Physiology Exercise 3
Activity 1
1. Explain why increasing extracellular K+ reduces the net diffusion of K+ out of the neuron through the K+ leak channels. Your answer: By increasing the extrecellular K+ the net diffiusion of K+ out the neuron through the eak channels is reduced. This is caused by the flow of ions following the concentration gradient of traveling from an area of high concentration to areas of low concentration. The higher concentration gradient would result in a reduced diffusions of K+ ions out of the neuron via the leak channels. 2. Explain why increasing extracellular K+ causes the membrane potential to change to a less negative value. How well did the results compare with your prediction? Your answer: By increasing the extracellular K+ the membrane potential will change to be a less negative value. By adding more extracellular K+ the intracellular K+ will decrease and cause the membrane potential to become less negative. This result in a increased postitve membrane was what we predicted during the experiment. 3. Explain why a change in extracellular Na+ did not significantly alter the membrane potential in the resting neuron? Your answer: A change in the extracellular Na+ did not significantly alter the membrane potential in the resting neuron because of the channels.
There are fewer sodium leak channels than compared to potassium leak channels and therefore the Na+ did not alter the membrane potential significantly. 4. Discuss the relative permeability of the membrane to Na+ and K+ in a resting neuron. Your answer: The permeability of the membrane to Na+ and K+ in a resting neuron is different because of the varying number of leak channels. A cell has more potassium leak channels and has a higher permeability compared to the fewer sodium channels resulting in a lower relative permeability. 5. Discuss how a change in Na+ or K+ conductance would affect the resting membrane potential. Your answer: The cells resting membrane potential is affected by a change in Na+ or K+ conductance. Conductance change would result in a change of the concentration gradient for the two ions, resulting in the ions flowing either into or out of the cell. This flow would result in a change in the cells resting membrane potential. Activity 2
1.Sensory neurons have a resting potential based on the efflux of potassium ions (as demonstrated in Activity 1). What passive channels are likely found in the membrane of the olfactory receptor, in the membrane of the Pacinian corpuscle, and in the membrane of the free nerve ending? Your answer: Passive channels that are likely to be found in the membrane of both the olfactory receptor and the free nerve ending would be Na+ ion channels. In the Pacinian Corpuscle it is most likely that K+ leak channels would be found.
2.What is meant by the term graded potential? Your answer: Graded potential refers to the changes in the membrane potential from stimulus intensity. The stimulus will result in the increase of the receptor potential which will eventually decrease again. These graded potentials do not usually refer to potassium channels or voltage-gated sodium channels.
3.Identify which of the stimulus modalities induced the largest amplitude receptor potential in the Pacinian corpuscle. How well did the results compare with your prediction? Your answer: The stiumuls that induced the largest amplitude receptor potential in the Pacinian corpuscle was the high-intensity pressure. This stimulus resulted in a peak value of -30mV and an amplitude of 40mV. Although I selected moderate-intesity pressure, which was not the highest amplitude value but second highest, this value was still higher than stimulus caused by chemicals, heat or light which were the other answers in the question.
4.Identify which of the stimulus modalities induced the largest amplitude receptor potential in the olfactory receptors. How well did the results compare with your prediction? Your answer: The stimulus that induced the largest amplitude receptor potential in the olfactory receptors was the high-intensity chemical stimulus. This resulted in a peak value of -45mV and an amplitude value of 25mV. I predicted that the moderate-intensity chemical would cause the highest amplitude value because high-intensity was not listed as a choice. Chemical stimulus over-all caused higher values than pressure, heat or light stimulus.
5.The olfactory receptor also contains a membrane protein that recognizes isoamylacetate and, via several other molecules, transduces the odor stimulus into a receptor potential. Does the Pacinian corpuscle likely have this isoamylacetate receptor protein? Does the free nerve ending likely have this isoamylacetate receptor protein? Your answer: Both the Pacinian corpuscle and the free nerve ending are both lacking the isoamylacetate receptor protein. This protein caused the olfactory receptor to respond to the chemical stimulus. Since neither the Pacinian corpuscle or the free nerve endings had any response to the chemical receptor we can conclude that they are lacking this receptor protein. 6.What type of sensory neuron would likely respond to the green light? Your answer: It is most likely that the free nerve ending would respond to the green light. Since the free nerve endings are not as specialized as the olfactory receptor or the Pacinian corpuscle it was able to respond to more stimulus modality that the others. Activity 3
1.Define the term threshold as it applies to an action potential. Your answer: The term threshold refers to a transmembrane potential. This applies to action potential being this is the starting point where action potential begins.
2.What change in membrane potential (depolarization or hyperpolarization) triggers an action potential? Your answer: A change in membrane potential will result in an action potential, this is triggered by the depolarization of the membrane potential which is making it less negative.
3.How did the action potential at R1 (or R2) change as you increased the stimulus voltage above the threshold voltage? How well did the results compare with your prediction? Your answer: The action potential at R1 or at R2 did not change as the stimulus voltage was increased above the threshold voltage. These results compare with my prediction that there would be no change in potential.
4.An action potential is an “all-or-nothing” event. Explain what is meant by this phrase Your answer: When a membrane is brought to the threshold by a stimuli the action potentials that are generated are always identical, as shown in this experiment. The action potential occuring is not dependent on the strength of the stimuli, meaning that the action either has the ability to occur as a result of depolarization or it does not. This result has lead to the phrase of an action potential as being “all-or-nothing”.
5.What part of a neuron was investigated in this activity? Your answer: Through this activity the axon was investigated by stimulating various voltage to then view the resulting action potential. Activity 4
1. What does TTX do to voltage-gated Na+ channels? Your answer: TTX is known as a receptor antagonist and blocks action potentials from occuring by
binding to the voltage-gated Na+ channels. This blocking prevents other nerve cells from firing off and stops the process from occuring.
2. What does lidocaine do to voltage-gated Na+ channels? How does the effect of lidocaine differ from the effect of TTX? Your answer: Lidocaine also binds to the voltage-gated Na+ channels in the channels open state as well as its inactive state. However, lidocaine is unable to bind in the resting state of the channels which is why it is used as an anesthetic in dental surgeries. TTX on the other hand irreversibly binds to these voltage-gates Na+ channels.
3.A nerve is a bundle of axons, and some nerves are less sensitive to lidocaine. If a nerve, rather than an axon, had been used in the lidocaine experiment, the responses recorded at R1 and R2 would be the sum of all the action potentials (called a compound action potential). Would the response at R2 after lidocaine application necessarily be zero? Why or why not? Your answer: No, the R2 response after the application of lidocaine would not necessarily be zero. In the experiment the peak values at both 8 seconds and 10 seconds was 0 but there were peak values at 2 seconds, 4 seconds and 6 seconds. Since we would be looking at the sum of all the action potentials it would therefore not be zero.
4.Why are fewer action potentials recorded at recording electrodes R2 when TTX is applied between R1 and R2? How well did the results compare with your prediction? Your answer: When TTX is applied between the R1 and R2 areas the result is fewer action potentials at R2. This is because the TTX binds and blocks the spreading of the signal to the R2 region. This was the predicted outcome during the experiment.
5.Why are fewer action potentials recorded at recording electrodes R2 when lidocaine is applied between R1 and R2? How well did the results compare with your prediction? Your answer: Similar to the effect TTX had on R1 and R2, when lidocaine was apllied between the R1 and R2 region fewer action potentials ocured at R2. This is also due to the binding that lidocaine does to the voltage-gated sodium channels. This result was also the predicted outcome during the experiment. 6.Pain-sensitive neurons (called nociceptors) conduct action potentials from the skin or teeth to sites in the brain involved in pain perception. Where should a dentist inject the lidocaine to block pain perception? Your answer: During a dental procedure a dentist should inject lidocaine into specific nerves in the mouth. These regions would depend on where the pain of the
procedure will occur and where the nociceptors are located. Activity 5
1.Define inactivation as it applies to a voltage-gated sodium channel. Your answer: Inactivation in a voltage-gated sodium channel refers to when the channel is closed. This idea is similar to a plug in the channel, stopping the flow of Na+ into the channels. This stop of inflow also stops the rise of the membrane potential. The membrane potential then decreases back to its resting potential.
2.Define the absolute refractory period. Your answer: Refractory period is the time that immediately follows stimulation of the nerve. In this time the nerve is unable to respond to any further stimulation, the membrane then rests and is able to undergo a second stimulus after its resting period.
3.How did the threshold for the second action potential change as you further decreased the interval between the stimuli? How well did the results compare with your prediction? Your answer: The threshold for the second action potential increased when the interval time between the stimuli was decreased. This was not what was predicted in the experiment, instead I predicted that the threshold would not change even if the intervals were decreased.
4.Why is it harder to generate a second action potential during the relative refractory period? Your answer: During the refractory period the neuron is more negative, or hyperpolarized, than it normally is. This extra negativing makes the graded potential more difficult to reach and thus harder to generate a second action while in this refractory period. The only way a second action would be possible is through an unusually strong signal which, would be able to overcome the extra negativity. Activity 6
1.Why are multiple action potentials generated in response to a long stimulus that is above threshold? Your answer: Multiple action potentials can be generates if there is a longer stimulus which is above the threshold. Having a longer stimuli allows time for recovery and being above the threshold allows the action potential to be capable of occuring after the relative refractory period.
2.Why does the frequency of action potentials increase when the stimulus intensity increases? How well did the results compare with your prediction? Your answer: When the stimulus intensity increases the frequency of action potentials also increase. Since the intensity is increasing there is more time allowed for the neuron to generate the action potential and recover and then generate a second action potential and so on.
3.How does threshold change during the relative refractory period? Your answer: The threshold changes during the relative refractory period by increasing. It is inn this time that a second action potential is able to be produced if the intensity of the stimuli has been increased. 4.What is the relationship between the interspike interval and the frequency of action potentials? Your answer: The frequency of the action potentials has a relationship to the interspike interval, it is its reciprocal. There is also a conversion from milliseconds to seconds as well.
Activity 7
1.How did the conduction velocity in the B fiber compare with that in the A Fiber? How well did the results compare with your prediction? Your answer: The conduction velocity of the B fiber was slower than that of the A fiber, comparing 10m/sec to 50m/sec. Since the B fiber has a smaller diameter this outcome was predicted during the experiment.
2.How did the conduction velocity in the C fiber compare with that in the B Fiber? How well did the results compare with your prediction? Your answer: The conduction velocity in the C fiber was slower than the B fiber, comparing the 1m/sec to 10m/sec. This was not predicted in the experiment, instead I predicted that there would be the same conduction velocity.
3.What is the effect of axon diameter on conduction velocity? Your answer: The axon diameter has an effect on conduction velocity because with an increase in diameter will result in an increase in the conduction velocity.
4.What is the effect of the amount of myelination on conduction velocity? Your answer: Myelination also has an effect on the conduction velocity. Having more myelination results in having a faster velocity as shown through this experiment.
5.Why did the time between the stimulation and the action potential at R1 differ for each axon? Your answer: The time between the stimulation and the action potential at R1 differ for each axon as a result of the amount of myelination. In this experiment the the 3 axons had either heavy, light or no myelination.
6.Why did you need to change the timescale on the oscilloscope for each axon? Your answer: In the experiement the timescale on the oscilloscope of each axon was changed so that the action potentials could be seen. Since the conduction velocity was being altered to become slower a time scale that was
longer was required.
Activity 8
1.When the stimulus intensity is increased, what changes: the number of synaptic vesicles released or the amount of neurotransmitter per vesicle? Your answer: When the stimulus is increased the number of synaptic vesicles released is increased with the intensity.
2.What happened to the amount of neurotransmitter release when you switched from the control extracellular fluid to the extracellular fluid with no Ca2+ ? How well did the results compare with your prediction? Your answer: When the amount of neurotransmitter released switched from the control extracellular fluid to no Ca2+ fluid no neurotransmitters were released. This occured because the exocytosis is Ca2+ dependent. This was predicted during the experiment. 3.What happened to the amount of neurotransmitter release when you switched from the extracellular fluid with no Ca2+ to the extracellular fluid with low Ca2+ ? How well did the results compare with your prediction? Your answer: When the amount of neurotransmitters released switched from the no Ca2+ to the fluid containing low Ca+ the amount increased slightly. This result was predicted during the experiment.
4. How did neurotransmitter release in the Mg2+ extracellular fluid compare to that in the control extracellular fluid? How well did the result compare with your prediction? Your answer: When using the Mg2+ fluid an amount of neurotransmitters was released, this amound being less than that of the control fluid. This result was predicted during the experiment. 5. How does Mg2+ block the effect of extracellular calcium on neurotransmitter release? Your answer: Mg2+ blocks the effect of extracellular calcium on neurotransmitter release because the added Mg will become oxidized. This results in Mg becoming the reducing agent, which Ca normally is. Activity 9
1.Why is the resting membrane potential the same value in both the sensory neuron and the interneuron? Your answer: The membrane potential is the same for both the sensory neuron and the interneuron because they both are at resting potential. Since they are at resting potential there is nothing to stimulate, therefore they have the same value.
2.Describe what happened when you applied a very weak stimulus to the sensory receptor. How well did the results compare with your prediction? Your answer: When a weak stimulus was applied to the sensory receptor responses occured at one of the locations. This was not what was predicted because only R1 responded while R2, R3 and R4 did not.
3.Describe what happened when you applied a moderate stimulus was to the sensory receptor. How well did the results compare with your prediction? Your answer: When a moderate stimulus was applied to the sensory recepto all 4 of the locations had a response, with R1 and R3 having the largest graded potential. However during the experiment it was predicted that a response would only occur at R1 and R2 with action potentials at R3 and R4.
4.Identify the type of membrane potential (graded receptor potential or action potential) that occurred at R1, R2, R3, and R4 when you applied a moderate stimulus (view Experiment Results to view the response to this stimulus). Your answer: When a moderate stimulus was applied a graded receptor potential occured at R1, R2, R3 and R4. 5.Describe what happened when you applied a strong stimulus to the sensory receptor. How well did the results compare with your prediction? Your answer: When a strong stimulus was applied to the sensory receptor action potentials were generated at all 4 of the locations. However, during the experiment it was predicted that this would occur at R2 and R4 but a depolarizing response would occur at R1 and R3.
