The Nervous System: Central Nervous System

Central Nervous System (CNS)

  • Comprises the brain and spinal cord.
  • Critical for homeostasis, perception, thought, and feeling.
  • Contains 10^{11} neurons and 10^{14} synapses.

Glial (Neuroglial) Cells

  • 75-90\% of CNS, non-excitatory.
  • Astrocytes: Star-shaped, involved in communication, neuron development, axon regeneration, blood-brain barrier formation, modulating synaptic activity, neurotransmitter removal (e.g., glutamate), protection from toxins/oxidative stress, and maintaining electrolyte balance (K^{+}).
  • Microglia: Small, protective phagocytes; remove bacteria, dead/injured cells, and protect from oxidative stress.
  • Ependymal cells: Line ventricles, synthesize cerebrospinal fluid (CSF).
  • Oligodendrocytes: Form myelin in CNS.
  • Schwann cells: Form myelin in PNS (mentioned in Ch. 7).
  • Clinical Connection: Glial cells produce inflammatory agents, implicated in neurodegenerative diseases like Multiple Sclerosis (loss of myelin), Alzheimer's (loss of cholinergic neurons), and Parkinson's (loss of dopaminergic neurons).

Physical Support of the CNS

  • Bone: Cranium (brain), vertebral column (spinal cord).
  • Meninges: Three protective layers:
    • Dura mater: "Tough mother".
    • Arachnoid mater: "Spider-web-like".
    • Pia mater: "Tender mother", closely adheres to CNS surface.
  • Cerebrospinal Fluid (CSF):
    • Clear fluid bathing CNS, found in cerebroventricles, central canal, and subarachnoid space.
    • Secreted by the choroid plexus (pia mater, ependymal cells, capillaries).
    • Circulates through ventricles to subarachnoid space, reabsorbed by arachnoid villi.
    • Total volume: 125-150\ \text{mL}; approximately 400-500\ \text{mL/day} produced, recycled thrice daily.
    • Functions: Neutral buoyancy, maintains stable interstitial fluid environment.
    • Differs in composition from plasma (e.g., lower protein, higher chloride).

Blood Supply to the CNS

  • CNS is 2\% of body weight but receives 15\% of blood supply.
  • High metabolic rate: Brain uses 20\% of body's oxygen and 50\% of glucose at rest.
  • Neurons depend on blood flow and glucose; no anaerobic respiration, no glycogen stores (minimal in glia), no fatty acid storage.
  • Cerebrovascular Accident (CVA)/Stroke: Ischemia due to:
    • Occlusive stroke: Atherosclerosis, arteriosclerosis, treated with anticoagulants or tissue plasminogen activator (tPA).
    • Hemorrhagic stroke: Aneurysm, hypertension, often requires surgical intervention.

Blood-Brain Barrier (BBB)

  • Formed by CNS capillaries (endothelium, basement membrane) and astrocytes.
  • Characterized by tight junctions between endothelial cells.
  • Allows passage of hydrophobic solutes (e.g., alcohol) by diffusion.
  • Requires specific transporters for hydrophilic solutes (e.g., GLUT-1 for glucose, insulin-independent).

Gray Matter & White Matter

  • Gray matter (40\% of CNS):
    • Composed of neuronal cell bodies, dendrites, unmyelinated axons, synapses.
    • Site of synaptic transmission and neural integration.
  • White matter (60\% of CNS):
    • Composed of myelinated axons.
    • Responsible for long-distance information transmission.
    • Projection fibers: Connect cerebral cortex with lower brain/spinal cord.
    • Association fibers: Connect two cortical areas on same side of brain.
    • Corpus callosum (commissural fibers): Connect same cortical regions on opposite sides of the brain.