Let’s delve into the basics of nervous transmission by looking at a motor neurone. Here is the structure of a myelinated motor neurone:
Labelled “insulating sheath”, the myelin sheath is responsible for protecting the electrical impulses that run across the neurone.
Dendrites carry electric signals i.e. nerve impulses towards the cell body (the soma), while the axon carries them away, often towards the receiving dendrites of other neurons.
The Myelin Sheath
This insulating sheath made up of Schwann cells is key in ensuring fast signal transmission. The signal is able to “jump” along the axon without losing its strength. Instead of the signal being gradually weakened by the resistance that an axon membrane exposed to the environment creates, this signal is shielded by the myelin sheath. The nodes of Ranvier in between the Schwann cells are the only points of axon membrane permeability to its environment.
Each pink cell is a Schwann cell. Due to the jump-like action, this conduction is termed saltatory conduction. Factors that affect conduction other than myelination and saltatory conduction (which allow speeds many times faster compared with no myelination) include temperature and axon diameter.
Since chemical movement (kinetic energy) relies on temperature, an optimal temperature maximises conduction. A temperature lower than this would slow it down. This is due to a slower opening of sodium channels for example, and also a slower inactivation resulting in a longer delay.
Axon diameter affects conduction in terms of resistance. The signal travelling along a thin axon encounters the resistance of the axon membrane, while for an axon with larger diameter, a smaller proportion of the signal is met with resistance in this way. The signal carried on the inner section of the axon has no resistance and can travel faster.