Neuro Intro: Meninges, CSF & BBB

Fig. 8.36 — Lateral view of the brain: frontal, parietal, temporal and occipital lobes, central and lateral sulci — the CNS structure protected within the cranium.

Gray's Anatomy for Students, 4e

The nervous system divides into the CNS (brain + spinal cord) and PNS (everything outside), but the more clinically important distinction is in how each handles damage. In the PNS, Schwann cells myelinate a single axon segment each and actively guide regeneration along endoneurial tubes after Wallerian degeneration at ~1 mm/day; in the CNS, oligodendrocytes myelinate multiple axons but the surrounding environment is actively inhibitory to regrowth (MAG, Nogo, CSPGs from reactive astrocytes and oligodendrocytes). This regeneration gap explains why a severed peripheral nerve can recover over months while a spinal cord injury rarely does. Grey matter in the CNS (cortex and nuclei — cell bodies, dendrites, synapses) corresponds to ganglia in the PNS, while white matter in both systems consists of myelinated axon tracts.

Neurons make up only about 10% of CNS cells — the remaining 90% are glia, each with a distinct role and, importantly, a distinct tumour. Astrocytes (GFAP-positive, most abundant) maintain the BBB through capillary end-feet, buffer extracellular K⁺, and clear glutamate; their injury response (reactive gliosis) is the dominant CNS repair mechanism but also inhibits axon regeneration. Oligodendrocytes myelinate CNS axons (one cell to many axons) and are the autoimmune target in multiple sclerosis — their destruction unmyelinates central tracts, slowing or blocking conduction. Microglia are the brain's resident macrophages, of mesodermal origin (uniquely non-neural), activated in HIV encephalitis, neurodegeneration, and injury. Ependymal cells lining the ventricles produce and circulate CSF; ependymoma in the fourth ventricle is the classical obstructive childhood CNS tumour.

Fig. 2.59 — Meninges: the three layers (dura, arachnoid, pia), the extradural, subdural and subarachnoid spaces, and their relationship to the cord and vertebral canal.

Gray's Anatomy for Students, 4e

The three meningeal layers — dura, arachnoid, and pia — form concentric coverings around the brain, each defining a distinct bleeding compartment. The tough outer dura mater creates four folds (falx cerebri between hemispheres, tentorium cerebelli between cerebrum and cerebellum with its notch for the brainstem, falx cerebelli, diaphragma sellae) and houses the venous sinuses between its two intracranial layers. Arachnoid granulations projecting from the arachnoid into the superior sagittal sinus are the main CSF absorption site. Three traumatic haemorrhages map precisely to these spaces: extradural (middle meningeal artery rupture at the pterion → biconvex CT lesion with classic lucid interval, does not cross sutures); subdural (bridging vein tears → crescent CT lesion crossing sutures, commonest in elderly/anticoagulated); subarachnoid (berry aneurysm rupture → thunderclap headache, xanthochromic CSF after 2 hours, hyperattenuating cisterns on CT).

CSF is produced at ~500 mL/day by the choroid plexus (mainly in the lateral ventricles) and follows a one-way highway: lateral ventricles → foramen of Monro → third ventricle → aqueduct of Sylvius (the narrowest segment, most vulnerable to obstruction) → fourth ventricle → foramina of Luschka (two lateral apertures) and Magendie (one midline aperture) → subarachnoid space → absorbed at arachnoid granulations into the superior sagittal sinus. Obstruction within the ventricular system causes non-communicating hydrocephalus (commonest site: aqueduct stenosis); impaired reabsorption after meningitis or SAH causes communicating hydrocephalus. Normal pressure hydrocephalus (NPH) in the elderly demonstrates the classic "Wet, Wacky, Wobbly" triad (urinary incontinence, dementia, gait apraxia) — the gait prominence distinguishes it from Alzheimer's, and temporary LP drainage confirms the diagnosis before VP shunting.

The blood-brain barrier is formed by tight junctions between cerebral capillary endothelial cells, maintained by astrocyte end-feet and pericytes, acting as a selective gateway that protects the brain while making CNS infections hard to treat. Lipid-soluble molecules (O₂, CO₂, ethanol, steroids, most anaesthetics) cross freely; hydrophilic drugs cannot — penicillin G only reaches therapeutic CNS levels when the BBB is inflamed by meningitis, justifying high-dose IV therapy. Dopamine cannot cross the BBB, so Parkinson's disease is treated with its precursor L-DOPA (crosses via amino acid transporters, then converted to dopamine inside the brain). The circumventricular organs — area postrema (chemoreceptor trigger zone), subfornical organ, pineal gland, neurohypophysis — deliberately lack a BBB to sample blood composition; the area postrema detects toxins and triggers vomiting, explaining why ondansetron (5-HT3 antagonist) acts there.

The BBB is formed by tight junctions between cerebral capillary endothelial cells, supported by astrocyte end-feet + pericytes. Restricts passage of molecules from blood to CNS.

Fig. 8.43 — Dural venous sinuses: superior & inferior sagittal, straight, transverse, sigmoid and cavernous sinuses draining to the internal jugular vein.

Gray's Anatomy for Students, 4e

The dural venous sinuses are blood-filled channels formed between the two layers of the dura mater, draining cerebral venous blood toward the internal jugular veins. The superior sagittal sinus (SSS), running along the top of the falx, receives cortical bridging veins and houses arachnoid granulations for CSF absorption; SSS thrombosis causes headache, papilloedema, and parasagittal cortical infarction (bilateral leg weakness), with the "empty delta sign" on contrast CT. The cavernous sinus flanks the sella turcica and is anatomically unique — CN III, IV, V1, V2 run in its lateral wall (mnemonic O TOM CAT: Oculomotor, Trochlear, Ophthalmic, Maxillary), while CN VI and the internal carotid artery float freely within the sinus itself; cavernous sinus thrombosis from facial infection causes proptosis, chemosis, and complete ophthalmoplegia. The transverse-sigmoid system drains laterally to the jugular foramen and is at greatest risk from mastoiditis (lateral sinus thrombosis — picket-fence fever).

LP is performed at L3/L4 or L4/L5 because the spinal cord ends at L1/L2 (conus medullaris) in adults, leaving only freely floating cauda equina roots below — identified using Tuffier's line (connecting the iliac crests crosses L4). Layers traversed from skin to CSF: supraspinous ligament → interspinous ligament → ligamentum flavum → epidural space → dura → arachnoid → subarachnoid space. CSF analysis then cleanly separates meningitis causes: bacterial (turbid, neutrophils >1000, glucose ↓↓, protein ↑↑↑); viral (clear, lymphocytes 10–1000, glucose normal); TB (clear/fibrin web, lymphocytes, glucose ↓, protein ↑↑). In suspected meningococcal septicaemia (non-blanching rash) or raised ICP, antibiotics must be given before LP — never delay treatment for a diagnostic procedure. Meningism signs (Kernig's, Brudzinski's, nuchal rigidity) confirm meningeal irritation clinically but have only ~50–60% sensitivity.

The Monro-Kellie doctrine governs intracranial pressure: the skull is rigid, so any volume increase in brain, blood, or CSF must be compensated by a decrease in another compartment or ICP rises. Cerebral perfusion pressure (CPP = MAP − ICP) must be kept ≥60 mmHg; when ICP rises unchecked, Cushing's triad appears — hypertension with widened pulse pressure, bradycardia, and irregular respirations — a terminal sign of impending brainstem compression. Uncal herniation (temporal mass pushing the uncus over the tentorial edge) compresses CN III first (ipsilateral fixed dilated pupil before ophthalmoplegia) then the posterior cerebral artery. Management escalates through steps: head elevation and sedation → osmotherapy (mannitol or hypertonic saline) → brief hyperventilation (CO₂-driven vasoconstriction, temporary) → surgical decompression; dexamethasone reduces vasogenic oedema around tumours but worsens outcomes in TBI and stroke. IIH (obese women of childbearing age — headache, pulsatile tinnitus, papilloedema, normal CSF composition) is treated with weight loss + acetazolamide; the chief danger is permanent visual loss from optic nerve damage.

Normal ICP: 5–15 mmHg. Raised ICP (>20 mmHg sustained) = neurological emergency. Monro-Kellie doctrine: intracranial volume is fixed (skull rigid) → any increase in one compartment (brain, blood, CSF) must be compensated by decrease in another, or ICP rises.

The cerebral circulation divides into anterior (ACA, MCA) and posterior (PCA, basilar, PICA, AICA) territories, each producing a recognisable syndrome when occluded. MCA is the most common stroke territory — contralateral face + arm weakness > leg, hemisensory loss, and homonymous hemianopia; dominant hemisphere involvement adds aphasia (Broca's for expression at the inferior frontal gyrus, Wernicke's for comprehension at the posterior superior temporal gyrus). PICA infarct (Wallenberg/lateral medullary syndrome) causes the classic crossed sensory loss — ipsilateral face pain/temperature loss + Horner's + dysphagia/hoarseness + contralateral body sensory loss, without hemiplegia (corticospinal tract is ventral and spared). Basilar artery occlusion causes locked-in syndrome: bilateral pontine infarct with bilateral hemiplegia and anarthria but preserved vertical eye movements and consciousness. Lacunar infarcts (small vessel lipohyalinosis in perforating arteries) produce pure motor, pure sensory, or ataxic-hemiparesis syndromes without cortical features; watershed infarcts from global hypoperfusion damage zones between adjacent territories.

15.9.1 — Circle of Willis & Cerebral Arteries

Fig. 8.38 — Arterial supply to the brain. A. The cerebral arterial circle (of Willis) linking the internal carotid and vertebrobasilar systems via the communicating arteries. B–C. MR/CT angiograms.

Gray's Anatomy for Students, 4e

The Circle of Willis (circulus arteriosus) is an anastomotic polygon at the base of the brain formed by: bilateral ACAs (joined by AComm), bilateral MCAs, bilateral PCAs (joined to ICA by PComms). Complete circle in only 20–25% — frequent anatomical variants. Functions as collateral bypass when one vessel occluded.

Artery	Territory	Stroke Syndrome
ACA (Anterior Cerebral)	Medial surface of frontal + parietal lobe; anterior corpus callosum. Leg area of motor + sensory cortex	Contralateral LEG > arm/face weakness + sensory loss. Urinary incontinence (medial frontal lobe). Behavioural changes (prefrontal cortex). Grasp reflex. Bilateral ACA infarct (from AComm aneurysm or ICA bifurcation occlusion) → akinetic mutism (alert but mute + immobile — bilateral supplementary motor cortex)
MCA (Middle Cerebral)	Lateral surface of frontal + parietal + temporal lobes; deep (lenticulostriate arteries) → putamen, caudate, internal capsule	Most common stroke territory. Face + arm >> leg weakness (contralateral). Hemisensory loss. Dominant hemisphere (L in R-handed): Broca's area (inferior frontal gyrus) → expressive aphasia; Wernicke's area (posterior STG) → receptive aphasia. Non-dominant: hemispatial neglect, anosognosia. Homonymous hemianopia (optic radiation). Internal capsule infarct (lenticulostriate) → pure motor hemiplegia or pure sensory stroke
PCA (Posterior Cerebral)	Occipital lobe (visual cortex) + posterior temporal + thalamus + midbrain	Contralateral homonymous hemianopia WITH macular sparing (dual MCA/PCA supply to macular cortex). Thalamic infarct: contralateral hemisensory loss ± thalamic pain (Déjérine-Roussy syndrome). Top-of-basilar syndrome: bilateral PCA occlusion → cortical blindness (Anton's syndrome — patient denies being blind) + altered consciousness
PICA (Post. Inferior Cerebellar)	Lateral medulla + inferior cerebellum	Wallenberg syndrome (lateral medullary syndrome): ipsilateral face pain/temperature loss (CN V nucleus) + Horner's syndrome (descending sympathetic) + dysphagia/hoarseness (CN IX/X nuclei) + ataxia (cerebellum/inf. cerebellar peduncle). Contralateral body pain/temperature loss (spinothalamic tract). No hemiplegia (corticospinal tract in ventral medulla, spared). "Crossed sensory loss" = lateral medullary infarct until proven otherwise
AICA (Ant. Inferior Cerebellar)	Lateral pons + inner ear (labyrinthine artery) + anterior cerebellum	Ipsilateral: facial sensory loss + CN VI (abducens) + CN VII palsy + sensorineural deafness + vertigo (inner ear ischaemia). Contralateral body sensory loss. Ipsilateral ataxia
Basilar artery	Pons + midbrain + supplies SCA + AICA	Basilar artery occlusion = locked-in syndrome: bilateral pontine infarct → bilateral hemiplegia + anarthria + dysphagia. Preserved: vertical eye movements + blinking (corticospinal fibres to CN III intact above). Patient conscious but cannot move; communicates by eye movement. Distinguish from vegetative state (vegetative = unconscious, no voluntary movement)

A ring-enhancing lesion — central non-enhancement (necrosis, pus, or cyst) surrounded by a contrast-enhancing ring with oedema — is non-specific and always demands a differential with the MAGIC mnemonic. Metastases are the most common cause in adults over 40, typically multiple at the grey-white junction with disproportionate oedema. A cerebral abscess has a smooth, thin, regular ring with the critical distinguishing feature of restricted diffusion centrally on DWI-MRI — pus is viscous and cellular, restricting water movement, unlike the free diffusion of necrotic GBM. GBM shows a thick, irregular ring, may cross the corpus callosum as a "butterfly glioma", and has free central diffusion. In AIDS patients with CD4 <100, multiple lesions in the basal ganglia are treated empirically for toxoplasmosis (pyrimethamine + sulfadiazine) — failure to respond in 2 weeks mandates biopsy to exclude primary CNS lymphoma. LP is contraindicated in all these conditions given the risk of herniation — MRI is the investigation of choice.