The Neuron Structure & intracellular signal transmission

• Neurons

  Cells specialised for the generation and transmission of electrical impulses
• Neurons‘ responsibilities

  • Electrical impulses reach specific targets
  • Modify activity of target cells
  • Allow selective control of specific target structures
  • Electrical activity modulated by integrated input from other cells
• Glia cells

  Cells that provide a protected environment for neurons to survive
• Types of glia cells

  • Astrocytes
  • Microglia
  • Oligodendroglia
• Astrocytes

  • Provide physical & nutritional support for neurons
  • Take part in neural signalling

  
• Microglia

  • Produce chemicals that aid repair of damaged neurons
  • Digest dead neurons
• Oligodendroglia

  • Consist of fatty substance, insulating the axon (myelin sheath)
• Neurons do not divide (develop from 'neural stem cells') and neurogenesis is almost complete ~5 months after conception
• Neuron death is part of normal brain development: 20-80% of all neurons die during maturation
• Neurons have virtually no possibility to store energy, so glucose (sugar) and oxygen must be constantly supplied
• Without supply of glucose and oxygen, neurons stop working within seconds, and die within minutes
• Resting potential

  Electrical potential maintained by ion gradients and protein channels in the cell membrane
• Electrical activity - Signal transmission

  1. Ion-specific channels in cell membrane open

  2. Positive ions enter or negative ions leave => membrane depolarises

  3. Negative ions enter or positive ions leave => membrane hyperpolarises
• Electrotonic transmission

  Electrical & concentration gradients sweep ions along the membrane, passive & graded
• Action potential

  Electrical & concentration gradients push/pull ions across the membrane, active & not graded, self-replicating with constant magnitude
• Sequence of events in action potential generation

  1. Membrane depolarised

  2. Na+ channels open

  3. Na+ ions enter the cell

  4. Membrane depolarises further
• Threshold potential

  If membrane potential at axon hillock remains below ~ -50mV, resting potential returns; if it depolarises beyond ~ -50mV, action potential is generated
• Electrochemical processes during an action potential

  1. Enough positive ions arrive that threshold is reached => complete depolarisation

  2. Na+ channels close, K+ channels open => membrane repolarises

  3. K+ channels close when resting potential is restored => brief hyperpolarisation

  
• Action potential

  • Does not decay during transmission
  • Always strong enough to depolarise next area of membrane ahead of it
  • 'All-or-nothing' phenomenon
  • Can not be produced continuously
• Saltatory conduction

  In myelinated axons, action potentials 'jump' from node to node, resulting in much faster transmission
• Signal transmission and information

  • Qualitative information determined by place in brain where signal is received
  • Quantitative information represented in neuron's 'firing rate'