Resting potential
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The resting potential of nerve cells is established by the movement of ions across the cell membrane. Sodium-potassium pumps actively transport sodium ions out of the cell and potassium ions into the cell. More sodium ions are transported out than potassium ions transported in, making the outside of the cell more positive than the inside. Gated channels then allow more potassium ions to diffuse out of the cell than sodium ions into the cell, further increasing the charge difference between the inside and outside of the cell. This electrical gradient causes potassium ions to be attracted to the inner cytoplasm and repelled from the outer fluid, resulting in no net movement of ions and establishing the cell's resting potential.
Resting potential
- 1. Resting Potential • Intrinsic proteins in the phospholipid membrane of the nerve cell contain channels which can be opened or closed to allow sodium and potassium ions in and out of the cytoplasm. These are called gated channels. Others are open all the time allowing the ions to diffuse through them unhindered. • Some proteins called sodium potassium pumps actively transport potassium ions into the cytoplasm and sodium ions out at the same time As a result of these controls the inside of the axon is negatively charged relative to the outside and this is the resting potential. It varies in different nerve cells. When the axon is at the resting potential it is polarised. How is this resting potential established in the first place? It is all down to the movement of ions. Both sodium and potassium ions carry the 1+ charge. Sodium ions are actively transported out of the cell and potassium ions are transported in. These are both transported by the sodium potassium pump. However more sodium ions are transported out for potassium ions transported in (3 sodium for every 2 potassium). There are therefore more sodium ions outside of the neurone than inside and more potassium ions inside than outside. The gated channels then open. More open for potassium ions than sodium ions so as a result the membrane allows lots more potassium ions out of it than sodium ions in. This
- 2. again increases the difference in charge between the positive outside and less positive inside of the cell. This sets up an electrical gradient. This means that after the initial exit of potassium ions from the cytoplasm of the cell it becomes harder for more to diffuse out. This is because they are positively charged and the fluid outside the neurone is positively charged. This means they are repelled (like magnets). However the cytoplasm is less positive so they are attracted to this. This results in no net movement of ions and establishes the resting potential.