brain protection Flashcards
(11 cards)
nervous tissue
Nervous tissue is soft and delicate, and even slight pressure can injure the irreplaceable neurons. As we saw in Chapter 5, nature tries to protect the brain and spinal cord by enclosing them within bone (the skull and vertebral column). Now, let’s focus on three additional protections for the CNS: the meninges, cerebrospinal fluid, and blood-brain barrier.
menigines
The three connective tissue membranes covering and protecting the CNS structures are meninges
dura matter
The outermost layer of the meninges is the dura mater.
Around the brain, the dura mater has two layers:
Periosteal layer – the outer layer, attached to the inside of the skull.
Meningeal layer – the inner layer, which continues down to cover the spinal cord.
These two layers are fused together in most places, but in some areas, they separate to form spaces called dural venous sinuses.
These sinuses collect venous blood from the brain and drain it into the veins.
, the meningeal layer of the dura mater folds inward to:
Anchor the brain to the skull,
Stabilize it so it doesn’t move too much,
And separate parts of the brain to prevent them from bumping into each other when the head moves or shakes.
the dural fold
Falx cerebri
Sits between the left and right cerebral hemispheres
Keeps them from rubbing against each other
Tentorium cerebelli
Separates the cerebrum from the cerebellum
Supports and protects both
anchoid layer
Arachnoid Mater (Middle Layer)
The arachnoid mater is the middle meningeal layer.
The name “arachnoid” comes from the Greek word for spider—because it looks like a web.
It’s delicate and has threadlike extensions that stretch across the subarachnoid space.
Subarachnoid Space:
The space beneath the arachnoid mater.
Filled with cerebrospinal fluid (CSF), which cushions the brain and spinal cord.
The web-like threads connect the arachnoid mater to the pia mater below.
piamatter
🧠 Pia Mater (Innermost Layer)
Very thin and delicate.
Clings tightly to the surface of the brain and spinal cord, following every fold and groove (sulcus and gyrus).
Called the “gentle mother” because it’s soft and nurturing in function.
csf fluid
Cerebrospinal Fluid (CSF) and the Subarachnoid Space
The subarachnoid space (between the arachnoid mater and pia mater) is filled with CSF.
CSF cushions and protects the brain and spinal cord.
💧 Where CSF Comes From
CSF is produced by the choroid plexuses.
These are special structures made of capillaries and ependymal cells.
Located in the ventricles (fluid-filled spaces in the brain).
🔁 How CSF Is Drained (Absorbed)
Arachnoid granulations (also called arachnoid villi) are tiny projections of the arachnoid mater.
These granulations protrude through the dura mater and stick into the dural venous sinuses (like the superior sagittal sinus).
Their job: to absorb CSF from the subarachnoid space and drain it into the bloodstream.
menigitis
Meningitis, an inflammation of the meninges, is a serious threat to the brain because bacterial or viral meningitis may spread into the nervous tissue of the CNS. This condition of brain inflammation is called encephalitis (en-sef-ah-li′tis). Meningitis is usually diagnosed by taking a sample of cerebrospinal fluid from the subarachnoid space surrounding the spinal cord.
csf production
What is CSF?
Cerebrospinal fluid (CSF) is a clear, watery fluid made from blood plasma, but:
It contains less protein
More vitamin C
Has a different ion balance
CSF cushions the brain and spinal cord, acting like a shock absorber.
It also helps the brain “float”, preventing it from being crushed by its own weight.
🏗️ Where CSF is Made
Choroid plexuses = Clusters of blood capillaries + ependymal cells.
Found in the roof of each ventricle (fluid-filled chamber in the brain).
Continuously produce CSF from filtered blood.
🌀 CSF Circulation Pathway
Lateral Ventricles
Two large chambers in the cerebral hemispheres
CSF is first produced here
Third Ventricle
In the diencephalon
CSF flows here via the interventricular foramina (small channels)
Cerebral Aqueduct
A narrow canal in the midbrain
Connects the third and fourth ventricles
Fourth Ventricle
Located behind the pons and medulla oblongata
From the fourth ventricle, CSF goes:
Down the central canal of the spinal cord, and/or
Into the subarachnoid space via:
Two lateral apertures
One median aperture
Subarachnoid Space
CSF flows around the brain and spinal cord
Provides cushioning and nutrient exchange
Arachnoid Granulations
Small projections of the arachnoid mater
Absorb CSF back into the dural venous sinuses (e.g., superior sagittal sinus)
CSF is returned to blood circulation
Ordinarily, CSF forms and drains at a constant rate so that its normal pressure and volume (150 ml—about half a cup) are maintained. Any significant changes in CSF composition (or the appearance of blood cells in it) could indicate meningitis or certain other brain pathologies (such as tumors or multiple sclerosis). A procedure called a lumbar (spinal) puncture can obtain a sample of CSF for testing. Because the withdrawal of fluid decreases CSF fluid pressure, the patient must remain horizontal (lying down) for 6 to 12 hours after the procedure to prevent an agonizingly painful “spinal headache.”
when csf is blocked from drainage
If something obstructs its drainage (for example, a tumor), CSF begins to accumulate and exert pressure on the brain. This condition is hydrocephalus (hi-dro-sef′ah-lus), literally, “water on the brain.” Hydrocephalus in a newborn baby causes the head to enlarge as the brain increases in size. This is possible in an infant because the skull bones have not yet fused. However, in an adult this condition is likely to result in brain damage because the skull is hard, and the accumulating fluid creates pressure that crushes soft nervous tissue and could restrict blood flow into the brain. Today hydrocephalus is treated surgically by inserting a shunt (a plastic tube) to drain the excess fluid into a vein in the neck or abdomen.
blood brain barrier
Why the Brain Needs Extra Protection
The brain is extremely sensitive to changes in its internal environment.
Other body tissues can handle small fluctuations in:
Ions (like sodium and potassium)
Hormones
Nutrients
But in the brain, even small changes in chemical composition can lead to uncontrolled neural activity, seizures, or damage—because:
Ions are critical for nerve impulses
Some amino acids act as neurotransmitters
🛡️ The Blood-Brain Barrier (BBB)
Function: Protects brain tissue from harmful substances in the blood.
Structure:
Made of the tightest capillaries in the entire body.
Endothelial cells in brain capillaries are joined by tight junctions, forming an almost leak-proof barrier.
Supported by astrocytes (glial cells) whose “feet” help regulate what gets through.
✅ What Can Cross the BBB:
Water
Glucose
Essential amino acids
Fat-soluble molecules, including:
Oxygen (O₂) and carbon dioxide (CO₂)
Alcohol
Nicotine
Anesthetics
❌ What Cannot Cross the BBB (or are pumped out):
Urea and toxins
Proteins
Most drugs
Nonessential amino acids
Potassium ions
In fact, some unwanted substances are actively pumped back out of the brain into the bloodstream.
