Meninges, ventricles and cerebrospinal fluid

Tight junctions seal neighboring cells together near their surface, so substances must go through the cells rather than between them. Gap junctions let ions pass cell to cell; desmosomes anchor cells against pulling.

• Correct: Seals the gap so substances cannot pass between the cells:
• Lets ions pass directly from one cell to the next:
• Anchors the cells' cytoskeletons together against stretching:

Ependymal cells line the brain's internal fluid spaces and the central canal of the spinal cord. Astrocytes support neurons and wrap capillaries; oligodendrocytes make myelin in the CNS.

• Astrocytes:
• Correct: Ependymal cells:
• Oligodendrocytes:

From top to bottom the brainstem is midbrain, pons, medulla oblongata, and the midbrain sits just below the diencephalon. The cerebellum lies behind the brainstem.

• The medulla oblongata:
• The cerebellum:
• Correct: The midbrain:

The dura mater is the thick outer meningeal layer against the inside of the skull. In the skull it has two layers, which split to enclose the venous channel in the midline.

• Correct: Dura mater: Correct. The thick layer against the bone is the dura mater.
• Arachnoid mater: The arachnoid mater is the thin layer just inside the dura, with web-like strands reaching down to the pia.
• Pia mater: The pia mater is the thin layer stuck to the brain surface, following it into the sulci.
• Web-like arachnoid strands: The web-like strands cross the subarachnoid space below the arachnoid; the pinned layer is the thick sheet against the bone.

CSF made in the lateral ventricles passes through the interventricular foramina to the third ventricle, down the cerebral aqueduct to the fourth ventricle, out through the apertures into the subarachnoid space, and back into venous blood at the arachnoid granulations.

• Correct order: 1. Choroid plexus of a lateral ventricle 2. Third ventricle 3. Cerebral aqueduct 4. Fourth ventricle 5. Subarachnoid space 6. Arachnoid granulations

A block at the cerebral aqueduct traps CSF made in the lateral and third ventricles. Those ventricles swell and pressure in the skull rises, while the fourth ventricle below the block stays normal and secretion continues: obstructive hydrocephalus.

• Size of the lateral ventricles: up. The choroid plexuses above the block keep secreting CSF, which cannot pass the aqueduct, so it builds up and swells the ventricles above it.
• Size of the fourth ventricle: no change. The fourth ventricle lies below the block. Its own CSF still drains out through the apertures, so it stays normal size.
• Pressure inside the skull: up. Fluid is added to a closed bony box faster than it leaves, so the swelling ventricles press the brain against the skull and pressure rises.
• Rate of CSF secretion by the choroid plexuses: no change. Secretion is driven by the choroid plexus cells' ion pumping and changes little as pressure rises, which is why the fluid keeps building up.

A blow to the thin temporal bone can tear the middle meningeal artery, which runs on the inside of the skull. Its arterial blood peels the dura off the bone, making a compact, lens-shaped epidural hematoma that stops at the sutures, where the dura is most firmly attached. Arterial pressure makes it grow fast.

• A bridging vein: Torn bridging veins cause a subdural hematoma, which spreads as a thin crescent and usually builds more slowly.
• An artery at the base of the brain: A burst artery at the base of the brain bleeds into the subarachnoid space, mixing blood with CSF rather than forming a lens-shaped clot against the skull.
• A capillary of the choroid plexus: The choroid plexus lies inside the ventricles, far from the skull, and does not cause a clot between skull and dura.
• Correct: The middle meningeal artery: Correct. A torn middle meningeal artery causes an epidural hematoma.

Brain capillary endothelial cells are sealed by tight junctions, so substances must pass through the cells. Small lipid-soluble molecules dissolve straight through the cell membranes, so an anesthetic gas crosses easily.

• A large protein molecule circulating in the plasma: Large proteins cannot pass between the sealed cells, and brain endothelium carries very few transport vesicles, so they barely enter.
• Dopamine injected into a vein: Dopamine is water-soluble and has no carrier protein in the barrier, so it stays in the blood. Its precursor levodopa, an amino acid, is carried across instead.
• Correct: A small, lipid-soluble anesthetic gas: Correct. Small lipid-soluble molecules cross the barrier freely.
• Sodium ions dissolved in the plasma: Charged ions cannot dissolve through the lipid membrane and cannot slip between the sealed cells, so they cross only slowly through specific channels and pumps.

The barrier is the brain capillary endothelium: cells sealed edge to edge by tight junctions, with no pores and few vesicles. Everything entering brain tissue has to pass through these cells.

• The layer of astrocyte end-feet that wraps the outside of each brain capillary: Astrocyte end-feet cover the capillaries and keep the endothelium in its barrier state, but molecules could slip between them; they are not the barrier itself.
• The pia mater covering the brain surface: The pia mater is a meningeal layer on the brain's surface, not part of every capillary wall.
• Correct: The capillary endothelial cells sealed by continuous tight junctions: Correct. The sealed endothelial cells are the barrier.
• The cells covering the choroid plexus: The choroid plexus covering cells form the blood–CSF barrier, a separate barrier between blood and CSF.

Ventricles above a block enlarge and those below stay normal. The third ventricle is enlarged and the fourth is not, so the block lies between them: the cerebral aqueduct.

• At the arachnoid granulations: A failure of return at the arachnoid granulations would back fluid up into all four ventricles, including the fourth.
• At the median and lateral apertures: A block at the apertures would enlarge the fourth ventricle too, since it lies above them.
• At one interventricular foramen: A block at one interventricular foramen would enlarge only that lateral ventricle, not the third ventricle.
• Correct: In the cerebral aqueduct: Correct. A block in the cerebral aqueduct enlarges the lateral and third ventricles and spares the fourth.