In: Anatomy and Physiology
On a night out with friends at the viaduct, john has too much to drink. He loses his balance and falls, hitting his head on the concrete floor. He is assessed by a paramedic for a possible concussion. Assume normal physiology and anatomy in addressing the specific questions below
Section1(200 words)
1-Describe the anatomical structures and locations that produce and remove cerebrospinal fluid from the central nervous system?
Section 2 (250words)
2A. Describe the structure of the blood brain barrier and explain how it is supposed to protect the brain?
2B-Why did the blood brain barrier not successfully prevent the alcohol from entering into johns’s brain?
2C-What major regions of John’s brain would you expect to have been affected by the alcohol to influence his somatic motor function?
Cerebrospinal fluid (CSF) is a clear, colorless body fluid found in the brain and spinal cord. It is produced by specialised ependymal cells in the choroid plexuses of the ventricles of the brain, and absorbed in the arachnoid granulations. There is about 125 mL of CSF at any one time, and about 500 mL is generated every day. CSF acts as a cushion or buffer, providing basic mechanical and immunological protection to the brain inside the skull. CSF also serves a vital function in the cerebral autoregulation of cerebral blood flow.
CSF occupies the subarachnoid space (between the arachnoid mater and the pia mater) and the ventricular system around and inside the brain and spinal cord. It fills the ventricles of the brain, cisterns, and sulci, as well as the central canal of the spinal cord. There is also a connection from the subarachnoid space to the bony labyrinth of the inner ear via the perilymphatic duct where the perilymph is continuous with the cerebrospinal fluid. The ependymal cells of the choroid plexuses have multiple motile cilia on their apical surfaces that beat to move the CSF through the ventricles.
A sample of CSF can be taken via lumbar puncture. This can reveal the intracranial pressure, as well as indicate diseases including infections of the brain or its surrounding meninges. Although noted by Hippocrates, it was only in the 18th century that Emanuel Swedenborg was credited with its rediscovery, and as late as 1914 Harvey Cushing demonstrated CSF was secreted by the choroid plexus.
There is about 125–150 mL of CSF at any one time. This CSF circulates within the ventricular system of the brain. The ventricles are a series of cavities filled with CSF. The majority of CSF is produced from within the two lateral ventricles. From here, CSF passes through the interventricular foramina to the third ventricle, then the cerebral aqueduct to the fourth ventricle. From the fourth ventricle, the fluid passes into the subarachnoid space through four openings – the central canal of the spinal cord, the median aperture, and the two lateral apertures.CSF is present within the subarachnoid space, which covers the brain, spinal cord, and stretches below the end of the spinal cord to the sacrum.There is a connection from the subarachnoid space to the bony labyrinth of the inner ear making the cerebrospinal fluid continuous with the perilymph in 93% of people.
CSF moves in a single outward direction from the ventricles, but multidirectionally in the subarachnoid space. Fluid movement is pulsatile, matching the pressure waves generated in blood vessels by the beating of the heart.[3] Some authors dispute this, posing that there is no unidirectional CSF circulation, but cardiac cycle-dependent bi-directional systolic-diastolic to-and-from cranio-spinal CSF movements.
ContentsEdit
CSF is derived from blood plasma and is largely similar to it, except that CSF is nearly protein-free compared with plasma and has some different electrolyte levels. Due to the way it is produced, CSF has a higher chloride level than plasma, and an equivalent sodium level.
CSF contains approximately 0.3% plasma proteins, or approximately 15 to 40 mg/dL, depending on sampling site.In general, globular proteins and albumin are in lower concentration in ventricular CSF compared to lumbar or cisternal fluid. This continuous flow into the venous system dilutes the concentration of larger, lipid-insoluble molecules penetrating the brain and CSF.CSF is normally free of red blood cells, and at most contains less than 5 white blood cells per mm³. Any white blood cell count higher than this constitutes pleocytosis.CSF contains nucleic acids, in particular cell-free DNA.
FunctionEdit
CSF serves several purposes:
Buoyancy: The actual mass of the human brain is about 1400–1500 grams; however, the net weight of the brain suspended in CSF is equivalent to a mass of 25-50 grams.The brain therefore exists in neutral buoyancy, which allows the brain to maintain its density without being impaired by its own weight, which would cut off blood supply and kill neurons in the lower sections without CSF.
Protection: CSF protects the brain tissue from injury when jolted or hit, by providing a fluid buffer that acts as a shock absorber from some forms of mechanical injury.
Prevention of brain ischemia: The prevention of brain ischemia is aided by decreasing the amount of CSF in the limited space inside the skull. This decreases total intracranial pressure and facilitates blood perfusion.
Homeostasis: CSF allows for regulation of the distribution of substances between cells of the brain,and neuroendocrine factors, to which slight changes can cause problems or damage to the nervous system. For example, high glycine concentration disrupts temperature and blood pressure control, and high CSF pH causes dizziness and syncope.
Clearing waste: CSF allows for the removal of waste products from the brain, and is critical in the brain's lymphatic system. Metabolic waste products diffuse rapidly into CSF and are removed into the bloodstream as CSF is absorbed.
The brain produces roughly 500 mL of cerebrospinal fluid per day at a rate of about 25 mL an hour.[1] This transcellular fluid is constantly reabsorbed, so that only 125–150 mL is present at any one time.
CSF volume is higher on a mL/kg basis in children compared to adults. Infants have a CSF volume of 4 mL/kg, children have a CSF volume of 3 mL/kg, and adults have a CSF volume of 1.5-2 mL/kg. A high CSF volume is why a larger dose of local anesthetic, on a mL/kg basis, is needed in infants. Additionally, the larger CSF volume may be one reason as to why children have lower rates of postdural puncture headache.
Most (about two-thirds to 80%) of CSF is produced by the choroid plexus.The choroid plexus is a network of blood vessels present within sections of the four ventricles of the brain. It is present throughout the ventricular system except for the cerebral aqueduct, and the frontal and occipital horns of the lateral ventricles.CSF is also produced by the single layer of column-shaped ependymal cells which line the ventricles; by the lining surrounding the subarachnoid space; and a small amount directly from the tiny spaces surrounding blood vessels around the brain.
CSF is produced by the choroid plexus in two steps. Firstly, a filtered form of plasma moves from fenestrated capillaries in the choroid plexus into an interstitial space,with movement guided by a difference in pressure between the blood in the capillaries and the interstitial fluid. This fluid then needs to pass through the epithelium cells lining the choroid plexus into the ventricles, an active process requiring the transport of sodium, potassium and chloride that draws water into CSF by creating osmotic pressure.Unlike blood passing from the capillaries into the choroid plexus, the epithelial cells lining the choroid plexus contain tight junctions between cells, which act to prevent most substances flowing freely into CSF.Cilia on the apical surfaces of the ependymal cells beat to help transport the CSF.
Water and carbon dioxide from the interstitial fluid diffuse into the epithelial cells. Within these cells, carbonic anhydrase converts the substances into bicarbonate and hydrogen ions. These are exchanged for sodium and chloride on the cell surface facing the interstitium.Sodium, chloride, bicarbonate and potassium are then actively secreted into the ventricular lumen.This creates osmotic pressure and draws water into CSF, facilitated by aquaporins.Chloride, with a negative charge, moves with the positively charged sodium, to maintain electroneutrality. Potassium and bicarbonate are also transported out of CSF As a result, CSF contains a higher concentration of sodium and chloride than blood plasma, but less potassium, calcium and glucose and protein.Choroid plexuses also secrete growth factors, iodine,vitamins B1, B12, C, folate, beta-2 microglobulin, arginine vasopressin and nitric oxide into CSF] A Na-K-Cl cotransporter and Na/K ATPase found on the surface of the choroid endothelium, appears to play a role in regulating CSF secretion and composition
Orešković and Klarica hypothesise that CSF is not primarily produced by the choroid plexus, but is being permanently produced inside the entire CSF system, as a consequence of water filtration through the capillary walls into the interstitial fluid of the surrounding brain tissue, regulated by AQP-4.
There are circadian variations in CSF secretion, with the mechanisms not fully understood, but potentially relating to differences in the activation of the autonomic nervous system over the course of the day.
Choroid plexus of the lateral ventricle produces CSF from the arterial blood provided by the anterior choroidal artery. In the fourth ventricle, CSF is produced from the arterial blood from the anterior inferior cerebellar artery (cerebellopontine angle and the adjacent part of the lateral recess), the posterior inferior cerebellar artery (roof and median opening), and the superior cerebellar artery.
ReabsorptionEdit
CSF returns to the vascular system by entering the dural venous sinuses via arachnoid granulations.These are outpouchings of the arachnoid mater into the venous sinuses around the brain, with valves to ensure one-way drainage.This occurs because of a pressure difference between the arachnoid mater and venous sinuses.CSF has also been seen to drain into lymphatic vessels, particularly those surrounding the nose via drainage along the olfactory nerve through the cribriform plate. The pathway and extent are currently not known] but may involve CSF flow along some cranial nerves and be more prominent in the neonate.CSF turns over at a rate of three to four times a day.CSF has also been seen to be reabsorbed through the sheathes of cranial and spinal nerve sheathes, and through the ependyma.
RegulationEdit
The composition and rate of CSF generation are influenced by hormones and the content and pressure of blood and CSF. For example, when CSF pressure is higher, there is less of a pressure difference between the capillary blood in choroid plexuses and CSF, decreasing the rate at which fluids move into the choroid plexus and CSF generation. The autonomic nervous system influences choroid plexus CSF secretion, with activation of the sympathetic nervous system increasing secretion and the parasympathetic nervous system decreasing it] Changes in the pH of the blood can affect the activity of carbonic anhydrase, and some drugs (such as frusemide, acting on the Na-Cl cotransporter) have the potential to impact membrane channels.