How Do Nerve Cells Communicate
Nerves Use Ion Flux to Make Information Flow
Nerves communicate with their targets, whether that is another nerve cell or some different tissue, using specific molecules known as neurotransmitters. The neurotransmitter molecule signals to the target cell to make a change in its physiologic behavior. All of these events take place because of minute changes in electrical currents that flow across cell membranes.
When a nerve cell decides to release some neurotransmitter molecules to act on one of its targets, it does so because its membrane has generated what is known as an ‘action potential’. In an action potential, infinitesimally small pores, or channels, are caused to sequentially open in the cell membrane, allowing for individual charged ions, such as sodium or potassium or calcium, to move across. This movement of charged ions (they are not electrically neutral) leads to a localized current flow across the membrane.
Electrically Active Cells Have a Membrane Voltage
Not every cell in the human body is electrically responsive in this way. But many are, including nerve cells, skeletal muscle, and some glandular cells amongst others. They are electrically active, first and foremost, because they can actively maintain a difference in the concentration of particular charged ions inside and outside of the cell. These charged ions include those listed above as well as chloride, magnesium, and several others. This gradient of ionic particles leads to the inside of the cell having a different net ‘charge’ as compared to the outside. This difference in so-called electrical potential leads to the generation of a small but measurable voltage across the cell membrane. This voltage can be typically anywhere from 50 to 90 millivolts (thousandths of a volt).
What Makes Current Flow in Nerve Cells
Current begins to flow in a nerve cell (or any electrically excitable cell for that matter), when a signal is received to open the pores, or ion channels, in the cell membrane. The initiating signal is most often that which is induced by a neurotransmitter, binding to a specific ion channel, causing it to open briefly for ions to flow. But that is not the only means by which this happens. Another important way for ions to flow is through another class of ion channels in the cell membrane that do not respond to a particular molecule, like a neurotransmitter, but rather, they respond to changes in the voltage across the cell membrane, or what is referred to as the ‘membrane potential’. These different classes of ion channels work together to allow all nerve cells to transmit information.
Information Transfer from Nerve to Nerve
A nerve cell receives a neurotransmitter signal at the tiny specialized communication site called the synapse, where another nerve cell tells it to activate. The neurotransmitter signal makes the recipient cell allow ions to flow across the membrane. This alters the membrane voltage in that region of the cell. The voltage-sensitive ion channels now open to let more ions flow through in adjacent membrane, now moving away from the site of the synapse. These voltage-sensitive channels are dispersed throughout the cell membrane and they now allow for ‘propagation’ of the signal away from the synaptic site. Once the signal reaches the point where the stimulated nerve cell contacts the next nerve cell in the circuit, the change in the membrane potential now causes the activated cell to specifically release its neurotransmitter for the next cell to be stimulated. And remarkably enough this all happens in fractions of thousandths of a second.
Repeated over and over and over again, this is how electrical signals allow nerve cells to transmit their signals. To see a wonderful animation of how nerve cells use ion channels check out this cartoon at www.wiley.com and click on Active Transport.
A Neuron
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