Week 3: Neurophysiology
In the animation's analogy of "Fire!" and "Don't Fire!" of the neuron, what physiological response is implied?
"Don't Fire!" implies the membrane potential becomes more negative. In this analogy, the postsynaptic neuron may respond with either "Fire!" or "Don't Fire!" The "Fire!" corresponds to an excitatory postsynaptic potential: here we will see the membrane potential become less negative (or more positive). Sodium and potassium ions cross chemically-gated ion channels once a neurotransmitter binds to the membrane receptor, producing depolarization. In the "Don't Fire!" analogy, we see an IPSP—an inhibitory postsynaptic potential. With an IPSP, the membrane potential is increased, or becomes more negative (or, less positive). The chloride and potassium chemically-gated ion channels are key role-players for an IPSP.
Drag and drop the descriptive labels of events into the correct sequence at the chemical synapse.
(1) Action potential sweeps down presynaptic axon (2) Calcium channels open in axon terminal (3) Synaptic vesicles fuse and exocytize neurotransmitter (4) Diffusion of neurotransmitter into extracellular fluid separating two neuron's membranes (5) Graded potential at postsynaptic membrane
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 (b) Neurotransmitter binds to receptors (c) Action potential arrives at axon terminal (d) Neurotransmitter is removed from the synaptic cleft (e) Neurotransmitter released into synaptic cleft (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
At which membrane potential, given the graphs shown, do a sufficient number of the voltage-gated Na+ channels start to open?
-60 mV
Assume you have a membrane with only potassium leakage channels. The RMP is -90mV. Predict the RMP if we add Na+ leakage channels.The most likely RMP value of Na+ is __________.
-70 mV
What is the value for the resting membrane potential for most neurons?
-70 mV
What is the resting membrane potential of the neuron used in the experiment?
-80 mV
Mr. M is a Type 2 diabetic with diagnoses of (poorly controlled) hypertension and chronic renal failure. He has come in for a routine checkup and claims to be feeling fine, just a little tired. He presents as an overweight 58-year-old man with a sunburn on his cheeks and ears and a BAND-AID® on one cheek (where he says he nicked himself shaving that morning), a heart rate of 66 bpm, blood pressure of 124/80 mm Hg, normal breath and heart sounds, and slightly puffy ankles, face, and hands. His weight has increased by 1.5 lbs since he was last seen eight months ago. Do you see anything to concern you in Mr. M's description? There are several worrying signs in Mr. M's presentation. Can you pick them out?
-His puffy ankles, face, and hands could indicate fluid retention. -His tiredness might indicate worsening of his diseases. -His blood pressure is low for someone with hypertension.
An important aspect of learning physiology is understanding the cause-and-effect relationships in pathways. This activity will test your understanding of the sequence of events in an action potential. Place the action potential events in the correct sequence.
1. Activation gates on Na+ channels open; Na+ enters cell. 2. Cell depolarizes. 3. Inactivation gates on Na+ channels close and activation gates on K+ channels open; K+ leaves cell. 4. Cell repolarizes. 5. Activation gates on K+ channels begin to close. 6. Cell experiences hyperpolarization.
Drag the labels to identify the sequence of events that occurs at a synapse.
1. An action potential arrives at the synaptic terminal. 2. Calcium channels open, and calcium ions enter the synaptic terminal. 3. Vesicles containing neurotransmitters fuse with the plasma membrane of the sending neuron. 4. Neurotransmitter molecules diffuse across the synaptic cleft. 5. The neurotransmitter molecules bind to receptors in the plasma membrane of the receiving neuron, causing ion channels there to open
Order each of the following events so that they occur in the proper order, from left to right, with respect to the function of the channels, ion permeability, and changes in membrane potential.
1. Opening of voltage-gated Na+ channels 2. Na+ ion flow into the neuron increases 3. Depolarization of the membrane to +30 mV 4. K+ ion flow out of the neuron increases 5. Repolarization of the membrane
Using the tracing above, drag each of the terms or values to correctly complete each sentence. Answers may be used once or not at all. Pay attention to the units!
1. This neuron is most depolarized at +30 mV. 2. This neuron spends approximately 2.5 msec in a hyperpolarized state. 3. The ion K+ is crossing the cell's plasma membrane at 1.5 msec. 4. This cell reaches threshold at approximately 0.6 msec.
What is the magnitude (amplitude) of an action potential?
100 mV 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.
Cl- is a common, negatively charged extracellular ion. Predict the effect on the RMP if many Cl− gated channels are suddenly opened.
A more negative RMP would result.
As mentioned in the case description, tetrodotoxin is a molecule that blocks voltage-gated sodium ion channels. What is a voltage-gated sodium ion channel and what is its function?
A voltage-gated sodium channel is protein located in the cell membrane of the neuron (usually in the axon hillock) that allows sodium to pass into the cell when it reaches a certain voltage.
What is happening to the electrical potential of a neuron when it generates an action potential? What is the function of the action potential in neurons?
Action potential causes different ions to exchange across the membrane and travel down the axon. When the three molecules of sodium move inside the neuron and two molecules of potassium comes out of the neuron it causes depolarization and vice versa in repolarization. At stimulus, the membrane potential rises above the threshold potential -55 mV and when the stimulus applied, the action potential rises to peak + 30mV and drops to -90mV and then finally reaches the resting membrane potential -70mV. Thus the function of the action potential is to transmits the electrical signal in the neuron.
How is an action potential propagated along an axon?
An influx of sodium ions from the current action potential depolarizes the adjacent area. Submit
After recovering, Dr. Westwood decided to learn more about neurotoxins. In conducting his research, he learned about a toxin called batrachotoxin (BTX). BTX also binds to voltage-gated sodium ion channels. Dr. Westwood reviewed experiments with neurons exposed to BTX, which showed that after depolarizing, the membrane potential remained positive for an extended length of time. Repolarization of these neurons was often extremely delayed. Which of the following could help explain this result?
BTX prevents voltage-gated sodium ion channels from inactivating.
Which of the following membrane regions would have significant numbers of voltage-gated ion channels?
C and D
What is the role of calcium in synaptic activity?
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.
Choose the correct response to this statement: Presynaptic neurons synapse on the ________ of the postsynaptic neuron.
Cell body or dendrites
What type of conduction takes place in unmyelinated axons?
Continuous conduction 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.
Drag and drop each of the terms at the left to the correct statement at the right.
Depolarize: The membrane potential is becoming more positive than the resting membrane potential. Repolarize: The membrane potential is moving from a more positive value toward resting membrane potential. Hyperpolarize: The membrane potential is more negative than the resting membrane potential.
Define the following phrases and terms associated with the signs and symptoms of Dr. Westwood's TTX poisoning: diaphoresis, motor dysfunction, paresthesias, cyanotic, hypoventilating, bradycardia, gastric lavage, oxygen saturation.
Diaphoresis - sweating, especially to an unusual degree as a symptom of disease or a side effect of a drug. Motor dysfunction - motor, pertaining to a muscle, nerve, or center that produces or affects movement. motor neuron, one of the various efferent nerve cells that transmit nerve impulses from the brain or from the spinal cord to muscular or glandular tissue. Paresthesias - Paresthesia is an abnormal sensation such as tingling, tickling, pricking, numbness or burning of a person's skin with no apparent physical cause. Cyanotic - Cyanosis refers to bluish discoloration of skin, nail beds and mucous membranes. Hypoventilating - In medicine, hypoventilation (also known as respiratory depression) occurs when ventilation is inadequate (hypo meaning "below") to perform needed gas exchange. By definition it causes an increased concentration of carbon dioxide (hypercapnia) and respiratory acidosis. Bradycardia - abnormally slow heart beat Gastric lavage (also commonly called stomach pumping or gastric irrigation) - is the process of cleaning out the contents of the stomach. It has been used for over 200 years as a means of eliminating poisons from the stomach. Oxygen saturation - Oxygen saturation is a term referring to the fraction of oxygen-saturated hemoglobin relative to total hemoglobin (unsaturated + saturated) in the blood. The human body requires and regulates a very precise and specific balance of oxygen in the blood. Normal blood oxygen levels in humans are considered 95-100 percent.
Place each of the descriptions into their appropriate bin: EPSP (excitatory post-synaptic potential) or IPSP (inhibitory post-synaptic potential) or both.
EPSP: Membrane potential becomes more positive May produce an action potential IPSP: Membrane potential moves away from threshold Influx of negative ions Both EPSP and IPSP: Is a type of graded potential
What must occur for threshold to be reached?
Enough excitatory potentials add up to open voltage-gated channels at the axon hillock.
Which description best fits graded potentials?
Graded potentials can result in either depolarization or hyperpolarization of the membrane and vary in strength. Both EPSPs and IPSPs are graded potentials. The signal strength of each varies depending on both temporal and spatial summation.
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.
Arrange the parts in order, from left to right, of a successful direct depolarization path within one neuron.
Incoming messages to the dendrites or cell body pass through neuroplasm to the axon hillock. If a threshold stimulus value is reached at the axon hillock, depolarization passes down the axon to the presynaptic terminal containing synaptic vesicles.
Where do most action potentials originate?
Initial segment
How would an increase in extracellular K+ affect repolarization?
It will decrease the concentration gradient, causing less K+ to flow out of the cell during repolarization.
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. 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.
Which of the following is the clearest example of a neuronal membrane's selective permeability?
K+ ions can diffuse across the membrane more easily than Na+ions. Submit
During an action potential, hyperpolarization beyond (more negative to) the resting membrane potential is primarily due to __________.
K+ ions diffusing through voltage-gated channels
The repolarization phase of the action potential, where voltage becomes more negative after the +30mV peak, is caused primarily by __________.
K+ ions leaving the cell through voltage-gated channels
Match the words in the left column to the appropriate phrases on the right. Make certain each word is matched with a phrase before submitting your answer.
K+ permeability:The primary determinant of the resting membrane potential Membrane potential of −70 mV: The resting membrane potential of the cell Na+ permeability: Although it plays a role, it is not a primary determinant of the resting membrane potential Membrane potential of −90 mV: The point at which there is no net movement of K + into or out of the cell
The membranes of neurons at rest are very permeable to _____ but only slightly permeable to _____.
K+; Na+
Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel?
Leak channels Leak channels for Na+ and K+ are ubiquitous, and they allow for the diffusion of these ions across plasma membranes.
Predict the possible effect of a drug that totally blocks the neurotransmitter receptor on the postsynaptic membrane. For example, curare is a neurotoxin used by several South American cultures. The primary effect of curare is that acetylcholine, a major neuromuscular neurotransmitter, cannot bind at its receptor because curare is blocking it. Predict the possible effects of curare on the postsynaptic membrane and muscle.
Local graded potential and action potential transmission is blocked, and there is no response by the postsynaptic cell, the muscle.
Which statement best describes exocytosis?
Membrane organelles fuse with the membrane and release contents out of the cell.
For each of the following, indicate whether the condition will cause the membrane potential to become more positive, more negative, or largely unchanged when compared to the normal physiological resting membrane potential. Drag the appropriate items to their respective bins.
More Positive: Triple the number of Na+ leak channels; Double the concentration of K+ outside the cell More Negative: Double the number of K+ leak channels; Decrease the concentration of Na+ outside the cell by half Largely Uncharged: Double the size of the cell, without adding channels; Double the number of closed channels for K+
The EKG is hooked up, and Mr. M's trace looks very abnormal. His heart rate is low. The T wave, which indicates repolarization of the ventricles, is unusually large and pointed and the QRS wave, which indicates the depolarization impulse's passage through the ventricles, is widened or slowed."That's a classic EKG for hyperkalemia (high blood potassium)!" The doctor gives Mr. M an injection of calcium gluconate.Calcium ions block the Na+ channels on nerve and muscle cells. How will this help counteract the effects of high blood K+? Choose the best answer.
Mr. M's cells are depolarizing too easily. Blocking Na+ channels will make it harder for them to depolarize, bringing their sensitivity back to normal. By using calcium ions to block the Na+ channels, this gives the nerves and muscles more time to reach less sensitivity by allowing the Na+ / K+ ATPase to focus on regulating the amount of K+ in the extracellular fluid to the inside of the cell, reaching normal levels, and allowing more K+ to leave the cells during repolarization instead of being stuck in depolarization from the constant fire of Na+ into the cell.
Paralysis is a term used to describe the loss of muscle function. If tetrodotoxin's effect is on neurons, why did Dr. Westwood experience paralysis?
Mr. Westwood experienced paralysis because of a chain reaction. Movement occurs when every single neuron in the muscles is working properly and receiving the right impulse from the brain. For movement to occur, sodium ions are necessary. If sodium ions are being blocked, neurons remain in resting potential and do not receive any impulse or message from the brain to move. An electrical impulse travels from cell to cell, so if a group of cells are poisoned with TTX, many other groups of cells are not receiving impulses, and many other cells after them. This chain reaction will eventually result in paralysis when a body parts cells are impaired.
In which type of axon will velocity of action potential conduction be the fastest?
Myelinated axons with the largest diameter
The diffusion of what ion, across the neuronal membrane, is responsible for the local currents that depolarize regions of the axon to threshold?
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.
The concentrations of which two ions are highest outside the cell.
Na+ and Cl-
What is happening to voltage-gated channels at this point in the action potential?
Na+ channels are inactivating, and K+ channels are opening.
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. 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.
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?
Na+ is pumped out of the cell and K+ is pumped into the cell.
What prevents the Na+ and K+ gradients from dissipating?
Na+-K+ ATPase 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.
The concentrations of which two ions are highest outside the cell?
Na+Na+ and Cl−
What is the direct role of neurotransmitter at a chemical synapse?
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.
What characterizes repolarization, the second phase of the action potential?
Once the membrane depolarizes to a peak value of +30 mV, it repolarizes to its negative resting value of -70 mV.
How do action potential propagation speeds compare in myelinated and unmyelinated axons?
Propagation is faster in myelinated axons.
Which step of nerve or muscle firing would be directly affected by a change in extracellular K+ ? Choose the best answer.
Repolarization
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)
Action potentials (nerve impulses) are changes in the membrane potential that, once started, will affect the entire excitable membrane. The first action potential is usually generated at the initial segment of the neuron's axon. This activity will test your understanding of the sequence of events that occur at the membrane of the initial segment of the axon during generation of an action potential. Place the events involved in generation of an action potential in the correct order of occurrence from left to right.
Return to resting state: 1. Threshold stimulus Na+ channels open 2. Na+ influx Depolarization 3. Na+ channels close, K+ channels open 4. K+ efflux Repolarization 5. Hyperpolarization K+ channels close
Sort the phrases into the appropriate bins depending on whether they describe similarities or differences between voltage-gated K+ channels and voltage-gated Na+ channels.
Similarities: The stimulus that triggers opening; The polarity of the substances they transport across the membrane; The placement of the channel protein relative to the membrane Differences: The direction of ion movement through the channel; The inactivation mechanism
The propagation of an action potential (AP) in an unmyelinated axon is called continuous propagation. This activity will test your understanding of the sequence of events that occur during continuous propagation. Place the events involved in the propagation of an action potential in the correct order of occurrence from left to right.
Stimulation of initial axon segment to current flows continuously: 1. Local current flows to axon segment 2. Axon segment depolarized to threshold 3. Voltage-gated Na+channels open 4. Influx of Na+ 5. AP regenerated in adjacent axon segment
Which of the stimuli in this experiment would be considered a threshold stimulus?
Stimulus 2 A minimum threshold stimulus occurs when a cell is stimulated and the stimulus is strong enough to elicit an action potential.
Now that you have addressed some of the basic biology of this case, explain why Dr. Westwood experienced numbness after eating the pufferfish meal.
TTX inhibits neurons that transmit sensory information to the brain. Since TTX prevents the generation of action potentials in sensory nerves, the ability of the neurons to transmit sensory information will be inhibited. This loss of sensory information would be experienced as a "numbness" in areas exposed to the toxin.
Place each statement into its correct category: temporal summation, spatial summation, or both temporal and spatial summation.
Temporal summation: One presynaptic neuron firing at high frequency to a postsynaptic neuron Spatial summation: Multiple neurons firing simultaneously at one postsynaptic neuron Both temporal and spatial summation: May result in an action potential; May be from EPSPs; May be from IPSPs
What would happen to a neuron if it was exposed to tetrodotoxin? Be specific regarding its effect on the ability of a neuron to communicate.
Tetrodotoxin blocks voltage-gated sodium ion channels. When these channels become blocked, the neuron cannot balance the charges and ion concentrations. This will result in the signal not being propagated down the nerve. The motor neuron will not be able to communicate/send information to skeletal muscle.
Which of the following best describes the Na+ and K+ concentrations across a neuron's plasma membrane?
The Na+ concentration is higher outside the cell compared to inside. The K+ concentration is higher inside the cell compared to outside.
Imagine that the cell membrane from the previous problem becomes more permeable to Na+Na+. Predict how this will affect the RMP.
The RMP will be more positive.
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.
Why does the action potential only move away from the cell body?
The areas that have had the action potential are refractory to a new action potential.
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 Na+Na+ and K+K+ are the major determinants of RMP in this cell, which of the following could best explain your results?
The cell's Na+Na+-K+K+ ATPase pumps have stopped functioning. 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.
The doctor is concerned about Mr. M's weakness. She orders an EKG. Why might an EKG be an appropriate test for this patient? Choose the best answer.
The heart is a muscle too, so whatever is making his skeletal muscles weak might be affecting it. The heart, like any other muscle, depolarizes and repolarizes. So if abnormal extracellular K+ levels have been interfering with Mr. M's skeletal muscle function, as indicated by his weak reflexes, they might be interfering with his heart function as well - a much more serious problem!
Why does regeneration of the action potential occur in one direction, rather than in two directions?
The inactivation gates of voltage-gated Na+ channels close in the node, or segment, that has just fired an action potential.
On average, the resting membrane potential is -70 mV. What does the sign and magnitude of this value tell you?
The inside surface of the plasma membrane is much more negatively charged than the outside surface.
What characterizes depolarization, the first phase of the action potential?
The membrane potential changes from a negative value to a positive value.
What event triggers the generation of an action potential?
The membrane potential must depolarize from the resting voltage of -70 mV to a threshold value of -55 mV.
What is the function of the myelin sheath?
The myelin sheath increases the speed of action potential conduction from the initial segment to the axon terminals.
In a myelinated axon, how do the nodes of Ranvier differ from other segments of the same axon?
The nodes are more permeable to ions.
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 concentration gradients and leak channels 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.
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 resting membrane potential is dependent upon two important factors: 1) differences in sodium and potassium concentrations across the membrane (electrochemical gradients) and 2) differences in sodium and potassium membrane permeability. This activity will help you to review how the resting membrane potential of cells are generated and maintained. Use the provided ions to correctly complete each sentence about the resting membrane potential. Ions may be used more than once, or not at all.
The resting membrane potential is maintained by Na+-K+ pumps that actively transport K+ into and Na+ out of the cell. The concentration of K+ is higher inside than outside the cell. The membrane is more permeable to K+. The concentration of Na+ is higher outside than inside the cell.
Hypothetically, what would be the most immediate effect of doubling the number of Na+ leakage channels in the plasma membrane?
The resting membrane potential would become less negative (more positive).
Suppose a drug is developed that blocks K+ leakage channels. The drug prevents ions from passing through those channels. If this drug was applied to a neuron, what would be the most immediate effect on that neuron?
The resting membrane potential would become less negative (more positive).
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?
The speed of action potential propagation would be slower.
Which of the following describes why Dr. Westwood experienced paralysis?
The tetrodotoxin prevented the motor neurons from sending signals to skeletal muscle.
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.
What opens first in response to a threshold stimulus?
Voltage-gated Na+ channels
What is the first change to occur in response to a threshold stimulus?
Voltage-gated Na+ channels change shape, and their activation gates open.
When an action potential arrives at the end of the axon terminal, a series of events take place that result in the release of neurotransmitter from the presynaptic axon. Select the answer that correctly describes the primary stimulus for vesicles to move toward the cell membrane and eventually release their contents.
Voltage-gated membrane channels open, and Ca+2 enters the cytoplasm, increasing intracellular calcium.
Describe the role of sodium ions and sodium channels in generating an action potential.
When the membrane is polarized it usually has a membrane potential of -70 to -90 mV. It is due to the cell membrane's increased permeability to the potassium ions. When a stimulus is given to the axonal membrane, its permeability for ion changes, and it becomes permeable for sodium ions as the sodium gated channels open. Sodium ions influx starts from extracellular fluid into the cell's interior, which causes an increase in the membrane potential from -70 to +30 mV, causing the membrane to depolarize. At this time, the membrane is completely impermeable to anions.
When nerve cells are at rest, there is an unequal amount of positive and negative charges on either side of a nerve cell membrane. This charge difference creates an electrical potential. Describe how the resting membrane potential (resting potential) is generated.
When the neuron is at rest, K+ ions can cross through the membrane, while Cl- and Na+ ions are not readily permeable.The Na+/K+ pump creates the ion concentration gradient by moving 3 Na+ out of the cell while moving 2 K+ into the cell, against their concentration gradient(s). Due to this, more sodium ions accumulate outside the cell leaving the cell inside more negative. This difference in the ion concentration creates a potential, called membrane potential. The resting membrane potential (of a neuron) is -70 mV, which means that the inside of the neuron is 70 mV less than the outside.
What type of channel on the postsynaptic membrane binds neurotransmitter?
a chemically gated channel
What is meant by an EPSP?
a localized change in the membrane potential resulting in a less negative membrane voltage
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.
Which of the following best describes a voltage-gated sodium ion channel?
a protein embedded in the cell membrane of a neuron
The velocity of the action potential is fastest in which of the following axons?
a small myelinated axon
The mechanism by which the neurotransmitter is returned to a presynaptic neuron's axon terminal is specific for each neurotransmitter. Which of the following neurotransmitters is broken down by an enzyme before being returned?
acetylcholine Acetylcholine is broken down by acetylcholinesterase before being returned to the presynaptic neuron's axon terminal.
What condition will increase the diffusion of molecules, such as neurotransmitters?
an increase in the amount of neurotransmitter exocytized by the presynaptic axon
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
Where are action potentials regenerated as they propagate along a myelinated 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.
Where in the neuron is an action potential initially generated?
axon hillock
During depolarization, which gradient(s) move(s) Na+ into the cell?
both the electrical and chemical gradients
If the membrane of a postsynaptic dendrite is setting up a graded potential, what must have happened after neurotransmitter was released by the presynaptic terminal?
bound at postsynaptic receptors to open postsynaptic ion channels
What kinds of gated channels are typically found in high concentrations at the dendrites?
chemically gated
Which component has a role in the postsynaptic cell during synaptic activity?
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.
Binding of a neurotransmitter to its receptors opens __________ channels on the __________ membrane.
chemically gated; postsynaptic
An action potential is self-regenerating because __________.
depolarizing currents established by the influx of Na+ flow down the axon and trigger an action potential at the next segment
During the action potential, when does sodium permeability initially DECREASE?
during the peak of depolarization
Binding of the neurotransmitter to its receptor causes the membrane to __________.
either depolarize or hyperpolarize
Neurotransmitter is released from presynaptic neurons through what mechanism?
exocytosis
When neurotransmitter molecules bind to receptors in the plasma membrane of the receiving neuron,
ion channels in the plasma membrane of the receiving neuron open.
What is the major role of the Na+-K+ pump in maintaining the resting membrane potential?
maintaining the concentration gradients for Na+ and K+ across the cell membrane
Which of the following would increase the membrane permeability to K+?
more K+ leakage channels
Complete the following sentence. The operation of the Na+−K+ ATPase pump __________.
moves 3 Na+ to the ECF and 2 K+ to the cytoplasm
Saltatory propagation occurs in _________ axons, in which action potentials _________.
myelinated; move from one node of Ranvier to another
When a neuron is at rest, the cytoplasmic (inside) side of the cell is ______.
negatively charged relative to the outside of the cell
A molecule that carries information across a synaptic cleft is a
neurotransmitter.
Let's consider a scenario in which the resting membrane potential changes from −−70 mV to +70 mV, but the concentrations of all ions in the intracellular and extracellular fluids are unchanged. Predict how this change in membrane potential affects the movement of Na+. The electrical gradient for Na+ would tend to move Na+ __________ while the chemical gradient for Na+ would tend to move Na+ __________.
out; in If the inside of the cell is positively charged compared to the outside, the electrical gradient will favor movement of positively charged sodium ions out of the cell.
Depolarization of the neuron occurs when ______.
positively charged sodium ions rush into the neuron
Which of the following choices best represents synaptic transmission?
presynaptic axon to synapse to dendrite or postsynaptic cell body Synaptic transmission uses a synapse and, therefore, two cells. Incoming messages are dendritic or somatic; outgoing messages are axonal.
In a synapse, neurotransmitters are stored in vesicles located in the __________.
presynaptic neuron
You are working on a new medication to selectively kill cancer cells. You test the drug on a culture of normal neurons to check for any side effects. Despite the presence of the drug, a normal resting membrane potential of −70 mV is maintained. Look at each of the figures below to analyze how the drug might be affecting the neuron. Figure A shows neuron activity under normal, or control, conditions (without the drug). Figure B shows neuron activity that has been altered due to exposure to the drug. Based on these results, the drug is most likely __________.
preventing Na+-gated channels from opening
In addition to diffusion, what are two other mechanisms that terminate neurotransmitter activity?
reuptake and degradation
The node-to-node "jumping" regeneration of an action potential along a myelinated axon is called __________.
saltatory conduction
Hyperpolarization results from __________.
slow closing of voltage-gated K+ channels
Transmission at the chemical synapse (i.e., release of neurotransmitters from the presynaptic axon and binding at the postsynaptic) occurs in response to an action potential. This action potential propagates down the length of the axon terminal as a spreading wave of depolarization from the axon hillock onward. This wave results from small, consecutive steps of Na+ movement from the extracellular fluid into the cytoplasm of the axon, which makes the membrane potential more positive. This change in membrane potential is called depolarization. Which of the following best characterizes depolarization?
small, consecutive steps of Na+ penetration into the axon along its length
The small space between the sending neuron and the receiving neuron is the
synaptic cleft.
Imagine you changed the concentration of K+ outside a neuron such that the resting membrane potential changed to -80 mV (from the normal resting value of -70 mV). What have you changed?
the electrical gradient for K+ and the concentration gradient for K+
An action potential in one segment of axon causes adjacent sections of axon membrane to reach threshold through what mechanism?
the generation of local currents
The repolarization phase of an action potential results from __________.
the opening of voltage-gated K+ channels
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. Submit
During an action potential, the rapid decrease in sodium permeability and simultaneous increase in potassium permeability is responsible for __________.
the repolarization phase
What is the electrochemical gradient of an ion?
the sum of the electrical and concentration gradients for that ion An ion's electrochemical gradient consists of two parts. The first part, the electrical gradient, is a consequence of the membrane potential. For negative membrane potential values, positively-charged ions (like K+ and Na+) tend to enter the cell. The second part, the concentration gradient, is the tendency of ions to diffuse from a region of high concentration to a region of low concentration. The electrochemical gradient determines the net movement of ions across the plasma membrane.
When calcium ions enter the synaptic terminal,
they cause vesicles containing neurotransmitter molecules to fuse to the plasma membrane of the sending neuron.
Which choice best characterizes K+ leakage channels?
transmembrane protein channels that are always open to allow K+ to cross the membrane without the additional input of energy
An action potential releases neurotransmitter from a neuron by opening which of the following channels?
voltage-gated Ca2+ channels
The depolarization phase of an action potential results from the opening of which channels?
voltage-gated Na+ channels
The generation of an action potential in a neuron requires the presence what type of membrane channels?
voltage-gated channels
