Impulses move from one nerve cell to another because of a difference in electrical "action potential" caused by ions inside and outside the cell. The cell membrane is selectively permeable to potassium ions, K⁺, and highly impermeable to sodium ions, Na⁺.

These are the basic steps:

1. Resting state:

• A neuron is not conducting an impulse.
• The K⁺ concentration is higher inside the cell than out.
• The Na⁺ concentration is higher outside the cell than in.

2. Depolarization:

• A nerve cell is stimulated.
• At the point of stimulation, the membrane becomes permeable to Na⁺ for an instant and they quickly move into the cell.
• The inner surface of the cell membrane is now more positively charged than the outside.

3. Repolarization:

• When the cell membrane becomes depolarized, K⁺ automatically leave the cell until the cell is back to its resting state.

4. The impulse travels:

• This quick movement of ions causes a similar change or wave all across the cell and down the axon.
• Vertebrate nerves are covered by a myelin sheath with openings called nodes. The myelin sheath is an insulator and causes the ion exchange to occur only at the nodes. This speeds up the process by several orders of magnitude.

5. Transmission across a synapse:

• Neurons to not actually touch. The axon terminals of one neuron stop before reaching the dendrites of the next neuron. This gap between the two cells is called a synapse.
• Impulses are carried across a synapse by chemical messengers called neurotransmitters.
• Approximately 30 different neurotransmitters have been identified, but they all do one of two things:
   
  1. Stimulate the action potential in the next neuron cell.

     Speed up the impulse.

  2. Inhibit the action potential in the next neuron.

     Slow down or completely stop the impulse.

6. Refractory period:

• The period of time it takes a neuron to return to its resting potential after being stimulated.
• A neuron cannot be stimulated during its refractory period.
• This period of time is about 0.004 of a second.

 

Action potentials can travel along axons at speeds of 0.1 to 100 m/s. This means that nerve impulses can get from one part of a body to another in a few milliseconds, which allows for fast responses to stimuli.

Impulses are MUCH slower than electrical currents in wires, which travel at close to the speed of light, 3x10⁸ m/s.

The speed is affected by 3 factors:

• Temperature - The higher the temperature, the faster the speed. So warm-blooded animals have faster responses than cold-blooded ones.
• Axon diameter - The larger the diameter, the faster the speed. So marine invertebrates, who live at temperatures close to 0^oC, have developed thick axons to speed up their responses. This explains why squid have their giant axons.
• Myelin sheath - Only vertebrates have a myelin sheath surrounding their neurones. The voltage-gated ion channels are found only at the nodes of Ranvier, and between the nodes the myelin sheath acts as a good electrical insulator. The action potential can therefore jump large distances from node to node (1 mm), a process that is called saltatory propagation. This increases the speed of propagation dramatically, so while nerve impulses in unmyelinated neurones have a maximum speed of around 1 m/s, in myelinated neurones they travel at 100 m/s.