intro to nervous system

• Central nervous system


  • Formed by the brain & spinal cord
  • Function: integration & command centre

  
• Peripheral nervous system


  • Has SENSORY (information IN) & MOTOR divisions (information OUT)
  • SENSORY Function is to detect & report changes and to stimulate effectors
  • Somatic (body) & Visceral sensory (afferent from organs in the ventral body cavity)

  
• Motor division


  • Voluntary (somatic)
  • Involuntary (autonomic)
• Autonomic


  • Sympathetic prepares you for danger "fight or flight"
  • Parasympathetic actions oppose the effects of sympathetic, Rest/RELAX
• Neurons or nerve cells

  • Make and conduct the nerve message (action potential)
  • Are long lived: live as long as you do!
  • Are amitotic, they do not mitose or replace themselves
  • Have a very high metabolic rate and use a lot of oxygen to make a lot of energy needed to make action potentials
• Neurons or nerve cells: Functional areas

  • AXON transmits or carries message to next cell
  • AXON HILLOCK where action potential or message made
  • Dendrites receive message from other nerve cells
  • Cell body regulates cell
  • AXON TERMINAL transfers message across synaptic cleft to next cell

  
• Glial cells


  • Are support cells and do not make action potentials
  • Control environment around the nerve cells
  • Phagocytes: immune response
  • Make cerebrospinal fluid to "float" and support the brain
  • Both oligodendrocytes (central nervous system) and Schwann Cells (peripheral nervous system) Make insulation for nerves called myelin and increase the rate of nerve cell message travel
• Nerve cells or neurons

  Are EXCITABLE cells. This means their cell membrane has an electrical charge. The charge is due to the relative amounts of ions inside versus outside of the cell membrane

  
• Na/K pump


  • Needed because the cell has less Na+ and more K+ than extracellular fluid and needs to maintain these concentrations
  • Because the cell membrane is differentially/selectively permeable, thus more K+ and more leakage out with Na leaking in
  • Na+/K+ pump (3Na+ out, 2K+ in)
• Action potential (AP) = Nerve impulse


  • Excitable cells (nerves) only: AXON generates AP
  • How: opening & closing gates for ions (due to voltage)
• Generation of an action potential (AP)

  1. All gated K+ and Na+ channels closed

  2. Depolarisation: Na+ flows in

  3. Repolarisation: Na+ channels close, K+ channel open

  4. Undershoot (too many K+ lost)
• This impulse travels down nerve due to local current flow within axon
• All or none: if depolarisation not big enough, NO nerve message or action potential is sent
• Depolarisation


  Reduction in membrane potential, from -70 to -40 mv (positive charge of the cytoplasm increased)
• Hyperpolarisation


  Increase in membrane potential, from -70 to -90mv (positive charge of the cytoplasm reduced)

  
• Events of an action potential

  1. All gated channels shut, then stimulus opens Na gates

  2. Depolarisation as Na ENTERS nerve cell

  3. When voltage gets to +30, Na gates shut but K open, thus K EXITS the nerve cell (Repolarisation)
• Action potential travels down nerve

  1. Makes membrane permeable to Ca++

  2. Ca++ causes migration of synaptic vesicles

  3. Vesicles release chemical messenger (acetylcholine)

  4. This diffuses across cleft & depolarises next cell (post-synaptic membrane (nerve cell, muscle cell or gland cell)