Module 5

• Myelination


  Schwann cells wrap around the axon of neurons to create a myelin sheath
• Myelin sheath


  Acts as an electrical insulator because it is impermeable to ions
• Ions


  • Na+
  • K+
• Depolarisation and action potentials cannot occur at the myelinated parts of the axon
• Depolarisation and action potentials can only occur in the gaps between (nodes of Ranvier)
• Saltatory conduction


  The nervous impulse jumps from one node to the next
• An increase in temperature increases kinetic energy
• Ions move across the membrane more rapidly when they have more kinetic energy
• Giant axons are found in the giant squid
• Greater axon diameter


  Means there is a greater surface area for the movement of ions across the cell membrane
• Neurones are cells that transmit information from receptors to effectors
• Sensory neurones


  Carry nervous impulses from receptors into the CNS
• Motor neurones


  Carry impulses from the CNS to effector organs
• Relay neurones


  Intermediate neurones that receive impulses from a sensory neurone and relay them to motor neurones
• The structure of neurones is important in ensuring information is transmitted correctly and efficiently
• Basic structure of neurones

  • Dendrites
  • Axons
  • Cell body
• Dendrites


  Carries nervous impulses towards a cell body
• Axons


  Carries nervous impulses away from the cell body
• Cell body


  Where the nucleus is normally located
• Motor neurones in invertebrates are usually myelinated
• Gaps between adjacent Schwann cells are called nodes of Ranvier
• Myelin


  Increases the speed of the electrical impulse travelling along neurons
• When a neurone has not been stimulated, it is at resting state
• Potential difference


  The difference in charge across the neurone membrane
• At resting state, the inside of the neurone is more negatively charged than outside
• The difference in charge is called a potential difference
• Sodium-potassium pumps


  Maintain the resting potential in the neurone membrane
• Three Na+ ions are actively transported out of the neurone for every two K+ ions that are transported in
• This leads to a build-up of positive ions outside the cell
• Potassium ion channels


  Make the neurone membrane permeable to K+ ions
• When K+ ions are transported into neurones, they can diffuse back out
• The neurone membrane is impermeable to Na+ ions
• Together the action of sodium-potassium pumps and potassium ion channels leads to a potential difference across the neurone membrane
• Resting potential


  The potential difference is called the resting potential
• The neurone is said to be polarised
• Resting potential is about -70mV
• Depolarisation


  A change in potential difference when a resting neurone is stimulated
• Stimulation


  Na+ ion channels in the cell membrane open when a neurone is stimulated
• Na+ ions flood into the neurone
• The potential difference across the membrane changes to become more positive inside the neurones