Membranes, neurons and potentials

• Membrane potential


  Separation of opposite charges across the membrane

  
• Equilibrium potential


  Nernst equation
• Equilibrium potential


  1. Both compartments contain KCl at higher concentration in 1

  2. If the membrane allowed KCl to cross, the constituent ions K+ and Cl- would diffuse from compartment 1 to 2

  3. Suppose the membrane is permeable only to K+ ions

  4. K+ will tend to diffuse from compartment 1 to 2, but Cl- ions cannot because the membrane is not permeable to them, so there will be a net transfer of positive charge from compartment 1 to 2 (carried by the K+ ions) and compartment 2 will become electrically positive with respect to compartment 1

  
• Electrical gradient


  Tends to push K+ ions from compartment 2 to 1
• Nernst potential


  Electrical potential difference at which the electrical difference will be just large enough to move K+ ions to the left at the same rate as they tend to diffuse to the right due to the concentration gradient

  
• Equilibrium does not exist because ions do not occur alone in cells
• Membrane


  • 30 x more permeable to K+ than to Na+
  • Concentration differences maintained through a variety of mechanisms
• Resting potential


  Constant electrical potential due to uneven distribution of Na+ and K+
• Polarisation


  Membrane is in a state of polarisation because of different concentrations of charged particles on inside & outside of the neuron

  
• Depolarisation


  Change in potential that decreases membrane potential, moving it closer to 0 mV

  
• Hyperpolarisation


  Change in potential further polarising (moving further from 0 mV) the membrane
• Leak channels


  Open all the time, allowing for ionic movement between cell and extracellular fluid (ECF)
• Gated channels


  • Open in response to a triggering event (e.g., stimulus, such as noise, or synaptic communication)
  • Voltage-gated: open/close in response to changes in membrane potential
  • Chemically (ligand)-gated: open/close in response to chemical messengers (e.g., chemical neurotransmitters)
• Graded potentials


  • Depolarisation is restricted to area with open channels
  • Current: flow of electrical charges (in the direction in which positive charges are moving)
  • Triggering event
  • Ion (Na+) channels open
  • Na+ enters neuron
  • Neuron depolarises (graded potential)
  • Extent of depolarisation dependent on number of open channels (= stimulus strength)

  
• Because membranes have leak channels, the strength of current diminishes as ions escape through them. Therefore these potentials only travel short distances
• Action potential


  • Electrical waveform consisting of depolarisation, repolarisation and hyperpolarisation
  • Brief (< 1 millisecond)
  • Fast - the larger the diameter of the axon the faster the conductance
  • Large (100 mV) - reverses from negative to positive inside neuron
  • Does not diminish in strength - long distance signal
  • All or nothing - neuron only 'fires' or 'spikes' if the triggering event changes membrane potential to its threshold. At this point it fires maximally. If threshold is not reached the neuron does not fire
  • Stimulus strength is instead coded by the frequency of action potentials

  
• Voltage gated channels


  • Sensitive to changes in membrane potential
  • Na+ channels have two 'gates': activation & inactivation
  • Can be in one of three conformations

  
• K+ channels


  Can be either: (K+ out of the cell)
• Resting potential: inside cell is relatively negative along membrane & outside relatively positive. Electrical gradient

  
• More K+ inside (despite leaking through leak channels) & more Na+ outside (membrane relatively impermeable to Na+). Concentration gradient
• Action potential


  1. At resting potential both K+ & Na+ channels closed (Na+ channel closed but capable of opening mode)

  2. Triggering event slightly depolarises membrane & causes activation gates to open, favouring Na+ into cell through concentration & electrical gradients

  3. Cell becomes more depolarised - more Na+ enters until threshold is reached -> lots of Na+ gates open fast & inside of cell becomes positive. At the same time inactivation gates begin to close with delay & K+ gates open with delay

  4. K+ gates open and consequently allow K+ to exit cell fast up both electrical (now cell is highly positive due to Na+ inside) and concentration (less K+ outside) gradients

  5. No more Na+ enters (gates closed) and K+ (gates open) leaving cell. Restores internal negativity of cell

  6. K+ gates then close, but because they are slow, neuron tends to hyperpolarise
• Axon hillock


  Neuron's 'trigger zone' (large density of voltage-gated channels)
• Dendrites send signals towards the cell body & axons carry signals away from the cell body
• Dendrites or soma receive 'triggering event'
• Once a spike has been initiated a self-perpetuating cycle begins & propagates the action potential automatically down the axon
• Action potential occurs in a localised area of cell. Must then propagate down cell through axon
• Key topics


  • Equilibrium, Nernst
  • Membrane potential
  • Graded potentials
  • Action potentials
  • Threshold potential
  • K+ and Na+ channels