Week 3: Action Potential Propagation

Week 3: Action Potential Propagation

1. How electrical signals spread in nerve cells - passive spread

   • Nerve fibres have electrical properties that allow electrical signals to spread passively over short distances
   • All current in nerve cells are carried by movement of ions in solutions
   • Nerve fibres have membrane resistance due to impermeable phospholipid bilayer, with ion channels that conduct when open
   • Cytoplasm of the axon has a resistance to current flow - called Ri, Ri is lower in big axons and high in small-diameter axons
   • Signal at one end of an axon faces two resistances. The ratio of Rm:Ri determines how much of charge leaks out of axon and show much spreads down it
   • The distance over which it decreases to 37% of its starting value of space constant
   • Membrane capacitance slows the rise of signal as it spreads - the time it takes a signal to reach 63% of its maximum size is the time constant of membrane

2. Action potential propagation

   • Voltage decays over adjacent regions of fibre with a decay curve determined by space constant
   • Action potential conduction velocity is faster with longer space constants

3. Threshold Potential

   • All signals at one point in a neuron will spread passively from the point - 'heat signal' on a steel bar will spread from heating point
   • Large depolarising signal reach the threshold for generating action potential leading to active propagation of signals
   • The threshold is the potential at which the opening of voltage-sensitive Na+ channls is initiated, leading to generation of action potential
   • Membrane of excitable cell may be brought to threshold potential
     • Action of neurotransmitter on receptors at a synapse
     • Action of hormone or drug on receptors
     • Applied electric shock or injected current pulse
     • Transient damage to nerve cell membrane
     • Sensory stimulus
     • Action potential spreading from adjacent region of cell
     • raising extracellular concentration of K+

4. Action Potential

   • Shape of action potential is determined by the underlying conductances
   • Few neurons and other excitable cells, part of inward current during the rising phase of action potential is carried by Ca2+ ions, via voltage-operated calcium channels
   • Calcium influx can activate calcium activated potassium currents - which give a very long after hyperpolarisation
   • Calcium influx is important in action potentials of some neurons, cardiac muscle, smooth muscle

5. Action Potential Propagation

   • If a cell depolarised at one point, charge spreads
   • As charge moves along the membrane, it must charge the membrane capacitance
   • Charge leaks through the membrane Rm so charge leaks out as it spreads along
   • Passive spread of charge through the nerve cell is important for action potential and synaptic integration

6. Speed of propagation of action potentials

   • Current spreads passively along the axon according to cable properties
   • In thin axons, space constant is short and action potential propogrates slowly
   • Large diameter axons have lower Ri therefore longer space constants - current spreads further and action potentials propograte faster
   • Insulating the axon, Ri is unaffected but Rm is increased - this means current spreads

7. Myelinated axons

   • Myelin is produced by oligodendroglia and Schwann cells in peripheral nervous system
   • Consists of spiral wrapping a cell leaflet around the axon, with very little cytoplasm between the two membranes
   • Gaps in wrapping called "Nodes of Ranvier" where ion channels are concentrated
   • Insulating the axons, transmembrane current flow is reduced membrane capacitance is reduced and current spreads further and with faster-time course along axon
   • This causes rapid depolarisation of next Node of Ranvier, reaching threshold and triggering a regenerative action potential which then causes depolarisation to spread down the axon
   • Some nerve fibre are wrapped in a sheath of myelin which acts as an electrical insulator, so little ionic current crosses the membrane in myelinated regions
   • Myelination increases the resistance of membrane and also decreases its capacitance
   • Current flow of confined to regions where there are gaps in myelin called 'Nodes of Ranvier" - where Na+ channels are concentrated

8. Action potential conduction velocity

   • Conduction velocity is determined first by axon diameter, and second by myelination
   • Invertebrates have not evolved myelination where rapid responses are needed
   • In mammals, insulation with myelin-filled Schwann cells increases conduction velocity, without increasing overall diameter to same extent
   • Conduction velocity is proportional to square root of diameter for unmyelinated nerve fibres
   • Conduction velocity is directly proportional to overall fibre diameter in myelinated axons

9. Action Potential Waveform

   • If action potential lasts for 1 msec and conducts at 100m/sec, the waveform of ATp occupies
   • After hyperpolarisation that follows action potential can last much longer and cover more of axons
   • In nerve cell bodies an influx of calcium ions during action potentials can trigger an after hypolarisation lasting 10 seconds

10. Action potentials and conduction

    • Action potential normally travel in a set direction along the axon - orthodromic conduction
    • in sensory, orthodromic is towards the CNS: in motor neurons, it is away from the cell body in CNS
    • Normally action potentials are initiated at paritcular sites in neuron then propagate from there. In mos cells action potential are initiated at axon hillock
    • Action potential is initiated in middle of axon it will propagate in both directions