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 membrane:inner pia-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 fibre,while innermost vascular piamater(nutritive) made up of loose areolar connective tissue.Between dura and arachnoid mater presence of sub dural space(no CSF 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 is called falx cerebri. Tentorium,an extension of duramater between cerebrum and cerebellum.
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 plexuse 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 towards the fourth ventricle by secretion pressure and passes into the spinal cord.Some fluid escapes into the subarachnoid space 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 space, the cerebrospinal fluid is transferred to the blood of the venous sinuses.Nervous tissue is without lymphatic vessels.
The cerebro-spinal fluid(CSF) provides:-
1.Protection to brain from mechanical shocks,physical injury.
2.Optimum physiological fluid environment for neural functions e.g. conduction of nerve impulses,transport of aminoacids,sugars,O2 etc.
3.'Relief' mechanism for the increase in intracranial pressure that occurs with each arterial pulse of blood to brain.
4.'Sink' like facility for metabolites of brain.
5.The blood CSF barrier for selective transport process between blood and CSF.
6.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/h(480 ml/day) 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-and some WBC.
Blood brain barrier facilitate maintenance of stable internal environment.It acts as physiological and pathological barrier.
The enlargement of head,a pathological condition characterized by an abnormal accumulation of cerebrospinal fluid resulting in headache,vomiting,pain and stiffness of the neck.*
increased cerebrospinal fluid may result in Meningitis.*
Meningitis may occur due to infection and inflamation of meninges or injury of meninges.*
Infection may be viral,bacterial or both.The most common cause of meningitis is the infection of Streptococcus pneumoniae,
Neisseria meningitidis and Haemophilus influenzae.*
Lumbar puncture is done for drainage of excess of cerebrospinal fluid during meningitis.*
Cerebrospinal fluid is formed by choroid plexus(ACP and PCP).
There are three choroid plexus in humans:-
1.Lateral choroid plexus:
It is in the roof of I and II ventricle.
2.Anterior choroid plexus:
It is in the roof III ventricle(diocoel).
3.Posterior choroid plexus or pelochoroid:
It is in the roof of IV ventricle.
Oxygen and glucose requirements:
Brain controls the functions of our body organs and also provides the qualities of mind-learning,reasioning,and memory.For the these acitivites,brain needs a large and constant energy supply 20% of the body's consumption of oxygen and 15% of its consumption of blood glucose.Brain deprived of oxygen for just 5 minutes is permanently damaged .Mental confusion results if brain is deprived of glucose.