Nervous System: Nerve Cells and Nerve Impulses

Evolutionary Perspective

  • Nervous tissue exists in most multicellular animals.
  • Some single-celled organisms use electrical signals for information transmission.
  • Sponges (600 million years ago) lack neurons but possess synaptic junctions.
  • Jellyfish have diffuse nerve nets without a central nervous system.
  • Bilaterians (animals with bilateral symmetry) feature:
    • Nerve cord.
    • Brain.
    • Anus.
    • Mouth.
    • Gut.
  • Cephalisation: Nervous tissue concentrates toward one end of the organism, forming a brain.

Nervous System

  • Central Nervous System (CNS): Brain and spinal cord.
  • Peripheral Nervous System (PNS): All nervous system components outside the brain and spinal cord (e.g., nerves from sense organs to the CNS).

Microstructure of the Nervous System

  • Nerve cells (neurons) share a basic architecture and function.
  • Glia cells support neurons.
  • Santiago Ramón y Cajal and Camillo Golgi (Nobel Prize 1906) established that neurons are separable with small gaps between them, indicating that the nervous system consists of individual neurons.

Cells in the Nervous System

  • Neurons / nerve cells
    • Cerebellum: 70 billion neurons.
    • Cerebral cortex: At least 12-15 billion neurons.
    • Spinal cord: 1 billion neurons.
  • Glia / glial cells / neuroglia
    • Smaller than neurons but outnumber them (estimates vary, recent estimate: x 1.2).
    • Most common types: oligodendrocytes (76%), astrocytes (17%), and microglia (6%).

Glia Functions

  • Support cells with vital roles:
    1. Provide structure by surrounding and holding neurons in place (astrocytes).
    2. Insulate nerve cells with myelin sheaths (oligodendrocytes in the CNS, Schwann cells in the PNS).
    3. Supply nutrients and oxygen to neurons (astrocytes).
    4. Remove dead neuronal tissue and provide immune defense in the CNS (microglia: phagocytes).
    5. During development, provide scaffolds for neuronal migration (radial glia).
    6. Modulate neurotransmission in synapses.

Multiple Sclerosis (MS)

  • A neurological condition affecting approximately 100,000 people in the UK.
  • Common symptoms: vision problems, fatigue, difficulties with walking.
  • Cause: demyelination of axons in the brain and spinal cord.

Neuron Structure

  • Neurons specialize in information processing.
  • Each neuron comprises a cell body (soma) and fibers (dendrites and an axon).

Soma (Cell Body)

  • Contains the cell nucleus and