Physiolec: Module 1

• Graded Potential


  Leaky channels, short distance/incremental response
• Action Potential


  Voltage-gated channel, all-or-none response
• Resting Membrane Potential


  Gated channels are inactive, -70 mV
• Depolarization


  1. Opening of sodium channels

  2. High conduction with respect to time

  3. Voltage-gated

  4. Activation & inactivation

  5. Open because of the concentration gradient

  6. Na+ goes in making it positive

  7. No conformational change in leaky channels

  8. High sodium = goes in at a faster rate
• Repolarization

  1. Membrane potential goes back to resting

  2. Extracellular environment is positive (+) while intracellular environment is negative (-)

  3. Potassium channels slowly start to open

  4. More negative than the resting membrane potential

  5. May approach the K+ equilibrium potential
• Hyperpolarization

  1. Na+-gated channels are closed

  2. Voltage-gated channels are still open

  3. K+-gated channels are still open
• Relationship of Voltage-Gated Na+ and K+ Channels During an Action Potential


  • Feedback mechanisms establishes homeostasis
  • A suprathreshold graded potential stimulates both Na+ and K+ channels to open
  • Na+ channels open immediately, and the resulting influx of Na+ causes even more Na+ channels to open, in a positive feedback loop
  • K+ channels open more slowly, becoming fully opened around the time that the Na+ channels close and causing an efflux of K+ ions that repolarizes the membrane
  • K+ ions may continue to leave the cell and cause the membrane to hyperpolarize
  • Repolarization and hypolarization remove the stimulus to open K+ channels, causing them to close
• Absolute Refractory Period


  Period where additional stimulus cannot generate action potential
• Relative Refractory Period


  Higher than threshold is needed to generate another action potential
• From depolarization to repolarization is the absolute refractory period
• Hyperpolarization is the relative refractory period, above the threshold
• Cannot produce action potential, counter intuitive, unidirectional signal while stimulation of action potential is bidirectional
• Cell Body


  • Bidirectional impulses due to absence of refractory period
• Axon Hillock


  • Unidirectional impulse due to presence of refractory period
• Specificity


  • Signals specific organs
• Energy


  • Produces heat
• Myelination


  Insulating layer of lipid-rich Schwann cells (peripheral nervous system) or Oligodendrocytes (central nervous system) wrapped around axons
• Schwann cells and Oligodendrocytes are types of Glial cells which are cells other than neurons that support neuron function
• Myelination


  • Reduces "leakage" of charge across membranes
  • Prevents leaking (for insulation) and increase velocity (allows signal to travel from node to node = saltatory conduction in Nodes of Ranvier)
• Conduction Velocity


  • Axon Diameter
  • Myelination
  • Myelinated > unmyelinated
• Type of Conduction


  Continuous < saltatory = higher velocity
• Electrical Synapse


  • Connected via gap junctions (relay signals faster)
  • Abundant in embryonic brain
  • Rapid transmission
  • Bi-directional
• Chemical Synapse


  • Utilize neurotransmitters
  • Abundant in adult human brain
  • Slower than electrical
  • Has synaptic delay (0.3 to 5.0ms)
  • Unidirectional
• Presynaptic cell


  Synaptic Cleft
• Neurotransmitters


  • Bind with receptor to establish reaction
  • Receptor can bind with others but no reaction
  • Na+ - ligand-gated channel depolarize postsynaptic cell
  • Enzyme degradation = acetylcholinesterare
  • Stop continuous excitation