The Spark of Life: Electricity in the Human Body by Frances Ashcroft Page B
Authors: Frances Ashcroft
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difference across their membrane, the inside of the cell usually being between 60 and 90 millivolts more negative than the outside. This resting potential arises because of a tug of war between the concentration and electrical gradients across the cell membrane that the potassium ion experiences.
At rest, many potassium channels are open in the cell membrane. As potassium ions are high inside the cell but low outside, they rush out of the cell down their concentration gradient and, because potassium ions carry a positive charge, their exodus leads to a loss of positive charge – or, to put it another way, the inside of the cell becomes gradually more negative. At some point, the exit of potassium ions is impeded by the increasing negative charge within the cell, which exerts an attractive force on the potassium ion that counteracts further movement. The membrane potential at which the chemical force driving the potassium ions out of the cell and the electrical force holding them back exactly balance one another is known as the equilibrium potential.
If the membrane were only permeable to potassium ions, the resting membrane potential would be exactly the same as the potassium equilibrium potential. However, the real world is not so simple and in most cells a few other types of ion channel are open that allow positively charged ions to sneak into the cell, pushing the resting potential to a more positive level.
The importance of the resting potential is that it acts like a tiny battery in which electric charge (in the form of ion gradients) is separated by the insulating properties of the lipid membrane. This stored energy is used to power the electrical impulses of our nerve and muscle fibres.
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Ions take the path of least resistance and move down their concentration gradient from an area of high concentration to one of low concentration. The number of sodium ions is much higher outside the cell than inside, so that sodium ions flood into the cell when the sodium channel gates open. Conversely, as there are many more potassium ions inside than out, potassium ions tend to leave the cell when the potassium channels open. Because ions are charged, their flow produces an electric current. It is such currents, carried by ions surging through ion channels, that underlie all our nerve and muscle impulses, and that regulate the beating of our hearts, the movement of our muscles and the electrical signals in our brains that give rise to our thoughts. This, in essence, is how the energy stored in the concentration gradients is used to power the electrical impulses of our nerve and muscle fibres.
Suck it and See
Given the importance of ion channels, it may seem surprising that their very existence was not dreamt of until the middle of the last century and that even by the early 1970s the idea that ions crossed the membrane via specialized protein pores was still a matter of speculation. To demonstrate their existence directly it was necessary to measure the current that flows through a single channel when it opens. This was far from easy, because the current is extremely tiny and can only be measured with highly specialized electronic equipment. If you consider that the currents flowing through a single ion channel when it is open are about a million millionth of the current needed to power your kettle – a few picoamps only – you will get some idea of just how infinitesimally small they are.
The problem was solved using an elegant technique invented by two German scientists, Erwin Neher and Bert Sakmann, which won them a Nobel Prize. Truly innovative science often arises at the interface of different disciplines and their prize-winning combination of talent illustrates this perfectly. Neher was trained as a physicist and Sakmann in medicine, so they brought complementary skills to the problem; together they provided the breadth of vision to see where this new technology could take them and the attention to detail needed
At rest, many potassium channels are open in the cell membrane. As potassium ions are high inside the cell but low outside, they rush out of the cell down their concentration gradient and, because potassium ions carry a positive charge, their exodus leads to a loss of positive charge – or, to put it another way, the inside of the cell becomes gradually more negative. At some point, the exit of potassium ions is impeded by the increasing negative charge within the cell, which exerts an attractive force on the potassium ion that counteracts further movement. The membrane potential at which the chemical force driving the potassium ions out of the cell and the electrical force holding them back exactly balance one another is known as the equilibrium potential.
If the membrane were only permeable to potassium ions, the resting membrane potential would be exactly the same as the potassium equilibrium potential. However, the real world is not so simple and in most cells a few other types of ion channel are open that allow positively charged ions to sneak into the cell, pushing the resting potential to a more positive level.
The importance of the resting potential is that it acts like a tiny battery in which electric charge (in the form of ion gradients) is separated by the insulating properties of the lipid membrane. This stored energy is used to power the electrical impulses of our nerve and muscle fibres.
----
Ions take the path of least resistance and move down their concentration gradient from an area of high concentration to one of low concentration. The number of sodium ions is much higher outside the cell than inside, so that sodium ions flood into the cell when the sodium channel gates open. Conversely, as there are many more potassium ions inside than out, potassium ions tend to leave the cell when the potassium channels open. Because ions are charged, their flow produces an electric current. It is such currents, carried by ions surging through ion channels, that underlie all our nerve and muscle impulses, and that regulate the beating of our hearts, the movement of our muscles and the electrical signals in our brains that give rise to our thoughts. This, in essence, is how the energy stored in the concentration gradients is used to power the electrical impulses of our nerve and muscle fibres.
Suck it and See
Given the importance of ion channels, it may seem surprising that their very existence was not dreamt of until the middle of the last century and that even by the early 1970s the idea that ions crossed the membrane via specialized protein pores was still a matter of speculation. To demonstrate their existence directly it was necessary to measure the current that flows through a single channel when it opens. This was far from easy, because the current is extremely tiny and can only be measured with highly specialized electronic equipment. If you consider that the currents flowing through a single ion channel when it is open are about a million millionth of the current needed to power your kettle – a few picoamps only – you will get some idea of just how infinitesimally small they are.
The problem was solved using an elegant technique invented by two German scientists, Erwin Neher and Bert Sakmann, which won them a Nobel Prize. Truly innovative science often arises at the interface of different disciplines and their prize-winning combination of talent illustrates this perfectly. Neher was trained as a physicist and Sakmann in medicine, so they brought complementary skills to the problem; together they provided the breadth of vision to see where this new technology could take them and the attention to detail needed
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