If any of the structures sound unfamiliar to you, please review the relevant anatomy post: Protection for the Brain: Meninges, CSF, Blood-Brain Barrier
Formation of the CSF
Cerebrospinal Fluid is produced by the choroid plexus within the ventricles of the brain. Our first thought is, what is the choroid plexus? It’s a vascularized membrane, a membrane with a lot of blood vessels. Now where is this membrane? Inside the ventricles (chambers) of the brain.
Remember, CSF has a chemical composition similar to that of tissue fluid since it comes from blood vessels, just like tissue fluid does.
Circulation of the CSF
We see a mid-sagittal section of the brain and spinal cord. Right around the middle, you see it says choroid plexus of lateral ventricle. CSF squirts out of it. (In a sinovial joint, like an elbow or knee joint, fluid squirts out called sinovial fluid. In this case, it’s CSF.) There’s plenty of these chambers that form CSF, such as the Choroid plexus of Ventricle #3 (C.P.V.3).
This CSF circulates through a canal labeled Aqueduct of Sylvius (AS). Right at the back there is another ventricle labeled Choroid Plexus of Ventricle #4 (C.P.V.4). Ninety percent of the fluid is going to leave the brain through a small hole called the Foramen of Magendie and enter the subarachnoid space. (Magendie was the name of a very famous French neurosurgeon). The other 10% of the CSF actually continues downward through the spinal cord. There’s a canal through which this fluid flows called the Central Canal that goes to the bottom of the spinal cord. There’s an opening at the bottom of the subarachnoid space that allows the fluid from the central canal to go through. This CSF is flowing upwards to the top of the brain (in the shaded area). In the top you see Subarachnoid Space (S-AS) and then right above that you see Superior Sagittal Sinus (SSS). A sinus is a very thin walled vein. This CSF that circulates to the top of the S-AS is reabsorbed into the blood stream through the Superior Sagittal Sinus.
Let’s summarize: CSF is squirted out of the blood stream from the ventricles. It circulates, goes to the S-AS and returns back to the blood stream through the SSS. The CSF goes into the cranial venous sinuses and goes through the hole called the jugular foramen and becomes the internal jugular vein. When you look at the inside of the skull, where the brain sits, remember the groove for the sigmoid sinus and that led right to the jugular foramen.
What’s the point of all this? The CSF cushions the brain and spinal cord against trauma. :bangs head against wall: Over the years, kids get shoved, they punch each other, they get in fights. Instead of the brain getting slammed against the wall, the CSF cushions the blow and reduces the trauma. As long as there is a constant supply of CSF, there will be a constant pressure within the cranial cavity. If there is trauma and injury to the brain and there is too much fluid, it will increase intracranial pressure and doctors may have to make a hole to drain the pressure.
Clinical Considerations regarding CSF
Hydro- means water and -cephaly means brain, so it means water in the brain. If you have an accumulation of this fluid, that would be a problem and the fluid would need to be drained.
Rate of reabsorption of CSF < Rate of production of CSF
2) Lumbar puncture / Spinal tap.
Sometimes doctors want to insert a needle into where CSF is located. They typically insert the needle where the spinal cord ends so the needle doesn’t hit the spinal cord. That would be in the subarachnoid space between L3 and L4 (spinal cord ends at L2). Why would they do that? The only way to know if someone has meningitis is to take CSF and have a lab analyze it.
What are the two classic symptoms of meningitis? High fever and a stiff neck. That doesn’t mean it’s spinal meningitis but it’s an indicator to find out what it is. In an adult when they do a spinal tap, they usually apply a topical anesthetic like lidocaine. In children, they just do it right away. On a medical chart for spinal fluid, there would be a column that tests for WBC’s, Lymphs, Blood and PMN’s because those things shouldn’t be in the CSF. If there’s blood in the CSF for example, then that means it’s leaking from somewhere.
3) Regional Anesthesia.
Esthesia means sensation. When you put the prefix An- in front it means without sensation. Don’t confuse that with analgesic. Alge means pain and an analgesic reduces pain. Anesthesia is a loss of all sensation, not just pain. As far as the drugs that are used for it, whether it’s for the dentist or an epidural block, it’s the same class of drug called anesthetics.
They used to use Novocaine 35 years ago. Novocaine is the brand name for procaine. They stopped using it because its duration of action was too short of about 30 minutes compared to lidocaine which is a couple hours. Nowadays they use lidocaine which is a general name and there’s a lot of brand names, such as Xylocaine. The other anesthetic is benzocaine, which is the active ingredient in many OTC anesthetics such as “solarcaine” and “bactine” which is for sun burns and also a topical antiseptic.
Notice the pattern here? Lidocaine, novocaine, benzocaine, procaine, solarcaine, xylocaine… So what about cocaine? Cocaine was originally used as an anesthetic. The problem with it is that it stimulates your nervous system and wires you. People who snort cocaine with the nose will have a numb nose, or wherever it applies.
Local anesthetics temporarily slow down and block the conduction of action potentials (electrical impulses) in excitable cells. What are excitable cells? They are cells that generate action potentials, such as nerve cells, all muscle cells (skeletal, smooth and cardiac).
Lidocaine is used as a cardiac anti-arrhythmic. These drugs slow down these electrical patterns. One of the things that could cause arrhythmia is a heart attack. The most common arrythmia is a PVC (premature ventrical contraction or extrasystolys) and they control it with lidocaine. All drugs have multiple uses and multiple side effects. Any drug that affects electrical activity of the nervous system will also affect the muscles, including the cardiac muscle. Anyone ever heard of phenytoin (Dilantin)? It’s for seizures and also used for cardiac arrhythmia, because it’s effective at multiple things.
By the way, the lidocaine dosage for cardiac arryhtmia is 100 times greater than what is used in dental use.
4) Regional anesthesia during an epidural.
In this case we are giving an anesthetic to block spinal pain or the epidural pain.
When the local anesthetic is given to a pregnant woman, it will not only cause a loss of sensation, it will also cause paralysis because it’s going to block the action potentials of sensory neurons and motor neurons associated in an epidural.
There’s two ways of administering this: A spinal saddle block goes through the dura mater. A continuous lumbar epidural goes just outside the dura mater and it relies on the diffusion of the fluid across the dura mater into the CSF. The spinal saddle block works better but the reason you haven’t heard of that and have only heard of epidural is because that’s safer.
This is commonly used in woman during childbirth. This is commonly used during a C-section meaning the baby will not be born through the birth canal but an incision will be made to open it through the uterus. A problem with general anesthesia is that it knocks out the mother unconscious. When the baby is delivered, she will wake up a few hours later and nauseous. Today over 90% of the women who delivery vaginally, choose an epidural. They initially don’t want to but the pain of labor makes them change their mind. Labor is really painful. After 3 hours of severe labor, they change their mind. After they give birth with an epidural, they will never have another birth in any other way again. They don’t even need to be awake to push the baby because the uterus is the one that’s pushing, not the muscle. Remember since the epidural is occurring in the spinal cord and that’s separate from the blood stream, it isn’t going to affect the baby. That’s why you can’t give demerol or any other narcotic because that will go in the blood stream and affect the baby.