The brain is the best protected organ in the body. It has multiple layers of
protection starting with the first layer of protection being the
skull or cranium, which acts as armor shielding the brain from
blows. The next layer of protection is the meninges
which has three membranes that surround the brain and spinal cord to keep it
from being damaged by contact with the inside of the skull. The final layer of
protection is the cerebrospinal fluid (CFS) that
the brain and spinal cord basically float in. |
This protection does have its limits. The greater the impact or trauma, the more
likely it will be that the brain will be injured. A traumatic brain injury (TBI)
is defined as a blow or jolt to the head or a penetrating head injury that
disrupts the function of the brain. Not all blows or jolts to the head result in
a TBI. The severity of such an injury may range from "mild" (causing a brief
change in mental status or consciousness) to "severe" (causing an extended
period of unconsciousness or amnesia after the injury. A TBI can result in short
or long-term problems with independent function).
The brain is housed inside of a bony covering called the cranium. The cranium
protects the brain from injury and along with the bones that protect the face
are called the skull. Between the skull and brain is the meninges, which
consist of three layers of tissue that cover and protect the brain and spinal
cord. These membranes, however, can become infected when someone gets
meningitis, and because the meninges are in direct contact with the brain, it
can be dangerous. From the outermost layer inward they are the
dura mater, arachnoid
and pia mater.
The dura mater or pachymeninx is made up of two
layers of whitish membrane. The outer layer (endosteal) is called the periosteum.
An inner layer (meningeal), lines the inside of the entire skull and creates
little folds or compartments in which parts of the brain are protected and secured.
Because the two layers are indistinguishable except in a few areas, it's simpler
to consider the dura as one layer, which serves as both endocranium and meninx.
Instead of being considered as separating two layers, the venous sinuses are
usually described as being situated within a single dura.
The two special folds of the dura in the brain are called the falx and the
tentorium. The falx separates the right and left half of the brain and the
tentorium separates the upper and lower parts of the brain.
The dura, like the scalp, is supplied by both cranial (chiefly the trigeminal)
and cervical nerves. The brain itself is normally insensitive, and headaches are
commonly either of vascular (intracranial or extracranial) or dural origin.
The leptomeninges include the arachnoid and pia
mater. These layers bound the subarachnoid space, which is limited externally
by a water-tight layer of connective tissue, the arachnoid, and internally by
a thinner layer, the pia mater. The pia mater adheres intimately to the surface
of the brain and spinal cord. There is a trabecular structure of connections
between the arachnoid and the pia that bridges the subarachnoid space which is
otherwise full of circulating CSF.
The arachnoid is a thin and delicate membrane
that surrounds the brain loosely and is separable from the dura by a potential
space into which subdural hemorrhage may occur. There is a space between the dura
and the arachnoid membranes that is called the subdural space. The arachnoid is
made up of delicate, elastic tissue and blood vessels of varying sizes.
The arachnoid dips into the longitudinal interhemispheric fissure but not into
the sulci. Near the dural venous sinuses, the arachnoid has microscopic projections,
called arachnoid villi, which are believed to be concerned with the absorption
of CSF. Enlargements of the villi, known as arachnoid granulations, enter some
of the sinuses and their associated lateral lacunae and are visible to the naked
eye. Both the granulations and the lacunae lie in granular pits on the internal
aspect of the calvaria.
The pia mater is the layer of meninges closest
to the surface of the brain. It covers the entire surface of the brain intimately
and follows the brain into the gyri of the cerebral hemispheres and the folia of
the cerebellum. The pia mater has many blood vessels that reach deep into the
surface of the brain. The major arteries supplying the brain provide the pia
with its blood vessels. The space that separates the arachnoid and the pia is
called the subarachnoid space. It's within this area that the CSF flows.
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The Ventricular System
The ventricular system is divided into four cavities called ventricles, which
are connected by a series of holes called foramen, and tubes. Two ventricles
enclosed in the cerebral hemispheres are called the lateral ventricles (first
and second). They each communicate with the third ventricle through a separate
opening called the Foramen of Munro. The third ventricle is in the center of the
brain, and its walls are made up of the thalamus and hypothalamus.
The third ventricle connects with the fourth ventricle through a long tube
called the Aqueduct of Sylvius. CSF flowing through the fourth ventricle flows
around the brain and spinal cord by passing through another series of openings.
The space between the arachnoid and the pia mater, the subarachnoid space, contains
CSF. The ventricles of the brain contain vascular choroid plexuses, from which CSF
is formed. This fluid circulates through the ventricles, enters the subarachnoid
space, and eventually filters into the venous system. CSF protects the brain which
basically floats. It serves to minimize damage from blows to the head and neck.
CSF surrounds or bathes the brain and the spinal cord. It's a clear, watery
and almost protein-free liquid that acts as a fluid buffer for the protection
of the nervous tissue. It also compensates for changes in blood volume within the
CSF circulates between the pia mater and the arachnoid mater of the meninges.
In addition to cushioning blows, this fluid reduces the pressure at the base
of the brain by causing the nerve tissue to "float". The brain normally maintains
a balance between the amount of CSF that is absorbed and the amount that is
produced causing the pressure around the brain to be constantly balanced.
The CSF is produced and circulates through a series of communicating cavities
or channels called ventricles. The ventricles allow the CSF to flow around the
spinal cord and brain, while constantly being absorbed and replenished. A
specialized structure within each ventricle, called the choroid plexus, is
responsible for the majority of CSF production.
CSF is secreted by the choroid plexus in the upper ventricles and absorbed by
the venous system at the base of the brain. As this fluid flows downward, it
carries away toxic wastes and moves hormones between widely separated regions
of the brain. The arachnoid villi and arachnoid granulations are responsible
for the drainage of CSF into the venous sinuses of the cranial dura and the
The CSF may be examined by means of lumbar puncture (LP) or spinal tap.