What is a nerve impulse?

A nerve impulse is an electrochemical change that travels along the membrane of a neuron. Nerve impulses are transmitted rapidly, to allow for a rapid response to occur to any change in the internal or external environment. What are the conditions in a neuron at rest?

• A resting neuron is said to be polarised, as it has different charges on either side of the cell membrane

• The exterior of the cell is positively charged due to the larger number of sodium ions present outside of the cell in comparison to the inside of the cell

• The interior of the cell is negatively charged as it contains a low concentration of sodium ions and more negative organic molecules

• There is also a concentration gradient present with a high concentration of potassium ions inside the cell compared to the outside

• Due to these differences in charge, there is an electro-chemical difference or resting membrane potential (mV) of about -70 millivolts (-70mV)

• The sodium-potassium pump uses ATP molecules to actively maintain this difference by removing sodium ions from inside the cell back and drawing potassium ions into the cells

• The pump moves three sodium ions for every two potassium ions, and therefore there is a higher concentration of sodium outside compared to potassium inside

How is a nerve impulse generated? When a stimulus that is strong enough is detected by a neuron of at least 15mV to reach the threshold level of -55mV, a nerve impulse is generated in that neuron. This process is referred to as an 'all-or-none' response, and the stimulus triggers both chemical and electrical changes in the neuron.