Meninges and Cerebrospinal fluid

Meninges : 

 The meninges are connective tissue membranes which surround the brain and spinal cord of CNS. In the fishes, there is only one meninx called meninx primitiva (piamater). In amphibians, reptiles and birds, the brain is covered by two meninges or membranes: inner pia,middle arachnoid and outer duramater. In mammals, CNS is covered by three meninges or membranes or cranial meninges. Brain meninges are continuous with spinal meninges

The three layers of cranial meninges in order from superficial to deeper duramater, arachnoid and piamater.  Duramater is nonvascular, tough made up of fibrous connective tissue.
Arachnoid mater made up of reticular connective tissue with Collagen and elastin fiber, while innermost vascular piamater (nutritive) made up of loose aerolar connective tissue.
Between dura and arachnoid mater presence of sub dural Space (no CSF in mammals here), between Arachnoid and piamater presence of sub-arachnoid space (with CSF in mammals, CSF also found in ventricles and central canal).
 Between duramater and periosteum presence of epidural space. An extension of duramater between two cerebral hemispheres called falx cerebri.
 Tentorium, an extension of duramater between cerebrum and cerebellum.

 Cerebrospinal fluid :

All the ventricles of the brain, central canal of spinal cord are continuous and lined by a columnar, ciliated epithelium, the ependyma.
 They contain lymph-like extracellular fluid called the cerebrospinal fluid (C.S.F.).
This fluid is secreted by the choroid plexuses by filtration of blood.
The choroid plexuses consist of loose connective tissue of pia mater covered internally by a simple cuboidal epithelium of secretory (glandular) nature.
 The cerebrospinal fluid slowly flows toward the fourth ventricle by secretion pressure and passes into the spinal cord.
 Some fluid escapes into the subarachnoid spaces through three pores a median aperture (of magendie) and a paired lateral aperture (of Luschka) in the roof of the fourth ventricle in the medulla.
 From the subarachnoid spaces, the cerebrospinal fluid is transferred to the blood of the venous sinuses.
Nervous tissue is without lymphatic vessels.
(a) The cerebro-spinal fluid (CSF) provides —

• Protection to brain from mechanical shocks, physical injury.
•  Optimum physiological fluid environment for neural functions, e.g. conduction of nerve impulses, transport of aminoacids, sugars, oxygen etc.
•  'Relief’ mechanism for the increase in intracranial pressure that occurs with each arterial             pulse of blood to brain.
• 'Sink like facility for metabolites of brain.
•  The blood CSF barrier for selective transport process between blood and CSF.
•  Nourishment to CNS.

Major site of CSF formation is choroid plexus, and mid ventricular wall and sub-arachnoid wall also contribute. CSF is cell free, slightly alkaline, and is isotonic to plasma. Rate of formation of C.S.F is 20 ml/ht) (480 ml/day) 20 ml/hour approx, 1/2 litre per day. Total amount present in and around CNS is 80- 150 ml it means there is atleast 3 times renewal of C.S.F. every day. CSF contains glucose, proteins, lactic acid, urea, Na+, K+, Ca2+, Mg2+, Cl-  , HCO3-  and some WBC. Blood brain barrier facilitate maintenance of stable internal environment. Its acts as physiological and pathological barrier.
There are three chcroid plexus in humans —
Lateral chorold plexus:  It is in the roof of l and  II ventricle.Anterior choroid plexus :  It is in the roof of III ventricre (diaccoel) Posterior choroid plexus or pelochoroida:  It is in the roof of IV ventricle.