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Resting Membrane Potential |
POSITIVE on the outside NEGATIVE in the inside The separation of charges creates a voltage (electrical potential difference), which can be measured using a voltmeter. The resting membrane potential of a neuron averages -70mV (millivolts). All neural activities begin with a change in the resting membrane potential of a neuron. The resting membrane potential is maintained by Na+-K+ pumps that actively transport K+ into and Na+ out of the cell. The concentration of Na+ is higher outside than inside the cell. The membrane is more permeable to K+. The concentration of K+ is higher inside than outside the cell. All living cells have a membrane potential that varies depending on its cellular activities. |
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Which choice best characterizes leak channels? transmembrane channels that use energy to allow the movement of across the membrane common transmembrane channels that are always open for any ion to move through in the presence of chemically gated channels that open and close according to the binding of other molecules transmembrane protein channels that are always open to allow to cross the membrane without the additional input of energy |
transmembrane protein channels that are always open to allow to cross the membrane without the additional input of energy |
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Assume you have a membrane with only potassium leak channels. The RMP is -90mV. Predict the RMP if we add leak channels. The most likely RMP value of is __________. +70 mV |
-70 mV |
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Imagine that the cell membrane from the previous problem becomes more permeable to . Predict how this will affect the RMP. The RMP will be unaffected. |
The RMP will be more positive. |
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Complete the following sentence. The operation of the ATPase pump __________. releases 1 to the ECF and 1 to the cytoplasm |
moves 3 to the ECF and 2 to the cytoplasm |
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You are going to record RMP from a cell using an electrode. You place your electrode and record a resting membrane potential every millisecond. You record an initial value of -70mV; however, over time you notice that your recordings become more and more positive until the RMP reaches 0mV. Assuming that and are the major determinants of RMP in this cell, which of the following could best explain your results? The cell is becoming depleted of . |
The cell’s – ATPase pumps have stopped functioning. Yes! Since the RMP eventually becomes zero, the concentration of ions on either side of the membrane would be roughly equal. Without active processes to maintain concentration gradients, we would expect the concentration of ions on either side of the membrane to equilibrate. |
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is a common, negatively charged extracellular ion. Predict the effect on the RMP if many gated channels are suddenly opened. There would be no change in the RMP. The membrane would become hypopolarized or have less charge separation across the membrane. The RMP would become more positive. A more negative RMP would result. |
A more negative RMP would result. Correct! is negatively charged and has a higher concentration in the ECF. The opening of channels would allow more negative charge to attempt to enter the cytoplasm. If the cell’s normal RMP were -70mV, it would now become more negative. The farther away the RMP is from zero, either in the positive or in the negative direction, the greater the separation of charges is. This is called hyperpolarization. When the RMP moves closer to 0 mV, depolarization occurs. |
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The small space between the sending neuron and the receiving neuron is the neurotransmitter. |
synaptic cleft. The synaptic cleft is the small space between the sending neuron and the receiving neuron. |
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A molecule that carries information across a synaptic cleft is a neurotransmitter. |
Neurotransmitter molecules carry information across a synaptic cleft. |
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When calcium ions enter the synaptic terminal, the inside of the receiving neuron becomes more negative. they cause an action potential in the sending neuron. they cause vesicles containing neurotransmitter molecules to fuse to the plasma membrane of the sending neuron. neurotransmitter molecules are quickly removed from the synaptic cleft. the inside of the receiving neuron becomes more positive. |
they cause vesicles containing neurotransmitter molecules to fuse to the plasma membrane of the sending neuron. |
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When neurotransmitter molecules bind to receptors in the plasma membrane of the receiving neuron, the receiving neuron becomes more positive inside. the receiving neuron becomes more negative inside. ion channels in the plasma membrane of the receiving neuron open. ion channels in the plasma membrane of the sending neuron open. vesicles in the synaptic terminal fuse to the plasma membrane of the sending neuron |
ion channels in the plasma membrane of the receiving neuron open |
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If a signal from a sending neuron makes the receiving neuron more negative inside, the receiving neuron is less likely to generate an action potential. the receiving neuron is more likely to generate an action potential. the receiving neuron immediately generates an action potential. the sending neuron becomes more positive inside. the sending neuron becomes more negative inside. |
the receiving neuron is less likely to generate an action potential. If the receiving neuron is more negative inside, it is less likely to generate an action potential. |
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The generation of an action potential in a neuron requires the presence what type of membrane channels? voltage-gated channels |
voltage-gated channels |
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Place the events involved in generation of an action potential in the correct order of occurrence from left to right. |
Threshold stimulus Na+ channels open Na+ influx Depolarization Na+ channels close K+ channels open K+ efflux Repolarization Hyperpolarization K+ channels close |
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What type of conduction takes place in unmyelinated axons? Synaptic transmission |
Continuous conduction Yes! An action potential is conducted continuously along an unmyelinated axon from its initial segment to the axon terminals. The term continuous refers to the fact that the action potential is regenerated when voltage-gated Na+ channels open in every consecutive segment of the axon, not at nodes of Ranvier. |
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An action potential is self-regenerating because __________. depolarizing currents established by the influx of K+ flow down the axon and trigger an action potential at the next segment currents established by the efflux of K+ flow down the axon and trigger an action potential at the next segment depolarizing currents established by the influx of Na+ flow down the axon and trigger an action potential at the next segment repolarizing currents established by the efflux of Na+ flow down the axon and trigger an action potential at the next segment |
depolarizing currents established by the influx of Na+ flow down the axon and trigger an action potential at the next segment Yes! The Na+ diffusing into the axon during the first phase of the action potential creates a depolarizing current that brings the next segment, or node, of the axon to threshold. |
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Why does regeneration of the action potential occur in one direction, rather than in two directions? The activation gates of voltage-gated Na+ channels close in the node, or segment, that has just depolarized. |
The inactivation gates of voltage-gated Na+ channels close in the node, or segment, that has just fired an action potential. Yes! At the peak of the depolarization phase of the action potential, the inactivation gates close. Thus, the voltage-gated Na+ channels become absolutely refractory to another depolarizing stimulus. |
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What is the function of the myelin sheath? The myelin sheath decreases the speed of action potential conduction from the initial segment to the axon terminals. The myelin sheath decreases the resistance of the axonal membrane to the flow of charge. The myelin sheath increases the insulation along the entire length of the axon. The myelin sheath increases the speed of action potential conduction from the initial segment to the axon terminals. |
The myelin sheath increases the speed of action potential conduction from the initial segment to the axon terminals. Yes! The myelin sheath increases the velocity of conduction by two mechanisms. First, myelin insulates the axon, reducing the loss of depolarizing current across the plasma membrane. Second, the myelin insulation allows the voltage across the membrane to change much faster. Because of these two mechanisms, regeneration only needs to happen at the widely spaced nodes of Ranvier, so the action potential appears to jump. |
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What changes occur to voltage-gated Na+ and K+ channels at the peak of depolarization? Inactivation gates of voltage-gated Na+ channels close, while activation gates of voltage-gated K+ channels open. Activation gates of voltage-gated Na+ channels close, while activation gates of voltage-gated K+ channels open. Inactivation gates of voltage-gated Na+ channels close, while inactivation gates of voltage-gated K+ channels open. Activation gates of voltage-gated Na+ channels close, while inactivation gates of voltage-gated K+ channels open. |
Inactivation gates of voltage-gated Na+ channels close, while activation gates of voltage-gated K+ channels open. Yes! Closing of voltage-gated channels is time dependent. Typically, the inactivation gates of voltage-gated Na+ channels close about a millisecond after the activation gates open. At the same time, the activation gates of voltage-gated K+ channels open. |
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In which type of axon will velocity of action potential conduction be the fastest? Unmyelinated axons with the largest diameter Myelinated axons with the largest diameter Unmyelinated axons of the shortest length Myelinated axons with the smallest diameters |
Myelinated axons with the largest diameter Yes! The large diameter facilitates the flow of depolarizing current through the cytoplasm. The myelin sheath insulates the axons and prevents current from leaking across the plasma membrane. |
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Cold sores on the skin of the mouth occur when herpes simplex viruses that are dormant in neural ganglia become active and travel to the skin of the mouth. Which of the following is the mechanism by which these viruses travel from the ganglia (located within the head) to the skin of the mouth? transport along nerve impulses that travel down the axons anterograde axonal transport retrograde axonal transport travel of the viruses along neurofibrils |
anterograde axonal transport |
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How do neurofibrils differ from nerve fibers? There is no real difference, since they are both capable of impulse conduction away from a neuron. Neurofibrils are cytoskeletal intermediate filaments maintaining cell shape, but they do not conduct impulses. Neurofibrils are confined to dendrite receptor areas and are consequently not found in nerve fibers. Neurofibrils are axon subunits that are bundled together to make up a single nerve fiber. |
Neurofibrils are cytoskeletal intermediate filaments maintaining cell shape, but they do not conduct impulses. |
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In multiple sclerosis, the cells that are the target of an autoimmune attack are the _________. neurons muscle cells Schwann cells oligodendrocytes |
oligodendrocytes Oligodendrocytes are a type of neuroglial cell that function to form the myelin sheath around the axons of neurons within the central nervous system. |
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EFFERENT |
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PNS |
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CNS |
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Afferent |
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Somatic nervous system |
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Autonomic nervous system ANS |
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Sympathetic division |
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Parasympathetic division |
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Which of the following is NOT one of the basic functions of the nervous system? Decode sensory information from the environment. Generate direct, electrical signals. Release hormones into the bloodstream to communicate with other cells in the body. Integrate sensory input for decision making. |
Release hormones into the bloodstream to communicate with other cells in the body. Hormones are released by endocrine organs. The nervous system does have some control over endocrine function, but the endocrine system is considered a separate signaling system. |
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As you start working out, you notice that your heart rate and breathing rate start to increase. Which division of your nervous system is generating this response? Be as specific as possible. the somatic nervous system the parasympathetic division of the sympathetic division of the autonomic nervous system the afferent division of the nervous system |
the sympathetic division of the autonomic nervous system The sympathetic division of the autonomic, or involuntary, nervous system consists of visceral motor nerve fibers that regulate the activity of smooth muscles, cardiac muscles, and glands. This division is responsible for generating actions required during activity. |
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What division of the nervous system is most specifically responsible for voluntary motor control? parasympathetic nervous system somatic nervous system central nervous system sympathetic nervous system |
somatic nervous system Correct. The somatic nervous system is composed of somatic motor nerve fibers that conduct impulses from the central nervous system to skeletal muscles. It is often referred to as the voluntary nervous system because it allows us to consciously control our skeletal muscles. |
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Which of the cell types shown helps determine capillary permeability? A |
B Extensions from these cells wrap capillaries and provide extra control over which materials enter or leave the intercellular fluid of the CNS. |
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Name the glial cell at F. oligodendrocytes |
Schwann cells carry out myelination of the peripheral nervous system (PNS). |
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Which of the cell types shown is most associated with the production and flow of cerebrospinal fluid (CSF)? A |
D These cells line central cavities of the CNS and, in certain places, produce CSF. The cilia of these cells help circulate the CSF that nourishes and cushions the brain and spinal cord. |
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Which of the neuroglial cell types shown form myelin sheaths within the CNS? A |
A These cells form myelin sheaths within the white matter of the CNS. |
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Which of the neuroglial cell types shown are found in the peripheral nervous system (PNS)?
A |
E The cell shown in E wraps and insulates the soma of neurons within ganglia in the PNS. |
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Which of the following peripheral nervous system (PNS) neuroglia form the myelin sheaths around larger nerve fibers in the PNS? satellite cells |
Schwann cells Schwann cells (also called neurolemmocytes) surround all nerve fibers in the PNS and form myelin sheaths around the larger fibers. Myelin protects and electrically insulates nerve fibers, and it increases the transmission speed of nerve impulses. |
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Ependymal cells line many open cavities in the central nervous system (CNS). Ependymal cells have cilia on the side of the cell that face these openings. What is the most likely function of these ciliated cells? exchange of nutrients between the circulatory system and neurons create myelin sheaths for CNS cells movement and circulation of cerebrospinal fluid act as macrophage cells to destroy microorganisms or neuronal debris |
movement and circulation of cerebrospinal fluid Yes, cilia are an indication that the cells they appear on are moving fluids past the surface of the cells. |
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Which glial cells have the most diversity of function? ependymal cells |
astrocytes Astrocytes are the most abundant and diversely functioning glial cells. Examples of their many functions include: anchoring neurons to capillaries, aiding in the exchanges between neurons and blood, guiding the migration of young neurons, and helping control the chemical environment around neurons. |
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Which of the following types of glial cells monitors the health of neurons, and can transform into a special type of macrophage to protect endangered neurons? microglia |
Microglial cells are small and ovoid with relatively long "thorny" processes. Their processes touch nearby neurons, monitoring their health, and when they sense that certain neurons are injured or are in other trouble, the microglial cells migrate toward them. Where invading microorganisms or dead neurons are present, the microglial cells transform into a special type of macrophage that phagocytizes the microorganisms or neuronal debris. |
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Art-labeling Activity: Figure 11.5 (2 of 2) |
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Art-labeling Activity: Figure 11.5 (1 of 2) |
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What is the structure at A? soma |
dendrite |
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Which of these materials or structures would be found in greatest amounts or numbers at E? chromatophilic substance (Nissl bodies) chemically gated sodium ion channels vesicles containing neurotransmitter the nucleolus |
vesicles containing neurotransmitter Neurotransmitters are released by secretion from the ends of axonal terminals. |
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What structural classification describes this neuron? unipolar |
multipolar D only E only both A and B both A and EThe neuron shown has a many processes (axon and dendrites) that emerge from the cell body. Such neurons typically function as motor neurons or interneurons. |
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Which areas of this neuron would be classified as receptive regions? D only |
both A and B The dendrites and soma of the cell receive signals from other neurons. |
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In which area of the neuron is an action potential initially generated? A |
C Graded potentials originating in the dendrites and cell body are integrated (summated) at the axon hillock (C). Membrane potentials above threshold at the hillock will open voltage-gated Na+ channels found in the "trigger zone," producing an action potential that proceeds down the axon. |
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Which of the following membrane regions would have significant numbers of voltage-gated ion channels? A only |
C and D Voltage-gated Na+ and K+ channels allow for the triggering of an action potential at the axon hillock (C) and its propagation down the axon (D). |
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Which of the following is true of axons? Neurons can have multiple axons but only one dendrite. Axons use chemically gated ion channels to generate graded potentials. Smaller (thinner) axons are more likely to bear myelin sheaths than larger (thicker) axons. A neuron can have only one axon, but the axon may have occasional branches along its length. |
This is true; a neuron can have only one axon, but the axon may have occasional branches along its length. |
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Which criterion is used to functionally classify neurons? the number of processes extending from the cell body neuron the direction in which the nerve impulse travels relative to the central nervous system whether the nerve fibers are myelinated or unmyelinated whether the neurons are found within the CNS or the PNS |
the direction in which the nerve impulse travels relative to the central nervous system Functional classification groups neurons according to the direction in which the nerve impulse travels relative to the central nervous system. Based on this criterion, there are sensory neurons, motor neurons, and interneurons. |
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Many neurons have many short, branching extensions called dendrites. What is the benefit of these structures for a neuron? There is a large area for production of chemicals used to signal other neurons. |
The dendrites provide a large surface area for connections from other neurons. Yes, because of the branching and extensive membrane surface area, there is a large amount of membrane dedicated to synapses with other neurons. |
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Bundles of neurons in the central nervous system are called tracts. |
True Bundles of neurons are called tracts in the central nervous system. In the peripheral nervous systems, bundles of axons are called nerves. |
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Which is the main receptive portion of the neuron? the dendrite |
Dendrites are the main receptive or input regions, providing an enormous surface area for receiving signals from other neurons. |
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Which of the following pairings does NOT fit? afferent neurons: sensory neurons |
multipolar neurons: peripheral nervous system sensory neurons Unipolar neurons makeup the sensory neurons in the peripheral nervous system. |
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The membranes of neurons at rest are very permeable to _____ but only slightly permeable to _____. K+; Na+ |
K+; Na+ Yes, more K+ moves out of the cell than Na+ moves into the cell, helping to establish a negative resting membrane potential. |
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During depolarization, which gradient(s) move(s) Na+ into the cell? Na+ does not move into the cell. Na+ moves out of the cell. |
both the electrical and chemical gradients Yes, a positive ion is driven into the cell because the inside of the cell is negative compared to the outside of the cell, and Na+ is driven into the cell because the concentration of Na+ is greater outside the cell. |
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What is the value for the resting membrane potential for most neurons? +30 mV |
-70 mV Yes, the resting membrane potential for neurons depends on the distribution of both Na+ and K+ across the cell membrane. The potential is closer to the equilibrium potential of K+ because the cell is more permeable to K+. |
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The Na+-K+ pump actively transports both sodium and potassium ions across the membrane to compensate for their constant leakage. In which direction is each ion pumped? Both Na+ and K+ are pumped out of the cell. Both Na+ and K+ are pumped into the cell. K+ is pumped out of the cell and Na+ is pumped into the cell. Na+ is pumped out of the cell and K+ is pumped into the cell. |
Na+ is pumped out of the cell and K+ is pumped into the cell. Yes, Na+ is pumped out of the cell against its electrochemical gradient and K+ is pumped into the cell against its concentration gradient. |
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The concentrations of which two ions are highest outside the cell. K+ and A- (negatively charged proteins) Na+ and Cl- K+ and Cl- Na+ and A- (negatively charged proteins) |
Na+ and Cl- |
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Ions are unequally distributed across the plasma membrane of all cells. This ion distribution creates an electrical potential difference across the membrane. What is the name given to this potential difference? Resting membrane potential (RMP) |
Resting membrane potential (RMP) Yes! The resting membrane potential is the baseline potential that can be recorded across the plasma membrane of an excitable cell prior to excitation. |
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Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel? Leak channels |
Yes. Leak channels for Na+ and K+ are ubiquitous, and they allow for the diffusion of these ions across plasma membranes. |
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On average, the resting membrane potential is -70 mV. What does the sign and magnitude of this value tell you? There is no electrical potential difference between the inside and the outside surfaces of the plasma membrane. |
The inside surface of the plasma membrane is much more negatively charged than the outside surface. Yes! The inside surface of the plasma membrane accumulates more negative charge because of the presence of Na+ and K+ gradients and the selective permeability of the membrane to Na+ and K+. |
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The plasma membrane is much more permeable to K+ than to Na+. Why? There are many more K+ leak channels than Na+ leak channels in the plasma membrane. |
There are many more K+ leak channels than Na+ leak channels in the plasma membrane. Yes! More leak channels translates into more leakiness. Thus the outward flux of K+ is greater than the inward flux of Na+. |
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The resting membrane potential depends on two factors that influence the magnitude and direction of Na+ and K+ diffusion across the plasma membrane. Identify these two factors. The presence of a resting membrane potential and leak channels |
The presence of concentration gradients and leak channels Yes! The concentration gradient and the large number of K+ leak channels allow for rather robust K+ diffusion out of a cell. In contrast, the concentration gradient and the relatively few Na+ leak channels allow for much less Na+ diffusion into a cell. |
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What prevents the Na+ and K+ gradients from dissipating? Na+-K+ ATPase |
Na+-K+ ATPase Yes! Also known as the Na+-K+ pump, or simply the pump, this transporter moves three Na+ out of the cell and two K+ into the cell for every ATP it hydrolyzes. This pumping action prevents the Na+ and K+ gradients from running down as these ions passively move through leak channels. |
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BioFlix Activity: How Synapses Work — Events at a Synapse |
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BioFlix Activity: How Synapses Work — Synapse Structure |
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What is the basic condition when a neuron is described as polarized? Na+ is found outside of the neuron and K+ is found inside of the cell. |
There is a separation of positive and negative charges across a membrane. Polarization describes a condition where there is some sorting into poles. In a neuron, the inside of the cell is slightly negative compared to the positive charge (provided primarily by Na+) outside of the cell. |
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Na+ leaks through neuron membranes faster than K+ because of the properties of the non-gated leak channels. True |
False K+ leak channels allow K+ to leak through the neuron membrane almost 25 times faster than Na+. |
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Which of the following statements is correct regarding the polarization of a neuronal membrane and the formation of a resting membrane potential? As Na+ leaks across the membrane, that establishes a negative charge inside the membrane. |
Sodium/Potassium pumps maintain concentration gradients; sodium and potassium move down their concentration gradients through leakage channels. Correct. As Na+ leaks into the cell, the Na+/K+ pump actively transports Na+ back out of the neuron to maintain a gradient for Na+. |
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Which of the following does NOT describe conditions that occur during an action potential? Which of the following does NOT describe conditions that occur during an action potential? |
The Na+/K+ pump reestablishes resting concentration gradients. Na+ is used to repolarize the membrane. This is false. Initially, K+ leaves the cell through voltage-gated channels. As K+ leaves the cell, the inside of the cell becomes more negative, repolarizing the cell. |
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Where in the neuron is an action potential initially generated? soma and dendrites |
axon hillock Yes, this region (first part of the axon) receives local signals (graded potentials) from the soma and dendrites and has a high concentration of voltage-gated Na+ channels. |
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The depolarization phase of an action potential results from the opening of which channels? voltage-gated K+ channels |
voltage-gated Na+ channels Yes, when the voltage-gated Na+ channels open, Na+ rushes into the cell causing depolarization. |
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The repolarization phase of an action potential results from __________. the opening of voltage-gated K+ channels |
the opening of voltage-gated K+ channels Yes, as the voltage-gated K+ channels open, K+ rushes out of the cell, causing the membrane potential to become more negative on the inside, thus repolarizing the cell. |
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Hyperpolarization results from __________. fast closing of voltage-gated K+ channels |
slow closing of voltage-gated K+ channels Yes, the slow closing of the voltage-gated K+ channels means that more K+ is leaving the cell, making it more negative inside. |
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What is the magnitude (amplitude) of an action potential? 70 mV |
100 mV Yes, the membrane goes from -70 mV to +30 mV. Thus, during the action potential, the inside of the cell becomes more positive than the outside of the cell. |
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How is an action potential propagated along an axon? An influx of sodium ions from the current action potential depolarizes the adjacent area. Stimuli from the graded (local) potentials from the soma and dendrites depolarize the entire axon. An efflux of potassium from the current action potential depolarizes the adjacent area. |
An influx of sodium ions from the current action potential depolarizes the adjacent area. Yes, the influx of sodium ions depolarizes adjacent areas, causing the membrane to reach threshold and cause an action potential. Thus, the action potential is regenerated at each new area. |
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Why does the action potential only move away from the cell body? The flow of the sodium ions only goes in one direction—away from the cell body The areas that have had the action potential are refractory to a new action potential. |
The areas that have had the action potential are refractory to a new action potential. Yes, sodium channels are inactivated in the area that just had the action potential. |
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The velocity of the action potential is fastest in which of the following axons? a small myelinated axon |
a small myelinated axon Yes, the myelination acts as insulation and the action potential is generated only at the nodes of Ranvier. Propagation along myelinated axons is known as saltatory conduction. |
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Where do most action potentials originate? Axon terminal |
Initial segment Yes! The first part of the axon is known as the initial segment. The initial segment is adjacent to the tapered end of the cell body, known as the axon hillock. |
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What opens first in response to a threshold stimulus? Voltage-gated K+ channels |
Voltage-gated Na+ channels Yes! The activation gates of voltage-gated Na+ channels open, and Na+ diffuses into the cytoplasm. |
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What characterizes depolarization, the first phase of the action potential? The membrane potential changes to a less negative (but not a positive) value. |
The membrane potential changes from a negative value to a positive value. Yes! The plasma membrane, which was polarized to a negative value at the RMP, depolarizes to a positive value. |
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What characterizes repolarization, the second phase of the action potential? Before the membrane has a chance to reach a positive voltage, it repolarizes to its negative resting value of approximately -70 mV. |
Once the membrane depolarizes to a peak value of +30 mV, it repolarizes to its negative resting value of -70 mV. Yes! The plasma membrane was depolarized to a positive value at the peak of the first phase of the action potential. Thus, it must repolarize back to a negative value. |
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What event triggers the generation of an action potential? The membrane potential must hyperpolarize from the resting voltage of -70 mV to the more negative value of -80 mV. |
The membrane potential must depolarize from the resting voltage of -70 mV to a threshold value of -55 mV. Yes! This is the minimum value required to open enough voltage-gated Na+ channels so that depolarization is irreversible. |
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What is the first change to occur in response to a threshold stimulus? Voltage-gated K+ channels change shape, and their activation gates open. |
Voltage-gated Na+ channels change shape, and their activation gates open. Yes! The activation gates of voltage-gated Na+ channels open very rapidly in response to threshold stimuli. The activation gates of voltage-gated K+ channels are comparatively slow to open. |
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During the action potential of a neuron, which ion is primarily crossing the membrane during the depolarization phase, and in which direction is the ion moving? Na+ is entering the cell. |
Na+ is entering the cell. During the depolarization phase of the action potential, open Na+ channels allow Na+ ions to diffuse into the cell. This inward movement of positive charge makes the membrane potential more positive (less negative). The depolarization phase is a positive feedback cycle where open Na+ channels cause depolarization, which in turn causes more voltage-gated Na+ channels to open. |
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The figure shows the phases of action potential. The arrow points to the peak potential. Na+ channels are inactivating, and K+ channels are opening. Both Na+ and K+ channels are opening. Na+ channels are opening, and K+ channels are closing. Na+ channels are inactivating, and K+ channels are closing. |
Na+ channels are inactivating, and K+ channels are opening. As voltage-gated Na+ channels begin to inactivate, the membrane potential stops becoming more positive This marks the end of the depolarization phase of the action potential. Then, as voltage-gated K+ channels open, K+ ions rush out of the neuron, following their electrochemical gradient. This exit of positively-charged ions causes the interior of the cell to become more negative, repolarizing the membrane. |
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During what part of the action potential do voltage-gated Na+ channels begin to inactivate (their inactivation gates close)? at the end of the depolarization phase, as the membrane potential approaches its peak value at the beginning of an action potential, as the membrane potential reaches threshold at the end of the hyperpolarization phase of an action potential, as the membrane potential returns to its resting value at the end of the repolarization phase, as the membrane potential briefly passes its resting value |
at the end of the depolarization phase, as the membrane potential approaches its peak value Voltage-gated Na+ channels are opened by depolarization and then quickly inactivated. Once inactivated, these channels cannot pass Na+ ions. At the peak of the action potential, a large number of Na+ channels are open, but they are rapidly inactivating. As the action potential enters the repolarization phase, the number of open Na+ channels continues to decrease because more and more inactivation gates close. The number of inactivated Na+ channels is greatest towards the end of the repolarization phase. |
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The repolarization phase of the action potential, where voltage becomes more negative after the +30mV peak, is caused primarily by __________. Na+ ions leaving the cell through voltage-gated channels |
K+ ions leaving the cell through voltage-gated channels The large number of voltage-gated K+ channels opening during the repolarization phase quickly makes the membrane potential more negative as positively-charged K+ ions leave the cell. K+ ions continue to leave through open channels as the membrane potential passes (becomes more negative than) the resting potential. This hyperpolarization phase of the action potential is therefore due to K+ ions diffusing through voltage-gated K+ channels. The membrane potential remains more negative than the resting potential until voltage-gated K+ channels close. This period of hyperpolarization is important in relieving voltage-gated Na+ channels from inactivation, readying them for another action potential. |
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During an action potential, hyperpolarization beyond (more negative to) the resting membrane potential is primarily due to __________. Na+ diffusing through voltage-gated channels |
K+ ions diffusing through voltage-gated channels The large number of voltage-gated K+ channels opening during the repolarization phase quickly makes the membrane potential more negative as positively-charged K+ ions leave the cell. K+ ions continue to leave through open channels as the membrane potential passes (becomes more negative than) the resting potential. This hyperpolarization phase of the action potential is therefore due to K+ ions diffusing through voltage-gated K+ channels. The membrane potential remains more negative than the resting potential until voltage-gated K+ channels close. This period of hyperpolarization is important in relieving voltage-gated Na+ channels from inactivation, readying them for another action potential. |
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During the hyperpolarization phase of the action potential, when the membrane potential is more negative than the resting membrane potential, what happens to voltage-gated ion channels? K+ channels close. Na+ channels go from an inactivated state to a closed state. |
K+ channels close. Na+ channels go from an inactivated state to a closed state. Voltage-gated K+ channels are opened by depolarization. This means that as the membrane potential repolarizes and then hyperpolarizes, these K+ channels close. With the closing of voltage-gated K+ channels, the membrane potential returns to the resting membrane potential via leakage channel activity. Resetting voltage-gated Na+ channels to the closed (but not inactivated) state prepares them for the next action potential. |
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Tetraethylammonium (TEA) blocks voltage-gated K+ channels such that K+ cannot pass even when the channels are open. However, TEA leaves K+ leakage channels largely unaffected. How would you expect the action potential to change if you treated a neuron with TEA? The action potential would depolarize as usual, but the repolarization phase would take longer, causing the action potential to be more broad in time. |
The action potential would depolarize as usual, but the repolarization phase would take longer, causing the action potential to be more broad in time. The passage of K+ ions through open voltage-gated K+ channels is an important component of the repolarization phase of the action potential. However, repolarization would still occur (albeit more slowly) in the presence of TEA. Once voltage-gated Na+ channels open during the depolarization phase, those same Na+ channels quickly inactivate. In other words, open Na+ channels inevitably inactivate regardless of whether K+ channels open or not. This means that the depolarization phase of the action potential stops on its own. Once Na+ channels inactivate, the membrane potential is set by other open channels. If voltage-gated K+ channels are blocked by TEA, then the membrane will be (slowly) set, by leakage channels, to the resting membrane potential. |
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The diffusion of what ion, across the neuronal membrane, is responsible for the local currents that depolarize regions of the axon to threshold? K+ (potassium) |
Na+ (sodium) Sodium enters the cell during the beginning of an action potential. Not only does this (further) depolarize the membrane where those channels are located, but it also sets up local currents that depolarize nearby membrane segments. In the case of myelinated axons, these local currents depolarize the next node, some 1 to 2 millimeters away. |
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An action potential in one segment of axon causes adjacent sections of axon membrane to reach threshold through what mechanism? Na+ ions diffusing across the membrane through leakage channels |
the generation of local currents An action potential at the axon hillock generates local currents that depolarize nearby sections of axon to threshold. This change in membrane potential causes voltage-gated Na+ channels to open in the adjacent axon segment. This (re)generates the action potential in the adjacent segment, causing the action potential to propagate away from the axon hillock. |
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During action potential propagation in an unmyelinated axon, why doesn’t the action potential suddenly "double back" and start propagating in the opposite direction? The previous axonal segment is in the refractory period. |
The previous axonal segment is in the refractory period. A propagating action potential always leaves a trail of refractory membrane in its wake. The trailing membrane takes some time to recover from the action potential it just experienced. This is largely because its voltage-gated sodium channels are inactivated. By the time this membrane segment is ready to (re)generate another action potential, the first propagating action potential is long gone. |
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In a myelinated axon, how do the nodes of Ranvier differ from other segments of the same axon? The nodes are longer segments of the axon. |
The nodes are more permeable to ions. In myelinated axons, voltage-gated channels are largely confined to the nodes of Ranvier. This means the nodes are much more permeable to ions than the myelinated segments where voltage-gated channels are absent. |
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Where are action potentials regenerated as they propagate along a myelinated axon? at every segment of the axon |
at the nodes of Ranvier In myelinated axons, voltage-gated sodium channels are largely restricted to the nodes between myelinated segments. This means that action potentials can only be regenerated at these locations. Myelin prevents leakage of charge from the axon and ensures that currents generated at one node will quickly bring the next node to threshold even though it may be a millimeter away. |
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How do action potential propagation speeds compare in myelinated and unmyelinated axons? Propagation speeds are similar in both axon types. |
Propagation is faster in myelinated axons. The insulated segments of myelinated axons allow local currents to travel quickly between nodes where the action potential is regenerated. This "leaping" of action potentials from node to node is several-fold faster than the continuous propagation found in unmyelinated axons. In addition, myelinated axons tend to have larger diameters, which also enhances propagation speed. |
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The node-to-node "jumping" regeneration of an action potential along a myelinated axon is called __________. |
saltatory conduction Saltatory conduction is derived from the Latin word saltare, which means leaping. |
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The myelin on myelinated neurons can be degraded or destroyed in diseases such as multiple sclerosis-a process called demyelination. If a myelinated neuron was affected by demyelination, how would this affect action potentials in that neuron? Action potentials would propagate in both directions along the axon. |
The speed of action potential propagation would be slower. Because myelination allows for the fast saltatory propagation of action potentials, any reduction or removal of the myelin would slow nerve impulses along the axon. In extreme cases, the loss of myelin can lead to scarring that can prevent nerve impulses from being propagated at all. |
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Part A – Events Involved in Continuous Propagation of an Action Potential |
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Part A – Events Involved in Continuous Propagation of an Action Potential |
After depolarization, voltage-gated Na+ channels are inactivated and voltage-gated K+ channels open. This renders that segment of the axon temporarily refractory (insensitive) to another depolarizing stimulus and forces the action potential to move down the axon toward the next segment where the voltage-gated Na+ channels are closed and receptive to a depolarizing stimulus. |
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Saltatory propagation occurs in _________ axons, in which action potentials _________. unmyelinated; spread by depolarizing the adjacent region of the axon membrane unmyelinated; move from one node of Ranvier to another myelinated; move from one node of Ranvier to another myelinated; move continuously along the axon toward the axon hillock |
myelinated; move from one node of Ranvier to another Saltatory propagation is much faster than continuous propagation. The speed of propagation along an axon varies in two ways: 1) myelin sheaths limit the movement of ions across the axon membrane, thereby requiring the action potentials to "leap" from node to node during propagation, thus traveling at a greater speed; and 2) the diameter of the axon directly relates to the speed of propagation (i.e., the larger the diameter of the axon, the faster the speed of propagation). |
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At which point of the illustrated action potential would voltage-gated Na+ channels be mostly open but voltage-gated K+ channels be mostly closed?
A |
B Voltage-gated Na+ channels open when the membrane potential reaches threshold. Voltage-gated K+ channels would be mostly open near C. Transmembrane potential in an axon resulting from passage of an action potential. |
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Which of the following correctly states the direction followed by the specified ions when their voltage-gated channels open? Na+ ions move into the axon; K+ ions move out. |
Na+ ions move into the axon; K+ ions move out. Na+ ions move into the axon and K+ ions move out according to their concentration gradients. |
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Which of the following mechanisms is most significant in returning Na+ and K+ concentrations to resting ionic conditions (from point D to point E)? the opening of voltage-gated Na+ channels |
Na+ channels active transport by the Na+-K+ pump Maintenance (and restoration) of the resting ion concentrations depends on the Na+-K+ pump. Once gated ion channels are closed, the combined action of the pump and ion leakage (particularly that of K+) establishes a resting membrane potential in a typical neuron of around −70 mV. |
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What change in a neuron is being measured in the graph? the voltage measured across the axon membrane at a specific point as an action potential travels past |
the voltage measured across the axon membrane at a specific point as an action potential travels past the voltage measured between the neuron cell body and the axonal terminals It is important to recognize that an identical event will begin in the adjacent area of the axon membrane as the action potential observed here decays. |
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At which of the points along the illustrated action potential can a second action potential be produced, but only with a stimulus significantly greater than the one that produced the first? B |
D Point D would be considered within the relative refractory period. A significantly strong stimulus may create enough depolarization to return the membrane to threshold. An important consequence of this design is that greater stimulus intensity will result in an increase in nerve impulse frequency. |
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You discover that a new chemical compound interacts with K+ voltage-dependent channels. What would be the effect on a neuron if the chemical came into contact with the axonal membrane? The cell would be unable to generate a resting potential. The neuron would be unable to repolarize. The cell would die. The cell would be unable to depolarize. |
The neuron would be unable to repolarize. This is true; K+ is used to repolarize the membrane after Na+ rushes into the membrane. |
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Which of the following is NOT a difference between graded potentials and action potentials? Graded potentials can result from the opening of chemically gated channels; action potentials require the opening of voltage-gated channels. The magnitude of action potentials decrease as the impulse travels further away from the start of the impulse while graded potentials do not decrease in magnitude. Graded potentials occur along dendrites, whereas action potentials occur along axons. Greater stimulus intensity results in larger graded potentials, but not larger action potentials. |
The magnitude of action potentials decrease as the impulse travels further away from the start of the impulse while graded potentials do not decrease in magnitude. This is opposite of what is true. Graded potentials decrease in magnitude as the impulse travels. |
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Which of the following is a factor that determines the rate of impulse propagation, or conduction velocity, along an axon? length of the axon the number of axon collaterals extending from a truncated axon whether the axon is located in the central nervous system or in the peripheral nervous system degree of myelination of the axon |
degree of myelination of the axon the rate of impulse propagation depends largely on two factors, degree of myelination and axon diameter. The presence of a myelin sheath dramatically increases the rate of impulse (action potential) propagation. Heavily myelinated axons propagate impulses faster than lightly myelinated axons. Also, larger (thicker) axons conduct impulses faster than smaller (thinner) axons. |
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If a neuron had a mutation that prevented the production of voltage-gated Na+ channels, what function would the neuron NOT be able to accomplish? depolarization leading to action potentials |
depolarization leading to action potentials Correct. The voltage-gated Na+ channels cause the rapid depolarization phase of the action potential. |
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What type of stimulus is required for an action potential to be generated? multiple stimuli |
a threshold level depolarization The axolemma must be depolarized to threshold in order to generate an action potential. |
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Which membrane potential occurs because of the influx of Na+ through chemically gated channels in the receptive region of a neuron? inhibitory postsynaptic potential |
excitatory postsynaptic potential An excitatory postsynaptic potential, a type of graded potential, occurs because of the influx of Na+ through chemically gated channels in the receptive region, or postsynaptic membrane, of a neuron. Graded potentials are generated by chemically gated channels, whereas action potentials are produced by voltage-gated channels. |
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A postsynaptic cell can be a neuron, a muscle cell, or a secretory cell. What is an example of a presynaptic cell? |
a neuron A neuron is the only type of presynaptic cell. Neurons release neurotransmitters, effectively changing an electrical signal or action potential into a chemical signal that can communicate across the synaptic cleft to the postsynaptic cell. |
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Which component has a role in the postsynaptic cell during synaptic activity? calcium channels chemically gated channels Vesicles filled with neurotransmitter axon terminal |
chemically gated channels Neurotransmitter binds to receptors on the postsynaptic cell. These chemically gated channels open, allowing the transfer of the "signal" from a presynaptic neuron to the postsynaptic cell. |
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What is the role of calcium in synaptic activity? Calcium influx into the synaptic terminal causes vesicle fusion. |
Calcium influx into the synaptic terminal causes vesicle fusion. When an action potential reaches the synaptic terminal, voltage-gated channels open and calcium enters the cell. Calcium causes vesicles to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft. |
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What is the direct role of neurotransmitter at a chemical synapse? Neurotransmitter causes a graded potential in the postsynaptic cell. |
Neurotransmitter binds to receptors on the postsynaptic cell membrane and allows ions to diffuse across the membrane. Neurotransmitter leaves the presynaptic neuron by exocytosis and binds receptors on the postsynaptic cell membrane, opening the channels. When ions enter the postsynaptic cell, a graded potential takes place. |
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Neurotransmitter is released from presynaptic neurons through what mechanism? |
exocytosis Neurotransmitter molecules are released from vesicles that fuse with the plasma membrane through exocytosis. Note that "exo-" means "outside" and "cytosis" means "cell." Once released, neurotransmitter diffuses across the synaptic cleft. |
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What type of channel on the postsynaptic membrane binds neurotransmitter? a voltage-gated channel |
a chemically gated channel Chemically gated channels bind a specific chemical, which causes the channel to open. At chemical synapses, neurotransmitter molecules are released by the presynaptic neuron and bind to chemically gated channels on the postsynaptic cell membrane. The opening of these channels allows ions to diffuse across the membrane, causing a graded potential in the postsynaptic cell. |
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In addition to diffusion, what are two other mechanisms that terminate neurotransmitter activity? |
reuptake and degradation To terminate neurotransmitter effects, neurotransmitter molecules must be removed from the synaptic cleft. Reuptake does this by moving neurotransmitter back into the presynaptic neuron. Diffusion causes neurotransmitter to drift away from the synaptic cleft. Degradation occurs when enzymes break down neurotransmitter. As long as neurotransmitter molecules remain in the synaptic cleft, the chemically gated channels on the postsynaptic cell will continue to bind them and cause graded potentials. |
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Events that occur during synaptic activity are listed here, but they are arranged in an incorrect order. Choose the correct order of these events below. (a) Voltage-gated calcium channels open (e) Neurotransmitter released into the synaptic cleft (c) Action potential arrives at axon terminal (b) Neurotransmitter binds to receptors (f) Graded potential generated in postsynaptic cell (d) Neurotransmitter is removed from the synaptic cleft (c) Action potential arrives at axon terminal (a) Voltage-gated calcium channels open (e) Neurotransmitter released into synaptic cleft (b) Neurotransmitter binds to receptors (f) Graded potential generated in postsynaptic cell (d) Neurotransmitter is removed from the synaptic cleft (d) Neurotransmitter is removed from the synaptic cleft (b) Neurotransmitter binds to receptors (f) Graded potential generated in postsynaptic cell (c) Action potential arrives at axon terminal (a) Voltage-gated calcium channels open (e) Neurotransmitter released into the synaptic cleft (c) Action potential arrives at axon terminal (a) Voltage-gated calcium channels open (e) Neurotransmitter released into the synaptic cleft (d) Neurotransmitter is removed from the synaptic cleft (b) Neurotransmitter binds to receptors (f) Graded potential generated in postsynaptic cell |
(c) Action potential arrives at axon terminal (a) Voltage-gated calcium channels open (e) Neurotransmitter released into synaptic cleft (b) Neurotransmitter binds to receptors (f) Graded potential generated in postsynaptic cell (d) Neurotransmitter is removed from the synaptic cleft The correct sequence starts with a neural signal at the presynaptic cell, followed by the release of neurotransmitter, the creation of a graded potential in the postsynaptic cell, and degradation of the neurotransmitters. Neurotransmitters transfer information between a neuron and a postsynaptic cell. This process gets a "message" across a physical separation much like sending a text to your friend who is across town. |
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What ion is entering the axon terminal at A, and what effect does it have? Na+, which then causes repolarization of the axon terminal’s membrane K+, which then causes increased production of neurotransmitter neurotransmitter, which then causes the presynaptic neuron to form an action potential Ca2+, which then causes release of neurotransmitter from the axon terminal |
Ca2+, which then causes release of neurotransmitter from the axon terminal Calcium ion channels open when the membrane is depolarized, and the inflow of Ca2+ leads to the release of neurotransmitters from synaptic vesicles. |
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By which method does the structure at B release neurotransmitter? exocytosis |
exocytosis The influx of Ca2+ triggers the release of neurotransmitters stored in synaptic vesicles (B) by exocytosis. |
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How would the receptors at C best be classified? mechanically gated |
chemically gated The receptors at C are affected by the binding of a chemical neurotransmitter. |
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Which of the following statements most accurately describes the effects caused by binding of the neurotransmitter (green dots) to the structure labeled C? The membrane potential of the presynaptic membrane changes. The neurotransmitter is transported into the postsynaptic neuron. an action potential is generated The membrane potential of the postsynaptic membrane changes. |
The membrane potential of the postsynaptic membrane changes. Binding of the neurotransmitter to the receptor at C causes the opening of receptor-associated ion channels. Depending on the particular types of channels that open, diffusion of certain ions (typically Na+, K+, or Cl−) will cause a depolarizing or hyperpolarizing effect on the postsynaptic membrane. |
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The box labeled D illustrates three mechanisms by which the effects of a neurotransmitter may be terminated. Which of the following mechanisms is NOT included in the figure? reuptake of the neurotransmitter by transport into the postsynaptic cell |
reuptake of the neurotransmitter by transport into the postsynaptic cell Neurotransmitters are generally not transported into the postsynaptic cell upon which they exert their effects. |
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Which description of synapses is NOT correct? Excitatory synapses cause depolarization. |
Direct signaling involves the activation of G Proteins. Indirect signaling involves G Proteins. Direct signaling involves opening ion channels that affect the polarization of the membrane. |
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Suppose that both stimuli seen in these graphs happened equally at the same time on a postsynaptic membrane as a result of two different synapses. Which of the following best describes the result? An action potential would result. |
There would be little or no graded potential. In order for a graded potential to occur, there must be an overall change in membrane potential. Due to spatial summation, these stimuli would cancel each other. |
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Which of the following statements is true of both membrane potential responses shown in the graphs? Both responses are examples of graded potentials. |
Both responses are examples of graded potentials. |
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Which of the following stimuli caused the reaction in the graph on the left? opening of gated Ca2+ channels |
opening of gated Na+ channels Opening of gated Na+ channels allows Na+ to diffuse into the neuron, causing an increase in the local membrane potential. |
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Which of the following is expected to occur first if the membrane potential decrease shown in the graph on the left were to reach the threshold value indicated at ~ -55 mV? opening of voltage-gated K+ channels |
opening of voltage-gated Na+ channels The threshold value is the point at which voltage-sensitive Na+ channels open. This leads to the characteristic rapid depolarization phase of the action potential. |
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Opening K+ or Cl- channels in a postsynaptic membrane would produce an inhibitory postsynaptic potential (IPSP). True |
True Opening K+ or Cl- channels in a postsynaptic membrane, resulting in K+ efflux or Cl- influx, respectively, would induce hyperpolarization. As the membrane potential increases and is driven farther from the axon’s threshold, the postsynaptic neuron becomes less and less likely to "fire," and larger depolarizing currents are required to induce an action potential (AP). Hyperpolarizing changes in potential are called inhibitory postsynaptic potentials (IPSPs). |
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Which of the following does NOT describe the process of summation? Two EPSPs occur close enough in time to bring the cell to threshold. |
One EPSP fails to bring the cell to threshold. This statement is true, but summation considers the action of multiple synaptic potentials working on a cell simultaneously. |
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In a lab you are conducting tests with various chemicals and neurotransmitter receptors. You notice that exposed frog muscle cells depolarize when you add the chemical nicotine to the acetylcholine (ACh) receptors. What does this tell us about the relationship between neurotransmitters and their receptors? The effect of a neurotransmitter is based on the properties of the receptor more than the neurotransmitter. |
The effect of a neurotransmitter is based on the properties of the receptor more than the neurotransmitter. This is true; also consider that in the neuromuscular junction, ACh receptors are excitatory, but in the heart muscle, ACh receptors are inhibitory. |
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Art-labeling Activity: Figure 11.22 |
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What component of the reflex arc determines the response to a stimulus? effector |
The integration center receives sensory information (input), determines the proper response, and then signals the appropriate effector(s) to produce the response. |
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Which of the following circuit types is involved in the control of rhythmic activities such as the sleep-wake cycle, breathing, and certain motor activities (such as arm swinging when walking)? parallel after-discharge circuits |
Reverberating circuits are involved in the control of rhythmic activities such as breathing, the sleep-wake cycle, and repetitive motor activities such as walking. A signal travels through a chain of neurons, each feeding back to previous neurons in the pathway. |
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In the brain, vision originates in the rods and cones in the retina. Separate regions of the brain decode basic information, like color, shapes, intensity of light, and there are other regions that decode information like position in space, and awareness of patterns. As you use your visual system, all of these regions are working simultaneously. This simultaneous awareness of all regions working at the same time is due to which processing pattern listed below? oscillative processing |
parallel processing In parallel processing, inputs are segregated into many pathways, and different parts of the neural circuitry deal simultaneously with the information delivered by each pathway. This helps us understand all of the different types of information in our visual fields simultaneously. |
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Which of the following is NOT a type of circuit? reverberating circuits |
pre-synaptic circuits |
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Which neuron circuit pattern is involved in the control of rhythmic activities such as breathing? parallel after-discharge circuit |
Reverberating circuits are involved in the control of rhythmic activities such as breathing, the sleep-wake cycle, and repetitive motor activities like walking. A signal travels through a chain of neurons, each feeding back to previous neurons in the pathway. The oscillation of the signal can create cyclical activity in the pathway. |
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