The formation of an action potential can be divided into five steps. (1) A stimulus from a sensory cell or another neuron causes the target cell to depolarize toward the threshold potential. (2) If the threshold of excitation is reached, all Na+ channels open and the membrane depolarizes. (3) At the peak action potential, K+ channels open and K+ begins to leave the cell. At the same time, Na+ channels close. (4) The membrane becomes hyperpolarized as K+ ions continue to leave the cell. The hyperpolarized membrane is in a refractory period and cannot fire.
(5) The K+ channels close and the Na+/K+ transporter restores the resting potential. Action potentials are formed when a stimulus causes the cell membrane to depolarize past the threshold of excitation, causing all sodium ion channels to open. When the potassium ion channels are opened and sodium ion channels are closed, the cell membrane becomes hyperpolarized as potassium ions leave the cell; the cell cannot fire during this refractory period. The action potential travels down the axon as the membrane of the axon depolarizes and repolarizes.
Myelin insulates the axon to prevent leakage of the current as it travels down the axon. Nodes of Ranvier are gaps in the myelin along the axons; they contain sodium and potassium ion channels, allowing the action potential to travel quickly down the axon by jumping from one node to the next. Action Potential A neuron can receive input from other neurons via a chemical called a neurotransmitter. If this input is strong enough, the neuron will pass the signal to downstream neurons.
Transmission of a signal within a neuron (in one direction only, from dendrite to axon terminal) is carried out by the opening and closing of voltage-gated ion channels, which cause a brief reversal of the resting membrane potential to create an action potential (Figure 1). As an action potential travels down the axon, the polarity changes across the membrane. Once the signal reaches the axon terminal, it stimulates other neurons. Depolarization and the Action Potential When neurotransmitter molecules bind to receptors located on a neuron’s dendrites, voltage-gated ion channels open.
At excitatory synapses, positive ions flood the interior of the neuron and depolarize the membrane, decreasing the difference in voltage between the inside and outside of the neuron. A stimulus from a sensory cell or another neuron depolarizes the target neuron to its threshold potential (-55 mV), and Na+ channels in the axon hillock open, starting an action potential. Once the sodium channels open, the neuron completely depolarizes to a membrane potential of about +40 mV. The action potential travels down the neuron as Na+ channels open. Hyperpolarization and Return to Resting Potential
Action potentials are considered an “all-or nothing” event. Once the threshold potential is reached, the neuron completely depolarizes. As soon as depolarization is complete, the cell “resets” its membrane voltage back to the resting potential. The Na+ channels close, beginning the neuron’s refractory period. At the same time, voltage-gated K+ channels open, allowing K+ to leave the cell. As K+ ions leave the cell, the membrane potential once again becomes negative. The diffusion of K+ out of the cell hyperpolarizes the cell, making the membrane potential more negative than the cell’s normal resting potential.
At this point, the sodium channels return to their resting state, ready to open again if the membrane potential again exceeds the threshold potential. Eventually, the extra K+ ions diffuse out of the cell through the potassium leakage channels, bringing the cell from its hyperpolarized state back to its resting membrane potential. Myelin and Propagation of the Action Potential For an action potential to communicate information to another neuron, it must travel along the axon and reach the axon terminals where it can initiate neurotransmitter release (Figure 2).
The speed of conduction of an action potential along an axon is influenced by both the diameter of the axon and the axon’s resistance to current leak. Myelin acts as an insulator that prevents current from leaving the axon, increasing the speed of action potential conduction. Diseases like multiple sclerosis cause degeneration of the myelin, which slows action potential conduction because axon areas are no longer insulated so the current leaks. A node of Ranvier is a natural gap in the myelin sheath along the axon (Figure 3).
These unmyelinated spaces are about one micrometer long and contain voltage gated Na+ and K+ channels. The flow of ions through these channels, particularly the Na+ channels, regenerates the action potential over and over again along the axon. Action potential “jumps” from one node to the next in saltatory conduction. If nodes of Ranvier were not present along an axon, the action potential would propagate very slowly; Na+ and K+ channels would have to continuously regenerate action potentials at every point along the axon.
Nodes of Ranvier also save energy for the neuron since the channels only need to be present at the nodes and not along the entire axon. Much of what we know about how neurons work comes from experiments on the giant axon of the squid. This giant axon extends from the head to the tail of the squid and is used to move the squid’s tail. How giant is this axon? It can be up to 1 mm in diameter – easy to see with the naked eye. Neurons send messages electrochemically. This means that chemicals cause an electrical signal. Chemicals in the body are “electrically-charged” — when they have an electrical charge, they are called ions.
The important ions in the nervous system are sodium and potassium (both have 1 positive charge, +), calcium (has 2 positive charges, ++) and chloride (has a negative charge, -). There are also some negatively charged protein molecules. It is also important to remember that nerve cells are surrounded by a membrane that allows some ions to pass through and blocks the passage of other ions. This type of membrane is called semi-permeable. Resting Membrane Potential When a neuron is not sending a signal, it is “at rest. ” When a neuron is at rest, the inside of the neuron is negative relative to the outside.
Although the concentrations of the different ions attempt to balance out on both sides of the membrane, they cannot because the cell membrane allows only some ions to pass through channels (ion channels). At rest, potassium ions (K+) can cross through the membrane easily. Also at rest, chloride ions (Cl-)and sodium ions (Na+) have a more difficult time crossing. The negatively charged protein molecules (A-) inside the neuron cannot cross the membrane. In addition to these selective ion channels, there is a pump that uses energy to move three sodium ions out of the neuron for every two potassium ions it puts in.
Finally, when all these forces balance out, and the difference in the voltage between the inside and outside of the neuron is measured, you have the resting potential. The resting membrane potential of a neuron is about -70 mV (mV=millivolt) – this means that the inside of the neuron is 70 mV less than the outside. At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron. Action Potential The resting potential tells about what happens when a neuron is at rest. An action potential occurs when a neuron sends information down an axon, away from the cell body.
Neuroscientists use other words, such as a “spike” or an “impulse” for the action potential. The action potential is an explosion of electrical activity that is created by a depolarizing current. This means that some event (a stimulus) causes the resting potential to move toward 0 mV. When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold. If the neuron does not reach this critical threshold level, then no action potential will fire. Also, when the threshold level is reached, an action potential of a fixed sized will always fire… for any given neuron, the size of the action potential is always the same. There are no big or small action potentials in one nerve cell – all action potentials are the same size. Therefore, the neuron either does not reach the threshold or a full action potential is fired – this is the “ALL OR NONE” principle. Action potentials are caused when different ions cross the neuron membrane. A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron.
Remember, sodium has a positive charge, so the neuron becomes more positive and becomes depolarized. It takes longer for potassium channels to open. When they do open, potassium rushes out of the cell, reversing the depolarization. Also at about this time, sodium channels start to close. This causes the action potential to go back toward -70 mV (a repolarization). The action potential actually goes past -70 mV (a hyperpolarization) because the potassium channels stay open a bit too long. Gradually, the ion concentrations go back to resting levels and the cell returns to -70 mV. And there you have it… the Action Potential