Cell and cell membranes 2

• Biological electricity


  The foundation of life and behaviour
• Biological electricity recordings


  • Electrocardiogram (ECG) - heart
  • Electroencephalogram (EEG) - brain
  • Electromusculogram (EMG) - muscle
• Electrophysiological recordings


  Measurements of electric changes on a wide scale from single ion channel proteins to the whole organs
• Parameters recorded by using electrophysiological techniques

  • Voltage
  • Current
  • Resistance
  • Conductance
  • Capacitance
  • Frequency
• Ohm's Law


  Explains behaviour of biological electricity: I = V/R
• Georg Simon Ohm (1789 -1854) discovered Ohm's Law
• Voltage (V)


  Electrical potential difference
• Current (I)


  Flow of electric charge
• Resistance (R)


  Opposition to the flow of electric current
• Conductance (G)


  Measure of how easily electric current flows: G = 1/R = I/V
• Alan Lloyd Hodgkin, Andrew Huxley and John Carew Eccles won the Nobel Prize in Physiology or Medicine (1963) for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane
• Electrophysiological techniques


  • Extracellular recordings
  • Intracellular recordings
• Extracellular recordings


  Recording of field potentials (relative changes of voltage) using surface electrodes
• Intracellular recordings


  Recording of membrane currents and potentials using microelectrodes or patch pipettes
• Intracellular recording techniques


  • Single-unit
  • Multi-unit
• Current-clamp


  Membrane current controlled
• Voltage-clamp


  Membrane voltage controlled
• Electrophysiological recording apparatus includes a cell, amplifier, glass pipette and computer
• Ions contributing to membrane potential

  • Na+
  • Ca2+
  • K+
  • Cl-
• Membrane potential (Vm) is a sum of equilibrium potentials (Eion) of all contributing ions
• Resting conditions


  Membrane potential (Vm) is around -70mV
• Depolarization


  Reduction of charge difference between inside and outside the cell, resulting from increased influx of Na+ and/or Ca2+, increasing neuronal excitability
• Hyperpolarization


  Increase of charge difference between inside and outside the cell, resulting from increased efflux of K+, decreasing neuronal excitability
• Action potential is the change in voltage that occurs between the inside and outside of an excitable cell (neuron or muscle cell or fibre) spontaneously or as a result of stimulation
• Spike characteristics of action potential


  • Resting membrane potential
  • Threshold potential (~-40mV)
  • Overshoot (~+20mV)
  • Afterhyperpolarization (AHP; ~-80mV)
  • Amplitude (spike height) of action potential (~100mV)
  • Depolarization-rate (upstroke or upward phase ~90V/s)
  • Repolarization-rate ('downstroke' or 'falling phase' ~60V/s)
  • Half-width of action potential (~2-3ms)
• Voltage-gated Na+ and K+ channels contribute to the development of action potential
• Subthreshold pacemaking leak (Na+) current through Hyperpolarization and Cyclic Nucleotide (HCN) sensitive channels also contributes to action potential
• Action potentials in different excitable tissues

  • Neuronal action potential
  • Cardiac (ventricular) action potential
  • Smooth muscle action potential
  • Skeletal muscle fibre action potential
• Anatomy of neuronal action potential involves voltage-gated Na+ and K+ channels, as well as HCN channels
• Action potential


  Electrical signal that propagates along the membrane of an excitable cell (neuron or muscle cell)
• Cardiac (ventricular) action potential and transmembrane currents

  1. K+ current

  2. Na+ current

  3. Ca2+ current

  4. Q

  5. R

  6. S

  7. T

  8. P

  9. ECG

  10. Outward current

  11. Inward current
• Smooth muscle action potential and transmembrane currents

  1. K+ current

  2. Ca2+ current

  3. Outward current

  4. Inward current
• Skeletal muscle fibre action potential and transmembrane currents

  1. K+ current

  2. Na+ current

  3. Outward current

  4. Inward current
• Anatomy of neuronal action potential

  1. K+ current

  2. Na+ current

  3. c3waw

  4. Nav - Voltage-gated sodium channel

  5. Kv - Voltage-gated potassium channel

  6. HCN - hyperpolarization and cyclic nucleotide gated channels

  7. 2K+3Na+

  8. Na+/K+ ATPase

  9. Phase 0 - Resting membrane potential, slow depolarisation

  10. Phase 1 - Fast depolarisation

  11. Phase 2 - Repolarisation

  12. Phase 3 - Afterhyperpolarisation

  13. Phase 4 - Resting membrane potential, slow depolarisation
• Resting membrane potential and slow depolarisation (membrane is polarised)


  • Phases 0 and 4
• Fast depolarisation


  • Phase 1
• Repolarisation


  • Phase 2
• Afterhyperpolarisation (maximal polarisation of membrane)

  • Phase 3
• Refractory period


  Period of time during which an excitable cell (neuron or muscular cell) is incapable to respond to further stimulation
• Absolute refractory period


  Phase when excitable cell does not respond for stimulation no matter how great stimulus is applied