Electrical signaling in neurons

Cell membrane

• is polarized = electrical difference across the membrane

  • inside of the neuron more negative than the outside - when neuron is at rest (i.e. not carrying a signal)

• contains ion channels

  • important channels: potassium (K+ ), sodium (Na+ ), calcium (Ca2+), chloride (Cl- )

  • channels can open & close

Resting membrane potential

= when neuron is not sending a signal

• membrane potential is about -70 millivolts

• Membrane potential is maintained by K+ & Na+ pumps

• membrane is leaky to K+ via channels that are always open, K+ diffuses out

Signal transfer along axon via APs

• Dendrites receive signals from previous neurons

→ changes the potential of the cell membrane at the target cell

→ this will generate an Action Potential at the point where axon leaves the cell body (Axon hillock)

→ Action Potential spreads along the Axon to axon terminals

Action potential (AP)/Nerve impulse

• ‘all or nothing’ response - either occurs or doesn’t

• size is always the same

• spreads along the axon to the axon terminals

• starts at the axon hillock of the axon

• 2 phases: depolarisation, repolarisation

• to be triggered, cell must be depolarised to -55mV (threshold)

Depolarisation

inside of cell becomes more positive

1. sodium/Na+ channels open → positive sodium ions flow into the cell

   • the massive influx changes membrane potential, i.e. becomes positive at that area where sodium enters the cell

2. when sodium ions are more or less equal at each side of the membrane, Na+ channels close - sodium influx stops

Repolarisation

inside of cell becomes more negative

3. while Na+ channels close, potassium/K+ channels open

4. positive ions rush out of the cell

   → inside of the cell becomes negative again

5. once K+ ions are more or less the same inside & outside the cell, K+ channels close

   → membrane potential even more negative than when it is at rest (Hyperpolarization)

Sodium-potassium pump

• used to reestablish resting potential of -70mV

• each cycle transfer 3 Na+ out of cell and 2 K+ into cell

• restores balance of ions present at resting potential

  • Outside cell: high Na+

  • Inside cell: high K

Propagation AP

• Influx of positive sodium/Na+ ions depolarizes the membrane at this region

• these Na+ ions will diffuse to the neighboring areas triggering APs down the axon

  → causing depolarization of the membrane ahead of the AP

  → causes Na+ channels along this part of the membrane to open

  → again Na+ ions rush in & another AP occurs (De- & Repolarization)

  → Na+ ions diffuse to neighboring areas …

Refractory period

Once an AP has occurred, a 2nd AP can’t occur until the membrane has recovered

2 phases:

• Absolute period

  • not possible to trigger another AP

• Relative period

  • AP can be generated by needs to be a stronger stimulus

→ AP propagates forward

Unmyelinated axons

• continuous impulse propagation = voltage changes along the whole axon membrane

• passive depolarisation spreads

• time consuming

Myelinated axons

Saltatory conduction

• APs only generated at nodes of Ranvier (unmyelinated)

• ions can pass the membrane

  • Sodium rushes into the cell → positivity diffuses along the axon to next node → causes depolarization there, etc.

  • AP jumps from node to node

→ impulse transfer significantly faster

Application to Multiple Sclerosis (MS)

• demyelinating disease = loss of myelin

• loss of Saltatory conduction

• signal transfer significantly slower