CAM102 Lecture Notes - Lecture 18: Depolarization, Membrane Potential, Electric Field
Learning Objectives
• Describe how equilibrium potentials combines to determine the membrane potential
• Define graded potentials (hyperpolarising, depolarising) and how they are produced
• Describe the triggering and sequence of the action potential in terms of channels
• Describe action potential propagation (depolarisation, threshold, refractory periods)
• Describe the effect of myelin on propagation
What are Neurons for?
• Communication - Rapid and specific
• Detecting events
• Organising appropriate responses or initiating useful behaviour
o Sensory systems
o Internal networks
o Effectors
• To make efficient, useful responses to stimuli, neurons need certain qualities:
o Consistency and reliability of signalling
o Rapid integration of multiple influences
o Delivering information where it's needed
o Processing information in ways which are most useful for controlling responses
Electrical Properties of the Bilayer
• Excellent insulator: 100mV/10nm = 106 V/cm
• Behaves as a capacitor (1µF/cm2)
• This capacitance shapes the time course of changes in membrane potential (gradual)
• The charge is pulled away and replaced
• There is an intense electric field in the membrane wall
Typical Values
• These vary from cell to cell but retain the same basic relationships
Different Possible Equilibrium Potentials
• If resting cell were permeable to K+ only: Vm=Ek (-81mV)
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• If resting cell were permeable to Na+ only: Vm=ENa (+58mV)
Range and Change of Membrane Potential
• Equilibrium potentials of potassium and sodium (Ek and ENa) set limits for the range of
membrane potential (Vm)
• These values are fixed by intra/extracellular concentrations
• Opening channels for a particular ion, moves membrane potential toward the equilibrium for
that ion
• The cell overall will never be at an equilibrium due to the charge that is against the
concentration gradient
Gated Ion Channels Control Permeability
• How the cell can control its membrane permeability
• Some channels are always open (leak channels, porins)
• Others are gated - they open in response to specific things
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Graded Potentials
• Opening channels can change membrane potential, depending on:
o How many are open
o What ion(s) can pass
o The equilibrium for that ion
• Hyperpolarisation:
o Potential moves further from zero (=inhibition)
• Depolarisation
o Membrane potential moves towards zero (=excitation)
• Resting membrane potential is dominated by potassium ions therefore negative
Triggering the Membrane
• If the potential is large enough, it can set off voltage-gated sodium channels (threshold)
• Suddenly lots of sodium can cross the membrane, reversing the sodium-potassium balance
• The membrane briefly flips from the potassium equilibrium to the sodium equilibrium
• This rapid flip of membrane voltage is called an action potential
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Document Summary
What are neurons for: communication - rapid and specific, detecting events, organising appropriate responses or initiating useful behaviour, sensory systems. Typical values: these vary from cell to cell but retain the same basic relationships. If resting cell were permeable to k+ only: vm=ek (-81mv) If resting cell were permeable to na+ only: vm=ena (+58mv) Gated ion channels control permeability: how the cell can control its membrane permeability, others are gated - they open in response to specific things. Some channels are always open (leak channels, porins) If the potential is large enough, it can set off voltage-gated sodium channels (threshold) Suddenly lots of sodium can cross the membrane, reversing the sodium-potassium balance: the membrane briefly flips from the potassium equilibrium to the sodium equilibrium, this rapid flip of membrane voltage is called an action potential. Repolarisation: voltage gated sodium channels deactivate (separate mechanism to opening) Slower voltage gated k+ channels open (vm moves toward ek)
