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protective structures of the central nervous system
there are four major categories for protection in the central nervous system
the bones, the membrane
fluid cushioning and chemical protection
the bones of the skull and vertebrate offer physical protection for the
delicate nervous tissue of the brain and spinal cord
it's around the nervous tissue
in a case of hard bone that protect it from physical damage
The meninges
are connective tissue membranes covering and protecting structures of the
CNS and offer physical protection of the nervous system
the first layer of the meninges that clings to the surface of the
brain is the pia mater. It's surrounded
by the arachnoid mater which is further surrounded
by the meningeal and periosteal layers of the dura mater
The dura mater is the toughest and most durable of the meningeal layers
it offers two layers of protection
The periosteal layer clings to the skull and acts as the periosteum of the skull bones
the meningeal layer clings to the arachnoid mater
The arachnoid mater
is more delicate than the dura mater and contains many finger-like
projections that cling to the pia mater underneath
cerebrospinal fluid
circulates in the subarachnoid space underneath the arachnoid mater
the arachnoid mater also creates arachnoid villi which project into the
space between the layers
of the dura mater. The pia mater
is a very delicate where that clings tightly to the surface of the brain
following its contours. The cerebrospinal fluid is a watery solution
containing a delicately balanced mixture of ionic
salts, proteins and vitamin C. It is formed from the blood plasma
in the choroid plexuses. Most of the cerebrospinal fluid draines into the
subarachnoid space and into the blood
via the dural venous sinuses through the arachnoid villi
the blood-brain barrier offers chemical protection
for the nervous system neurons and glial cells require a very specific
strictly controlled extracellular environment. They require specific ions
in ion concentrations for action potential propagation
and some amino acids
act as neurotransmitters therefore must be strictly controlled
the blood-brain barrier keep the fluctuating concentrations of
substances in the blood
from affecting the delicately balanced environment of the CSF
the blood-brain barrier features very densely woven capillaries
which are the least permeable capillaries in the body
tight junctions between capillary cells
and the feet of astrocytes surround the capillaries
The blood-brain barrier allows the passage of water
glucose essential amino acids and lipid soluble molecules
but excludes urea, toxins,
proteins, most pharmaceuticals, nonessential amino acids,
and potassium ions which are actively pumped out at the CSF