High Yield Review for Nervous Tissue

Nervous Tissue Terminology

• Two cell types:
  • Neurons: conduct and transmit action potentials.
  • Glial cells: support and maintain neurons in the extracellular environment; do not conduct or transmit action potentials.

Key Glial Cells and Their Functions

• Astrocytes: set up and maintain the blood-brain barrier.
• Oligodendrocytes (dendrocytes): found in the central nervous system (CNS); responsible for myelin production.
• Schwann cells: found in the peripheral nervous system (PNS); responsible for myelin production.
  • Oligodendrocytes and Schwann cells both produce myelin, but are located in different parts of the nervous system (CNS vs. PNS).

Potentials Review: Membrane, Resting, and Action Potentials

• Membrane Potential:
  • Voltage difference across membranes.
  • Due to the difference in ion distribution from one side of the membrane to the other.
• Resting Membrane Potential:
  • The membrane potential when the cell is at rest.
  • Inside the membrane has more negative charges than outside.
  • Key ion channels:
    • Sodium-potassium pump (Na+/K+ ATPase).
      • Actively transports 3 sodium ions out of the cell and 2 potassium ions into the cell, maintaining the electrochemical gradient and contributing to the negative resting membrane potential.
      • 3Na^+ \text{ out} : 2K^+ \text{ in}
    • Sodium and potassium leak channels.
      • Main driver of the resting membrane potential is the potassium leak channels, allowing potassium ions to diffuse out of the cell down their concentration gradient.
• Action Potentials:
  • Changes in voltage from the resting membrane potential, where the inside becomes more positive than the outside.
  • Charge flips or shifts.

Channels Associated with Action Potentials

• Voltage-gated channels:
  • Sodium voltage-gated channels.
  • Potassium voltage-gated channels.

Phases of the Action Potential

• Polarized: at rest, resting membrane potential (negative inside, positive outside).
• Depolarized: positive on the inside, negative on the outside.
  • Mediated by voltage-gated sodium channels opening, allowing sodium ions to rush in.
  • Voltage Gated Sodium Channels: VG
• Repolarized/Hyperpolarized: back to negative on the inside, positive on the outside.
  • Mediated by voltage-gated potassium channels opening, allowing potassium to rush out.
  • If hyperpolarized, sodium-potassium ATPase is used to return to the resting membrane potential.

Voltage-Gated Sodium Channels: Activation and Inactivation Gates

• Two gates:
  • Inactivation gate (bottom).
  • Activation gate (top).
• Configuration varies depending on the phase.
  • Rest:
    • Activation gate: closed.
    • Inactivation gate: open.
  • Depolarization:
    • Activation gate: open.
    • Inactivation gate: open.
    • Ions (sodium) rush in.
  • Repolarization:
    • Activation gate: open (eventually closes).
    • Inactivation gate: closed.

Refractory Periods

• Related to the state of the sodium channels.
• Absolute Refractory Period:
  • No message can get through, regardless of stimulus strength.
  • Inactivation gate is closed, and activation gate is open; the channel is blocked.
• Relative Refractory Period:
  • Sodium channels are at rest.
  • Inactivation gate is open, and activation gate is closed.
  • A large enough stimulus can open the gates, allowing the message to get through.

Factors Affecting Action Potential Speed

• Slowing or Blocking Action Potentials:
  • Increases in calcium ions.
  • Anesthetics (sodium channel blockers).
  • Tetrodotoxin (puffer fish).
  • Decreases in temperature.
• Speeding Up Action Potentials:
  • Myelination: allows action potential to jump from node to node (saltatory conduction).
    • Nodes of Ranvier: the gaps between myelin sheaths where the action potential jumps.
  • Large diameter of the neuron (axon).
    • Increases conduction velocity.