Nerve Impulses

• Structure of general motor Neuron

  • Cell body - contains organelles & high proportion of RER
  • Dendrons - branch into dendrites which carry impulses towards cell body
  • Axon - long, unbranched fibre carries nerve impulses away from cell body
• Myelinated motor neuronstructure:

  • Schwann cells - wrap around axon
  • Myelin Sheath - made from membranes of schwann cells
  • Nodes of Ranvier - Short gaps between schwann cells where there is no myelin sheath
• Resting potential

  In a resting axon, the inside of the axon always has a negative electrical potential compared to outside the axon. This potential difference when there are no impulses is usually about -70mV.
• Resting potential is established by

  • Higher concentration of k+ ions inside & higher conc. of Na+ ions outside (neurone)
  • Membrane is more permeable to K+ (leaving) than Na+ (entering)
  • Sodium-potassium pump actively transports 3Na+ out of cell & 2K+ into cell; establishes an electrochemical gradient
• 'All or nothing' principle

  Any stimulus that causes the membrane to reach threshold potential will generate an action potential. All action potentials have the same magnitude.
• How an action potential passes along an unmyelinated neuron

  1. Stimulus leads to an influx of Na+ ions; First Section Of membrane depolarises

  2. Local electrical currents cause sodium Voltage-gated channels further along the membrane to open. Meanwhile, the section behind begins to depolarize

  3. Sequential wave of depolarisation
• Why myelinated axons conduct impulses faster than unmyelinated axons

  Saltatory conduction: Impulse 'jumps' from one node of Ranvier to another.

  Depolarisation cannot occur where myelin sheath acts as electrical insulator - i.e. so impulses don't travel along whole axon length
• Damage to the myelin sheath of neurons

  • No saltatory conduction
  • More depolarization occurs along whole length of axon / area of membranes
• Importance of the refractory period

  • No action potential can be generated in hyperpolarized sections of membrane
  • Ensures unidirectional impulse & Ensures discrete impulses
  • Limits frequency of impulse transmission
• Stages in generating an action potential

  1. Depolarisation: Stimulus causes sodium ion channels to open; facilitated diffusion of Na+ ions into cell down electrochemical gradient. P.d. across the membrane becomes more positive. If the membrane reaches a threshold potential(-50mV), voltage-gated Na+ channels open. Significant influx of Na+ ions reverses p.d. to +40mV.

  2. Repolarisation: Voltage-gated Na+ channels close and voltage-gated K+ channels open. Facilitated diffusion of K+ ions out of the cell down their electrochemical gradient. p.d. across the membrane becomes more negative.

  3. Hyperpolarization: 'Overshoot' when K+ ions diffuse out = p.d. becomes more negative than resting potential. Refractory period: no stimulus is large enough to raise membrane potential to threshold. Voltage-gated K+ channels close & sodium-potassium pump re-establishes resting potential.
• Factors that affect the speed of conductance

  • Myelinated axon - Myelination provides (electrical) insulation; Saltatory conduction /depolarisation at nodes of Ranvier
  • Axon diameter - Greater diameter = faster; Less resistance to flow of ions (depolarisation & repolarisation); Less 'leakage' of ions (easier to maintain membrane potential)
  • Temperature - Higher temperature = faster: Faster rate of diffusion (depolarisation & repolarisation); Faster rate of respiration (enzyme-controlled) = more ATP for active transport to re-establish resting potential. Temperature too high = membrane proteins denature
• How an organism detects the strength of a stimulus

  Larger stimulus raises membrane to threshold potential more quickly after hyperpolarization = greater frequency of impulses