LESSON 2.2 WORKBOOK How do our axons transmit electrical signals?

LESSON 2.2 WORKBOOK How do our axons transmit electrical signals? This lesson introduces you to the action potential, which is the process by which axons signal electrically. In this lesson you will learn how our axons utilize energy stored in their membranes to send signals throughout our bodies. Signaling is organized in the same way in all neurons To produce a behavior, each participating neuron in a circuit produces, in the same sequence, four types of signals at different sites: The dendrites generate electrical input signals. The axon hillock (or initial segment) integrates the input signals into a single electrical signal the action potential. The axon transmits the electrical action potential down to the presynaptic terminal. The presynaptic terminals convert the electrical action potential into a chemical output signal. We will discuss each of these signals, but it s easiest to understand if we start with the action potential, even though it comes in the middle. Before we discuss any of the signals though, we need to review the electrical properties of the cell membrane that are important to understand how these signals are generated. What are the four types of signals generated within neurons and where are they generated? Lesson 2.2 44

Diffusion the net movement of molecules from areas of high concentration to areas of low concentration. Electrostatic Pressure the repulsion of like charges and the attraction of opposite charges Potential energy the energy a body has because of its position relative to others, electric charge and other factors. Resting membrane potential the steady membrane potential of a neuron at rest, usually about -70 mv. Neuronal membranes store energy in the form of membrane potentials Neuronal membranes are electrically charged. This means there is a difference in electrical charge across their cell membranes of about 70 millivolts (mv). As we shall see in a minute this difference in charge occurs because sodium ( ), and potassium ( ) ions and organic anions (A - ) are unevenly distributed across the membrane so that the inside of the axon is negatively charged relative to the outside (Figure 7). This electrical charge is called the resting membrane potential. The term potential refers to the energy stored in the membrane or its potential energy. Because the outside of the axon is arbitrarily defined as zero, we say that the resting membrane potential of the axon is -70 mv. The resting membrane potential is produced as a result of the forces of diffusion and electrostatic pressure that the ions inside and outside the membrane experience. Remember that: Diffusion is the net movement of molecules (such as ions) down a concentration gradient Electrostatic pressure is the repulsion of like charges (positive is repulsed by positive and negative with negative) and the attraction of opposite charges Understanding what produces the membrane potential therefore requires that we know the concentration of various ions inside and outside the axon and what forces of diffusion and electrostatic pressure they are experiencing. Figure 7: Membrane potential. (A) When both electrodes are applied to the exterior of the axon in the extracellular fluid, no difference in potential is recorded. (B) When one electrode is inserted into the axon, a voltage difference between the inside and the outside is recorded. The graphs show the voltage change when one electrode is inside the axon. The force of diffusion, molecules moving down their concentration gradient, predicts the result of adding one drop of blue food coloring to a glass of water. Immediately after adding one drop of blue food coloring to a glass of water, that drop sits on the top of the water in an area of high concentration. What happens if you let the water sit for 5 minutes? The force of electrostatic pressure, attraction of opposite charges and repulsion of like charges, predicts what would happen if you had negatively charged ions at the top of a cup and positively charged ions at the bottom of a cup. Where do you predict the negatively charged ions will go? Lesson 2.2 45

Athough there are many types of ions inside and outside the axon, three are particularly important for the membrane potential (Figure 8): Organic anions (symbolized as A - ) Potassium ions ( ) Sodium ions ( ) Let s now consider how each of these important ions experiences the forces of diffusion and electrostatic pressure. Once we know this we will understand why each ion is located where it is when the axon membrane is at rest. Organic anions are negatively charged proteins and intermediary products of a cell s metabolism. They are unable to pass through neuron s membrane and so they are only found inside the axon. Therefore, they make the interior of the axon more negative and contribute to the negative membrane potential. Potassium ions ( ) are also concentrated within the axon, however they can move to the outside through special channels in the cell membrane. Thus, the force of diffusion will tend to push them out of the cell. However the high concentration of At rest negative organic anions makes the inside of the cell more negative relative to the outside. Because of this negative charge, electrostatic pressure tends to keep the potassium ions inside the cell. In the case of potassium ions the two forces of diffusion and electrostatic pressure oppose each other and balance each other out. As a result, potassium ions tend to remain where they are at high concentrations inside the axon. Sodium ions ( ) are concentrated outside the axon, in the extracellular fluid. Like potassium, there are sodium ion channels in the membrane. Therefore, the force of diffusion pushes these ions inwards. In addition, sodium ions are positively charged, so electrostatic pressure also attracts them into the negatively charged axon. However, if the sodium ions did enter the axon the charge difference across the membrane would break down and the potential energy in the membrane would be lost. Figure 8: Distribution of ions at resting membrane potential. ions (represented by blue circles) are more concentrated outside the neuron. ions (represented by red circles) and negatively charged proteins (represented by black stars) are more concentrated inside the neuron.axon, a voltage difference between the inside and the outside is recorded. What are organic anions, and where are they in highest concentration in our nervous systems? Where are potassium ions in highest concentration in our nervous systems? Where are sodium ions in highest concentration in our nervous systems? Lesson 2.2 46 - - Na - -

Sodium-potassium pump active transport mechanism that pumps sodium ( ) ions out of neurons and potassium ( ) ions into neurons. Voltage-gated channels channels that open or close in response to changes in voltage across the membrane. How then can overcome the two forces of diffusion and electrostatic pressure and stay on the outside the axon, preserving the resting membrane potential? The answer is this: there is another force provided by a pump that continuously pushes out of the axon, swapping an ion that might have leaked inside for a ion that might have leaked outside. Because the membrane is not very permeable to (there are fewer channels) the sodium-potassium pump ( / pump) is very effective at keeping the intracellular concentration of very low when the membrane is at rest. Just a quick side note: Sodium-potassium pumps use enormous amounts of energy up to 40% of a neuron s energy is used to operate them. The Action Potential We just saw that the forces of both diffusion and electrostatic pressure tend to attract into the axon. However, we also saw that the membrane is not very permeable to ions, and that the sodium/potassium pump continuously pumps out of the axon, keeping intracellular concentrations low. But imagine what would happen if the membrane suddenly became permeable to. The forces of diffusion and electrostatic pressure would cause to rush into the cell. This sudden influx of positively charged ions would drastically reduce the membrane potential. This is precisely what happens to cause the action potential: A brief increase in the permeability of the membrane to (which allows to enter the cell), is immediately followed by a transient increase in permeability of the membrane to (allowing to exit the cell). The question now is what is responsible for these transient increases in permeability? We already saw that there are two ways to move ions across the membrane, either through channels in the membrane or by hooking them up to pumps, like the sodium-potassium pump. Sometimes the passages or pores in the ion channels are always open, but usually they are closed and only open under specific conditions. When the channel pores are open they are only permeable to a particular type of ion, which can flow through the pore and thus enter or exit the cell. Some ion channels open or close depending on the cell s membrane potential. They are referred to as Voltage-gated ion Figure 9: The membrane s ion channels and pumps. Two ion channels are critical in the axon s conduction of the action potential: the voltage-gated channel and the voltage-gated How does sodium remain in highest concentration outside the axon? How do sodium channels open in response to changes in the cell s membrane potential? channels. K Lesson 2.2 channel. Additionally the / pump plays a critical role as well. between the inside and the outside is recorded. 47

Two voltage-gated channels are critical in the action potential (Figure 9): Voltage-gated channels open when the membrane potential reaches -50 mv, and close when the membrane potential reaches 40 mv. Voltage-gated channels open when the membrane potential reaches 40 mv, and close when the membrane potential reaches 70 mv. The following numbered paragraphs describe the movement of ions across the membrane through channels and pumps which underlies the action potential. The numbers in Figure 10 correspond to the numbers in the paragraphs below. Figure 10: Stage of the action potential. The opening and closing of voltage-gated and channels is responsible for the characteristic shape of the action potential. Refer to Figures 13-17 to see what is happening at each stage of the action potential. Describe the different types of voltage-gated ion channel. 1. The Resting Membrane potential: At rest the voltage-gated channels and voltage-gated channels are closed and the / pump is working hard, using ATP to sustain the resting membrane potential to move three ions out of the axon for every two ions moved into the axon. As a result the concentration of outside the axon is high, and the concentration of inside the axon is high (Figure 11). Because of the contribution of the organic cations (A - ) the inside of the axon is more negative than the outside, even though the is in high concentration there. As a result the membrane potential at rest is -70 mv, as we saw before. Figure 11: Resting membrane potential (Stage 1 in Figure 10). The resting membrane potential is maintained by the / pump. At rest, there is a slow leak of ions out of the cell, which the / pump corrects by pumping 3 ions out of the cell for every 2 ions it pumps into the cell. Describe where the and ions are when the axon s membrane is at rest. Why are they located there? Lesson 2.2 48

Absolute refractory period a brief period after the initiation of an action potential during which it is impossible to elicit another action potential in the same neuron. Depolarize to decrease the resting membrane potential. Decreasing membrane potential means that the membrane potential is becoming more positive. Local potentials small changes in voltage (membrane potential) due to dendritic signaling. Threshold the level of depolarization needed to generate an action potential. 2. Reaching Threshold: When the dendrites receive a signal just a few voltage-gated channels open and the charge across the dendritic membranes drops briefly causing small changes in voltage or local potentials. The forces of diffusion and electrostatic pressure then pull ions into the cell through the open channels (Figure 12). This inward flow of positive sodium ions starts to reduce or depolarize the membrane potential, meaning that the inside of the cell is becoming more positive relative to the outside. If enough channels open and enough ions enter the cell, then the membrane potential will decrease to the threshold (-50 mv) at which all the channels will open and large quantities of ions will enter the cell. Threshold is the critical level of membrane depolarization at which the cell can actively generate an action potential. As we see, whether threshold is reached depends on the strength of the dendritic signal. If the dendritic signal is strong then we are more likely to reach threshold. 3. Depolarization: When a threshold of -50 mv is reached, many more voltage-gated channels open allowing even more ions to quickly flow into the axon (Figure 13). This inward flow of into the axon further depolarizes the membrane, Na reducing the membrane potential even more so that eventually the inside of the axon becomes positive relative to the outside. 4. Hyperpolarization: When so many ions have entered the axon that the interior has reached 40 mv (relative to the external value of 0 mv) the voltage-gated channels close. This inactivates them, so they cannot open for a period of time. This is called the absolute refractory period. K Figure 12: Reaching threshold (Stage 2 in Figure 10). Local potentials open a few voltage-gated channels, allowing ions to enter the axon. K Na K Na Figure 13: Depolarization (Stage 3 in Figure 10). Once the membrane reaches threshold (-50 mv), many more voltage-gated channels open, allowing even more ions to enter the axon. Describe where the and ions are when the axon s membrane is reaching threshold. Why are they there? Describe where the and ions are when the axon s membrane is depolarizing. Why are they there? Lesson 2.2 49

Hyperpolarize to increase the resting membrane potential. Increasing membrane potential means that the membrane potential is becoming more negative. Relative refractory period period after the absolute refractory period during which a higher-thannormal amount of stimulation is necessary to make a neuron fire Remember that voltage-gated channels also open at 40 mv. This opening of channels allows ions to flow out of the axon (Figure 14). The ions flow out of the axon because the prior passage of ions into the cell has altered the forces of diffusion and electrostatic pressure that the ions now experience. First of all, is in higher concentrations inside the axon than outside the axon, so with the channel open the ion is forced down its concentration gradient and out of the cell by diffusion. Furthermore, with ions now inside the axon, the inside is now positive relative to the outside. So, electrostatic pressure also forces the positive ions outside the axon. This flow of outside of the axon decreases the positive charge on the inside, and has the effect of hyperpolarizing the axon membrane meaning that the inside of the membrane becomes more negative relative to the outside. Due to the huge flow of out of the cell, the membrane potential becomes higher than it is at rest (the inside of the axon is more negative relative to the outside). This period of higher membrane potential is called the relative refractory period because the sodium channels are now able to open, so if enough positive charge came along the axon could potentially reach threshold and depolarize again. However if you think about it, because the membrane potential is higher, more positive ions would be needed to reach threshold than if the cell was at rest, so depolarization during the relative refractory period is less Na Na Na likely to occur. 5. Returning to rest: To return the membrane to its resting membrane potential of -70mV, the / pump works hard and uses ATP to move three ions out of the cell for every two ions moved into the cell (Figure 15). Na Na Figure 14: Hyperpolarization (Stage 4 in Figure 10). At 40 mv the voltage-gated channels close and the voltage-gated channels open, allowing ions to exit the axon. Describe where the and ions are when the axon s membrane is hyperpolarizing. Why are they there? Describe where the and ions are when the axon s membrane is returning to rest. Why are they there? Lesson 2.2 ions out of the cell for every 2 ions it pumps into the cell. 50 K Figure 15: Returning to rest (Stage 5 in Figure 10). The voltage-gated channels close, and the / pump returns the membrane to rest by pumping 3

Conduction of the action potential movement of the action potential down the length of the axon Conduction of the Action Potential along the Axon Now we have a basic understanding of what the resting membrane potential is and how the action potential is produced, we can turn our attention to how this electrical message moves down the length of the axon to the presynaptic terminal. Axons do this in a process called the conduction of the action potential. The membrane depolarization that occurs during the action potential is localized to a small area of membrane where the ions and channels are localized. Meaning, this electrical signal does not move very far down the axon. Axons therefore need to use another method, called active conduction, to prevent the electrical signal from decaying. It does this by repeatedly generating action potentials along the length of the axon. Axons can use active conduction by stacking many voltage-gated channels along their membranes in close proximity to one another. When the dendrite signal causes the axon hillock to reach threshold and the channels to open, the depolarization of the membrane will cause adjacent channels to also open generating another Figure 16: Conduction of the action potential. An action potential is generated as ions flow in at one location along an axon. The depolarization spreads to the neighboring region of the membrane, initiating an action potential there. The original region repolarizes as ions flow out. The depolarization-repolarization process is repeated as the action potential is propagated down the length of the axon. action potential. This process is repeated until the action potential reaches the presynaptic terminal where it is converted to a chemical signal to cross the synaptic cleft (Figure 16). So, essentially conduction of an action potential down the length of the axon requires many individual action potentials along the length of the axon to be generated in sequence. Each individual action potential provides a depolarizing current which causes the next set of voltage-gated channels to reach threshold and trigger another action potential, causing a domino effect down the length of the axon. It is important to note that in order for the action potential to be conducted efficiently it is critical that the voltage-gated channels are stacked up along the entire length of the axon. If they are not, the depolarizing current from a single action potential will get smaller as it travels down the axon, either because the current leaks or because the proteins the axon is made of offer resistance to conduction. When voltagegated channels are in close proximity, the depolarizing current does not have enough space to decline before the next set of channels open and initiate a new action potential. How does an axon conduct the action potential down its length? You can watch a video about action potentials online see this unit on the student website or click below: Lesson 2.2 51 Video: Action Potentials

STUDENT RESPONSES Write a summary of what is happening at each stage of the action potential diagrammed below. Step 1: Remember to identify your sources Step 2: Step 3: Step 4: Step 5: Lesson 2.2 52