Nerve impulses

• Cell body


  • Contains normal organelles found in typical animal cells - nucleus
  • Proteins and neurotransmitter chemicals are made here
• Dendrites


  • Carry action potentials to surrounding cells
• Axon

  • Conductive long fibre
  • Carries nerve impulse along the motor neurone
• Schwann cells


  • Wraps around axon to form myelin sheath
  • Lipid and therefore does not allow charged ions to pass through it
  • Gaps between the myelin sheath are called nodes of Ranvier
• Neurone state at rest


  There is a difference between the electrical charge inside and outside the neurone
• Resting potential


  • The difference in electrical charge across a neurone membrane at rest
  • -70 mV
• How is the resting potential maintained?

  Sodium potassium pump
• Difference between sodium and potassium ions across the axon membrane


  • More sodium ions outside membrane
  • More potassium ions inside membrane
• Sodium-potassium pump

  1. Moves 2 K+ ions in

  2. 3 Na+ ions out

  3. Move by active transport

  4. Requires ATP
• What ion is the membrane more permeable to?


  • Membrane more permeable to K+ ions moving in making it more positive
  • More Na+ moves out
  • Greater concentration of sodium ions outside
• Action potentials

  • When the neurones voltage increases beyond a set point from the resting potential
  • Results in the generation of a nerve impulse
• Why does an action potential occur?

  Due to the neurone membrane becoming more permeable to Na+
• Stimulus

  1. Provides the energy needed for the voltage-gated sodium ion channels to open

  2. More Na+ to diffuse in
• At what point does depolarisation occur

  • Axon becomes more positive/ more sodium ions enter
  • Increases from -70 (resting potential) to -55
• What happens as depolarisation occurs?


  1. More voltage gated-sodium channels open

  2. So more Na+ diffuse into axon

  3. Becomes more positive
• At what point does depolarisation end?


  When it hits its peak at +40
• What happens when depolarisation ends?

  1. The voltage gated sodium ion channels close

  2. The voltage gated potassium ion channels open
• What happens as voltage gated potassium ion channels open after depolarisation?


  1. More potassium ions diffuse out of membrane

  2. Repolarised the axon
• What happens as more potassium ions move out?


  1. The axon becomes more negative than the resting potential of -70

  2. Hyperpolarised and enters refectory period
• How does the axon go back to it's resting potential after hyperpolarisation?


  1. Voltage gated potassium ion channels close

  2. Sodium potassium pump opens to restore normal activity - back to resting potential
• All or nothing principle


  If the depolarisation does not exceed the -55 threshold then an action potential is not produced
• Once the threshold is reached...


  • An action potential will always peak at the same maximum voltage
  • +40
• A bigger stimuli


  Increases the frequency of action potentials
• Importance of all or nothing principle

  • Animals will only respond to large enough stimuli
  • Rather than responding to every slight change in the enviroment
  • May overwhelm them
• Refractory period


  Describes the period after an action potential where the sodium channels are recovering and can therefore not be opened
• Importance of refractory period

  • Ensures discrete impulses
  • Ensures that action potentials are unidirectional
  • Limits the frequency of impulse transmissions
• Ensures discrete impulses


  • An action potential can not be generated immediately after another one
  • Each action potential is separate
• Ensures that action potentials are unidirectional


  • Travel in one direction
  • Stops action potentials from spreading out in two directions which may prevent a response
• Limits the frequency of impulse transmissions

  Prevents an overreaction to a stimulus and therefore overwhelming the senses
• Factors affecting the speed of conductance

  • Myelination and saltatory conduction
  • Axon diameter
  • Temperature
• Myelin sheath


  • Acts as an electrical conductor
  • Made up of Schwann cells
• Gaps between myelin sheath


  Nodes of Ranvier
• Saltatory conduction

  • In myelinated neurones depolarisation only occurs at these nodes
  • The neurones cytoplasm conducts enough electrical to depolarise the next neurone
  • An action potential jumps from node to node
  • Much faster
• Speed of conductance in non-myelinated neurone


  • Slower
  • As depolarisation has to occur across the entire length of the axon
• Axon diameter and speed of conductance

  • Bigger/wider the diameter - speed of conductance increases
  • Less resistance to the flow of ions compared to the cytoplasm in smaller axons
  • Depolarisation reaches other parts of the neurone quicker
• Temperature and speed of conductance


  • As temperature increases so does speed of conductance
  • Ions diffuse faster - greater kinetic energy
  • Enzymes involved in respiration work faster - more ATP for active transport of Na and K+