ch9. nervous system

• Nervous system

  • Central nervous system (CNS) – brain and spinal cord
  • Peripheral nervous system (PNS) – neurons carrying information to/from CNS
• Nervous system

  • Uses both electrical and chemical ways for efficient communication throughout the body
• Types of cells in nervous system

  • Neurons (or nerve cells)
  • Glia (glial cells or neuroglia)
• Glial cells

  • Microglia
  • Oligodendrocytes
  • Schwann cells
  • Ependymal cells
  • Astrocytes
• Neuron

  Transmit electrical currents called nerve impulses
• Glia


  The other non-neuronal cells in the nervous system

  --> def: a type of cell that porvides physical and chemical support to neurons and maintain their environemtn
• Parts of a neuron

  • Cell body (also called soma)
  • Dendrites
  • Axon
• Axons of some neurons are protected and insulated by a myelin sheath
• Nodes of Ranvier

  Gaps in myelin sheath
• Three classes of neurons

  • Sensory neurons (carry nerve impulses from sensory receptors ; most of it have cell body at the dorsal root ganglion ; found in PNS)
  • Interneurons (integrate all signals coming in from the sensory neurons ; found in CNS)
  • Motor neurons (carry nerve impulses to effector cells ; found in PNS)
• Nerve


  Bundle of axons (nerve fibers) in the PNS
• Tract


  Bundle of nerve fibers in the CNS
• Difficult to study in mammalian neurons as largest axons only have diameter of ~20 µm
• Relatively easy to study in the giant axons of the Atlantic squid, which are 0.5 to 1.5 mm in diameter
• Resting potential

  • -65 to -70 mV
• Charge difference (or polarity)

  Due to difference in the distribution of ions on inside and outside of cell
• Outside axon – higher concentration of Na+
• Inside axon - higher concentration of K+
• Sodium-potassium pumps


  Actively transport Na+ out of the axon and K+ into the axon
• Negative resting potential

  Due to there always being more positive ions outside the cell than inside AND the presence of negatively charged proteins inside
• Action Potential


  Nerve stimulated >> voltage gated Na+ channels in localised area open >> Na+ enters cell (depolarization) >> If enough Na+ enters to increase membrane potential from -70 mV to -55 mV (threshold), then this is point of no return for an action potential to start >> Depolarization continues (more Na+ enters cell) and membrane potential increases to +35 mV >> The influx of Na+ in one location stimulates neighbouring voltage-gated Na+ channels to open so action potential moves along axon >> Action potential ends when voltage-gated K+ channels open and K+ leaves cell (repolarization) >> Sodium-potassium pump then returns the ions to the resting locations >> Before resting potential (-70 mV) restored, the membrane potential briefly goes beyond this (e.g., to -85 mV) (refractory period)
• Scientists who worked on the squid giant axon and demonstrated the action potential were Alan Hodgkin and Andrew Huxley. Won Nobel Prize in Physiology or Medicine in 1963
• In myelinated axons, depolarization (movement of Na+) can only occur at Nodes of Ranvier
• Action potentials only occur along axons (not dendrites or cell bodies) and they start at axon hillock (trigger zone)
• Types of ion channels found in neurons

  • Resting (leak) K+ channels
  • Voltage-gated Na+ or K+ channels
  • Ligand-gated channel
  • Na+/K+ pump
• Number of neurons stimulated

  Differences in pain level
• Frequency of the action potentials

  Differences in pain level
• Anaesthetics


  Block Na+ channels and stop them from opening, which stops nerve transmission to pain centers of brain
• Fugu toxin

  A tetrodotoxin that blocks Na+ channels so action potentials cannot be transmitted. Deadly
• Multiple sclerosis


  Autoimmune disease where immune cells attack myelin sheath in neurons of CNS (and PNS)
• More common in women than men and typical age of diagnosis – 20 to 40 years
• Neuron to neuron and neuron to cell communication

  Action potential passes along axon of sending (pre-synaptic) cell >> Synapse >> Receiving (post-synaptic) cell
• Chemical synapse


  Use chemical messengers – neurotransmitters, which are stored in the pre-synaptic terminal in synaptic vesicles
• Electrical synapse


  Ions flow between cells via gap junctions
• Parts of a synapse

  • Pre-synaptic terminal (bouton)
  • Synaptic cleft
  • Post-synaptic terminal
• Transmission across synapse

  Action potential arrives at pre-synaptic axon terminal >> Ca2+ enters via voltage-gated Ca2+ channels >> Synaptic vesicles fuse with pre-synaptic membrane >> Neurotransmitters in vesicles released via exocytosis >> Diffuse across synaptic cleft >> Bind to ligand-gated receptors on post-synaptic membrane
• Excitatory neurotransmitters


  Stimulate influx of Na+ and an action potential in this new cell (e.g. glutamate and acetylcholine)
• Inhibitory neurotransmitters


  Open more K+ channels and prevent an action potential (e.g. GABA)
• One post-synaptic neuron might receive input from both excitatory and inhibitory neurons
• Synaptic integration

  All signals undergo to see if membrane potential increases to threshold and stimulates new action potential