Brain organisation and CSF Flashcards

Question

What substances are able or not able to cross the Blood Brain Barrier?

Answer 1

A few water-soluble sub- stances, such as glucose, cross the BBB by active transport. Other substances, such as creatinine, urea, and most ions, cross the BBB very slowly. Still other substances—proteins and most antibiotic drugs—do not pass at all from the blood into brain tissue. However, lipid-soluble substances, such as oxygen, carbon dioxide, alcohol, and most anesthetic agents, are able to access brain tissue

Answer 2

Cerebrospinal fluid (CSF) is a clear, colorless liquid composed primarily of water that protects the brain and spinal cord from chemical and physical injuries.

Answer 3

The total volume of CSF is 80 to 150 mL (3 to 5 oz) in an adult.

Answer 4

CSF contains small amounts of glucose, oxygen, proteins, lactic acid, urea, cations (Na , K , Ca2 , Mg2 ), and anions (Cl– and HCO3–); it also contains some white blood cells.

Answer 5

Anteriorly, the lateral ventricles are separated by a thin membrane, the septum pellucidum

Answer 6

The third ventricle is a narrow slitlike cavity along the midline superior to the hypothalamus and between the right and left halves of the thalamus.

Answer 7

Mechanical protection Homeostatic function Circulation

Answer 8

CSF serves as a shock-absorbing medium that protects the delicate tissues of the brain and spinal cord from jolts that would otherwise cause them to hit the bony walls of the cranial cavity and vertebral canal. The fluid also buoys the brain so that it “floats” in the cranial cavity.

Answer 9

The pH of the CSF affects pulmonary ventilation and cerebral blood flow, which is important in maintaining homeostatic controls for brain tissue. CSF also serves as a transport system for polypeptide hormones secreted by hypothalamic neurons that act at remote sites in the brain.

Answer 10

CSF is a medium for minor exchange of nutrients and waste products between the blood and adjacent nervous tissue.

Answer 11

The majority of CSF production is from the choroid plexuses, networks of blood capillaries in the walls of the ventricles.

Answer 12

Ependymal cells joined by tight junctions cover the capillaries of the choroid plexuses. Selected substances (mostly water) from the blood plasma, which are filtered from the capillaries, are secreted by the ependymal cells to produce the cerebrospinal fluid. This secretory capacity is bidirectional and accounts for continuous production of CSF and transport of metabolites from the nervous tissue back to the blood. Because of the tight junctions between ependymal cells, materials entering CSF from choroid capillaries cannot leak between these cells; instead, they must pass through the ependymal cells. This blood–cerebrospinal fluid barrier permits certain substances to enter the CSF but excludes others, protecting the brain and spinal cord from potentially harmful blood-borne substances. In contrast to the blood–brain barrier, which is formed mainly by tight junctions of brain capillary endothelial cells, the blood–cerebrospinal fluid barrier is formed by tight junctions of ependymal cells.

Answer 13

The CSF formed in the choroid plexuses of each lateral ventricle flows into the third ventricle through two narrow, oval openings, the interventricular foramina. More CSF is added by the choroid plexus in the roof of the third ventricle. The fluid then flows through the aqueduct of the midbrain (cerebral aqueduct), which passes through the midbrain, into the fourth ventricle. The choroid plexus of the fourth ventricle contributes more fluid. CSF enters the subarachnoid space through three openings in the roof of the fourth ventricle: a single median aperture and paired lateral apertures, one on each side. CSF then circulates in the central canal of the spinal cord and in the subarachnoid space around the surface of the brain and spinal cord.

Answer 14

CSF is gradually reabsorbed into the blood through arachnoid villi, fingerlike extensions of the arachnoid that project into the dural venous sinuses, especially the superior sagittal sinus. (A cluster of arachnoid villi is called an arachnoid granulation.) Normally, CSF is reabsorbed as rapidly as it is formed by the choroid plexuses, at a rate of about 20 mL/hr (480 mL/day). Because the rates of formation and reabsorption are the same, the pressure of CSF normally is constant. For the same reason, the volume of CSF remains constant.

Answer 15

Abnormalities in the brain—tumors, inflammation, or developmental malformations—can interfere with the circulation of CSF from the ventricles into the subarachnoid space. When excess CSF accumulates in the ventricles, the CSF pressure rises. **Elevated CSF pressure** causes a condition called **hydrocephalus**. The abnormal accumulation of CSF may be due to an **obstruction to CSF flow** or an **abnormal rate of CSF production** and/or **reabsorption**. In a **baby** whose fontanels have not yet closed, the **head bulges** due to the increased pressure. If the condition persists, the fluid buildup **compresses** and **damages the delicate nervous tissue**. Hydrocephalus is relieved by **draining the excess CSF**. In one procedure, called **endoscopic third ventriculostomy (ETV)**, a neurosurgeon makes a **hole in the floor of the third ventricle** and the CSF **drains** directly **into the subarachnoid space**. In adults, hydrocephalus **may occur after head injury, meningitis, or subarachnoid hemorrhage**. Because the adult skull bones are fused together, this condition can quickly become life-threatening and requires immediate intervention.

Brain organisation and CSF Flashcards

(44 cards)