Motor Neurones and Nerve Impulses

Structure of a Motor Neurone

A motor neurone is connected to many relay neurones within the central nervous system. The cell body of the motor neurone (containing the nucleus) lies within the central nervous system. This is branched into structures called dendrites which allow connections to be made to many other neurones. A single long extension of the cell body is called an axon. This transmits the action potential (nerve impulse) along its length until the motor neurone connects with a gland or a muscle. 

The axon is surrounded by accessory cells, called Schwann cells, which are present at intervals along its length. These secrete a fatty material called myelin, this insulates the axon and prevents movement of sodium and potassium ions into and out of the axon. Action potentials can only be generated where there are gaps between the Schwann cells, called the nodes of Ranvier. These gaps allow the movement of sodium and potassium ions across the membrane.

Nerve Impulses

A Nerve Impulse:

• Is a temporary reversal of potential difference across the axon membrane that travels as a self-propagating action potential
• The reversal of potential difference is caused by sodium and potassium ion movements across the membrane
• Action potentials can only be generated where the axon membrane is not insulated by myelin sheath, so they are able to jump from one node of Ranvier to the next adjacent node of Ranvier in myelinated neurones (saltatory conduction)

Establishment of a Resting Potential:

• In a resting (unstimulated) neurone the membrane is polarised
• The inside of the cell has a negative charge with respect to the outside of the cell
• Potential difference of the membrane is -70mV
• When the potential difference is reversed during an action potential, the only ions moving are positively charged sodium and potassium ions

The inside of the resting neurone is negatively charged with respect to the outside, which is positvely charged. This electrochemical gradient creates a resting membrane potential of -70mV. There are more potassium ions on the inside of the neurone than the outside and more sodium ions outside the neurone compared to the inside.

This distribution of…