physiology 4a

• Resting potential


  The difference between the recording and the reference electrodes when one electrode is inside the cell, typically around -70mV

  
• Ion behaviour


  Determined by diffusion and electric fields
• If the concentration of solutions are equal on both sides of a membrane, then flow left to right will equal flow right to left

  
• If the solutions start with different concentrations, ions follow a diffusion gradient from high to low

  
• If the barrier is selectively porous to only potassium then ions flow down a gradient until repulsed by an electrical difference as chlorides cannot diffuse across the membrane and hence a charge difference develops

  
• Bernstein's Hypothesis


  The resting potential arises from a concentration gradient of potassium ions across a membrane, requiring a high resting permeability of the membrane only to potassium

  
• Equilibrium Potential for Potassium (EK)


  The electrical repulsive force that deters the flow of potassium ions down their concentration gradient, determined by the difference in potassium ion concentration, measured in mV

  
• Bernstein's hypothesis does not fully explain the resting membrane potential as Vm deviates positively away from EK
• Goldman, Hodgkin and Katz Equation


  Calculates the resting membrane potential based on the relative permeabilities of sodium, potassium and chloride ions

  
• If the cell is placed into a new saline solution with 10X the concentration of potassium


  The resting membrane potential will become less negative as the diffusion gradient flattens

  
• If the cell is placed into a new saline solution with 1000X the concentration of potassium

  The resting membrane potential will become positive as the diffusion gradient and electrochemical gradient invert

  
• Action Potential


  An "all or nothing" event involving depolarisation, overshoot, and after-hyperpolarisation
• Sodium ions in the extracellular side of the membrane are essential for action potential generation

  
• When sodium is absent, action potentials cannot occur in cultured neurones
• Threshold Potential


  The minimum stimulus magnitude required to generate an action potential
• Action Potential


  • Rising Phase (depolarisation)
  • Falling Phase (repolarisation)
  • After-hyperpolarisation
• The rising phase of the action potential is due to a transient increase in membrane permeability to Na+
• The falling phase of the action potential involves increased membrane permeability to potassium ions

  
• Voltage gated Na+ channels and voltage gated K+ channels


  Mediate the changes in membrane permeability to their respective ions during the action potential

  
• Refractory Period


  A period when no new action potential can be initiated, ensuring unidirectional action potentials and preventing summation
• Action Potential Propagation

  1. Current flows through activated patch of membrane and depolarises adjacent patch

  2. Historic patch is repolarised and refractory

  3. Adjacent patch reaches threshold, current flows and next patch depolarises

  4. Historic patch repolarises and next patch depolarises
• Action Potential Parameters

  • Amplitude (invariant for a given neurone)
  • Frequency (variable but limited by refractory period)
  • Velocity (invariant for an individual neurone)
• Action Potential Velocity

  Proportional to the square root of the axon diameter
• Myelination


  A mechanism for increasing the velocity of action potential propagation without increasing axon diameter
• Synapse


  A junction between two neurones which permits the transmission of a signal from one nerve to another, either electrically or chemically
• Electrical Synapse


  • Allow rapid propagation of an action potential between neurones using Gap Junctions
• Chemical Synapse


  1. Exocytosis of neurotransmitters from pre-synaptic membrane-bound vesicles

  2. Neurotransmitter binds to receptors on the post-synaptic cell

  3. Neurotransmitter can be degraded or reused by storage back into the pre-synaptic neurone
• Excitatory Transmitters


  Cause depolarisation of the post-synaptic cell by triggering an excitatory post-synaptic potential (EPSP)
• Inhibitory Transmitters


  Cause hyperpolarisation of the post-synaptic cell by triggering an inhibitory post-synaptic potential (IPSP)
• Excitatory Neurotransmitters


  • Acetylcholine
  • Glutamate
• Inhibitory Neurotransmitters


  • Gamma-AminoButyric Acid (GABA)
  • Glycine
• Spatial Summation


  Summation of EPSPs from multiple pre-synaptic inputs onto a post-synaptic cell
• Temporal Summation


  Summation of EPSPs from repeated stimulation of a single pre-synaptic input onto a post-synaptic cell
• Summation of EPSPs enables post-synaptic cells to reach the threshold potential for action potential generation
• At the EPSP phase, ligand gated sodium channels cause the change in membrane potential, while in the Action Potential, voltage gated sodium channels take over to create the action potential