Structure and functions of Brain and Spinal cord :
The nervous system in animal kingdom shows a great variety. It has evolved from nerve net of hydra to ganglionated nervous system as in earthworm and cockroach to the superior mammalian brain. Amongst all the animals, human beings can show some activities which are unique to them solely due to presence of superior brain. The brain and spinal cord constitute central nervous system (CNS). CNS is derived from the embryonic ectoderm. The nerves given by CNS to connect all the body parts constitute peripheral nervous system (PNS). Autonomic Nervous System (ANS) keeps control on all the internal organs and are semi-independent in function.
The brain and spinal cord are surrounded by connective tissue membranes called meninges. They are as follows.
i) Dura mater : It is the outermost meninx which is tough, thick and fibrous. It is attached to the inner side of the cranium and is protective in function.
ii) Arachnoid mater : It is middle, thin and vascular, formed of reticular connective tissue. It is nutritive or protective having spider web appearance.
iii) Pia mater : It is the innermost thin membrane lying in contact with CNS. It is highly vascular and hence nutritive in function.
The arachnoid mater and dura mater are separated by sub-dural space filled with serous fluid. In between pia mater and arachnoid mater sub arachnoid space is present. It contains lymph-like, watery fluid called cerebrospinal fluid (CSF). It is secreted by choroid plexus and ependymal cells. (CSF) is alkaline and about 120 ml in adults. Infection of meaning is meningitis. Meninges form protective covering around the brain and spiral cord. CSF acts as a shock absorber and it protects the brain from mechanical injuries. CSF also maintains constant pressure inside cranium and regulates the temperature. It helps in exchange of nutrients and wastes between blood and brain tissue. It helps in the supply of O2 to brain. CSF protects the brain desiccation.