Cell and cell membranes 3- excitation

• Excitability


  The ability of the cell or tissue to respond to certain stimuli by generation and propagation of electrical signals known as action potentials
• Excitable cells


  • Neurons
  • Skeletal myofibres
  • Cardiomyocytes
  • Smooth muscle cells
• Excitation-Inhibition balance


  The balance between excitation and inhibition is crucial to healthy cognition and behaviour
• Neurologic disbalance between excitation and inhibition with consequent instability of the resting membrane potential results in various pathologies e.g.: seizures, convulsions, tetanus (epilepsy), neurodegeneration, anxiety, chronic pain (migraine) and other long term conditions
• Types of stimuli that can excite neurons and muscles

  • Chemical (neurotransmitters, hormones)
  • Electrical (electric transmission)
  • Mechanical (pressure, flow, etc.)
  • Thermal (low and high temperature)
  • Light (phototransduction retina)
• Factors on which excitability depends

  • Strength of stimulus
  • Duration of stimulus
  • Frequency of stimulus
• All-or-None Principle


  "If the stimulus exceeds the threshold potential, the nerve or muscle fibre will give a complete response; otherwise, there is no response"
• The magnitude of the action potential is independent of the strength of the exciting stimulus
• If the stimulus exceeds the threshold potential


  The nerve or muscle fibre will give a complete response
• If the stimulus does not exceed the threshold potential

  There is no response
• Accommodation


  A phenomenon when a neuron or a muscle cell is depolarised gradually or held at the depolarized level, the usual threshold potential may pass without an action potential
• Accommodation occurs as a result of the depolarization-dependent inactivation of voltage gated Na+ channels
• Accommodation is a reduction of the driving force for action potential
• Accommodation develops as a result of hyperkalaemia (increased [K+] in the blood)
• Sudden depolarisation


  Simultaneously activates critical number of fast voltage-gated sodium channels in the cell membrane, with consequent development of action potential
• Slow depolarisation


  Drives activation and inactivation of both voltage-gated sodium and potassium channels and never evokes action potential
• Propagation of action potential in unmyelinated axons

  • Action potential propagates continuously along the axon
  • Requires more energy to propagate
• Propagation of action potential in myelinated axons (saltatory propagation)

  • Action potential "jumps" from node to node
  • Provides rapid conduction of impulses
  • Conserves energy for the cell
• Pathology: multiple sclerosis - disease in which myelin sheath degrade; associated with poor muscle coordination