How is resting potential different from repolarization

There are many details, but go slow and look at the figures. 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.

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.

When a neuron is not sending a signal, it is "at rest. 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. 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 negative charge is localized in the large anions.

This voltage would actually be much lower except for the contributions of some important proteins in the membrane.

This may appear to be a waste of energy, but each has a role in maintaining the membrane potential. Resting membrane potential describes the steady state of the cell, which is a dynamic process that is balanced by ion leakage and ion pumping. Without any outside influence, it will not change. To get an electrical signal started, the membrane potential has to change. Because sodium is a positively charged ion, it will change the relative voltage immediately inside the cell relative to immediately outside.

This is known as depolarization , meaning the membrane potential moves toward zero. The electrical gradient also plays a role, as negative proteins below the membrane attract the sodium ion. These channels are specific for the potassium ion. Figure 7. Graph of Action Potential Plotting voltage measured across the cell membrane against time, the action potential begins with depolarization, followed by repolarization, which goes past the resting potential into hyperpolarization, and finally the membrane returns to rest.

What has been described here is the action potential, which is presented as a graph of voltage over time in Figure 7. It is the electrical signal that nervous tissue generates for communication. That can also be written as a 0. To put that value in perspective, think about a battery. An AA battery that you might find in a television remote has a voltage of 1.

The change seen in the action potential is one or two orders of magnitude less than the charge in these batteries. In fact, the membrane potential can be described as a battery. A charge is stored across the membrane that can be released under the correct conditions. What happens across the membrane of an electrically active cell is a dynamic process that is hard to visualize with static images or through text descriptions. View this animation to learn more about this process. And what is similar about the movement of these two ions?

The question is, now, what initiates the action potential? The description above conveniently glosses over that point. But it is vital to understanding what is happening. The membrane potential will stay at the resting voltage until something changes. Instead, it means that one kind of channel opens. Whether it is a neurotransmitter binding to its receptor protein or a sensory stimulus activating a sensory receptor cell, some stimulus gets the process started.

Sodium starts to enter the cell and the membrane becomes less negative. This is what is known as the threshold. Because of the threshold, the action potential can be likened to a digital event—it either happens or it does not. If the threshold is not reached, then no action potential occurs.

Also, those changes are the same for every action potential, which means that once the threshold is reached, the exact same thing happens. Stronger stimuli will initiate multiple action potentials more quickly, but the individual signals are not bigger. Thus, for example, you will not feel a greater sensation of pain, or have a stronger muscle contraction, because of the size of the action potential because they are not different sizes. The other gate is the inactivation gate , which closes after a specific period of time—on the order of a fraction of a millisecond.

When a cell is at rest, the activation gate is closed and the inactivation gate is open. Timed with the peak of depolarization, the inactivation gate closes. During repolarization, no more sodium can enter the cell. After that, the inactivation gate re-opens, making the channel ready to start the whole process over again.

It might take a fraction of a millisecond for the channel to open once that voltage has been reached. Potassium continues to leave the cell for a short while and the membrane potential becomes more negative, resulting in the hyperpolarizing overshoot. Figure 8. Stages of an Action Potential. All of this takes place within approximately 2 milliseconds Figure 8. While an action potential is in progress, another one cannot be initiated.

That effect is referred to as the refractory period. There are two phases of the refractory period: the absolute refractory period and the relative refractory period. During the absolute phase, another action potential will not start. Plotting voltage measured across the cell membrane against time as shown in Figure 8 , the events of the action potential can be related to specific changes in the membrane voltage.

The action potential is initiated at the beginning of the axon, at what is called the initial segment. Because of this, depolarization spreading back toward previously opened channels has no effect.

If the nodes were any closer together, the speed of propagation would be slower. Propagation along an unmyelinated axon is referred to as continuous conduction ; along the length of a myelinated axon it is referred to as saltatory conduction.

Along with the myelination of the axon, the diameter of the axon can influence the speed of conduction. This concept is known as resistance and is generally true for electrical wires or plumbing, just as it is true for axons, although the specific conditions are different at the scales of electrons or ions versus water in a river.

But when the level is far out of balance, the effects can be irreversible. Astrocytes can become reactive in cases such as these, which impairs their ability to maintain the local chemical environment. The glial cells enlarge and their processes swell. Visit this site to see a virtual neurophysiology lab, and to observe electrophysiological processes in the nervous system, where scientists directly measure the electrical signals produced by neurons.

Often, the action potentials occur so rapidly that watching a screen to see them occur is not helpful. These action potentials are firing so fast that it sounds like static on the radio. Electrophysiologists can recognize the patterns within that static to understand what is happening. Why is the leech model used for measuring the electrical activity of neurons instead of using humans? The nervous system is characterized by electrical signals that are sent from one area to another.

Whether those areas are close or very far apart, the signal must travel along an axon. The basis of the electrical signal is the controlled distribution of ions across the membrane. Transmembrane ion channels regulate when ions can move in or out of the cell, so that a precise signal is generated. This signal is the action potential which has a very characteristic shape based on voltage changes across the membrane in a given time period.

A stimulus will start the depolarization of the membrane, and voltage-gated channels will result in further depolarization followed by repolarization of the membrane. A slight overshoot of hyperpolarization marks the end of the action potential.

While an action potential is in progress, another cannot be generated under the same conditions. Once that channel has returned to its resting state, a new action potential is possible, but it must be started by a relatively stronger stimulus to overcome the state of hyperpolarization. The action potential travels down the axon as voltage-gated ion channels are opened by the spreading depolarization. In unmyelinated axons, this happens in a continuous fashion because there are voltage-gated channels throughout the membrane.

Saltatory conduction is faster than continuous conduction, meaning that myelinated axons propagate their signals faster. The diameter of the axon also makes a difference as ions diffusing within the cell have less resistance in a wider space.

View this animation to really understand the process. Sodium is moving into the cell because of the immense concentration gradient, whereas potassium is moving out because of the depolarization that sodium causes. However, they both move down their respective gradients, toward equilibrium.

The properties of electrophysiology are common to all animals, so using the leech is an easier approach to studying the properties of these cells. There are differences between the nervous systems of invertebrates such as a leech and vertebrates, but not for the sake of what these experiments study.

The conscious perception of pain is often delayed because of the time it takes for the sensations to reach the cerebral cortex. Why would this be the case based on propagation of the axon potential?

Skip to content Learning Objectives By the end of this section, you will be able to: Describe how movement of ions across the neuron membrane leads to an action potential Describe the components of the membrane that establish the resting membrane potential Describe the changes that occur to the membrane that result in the action potential.

External Website What happens across the membrane of an electrically active cell is a dynamic process that is hard to visualize with static images or through text descriptions. Homeostatic Imbalances — Potassium Concentration. Glial cells, especially astrocytes, are responsible for maintaining the chemical environment of the CNS tissue. The concentrations of ions in the extracellular fluid are the basis for how the membrane potential is established and changes in electrochemical signaling.

If the balance of ions is upset, drastic outcomes are possible. External Website Visit this site to see a virtual neurophysiology lab, and to observe electrophysiological processes in the nervous system, where scientists directly measure the electrical signals produced by neurons. Chapter Review The nervous system is characterized by electrical signals that are sent from one area to another. Interactive Link Questions What happens across the membrane of an electrically active cell is a dynamic process that is hard to visualize with static images or through text descriptions.

Review Questions. Critical Thinking Questions 1. If threshold is not reached, those channels do not open, and the depolarizing phase of the action potential does not occur, the cell membrane will just go back to its resting state.