Nervous coordination

• Myelinated motor neurone


  • Dendrites carry impulses to cell body
  • Axon carries impulses away from cell body
  • Myelin sheath provides electrical insulation
  • Node of Ranvier is a gap between adjacent Schwann cells where axon membrane is bare
• Cell body


  Contains the nucleus
• Some neurones are unmyelinated
• Refractory period


  When the membrane is hyperpolarised and then returning to resting potential, the ion channels are recovering and cannot be made to reopen for a brief period
• The refractory period acts as a time delay and makes sure action potentials cannot overlap and can pass along as discrete separate impulses
• The refractory period also means the action potentials are unidirectional (only pass along in one direction)
• All-or-Nothing Principle


  The events of an action potential will only be set into motion if the initial stimulus is big enough to cause a generator potential that reaches threshold value
• If the initial stimulus is not large enough, then not enough voltage-gated Na channels will open and the axon membrane will not become fully depolarised, i.e., no action potential will be generated
• If the stimulus is large enough, then the impulse will be generated at a constant size and speed
• Increasing the initial stimulus will not produce a larger or faster action potential
• A stronger stimulus will, however, increase the frequency of action potentials
• Wave of depolarisation


  1. Action potential fires, Na+ that entered the neurone diffuses sideways

  2. This stimulates voltage-gated Na channels to open in the adjoining region of the axon, depolarising the next section

  3. This results in a wave of depolarisation along length of neurone
• The wave of depolarisation moves away from parts of the membrane in refractory period (channels cannot open during the refractory period)
• Propagation of action potential


  This is called the nerve impulse
• Myelination of the axon

  • In non-myelinated neurones, the wave of depolarisation travels along the whole length of the axon membrane, resulting in relatively slow conduction
  • In myelinated neurones, the action potential 'jumps from node to node' (saltatory conduction), greatly increasing the speed of the nerve impulse
• Axon diameter


  • The greater the axon diameter, the faster the conduction of action potentials along the axon, due to less resistance to flow of ions
• Temperature


  • The higher the temperature (up to 40 °C), the faster the conduction of action potentials along the axon, due to increased rate of diffusion of ions and enzyme-controlled reactions
  • Above 40 °C, proteins begin to denature and the speed decreases
• Summation


  The effect of different impulses can be combined to build up enough neurotransmitter to generate an action potential
• Spatial summation


  • 2 or more presynaptic neurones release their neurotransmitter at the same time onto one postsynaptic neurone
• Temporal summation


  • Two or more impulses arrive in quick succession from the same presynaptic neurone, making an action potential more likely because more neurotransmitter is released into the synaptic cleft