Neuroscience: Nervous System Anatomy and Physiology Notes

Cerebrospinal Fluid

  • Nature's Protective Mechanism:
    • Inhibits neural regeneration in the CNS.
    • Produces a sheath that wraps around the axon, increasing action potential.
    • The sheath is composed of a white phospholipid layer that insulates myelinated axons.

Microglia

  • Function:
    • Small glial cells that migrate through neural tissue to clean up cellular debris, waste products, and pathogens via phagocytosis (cell eating).

Neuroglia of the Peripheral Nervous System

  1. Satellite Cells (Amphicytes):

    • Surround neuron cell bodies in ganglia.
    • Regulate the environment around neurons.

  2. Schwann Cells (Neurilemmacytes):

    • Form the myelin sheath (neurilemma) around peripheral axons.
    • One Schwann cell sheaths one segment of an axon, but many Schwann cells can sheath an entire axon.
    • Enable possible peripheral nerve regeneration.

Neural Response to Injury

  • Wallerian Degeneration:
    • Occurs when the axon distal to the injury site degenerates.
    • Macrophages migrate to clean up debris.
  • Schwann Cell Response:
    • Schwann cells do not regenerate; instead, they proliferate and form a cellular cord along the path of the axon.
    • As the neuron recovers, the axon grows into the injury site and the Schwann cells wrap around it, resuming normal synaptic activity.

Injury Site Steps

  1. Fragmentation of axon and myelin occurs at the distal stump.
  2. Schwann Cells Form Cord:
    • Schwann cells grow into the cut and unite stumps.
    • Macrophages engulf degenerating axon and myelin.
  3. Axon Growth:
    • Axon sends buds into the network of Schwann cells and then grows along the cord of Schwann cells.
    • The axon continues to grow into the distal stump and is enclosed by Schwann cells.

Resting Membrane Potential

  • Key Concept:
    • Minimum stimulus to generate action potential involves sodium (Na+) and potassium (K+) ions.
    • Resting membrane potential is typically around -70 mV due to ion concentration differences and selective permeability.

Requirements for Transmembrane Potential

  1. Concentration Gradient:
    • Higher concentration of sodium (Na+) outside the cell and potassium (K+) inside.
  2. Selective Permeability:
    • Ions move through channels, affecting membrane charge.
  3. Charge Difference:
    • Maintains resting potential of approximately -70 mV.

Electrochemical Gradient

  • Represents the sum of chemical and electrical forces acting on ions across a cell membrane, functioning as potential energy.

Active Forces Across Membrane

  • Sodium-Potassium ATPase:
    • Active transport that requires ATP to move 3 Na+ out and 2 K+ into the cell, balancing diffusion forces and maintaining resting potential (-70 mV).

Changes in Transmembrane Potential

  • Potential varies in response to temporary changes in membrane permeability, influenced by the opening/closing of ion channels (chemically, voltage, mechanically gated).

Sodium and Potassium Channels

  • Membrane Permeability:
    • Influenced by Na+ and K+ channels, which can be passive (always open) or active (gated and respond to stimuli).
    • Passive channels help maintain resting potential; active channels open/close in response to stimuli.

Gated Channels

  1. Chemically Regulated Channels:
    • Open in presence of specific chemicals (e.g., ACh) and are found in neuron cell bodies and dendrites.
  2. Voltage-Regulated Channels:
    • Respond to changes in membrane potential. Characteristic of excitable membranes, such as in neural axons and muscle cells.
  3. Mechanically Gated Channels:
    • Open in response to physical deformation of the membrane.