Csf And Meninges

Meninges, Cerebrospinal Fluid (CSF) and Blood Brain Barrier Meninges -are three connective tissue membranes that invest the spinal cord and brain. -consist of the pia mater and the arachnoid (together known as the leptomeninges) and the dura mater (pachymeninx).

A. Pia mater -is a delicate, highly vascular layer of connective tissue. -closely covers the surface of the brain and spinal cord. -is connected to the arachnoid by trabeculae.

1. Denticulate ligaments -consist of two lateral flattened bands of pial tissue. -attach to the spinal dura mater with 21 teeth.

2. Filum terminale -consists of a nonneural band of tissue that is a condensation of the pia mater. -extends from the conus medullaris to the end ofthe dural sac and fuses with it.

Lower part of spinal cord showing filum terminale and dura

B. Arachnoid -is a delicate. Nonvascular connective tissue membrane located between the dura mater and the pia mater. 1. Arachnoid granulations or arachnoid villi -enter the venous dural sinuses and permit the one-way flow of cerebrospinal fluid (CSF) from the subarachnoid space into the venous circulation. -are found in large numbers along the superior sagittal sinus but are associated with all dural sinuses.

2. Subarachnoid space -is located between the pia mater and the arachnoid. -contains CSF. -surrounds the entire brain and spinal cord. -extends, in the adult, below the conus medullaris to the level ofthe second sacral vertebra, the lumbar cistern

c. Dura mater -is the outer layer of the meninges and consists of dense connective tissue. -The supratentorial dura is innervated by the trigeminal nerve; the posterior fossa is innervated by the vagal and upper spinal nerves. -forms three major reflections and the walls of the dural venous sinuses: 1. Falx cerebri -lies between the cerebral hemispheres in the longitudinal cerebral fissure. -contains the superior and inferior sagittal sinuses between its two layers. 2. Tentorium cerebelli -separates the posterior cranial fossa from the middle cranial fossa. -separates the temporal and occipital lobes from the cerebellum and infratentorial brainstem. -contains the tentorial incisure, or notch, through which the brainstem passes. 3. Diaphragma sellae -forms the roof of the hypophyseal fossa. -contains an aperture through which the hypophyseal stalk (infundibulum) passes. 4. Dural sinuses -are endothelium-lined, valveless venous blood channels.

D. Meningeal spaces

1. Spinal epidural space -is located between the dura and the vertebral periosteum. -contains loose areolar tissue, venous plexuses, and lymphatics. -may be injected with a local anesthetic to produce a paravertebral nerve block.

2. Cranial epidural space -is a potential space between the periosteal and meningeal layers of the dura. -contains the meningeal arteries and veins. 3. Subdural space -is a potential space between the dura and the arachnoid. -intracranially transmits the superior cerebral veins to the venous lacunae of the superior sagittal sinus. Laceration of these ''bridging veins" results in subdural hemorrhage (hematoma). 4. Subarachnoid space -is located between the pia mater and the arachnoid. -contains CSF. -surrounds the entire brain and spinal cord. -extends, in the adult, below the conus medullaris to the level ofthe second sacral vertebra, the lumbar cistern (see Figure above).

5. Subarachnoid cisterns -are dilations of the subarachnoid space, which contains CSF. -are named after the structures over which they lie (e.g., pontine, chiasmatic, and interpeduncular cisterns).

Ventricles

-are lined with ependyma and contain CSF. -contain choroid plexus, which produces CSF at a rate of 500 ml/day. - consist of four fluid filled communicating cavities within the brain

A. Lateral ventricles - Are the two ventricles located within the cerebral hemispheres -communicate with the third ventricle via the interventricular foramina of Monro. -consist of five parts: 1. Frontal (anterior) horn - is located in the frontal lobe - lacks choroids plxus 2. Body -is located in the medial portion of the frontal and parietal lobes. -has choroid plexus. -communicates via the interventricular foramen of Monro with the third ventricle. 3. Temporal (inferior) horn -is located in the medial part of the temporal lobe. -has choroid plexus. 4. Occipital (posterior) horn -is located in the parietal and occipital lobes. -lacks choroid plexus. 5. Trigone (atrium) -is found at the junction of the body, occipital horn, and temporal horn of the lateral ventncIe.

B. Third ventricle (see Figure above) -is a slit-like, vertical midline cavity of the diencephalon. -communicates with the lateral ventricles via the interventricular foramina of Monro and with the fourth ventricle via the cerebral aqueduct. -contains a pair of choroid plexuses in its roof. C. Cerebral aqueduct (aqueduct of Sylvius) -lies in the midbrain. -connects the third ventricle with the fourth ventricle. -lacks choroid plexus. -Blockage leads to hydrocephalus (aqueductal stenosis).

D. Fourth ventricle -lies between the cerebellum and the brainstem. -contains a pair of choroid plexuses in its caudal roof. -expresses CSF into the subarachnoid space via the two lateral foramina of Luschka and single medial foramen of Magendie

Functions of CSF 1. Acts as a fluid buffer and protects the brain from shock; acts like a cushion and prevents brain from injury. 2. Regulates the volume of cranial contents- maintains normal intracranial pressure 3. Helps in absorption of nutritive substances and excretion of waste products 4. Is of diagnostic value in many diseases where pattern of deviation from normal values helps in diagnosis of diseases.

Clinical Importance

• Excessive accumulation of CSF in skull along with enlargement of head is known as hydrocephalous

Blood-Brain Barrier Barrier that provides selective permeability for substances entering as well as leaving brain. Blood-Brain Barrier (BBB) is formed by • Tight junctions of the endothelial cells • Foot processes of the astrocytes around the capillaries. Endothelial tight junctions associated with astrocytes form impermeable Blood brain barrier.

Remember- BBB is easily permeable for H2O, CO2, O2 and lipid soluble substances. Not well developed in infants- bile pigments which normally do not cross BBB are able to pass through BBB in infants.

Certain areas of brain are devoid of BBB: • Pituitary – helps in carrying hormones from pituitary to target organs via blood • Subfornical organs • Pineal region • Supra-optic area • Median eminence • Choroid plexus