3.6.2.1

• Cell body

  Contains all the usual cell organelles

  • Nucleus and large amounts of RER.
  • Associated with the production of proteins and neurotransmitters
• Schwann Cells

  Surround the axon, protecting it and providing electrical insulation.

  • Carry out phagocytosis (removal of cell debris)
  • Wrap around axon many times, so layers of their membranes build up around it.
• Why is action potential called all-or-nothing?

  Only happens if the stimulus reaches a threshold value (-55 mV)

  • All stimulus above this threshold value will generate the same size of AP, regardless of stimulus strength.
  • Only frequency changes.
• Action Potential is always the same size, all the way along the axon.
• The transmission of the action potential along the axon is the nerve impulse.
• Detecting the size of a stimulus

  The number of impulses in a given time

  • The larger the stimulus, the more impulses generated.
• Detecting the size of a stimulus

  By having neurones with different threshold values.

  • The brain interprets the number and type of neurons and thereby determines its size.
• Refractory Period

  A time after depolarisation where no new AP can start.

  • Time is needed to restore the proteins of voltage sensitive ions channels to their original resting conditions.
• The refractory period functions:

  • Action potential travels in one direction only
  • Produces discrete impulses
  • Limits the frequency of impulses.
• Factors that affect the speed of nerve impulse conduction

  • Myelination of axon
  • Axon diameter
  • Temperature
• What changes in the membrane when action potential is reached?

  Sodium channels close, potassium channels open

  • Potassium ions diffuse out of the axon, leading to repolarisation of the axon.
• Why does not all stimulus produce an action potential?

  • Refractory period so no signals can be sent
  • Greater stimulation is needed as the threshold is not reached to cause depolarisation.
  • Potassium channels are open; membrane is hyperpolarised
  • Sodium channels are inactive/closed/will not open.
• How does the charge of an axon change when an action potential is produced?

  • Sodium channels open; increasing permeability of axon membrane to sodium ions
  • Sodium ions diffuse into axon.
• Conduction along a non-myelinated axon

  Whole membrane is depolarised; every voltage gated sodium channel is opened

  • Makes transmission relatively slow
• Conduction along a myelinated axon.

  Depolarisation only occurs at nodes; impulse jumps from node to node

  • Saltatory conduction
  • Makes transmission faster.
• Function of Sodium-Potassium Pump

  Transports Sodium and Potassium ions

  • via active transport against concentration gradient
  • Restores ion balance after an action potential
• Permeability of a membrane is correlated to the number of channel proteins allowing the ions to be transported across.
• Dendrites

  Cell body extensions

  • Can connect to many other neurons and receive impulses from them
  • Form a network for easy communication
• Sensory neurones

  carry impulses from receptors to CNS
• Relay neurones

  found entirely within CNS

  • Connect sensory and motor neurones
• Motor neurones

  Carry impulses from the CNS to effectors
• Action potentials are caused by the rapid movement of sodium and potassium ions across the membrane of the axon.
• Threshold levels in receptors often increase with continued stimulation

  • greater stimulus is required before impulses are sent along sensory neurones.
• Axon

  A long fibre that conducts nerve impulses away from the cell body
• Coordinator

  Coordinates information from the receptors and sends instructions to the effectors.
• Dendrites

  Short branched extensions of the cell body that receive nerve impulses from other neurones.
• Dendrons

  Extensions of the cell body which branch into smaller fibres; dendrites.
• Effector

  An organ, tissue or cell that produces a response to a stimulus
• Myelin sheath

  An electrically insulating layer consisting of the membranes of Schwann cells.

  • Increases speed of nerve impulses as signal ‘jumps’ across Nodes of Ranvier
• Nodes of Ranvier

  Gaps between adjacent Schwann cells in the myelin sheath at which action potentials can occur.
• Factors affecting speed of conductance

  Myelination

  • Myelin sheaths insulate the axon so the signals ’jump‘ from node to node; only depolarises nodes, so signal travels faster
  • Called Saltatory conduction

  Axon Diameter

  • Wider axon = faster conduction
  • Less resistance to ion flow

  Temperature

  • Optimum higher temperature = faster conduction
  • Faster diffusion of ions