Human Anatomy: Meninges and CSF

Meninges of the Brain

• Dura mater: Outermost layer, thick and tough, with limited stretch ability.
• Arachnoid mater: Middle layer, thinner than dura mater, lacks blood vessels.
  • Subarachnoid space: Area below arachnoid containing cerebrospinal fluid and arachnoid trabeculae.
• Pia mater: Innermost layer, directly touching the brain, very delicate.
• Meningioma: A primary extrinsic brain tumour, often originating from arachnoidal cells and affecting venous sinuses. Causes displacement and compression of brain tissue. Symptoms vary but seizures are common. Treatment typically involves surgical removal.

Cerebrospinal Fluid (CSF)

• CSF: Clear, colorless bodily fluid produced in the choroid plexus, reabsorbed into venous blood. Circulates from brain ventricles to arachnoid granulations.
• Functions: Supplies nutrients, removes waste, cushions the brain.
• Lumbar tap: Needle inserted into the lumbar cistern to measure CSF pressure or to analyze for cellular components, antigens, or antibodies indicative of infection. A clear, colorless sample is normal.
• Hydrocephalus: Imbalance in CSF production and absorption leading to CSF buildup in ventricles.
  • Ovt hydrocephalus: Head enlargement.
  • Occult hydrocephalus: Head size remains normal.
  • Symptoms: Vary, potentially including headache, personality/consciousness changes, and motor/sensory deficits.

Vascular Supply

• Peripheral Nerves: Blood vessels accompany peripheral nerves for nourishment.
• Spinal Cord: Supplied by three spinal arteries: one anterior (2/3 anterior cord) and two posterior (1/3 posterior cord).
• Brain: Supplied by two internal carotid arteries and two vertebral arteries.
  • Internal Carotid Arteries: Supply anterior, superior, and lateral cerebral hemispheres.
  • Vertebral Arteries: Supply brainstem, cerebellum, and posteroinferior cerebrum. Merge to form the basilar artery.
  • Circle of Willis: Anastomosis (connection) of arteries supplying cerebral hemispheres, facilitating collateral blood flow.
  • Cerebral Arteries:
    • Anterior Cerebral Artery (ACA): Supplies medial frontal and parietal lobes.
    • Middle Cerebral Artery (MCA): Supplies internal capsule, globus pallidus, putamen, caudate, and most of lateral hemisphere. Most frequently involved in stroke.
    • Posterior Cerebral Artery (PCA): Supplies midbrain, occipital lobe, and parts of medial/inferior temporal lobe.
    • Brainstem supply: Superior, anteroinferior, and posteroinferior cerebellar arteries.
• Disorders of Vascular Supply: Interrupted blood flow results in focal functional loss. Effects range from transient to permanent damage.

  • Transient Ischemic Attack (TIA): Brief, focal brain function loss, full recovery within 24 hours, likely due to ischemia (reduced blood flow). Significant risk of CVA in next few months.

  • Cerebrovascular Accident (CVA): Lasting neurological deficit from vascular disruption lasting longer than a day – typically involves a larger artery.

    • Infarction: Vessel occlusion (blockage).
      • Embolus: Clot from elsewhere travelling to the brain.
      • Thrombus: Clot forming at the site of blockage.
    • Hemorrhage: Blood vessel rupture causing bleeding into the brain (about 12-15% of strokes).

  • Disorders of Vascular Formation:

    • Arteriovenous Malformation (AVM): Abnormal connections between arteries and veins usually asymptomatic until rupture, causing subdural hematoma or intracerebral hemorrhage.
    • Aneurysm: Weakening and dilation of artery/vein walls, increasing risk of rupture.

Fluid Dynamics

• Blood-Brain Barrier: Specialized membrane between capillaries and brain's extracellular space, preventing pathogen entry into the CNS.
• Cerebral Blood Flow: Constant blood supply needed as the brain cannot store glucose/oxygen. Oxygen consumption increases from brainstem to cortex due to higher energy needs of cortical functions.

CNS Imaging

• Positron Emission Tomography (PET): Radioactive material used to create images showing metabolic function and blood flow. Useful for assessing blood flow, oxygen, or glucose consumption over minutes.
• Computer Tomography (CT) Scan: Two-dimensional slices from x-rays. Safe and non-invasive, but with radiation exposure. Reveals bone/fractures well, limited soft tissue detail. May use contrast medium.
• Magnetic Resonance Imaging (MRI): Strong magnetic field and radio waves for detailed soft tissue imaging, especially useful for identifying tumors, infections, or multiple sclerosis. No known immediate adverse effects, but no metal should be present.
• Magnetic Resonance Angiography (MRA): Injected contrast enhances blood vessel visualization for identifying circulatory lesions in the brain.
• Blood Oxygen Level Dependent (BOLD) MRI: Detects oxygen consumption in different brain regions.
• Diffusion Tensor Imaging (DTI): Measures water diffusion in axons to provide detailed white matter tract imaging; useful for detecting ischemic strokes.
• Functional MRI (fMRI): Measures changes in oxygen consumption to assess brain function; no radioactive material required. Patients may be asked to perform tasks. Provides information about changes in blood flow in seconds.
• Electroencephalography (EEG): Records electrical brain activity with scalp electrodes; useful for detecting seizures.

Organization of the Spinal Cord & Brainstem

• (Omitting details of specific nerves and clinical connections where redundant* )

• Spinal Cord: Extends from foramen magnum to L1-L2, branching into the cauda equina below L1-L2. Cervical and lumbar enlargements for upper and lower limbs.

• Segmental levels: Cervical (8), Thoracic (12), Lumbar (5), Sacral (5), Coccygeal (1) paired spinal nerves, dorsal (sensory) and ventral (motor) roots.

• Gray Matter: "H" shape, containing cell bodies and neuronal capillary network.

• White Matter: Densely packed (myelinated) fibers.

• Ascending vs Descending Tracts: Vertical pathways, conducting sensory (afferent) and motor (efferent) information.

• Segmental vs Vertical Lesions: Segmental issues focus on dermatomal and myotomal areas with specific areas. Vertical issues interrupt tracts impacting areas below the lesion (loss of sensation and movement).

• Brainstem: Critical structure connecting brain to spinal cord with vital functions. Key features include cranial nerve attachment points, cerebellar peduncles. Contains reticular formation for crucial functions like arousal.