Meninges And Cerebrospinal Fluid
Question 1: Give a brief account of the meninges.
Answer. The brain and spinal cord are enclosed in three protective membranes called meninges. From outwards to inwards these are (1) dura mater, (2) arachnoid mater, and (3) pia mater.
The dura mater is mesodermal in origin, while the arachnoid and pia mater are ectodermal in origin.
Meninges Dura mater
The dura mater is the thick outermost covering of the brain and spinal cord. The part enclosing the brain is called cranial/cerebral dura, while the part enclosing the spinal cord is called spinal dura. It is the very tough, opaque, inelastic membrane of fibrous tissue (Greek, dura = tough, mater = mother). It is also called pachymeninx (pachy = thick).
Meninges Arachnoid mater
The arachnoid mater (Greek, arachnoid = cobweb-like, mater = mother) is a delicate avascular membrane deep to dura mater. Many thread-like trabeculae extend from its inner aspect to the pia mater.
Meninges Pia mater
The pia mater (Greek, pia = tender, mater = mother) is a thin, transparent, vascular membrane, closely adherent to the surface of the brain and spinal cord.
The arachnoid mater and pia mater together are termed leptomeninges (lepto = thin).
Question 2: Describe subarachnoid cisterns in brief.
Answer. The subarachnoid space is the space between the arachnoid mater and the pia mater. It surrounds the CNS. The subarachnoid space around the brain is continuous with the subarachnoid space around the spinal cord. In the region of the brain, it communicates with the ventricular system of the brain.
Subarachnoid Cisterns
In certain situations, the subarachnoid space around the brain enlarges to form pools of cerebrospinal fluid (CSF) called subarachnoid cisterns.
The principal subarachnoid cisterns:
Cerebellomedullary Cistern: It is the largest cistern (also called cisterna magna) and lies in a triangle formed by the cerebellum, medulla oblongata, and occipital bone. It receives the CSF from the 4th ventricle through a foramen Magendie and foramina of Luschka.
Pontine Cistern: It lies on the ventral aspect of the pons and contains a basilar artery.
Interpeduncular Cistern (basal cistern): It lies in the region of the interpeduncular fossa and contains a circle of Willis.
Cistern Of Lateral Sulcus (Sylvian cistern): It occupies the lateral sulcus and contains the middle cerebral artery.
Cistern Of Great Cerebral Vein (cisterna ambient): It occupies the interval between the splenium of the corpus callosum and the superior surface of the cerebellum.
Question 3: Describe the cerebrospinal fluid under the following headings: (1) formation, (2) circulation, (3) absorption, (4) functions and (5) applied anatomy.
Answer. The cerebrospinal fluid (CSF) is a modified tissue fluid similar to blood plasma and interstitial tissue fluid. It is present in the ventricles of the brain and subarachnoid space around the brain and spinal cord.
Formation Of Cerebrospinal Fluid
- The bulk of the CSF (80%–90%) is formed by choroid plexuses of lateral ventricles.
- The small amount is formed by the choroid plexuses of the 3rd and 4th ventricles.
- In an adult, the total quantity of CSF is about 150 mL, out of which only 30 mL lies in the ventricular system.
Read And Learn More: Selective Anatomy Notes And Question And Answers
Circulation Of Cerebrospinal Fluid It is given in the form of a flowchart given below:
Absorption Of Cerebrospinal Fluid
- The CSF is chiefly absorbed through the arachnoid villi and arachnoid granulations into the superior sagittal sinus.
- It is also absorbed partly by the perineural lymphatics around the cranial and spinal nerves and also via pial veins.
Functions Of Cerebrospinal Fluid
- Provides protection to the CNS.
- Provides nutrition to the CNS.
- Removes waste products from the CNS.
Applied Anatomy Of Cerebrospinal Fluid
- Withdrawal of CSF: The CSF can be obtained by lumbar puncture, cisternal puncture, and ventricular puncture for diagnostic and therapeutic purposes.
- Hydrocephalus: The excessive accumulation of CSF within the skull due to either obstruction to the flow of CSF in the ventricular system or impairment of its absorption into the intracranial dural venous sinuses through arachnoid granulations, leads to hydrocephalus in children and raised intracranial pressure in adults.
The characteristic clinical features of hydrocephalus are as follows:
- Abnormally large head
- Bossing of forehead
- Wide, tense anterior fontanelle
- ‘Setting-sun appearance’ of the eyes
- Thin scalp with dilated scalp veins
- Cracked-pot sound on skull percussion
- Dementia
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