|MadSci Network: Neuroscience|
Electricity, particuarly in the form of nerve impulses called action potentials, is the main mechanism of signalling within the central nervous system. It works because the brain (and the body as a whole) contains a lot of charged molecules such as Na+, K+, Ca2+, Cl- etc. Membranes such as those which surround cells do not readily let charged particles across them. Membranes do however contain two important types of proteins, ion channels and transporters.
Transporters use stored energy in order to actively transport particular ions across membranes in particular directions, against the concentration gradient. This enables there to be much more of e.g. K+ on one side of a membrane than the other. This creates both a chemical (more potassium on one side) and electrical (more positive charges on one side) gradient across the membrane. The ionic difference means that cells normally exist with a difference in voltage across the membrane (a potential gradient). The electro-chemical gradient is the basis of nerve cell signalling, because it is basically potential energy.
When a nerve cell receives a stimulus, it responds by opening some of its ion channels. These are usually relatively selective for some types of charged molecules e.g. K+ can pass through them but Na+ can't. Opening of these channels allows e.g. K+ to flow across the membrane down its electrochemical gradient. This results in a change in the potential gradient across the membrane. Normally neurones have a potential gradient of e.g. -70 mV between inside the cell and outside. Opening of K+ channels results in a rapid change in this, rapidly causing the potential gradient to be positive. This in turn causes some potential-sensitive ion channels to open e.g. Na+ channels. The flow of Na+, which transporters maintain at high levels on the opposite side of the membrane from K+, returns the potential gradient towards normal.
So, transporters create electrical gradients, which are used by nerve cell ion channels to create rapid, transient changes in potential, resulting in what we call action potentials. Action potentials travel along nerve cells as channels consecutively open and close along the length of the nerve axon. At the end of the cell, the change in potential causes the opening of ion channles selective for Ca2+. Elevated levels of Ca2+ inside the cell causes it to release molecules into the synapse that initiate signals in other cells. This is how electricity is basically the mechanism by which we think!
Peter Simpson, Postdoctoral Fellow
The signals used to carry information throughout the nervous system are electro-chemical, not truly electrical in the strict sense. Each neuron (in fact most cells) has ion pumps in its plasma membrane which use the energy in ATP to move ions across the membrane. By pumping specific ions into or out of the cell, these transporters generate ion gradients across the membrane, e.g. using a transporter which pumps sodium (Na +) out of the cell would eventually result in more Na+ outside the cell than in. Using several pumps to this end, resting neurons have the following ion gradients:
Na+(in) = 50 mM Na+(out) = 440 mM K+ (in) = 400 mM K+ (out) = 20 mM Cl-(in) = 100 mM Cl-(out) = 560 mM
As you can see, most of the Na+ is outside of the cell, while most of the K+ (potassium) is inside the cell. These gradients can only be maintained as long as there is minimal leakage across the membrane, since gradients create a strong chemical potential. If the membrane becomes permeable to the ions, then the ions instantly flow down the concentration gradient until there are the same number of each ion inside and out. If then the membrane becomes impermeable to the ions, the action of the pumps will restore the gradients to their original levels.
One consequence of making chemical gradients with charged particles, is that the concentration gradient results in a charge gradient with one side of the membrane becoming more positively charged and the other side becoming negatively charged. The result is that neuronal membranes are "polarized". Just as the concentration gradient defines a chemical potential, the charge gradient defines an electrical potential, which can be found by the Nernst Equation. By plugging in the numbers from the gradients, we find:
EK = -75 mV
ENa = +55 mV
So a polarized membrane has a measurable voltage gradient across it, based on the sum of the electrical potentials from each ion gradient. In otherwords, the electric energy in the nervous system is produced by chemical gradients across the membranes of the cells.
But how does the neuron use these voltage gradients to transmit signals along its axon? The initial signal that a neuron receives comes in the form of a neurotransmitter, a chemical produced by another cell which activates the nerve. Neurotransmitters bind to specialized proteins in the membrane which, upon binding to the neurotransmitter, open channels in the membrane, allowing the free movement of ions across the membrane. This releases the ion gradients, thus equilibrating the charge across the membrane, "depolarizing" it. This sudden change in voltage is detected by other channels which respond by openning, and allowing further depolarization. The axon is covered in voltage-gated Na + and K+ channels, which respond to depolarization of the membrane next to them, creating a wave of depolarization that travels along the axon at a rate limited by the flow of ions through the channels (which is much slower than the speed of electricity).
Recovery of the gradients is accomplished through Na+ and K+ ATPase pumps which quickly restore the original ion concentrations. Thus, the ion pumps behave as a constant current source by creating the ion gradients across the membrane, a process requiring large amounts of energy in the form of ATP. Whereas, the driving forces behind the depolarizing pulse are electrostatic and entropic, requiring no energy and occuring as a reaction wave traveling along the nerve fibers.
Stephen W. Kuffler, John G. Nicholls, and A. Robert Martin. From neuron to brain : a cellular approach to the function of the nervous system 2nd ed. Sunderland, Mass. Sinauer Associates, 1984.
Michael Onken, Graduate Student, Neurosciences
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