Meninges, Ventricular System & CSF

Overview

Lecture 21 covers the protective coverings of the brain and spinal cord (the meninges), the fluid-filled spaces within the brain (the ventricular system), and the production, circulation, and function of cerebrospinal fluid (CSF).

The Meninges

The meninges are three layers of protective connective tissue that surround the entire CNS (brain + spinal cord). From outermost to innermost:

1. Dura Mater ("Tough Mother")

• Outermost layer — dense, fibrous, tough.
• Has two layers in the brain (fused except where they separate to form venous sinuses).
• Inner layer forms the dural folds (reflections that stabilise the brain within the cranium).

Dural Folds (3):

| Fold | Location | Function |

|---|---|---|

| Falx cerebri | Median plane, vertical | Separates the two cerebral hemispheres |

| Falx cerebelli | Median plane, vertical | Separates the two cerebellar hemispheres |

| Tentorium cerebelli | Horizontal plane | Separates the cerebrum (above) from the cerebellum (below) |

Venous sinuses form where the two layers of dura mater separate. They collect:

• Venous blood draining from the brain.
• "Used" CSF after it circulates through the ventricular system (via arachnoid granulations).

2. Arachnoid Mater ("Spider-like")

• Middle layer — thin, web-like appearance.
• Lies beneath the dura mater but does not follow into sulci.
• Contains three special features:

1. Subarachnoid space: the space between the arachnoid and pia mater, filled with CSF and blood vessels (lying on top of the pia mater).

2. Arachnoid granulations: finger-like projections that perforate the inner layer of dura mater and transport old CSF from the subarachnoid space into the venous sinus.

3. Arachnoid trabeculae: collagen strands that span the subarachnoid space, connecting arachnoid to pia mater.

3. Pia Mater ("Delicate Mother")

• Innermost layer — transparent, delicate.
• Adheres closely to the brain surface, following all gyri and extending into sulci.
• Blood vessels in the arachnoid space sit on top of the pia mater.

Clinically Important Spaces

| Space | Location | Clinical significance |

|---|---|---|

| Epidural space | Between skull/vertebrae and dura mater | Epidural haematoma (arterial bleed — e.g. middle meningeal artery tear) |

| Subdural space | Between dura and arachnoid | Subdural haematoma (venous bleed — bridging veins; can be chronic) |

| Subarachnoid space | Between arachnoid and pia mater | Subarachnoid haemorrhage (arterial — "thunderclap" headache); site of lumbar puncture |

The Ventricular System

The ventricles are a series of interconnected fluid-filled spaces within the brain, lined by ependymal cells (whose cilia help circulate CSF):

| Structure | Location | Notes |

|---|---|---|

| Lateral ventricles (×2) | One in each cerebral hemisphere | The largest ventricles |

| Third ventricle (1) | In the diencephalon (between the two thalami) | Connected to lateral ventricles by interventricular foramina (foramen of Monro) |

| Cerebral aqueduct (1) | In the midbrain | Connects 3rd ventricle to 4th ventricle |

| Fourth ventricle (1) | At the level of the pons and cerebellum | Drains into the subarachnoid space |

| Central canal | Runs through spinal cord | Continuous with the 4th ventricle |

Cerebrospinal Fluid (CSF)

Production: CSF is produced by the choroid plexus — specialised epithelial structures within the lateral, third, and fourth ventricles. The blood/CSF barrier is formed by tight junctions between choroid plexus epithelial cells.

Circulation path:

Lateral ventricles → 3rd ventricle → cerebral aqueduct → 4th ventricle → subarachnoid space (flows around brain and spinal cord) → absorbed through arachnoid granulations into venous sinuses → returns to systemic circulation.

Functions of CSF:

1. Protection and cushioning — acts as a hydraulic shock absorber, protecting the brain from trauma.
2. Buoyancy — reduces the effective weight of the brain from ~1,400g to ~25g, preventing compression of blood vessels and nerve roots at the base of the skull.
3. Nutrient delivery and waste removal — transports nutrients to neural tissue and removes metabolic waste (including beta-amyloid during sleep — "brain-washing").
4. Chemical stability — maintains stable ionic environment for neurons.

Normal CSF volume: ~150 mL circulating at any one time; ~500 mL produced per day.

Lumbar puncture (spinal tap): a needle is inserted into the subarachnoid space at L3–L4 or L4–L5 (below the conus medullaris) to sample CSF for diagnosis (meningitis, subarachnoid haemorrhage, multiple sclerosis).

Hydrocephalus: obstruction of CSF flow (e.g. aqueduct stenosis) → accumulation of CSF → raised intracranial pressure.