3.6.2.1 Nerve Impulses

• Effector: Muscle or gland that produces a response to a stimulus
• CNS: Contains the brain and spinal cord. Unconscious response which provides the fastest route from receptor to effector. Contains only 3 neurones - Sensory, Relay, Motor
• Myelinated motor neurone: Connected to relay neurone in the CNS. Branched into dendrites which connect to other neurones. The cell body, the axon, is surrounded by Schwann cells that secrete mylein. Myelin insulates the axon to prevent movement of Sodium and Potassium. Action potentials can only be generated at the Nodes of Ranvier, uninsulated gaps between the myelin.
• Resting Membrane potential (-70mV): Inside the neuron is more negatively charged in comparison to outside, creating an electrochemical gradient. There are more K+ ions inside the neurone than outside, and more Na+ ions outside than in. The distribution is maintained by the Sodium-potassium pump (actively transports 3Na+ out & 2K+ in) and the diffusion of the ions down their respective gradient. The membrane is more permeable to K+.
• Action Potential generation:

  1. Resting potential - Na+/K+ ion pump maintains resting potential
  2. AP - Depolarisation (gated Na+ ion channel proteins open & K+ close) Rapid diffusion of Na+ ions into the axon raising membrane potential to +40mV
  3. Repolarisation - Gated Na+ ions close and K+ open. Rapid diffusion of K+ out of membrane making potential negative. (K+ stay open longer in hyperpolarisation making it more negative)
  4. Refractory period - Gated K+ channel proteins close. Membrane potential is restored to -70mV by Na+/K+ pump.
• Action Potential properties:

  • Unidirectional transmission - Previous part of axon in refractory period. Neurotransmitter only released at axon terminal.
  • Discrete - Don't overlap due to the refractonary period. Gated Na+ channel proteins cant open to cause depolarisation. Not enough Na+ outside and K+ inside
  • Saltatory conduction - Depolarisation jumps from node to node. In unmyelinated neurons the AP travels slower as it covers the whole length of the axon.
• All or nothing principle: A stimulus has to be above the threshold p.d to cause an action potential. All AP's have the same intensity/size regardless of the strength of the stimulus, a stronger stimulus will just produce more AP's.
• Factors affecting speed of nerve impulse transmission:

  • Myelination - insulates axon so AP's can only be generated at nodes of Ranvier. Faster impulse
  • Temperature - Higher temperature = faster diffusion of ions increasing speed of impulse. Too high and channel proteins denature
  • Axon diameter - Greater diameter = faster transmission as smaller SA:V. Less ions leakage and membrane is correctly polarised to allow it to be depolarised faster.