The structure of neurones is similar, as they all have a cell body composed of the nucleus as well as organelles such as mitochondria within the cytoplasm. Apart from the essential components, they also contain extensions called dendrites involved in conducting impulses towards the cell body , as well as axons which conduct them in the opposite direction, that is away from the cell body . The structure of neurones, that is the length of axons as well as the polarised nature of the neurone membrane in the resting state where the outside of the membrane is positively charged and the inside is negatively charged, enables the neurones to carry electrical impulses called action potentials . Resting Potential As previously mentioned, nerve cells are polarised in their resting state . This occurs as a result of an imbalance between sodium ions and potassium ions , thus giving the inside of the axon a negative charge in comparison to the external environment. As a result of the polarisation , there is a difference in the voltage across the axon membrane, with a value of -70mV known as the resting potential. This resting potential is generated as well as maintained with the help of the sodium- potassium pump which moves sodium ions out of the axon thus creating an electrochemical gradient with a higher concentration of sodium ions outside the axon . This is because the membrane is not permeable to sodium ions . The sodium-potassium pump is also involved in transporting potassium ions into the axon. However, the potassium ions move back in by facilitated diffusion due to the presence of potassium ion channels which are mainly open, compared to sodium ion channels which are mainly closed. As a result of that, the outside of the axon is positively charged due to the imbalance of positively charged ions. For every three sodium ions that are pumped out of the axon, two potassium ions are pumped in. The pumping of ions requires the use of ATP as this is active transport. Action Potential Upon stimulation , the axon membrane becomes depolarised . This occurs as follows: firstly, the excitation of the neurone cell, triggered by a stimulus, causes the sodium ion channels to open , as a result making it more permeable to sodium ions. These subsequently diffuse into the axon down the electrochemical gradient , as a result making the inside less negative . Upon reaching the threshold of -55mV , even more sodium ion channels open eventually giving a potential difference of +40mV. This is the end of depolarisation and now repolarisation starts. Repolarisation is achieved as a result of sodium ion channels closing and potassium ion channels opening . The potassium ions diffuse out of the neurone down the concentration gradient and eventually restore the resting potential . However, as the closing of potassium ion channels is slightly delayed , this leads to hyperpolarisation i.e. when the potential difference
becomes more negative than the resting potential. The resting potential is then achieved with the help of sodium-potassium pump
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