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Graded Potentials and Action Potentials

Pearson
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PRESENTER: Electrical signals allow rapid transmission of information within a single neuron. Neurons have two kinds of electrical signals; action potentials, also called nerve impulses, and graded potentials. Students generally find action potentials to be relatively straightforward, but graded potentials can seem a bit fuzzy. It will help you understand these potentials if you keep their overall purpose in mind. Action potentials transmit signals over long distances. Graded potentials determine whether or not an action potential is generated. Let's use a non-myelinated multipolar neuron as our example. Graded potentials usually occur in the dendrites and cell body. They travel only short distances-- less than a millimeter. Acting together at the axon's initial segment, they may be able to bring the membrane potential to threshold and generate an action potential. Action potentials occur in axons and travel from the initial segment to the axon terminals. This is usually a long distance-- sometimes as long as a meter. Action potentials are triggered by a change in voltage. If the graded potentials depolarizing the initial segment open enough voltage-gated sodium channels, an action potential occurs. Otherwise, it doesn't. In an axon, an action potential is always the same amplitude or size everywhere along the axon. It doesn't decay width distance. This is because the action potential is regenerated by voltage-gated sodium channels. Recall that this involves a positive feedback cycle. Depolarization opens voltage-gated sodium channels, which allow sodium to enter, which causes more depolarization, which opens more voltage-gated sodium channels, and so on until all of the voltage-gated sodium channels open. This positive feedback ensures that the action potential is an all or none phenomenon. Graded potentials, on the other hand, have different amplitudes, that is, they are graded or vary in size depending on the intensity of the stimuli that open their channels. These stimuli include chemicals, such as neurotransmitters, and sensory stimuli, such as stretch and light. Because voltage-gated channels are not involved in making graded potentials, no positive feedback cycle regenerates them. As a result, graded potentials die out over distance as ions leak across the membrane through leakage channels. Before we continue, let's briefly pause and consider the relationship between action potentials and graded potentials. Which of the following correctly describes the relationship between action potentials and graded potentials? Is it, (a) they occur on different parts of a neuron, and there is no interaction between them; (b) they are opposites of each other; (c) action potentials can directly initiate graded potentials; or (d) graded potentials can directly initiate action potentials? The correct answer is (d). Graded potentials can directly initiate action potentials by bringing the initial segment of the axon to threshold. So how do these potentials end? An action potential ends when the membrane repolarizes as voltage-gated sodium channels inactivate and potassium channels open. This process is voltage regulated. A graded potential only ends when the stimulus is no longer present, and this process is voltage-independent. Action potentials and neurons come in only one type. They are basically all the same. In contrast, there are several different kinds of graded potentials, and their names reflect where they occur. You learned about end plate potentials at the neuromuscular junction, and you'll learn in later chapters about receptor potentials and generator potentials that happen in sensory neurons. So let's focus on postsynaptic potentials that happen at a synapse between two neurons. Postsynaptic potentials come in two flavors-- excitatory postsynaptic potentials, or EPSPs, and inhibitory postsynaptic potentials, or IPSPs. EPSPs occur when neurotransmitters bind to and open chemically-gated channels that allow both sodium and potassium to pass. This depolarizes the membrane potential, moving it closer to the threshold for generating an action potential. IPSPs occur when neurotransmitters bind to and open chemically-gated channels that pass either potassium or chloride. This hyperpolarizes the membrane, moving the membrane potential away from threshold. Let's think about EPSPs for a minute. How is it that opening a channel that is permeable to both sodium and potassium can depolarize the membrane? Is it because, (a) both sodium and potassium enter the cell; (b) more potassium leaves the cell than sodium enters; or (c) more sodium enters the cell than potassium leaves? The correct answer is (c), more sodium enters the cell than potassium leaves. This is because at the resting membrane potential, the driving force or electrochemical gradient for movement of sodium is stronger than the driving force for potassium movement. In a neuron, all of the EPSPs and IPSPs sum together at the initial segment where, if threshold is reached, they trigger an action potential. You will learn about how summation works in the focus figure postsynaptic potentials and their summation. What about action potentials? They never sum. So how do you suppose these long distance signals convey information about the intensity of a stimulus? For a stronger stimulus, which of the following would be true? (A) Larger action potentials occur; (b) a higher frequency of action potentials occurs; or (c) both larger and more frequent action potentials occur. The correct answer is (b). Remember that there is no such thing as a larger action potential. All action potentials have the same amplitude. Instead, a stronger stimulus triggers a higher frequency of action potentials. Action potentials always depolarize the axon to about plus 30 millivolts. In contrast, graded potentials can either depolarize or hyperpolarize the membrane, generally by only a few millivolts. As you review all of the differences between graded potentials and action potentials that we've discussed, relate each one to the overall function of that electrical signal. Action potentials send signals over long distances, whereas graded potentials, when summed together, determine whether or not an action potential is generated.
PRESENTER: Electrical signals allow rapid transmission of information within a single neuron. Neurons have two kinds of electrical signals; action potentials, also called nerve impulses, and graded potentials. Students generally find action potentials to be relatively straightforward, but graded potentials can seem a bit fuzzy. It will help you understand these potentials if you keep their overall purpose in mind. Action potentials transmit signals over long distances. Graded potentials determine whether or not an action potential is generated. Let's use a non-myelinated multipolar neuron as our example. Graded potentials usually occur in the dendrites and cell body. They travel only short distances-- less than a millimeter. Acting together at the axon's initial segment, they may be able to bring the membrane potential to threshold and generate an action potential. Action potentials occur in axons and travel from the initial segment to the axon terminals. This is usually a long distance-- sometimes as long as a meter. Action potentials are triggered by a change in voltage. If the graded potentials depolarizing the initial segment open enough voltage-gated sodium channels, an action potential occurs. Otherwise, it doesn't. In an axon, an action potential is always the same amplitude or size everywhere along the axon. It doesn't decay width distance. This is because the action potential is regenerated by voltage-gated sodium channels. Recall that this involves a positive feedback cycle. Depolarization opens voltage-gated sodium channels, which allow sodium to enter, which causes more depolarization, which opens more voltage-gated sodium channels, and so on until all of the voltage-gated sodium channels open. This positive feedback ensures that the action potential is an all or none phenomenon. Graded potentials, on the other hand, have different amplitudes, that is, they are graded or vary in size depending on the intensity of the stimuli that open their channels. These stimuli include chemicals, such as neurotransmitters, and sensory stimuli, such as stretch and light. Because voltage-gated channels are not involved in making graded potentials, no positive feedback cycle regenerates them. As a result, graded potentials die out over distance as ions leak across the membrane through leakage channels. Before we continue, let's briefly pause and consider the relationship between action potentials and graded potentials. Which of the following correctly describes the relationship between action potentials and graded potentials? Is it, (a) they occur on different parts of a neuron, and there is no interaction between them; (b) they are opposites of each other; (c) action potentials can directly initiate graded potentials; or (d) graded potentials can directly initiate action potentials? The correct answer is (d). Graded potentials can directly initiate action potentials by bringing the initial segment of the axon to threshold. So how do these potentials end? An action potential ends when the membrane repolarizes as voltage-gated sodium channels inactivate and potassium channels open. This process is voltage regulated. A graded potential only ends when the stimulus is no longer present, and this process is voltage-independent. Action potentials and neurons come in only one type. They are basically all the same. In contrast, there are several different kinds of graded potentials, and their names reflect where they occur. You learned about end plate potentials at the neuromuscular junction, and you'll learn in later chapters about receptor potentials and generator potentials that happen in sensory neurons. So let's focus on postsynaptic potentials that happen at a synapse between two neurons. Postsynaptic potentials come in two flavors-- excitatory postsynaptic potentials, or EPSPs, and inhibitory postsynaptic potentials, or IPSPs. EPSPs occur when neurotransmitters bind to and open chemically-gated channels that allow both sodium and potassium to pass. This depolarizes the membrane potential, moving it closer to the threshold for generating an action potential. IPSPs occur when neurotransmitters bind to and open chemically-gated channels that pass either potassium or chloride. This hyperpolarizes the membrane, moving the membrane potential away from threshold. Let's think about EPSPs for a minute. How is it that opening a channel that is permeable to both sodium and potassium can depolarize the membrane? Is it because, (a) both sodium and potassium enter the cell; (b) more potassium leaves the cell than sodium enters; or (c) more sodium enters the cell than potassium leaves? The correct answer is (c), more sodium enters the cell than potassium leaves. This is because at the resting membrane potential, the driving force or electrochemical gradient for movement of sodium is stronger than the driving force for potassium movement. In a neuron, all of the EPSPs and IPSPs sum together at the initial segment where, if threshold is reached, they trigger an action potential. You will learn about how summation works in the focus figure postsynaptic potentials and their summation. What about action potentials? They never sum. So how do you suppose these long distance signals convey information about the intensity of a stimulus? For a stronger stimulus, which of the following would be true? (A) Larger action potentials occur; (b) a higher frequency of action potentials occurs; or (c) both larger and more frequent action potentials occur. The correct answer is (b). Remember that there is no such thing as a larger action potential. All action potentials have the same amplitude. Instead, a stronger stimulus triggers a higher frequency of action potentials. Action potentials always depolarize the axon to about plus 30 millivolts. In contrast, graded potentials can either depolarize or hyperpolarize the membrane, generally by only a few millivolts. As you review all of the differences between graded potentials and action potentials that we've discussed, relate each one to the overall function of that electrical signal. Action potentials send signals over long distances, whereas graded potentials, when summed together, determine whether or not an action potential is generated.