th ventricle (foramen of Luschka) - Enters central canal of spinal cord
Does CSF cross ventricular linings into the interstitial space?
Yes
