Neurophysiology of Nerve Impulses. Physioex 3

1. Explain why increasing extracellular K+ reduces the net diffusion of K+ out of the neuron through the K+ leak channels.

Increasing the extracellular K+ reduces the steepness of the concentration gradient and so less K+ diffuses out of the neuron.

2. Explain why increasing the extracellular K+ causes the membrane potential to change to a less negative value.

The membrane potential became less negative because less K+ diffused out. If more K+ stays in, it is more positive of less negative.

3. Explain why a change in extracellular Na+ did not significantly alter the membrane potential in the resting neuron?

There are less Na+ leakage channels than K+ leakage channels, and more K+ leakage channels are open.

4. Discuss the relative permeability of the membrane to Na+ and K+ in a resting neuron.

The resting neuron is approximately 4-5 times more permeable to K+ because of the increased number of K+ leakage channels.

5. Discuss how a change in Na+ or K+ conductance would affect the resting membrane potential.

A change in K+ conductance would have a greater effect of the resting membrane potential than a change in Na+ conductance would due to the greater number of K+ channels.

1. Sensory neurons have a resting potential based on the efflux of potassium ions. 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 endings?

The efflux of potassium is a key factor of resting potential, so there will be a potassium channel.

2. What is meant by the term graded potential?

Graded potentials are brief, localized changes in the membrane potential than can be either depolarizing or hyperpolarizing.

3. Identify which of the stimulus modalities induce the largest amplitude receptor potential in the Pacinian corpuscle.

The moderate intensity pressure modality induced a receptor potential of the largest amplitude in the Pacinian corpuscle.

4. Identify which of the stimulus modalities induced the largest-amplitude receptor potential in the olfactory receptors.

The moderate intensity chemical modality causes a receptor potential in the olfactory receptors.

5. The olfactory receptor also contains a membrane protein that recognizes isoamyl acetate and, via several other molecules, transduces the odor stimulus into a receptor potential. Does the Pacinian corpuscle likely have this isoamyl acetate receptor protein? Does the free nerve ending likely have this isoamyl acetate receptor protein?

The Pacinian corpuscle and the free nerve ending are not likely to have the isoamyl acetate receptor because they did not respond to chemical stimuli.

6. What type of sensory neuron would likely respond to a green light?

Photosensory neurons.

1. Define the term threshold as it applies to an action potential.

Threshold is the voltage that must be reached in order to generate an action potential.

2. What change in membrane potential (depolarization or hyperpolarization) triggers an action potential?

A depolarization in the membrane potential results in an action potential. The membrane potential must become less negative to generate an action potential.

3. How did the action potential at R1 (or R2) change as you increased the stimulus voltage above the threshold voltage?

The action potential didn’t change as the stimulus voltage increased. This is because once threshold is met, the event it all or none, not graded.

4. An action potential is an "all or nothing" event. Explain what is meant by this phrase.

This means that once threshold is met, an action potential occurs. If the stimulus is too small, an action potential does not occur.

5. What part of a neuron was investigated in this activity?

The trigger zone was investigated. This is where the axon hillock and the initial segment come together.

1. What does TTX do to voltage- gated Na channels?

TTX blocks the diffusion of sodium through the voltage-gated sodium channels.

2. What does lidocaine do to voltage-gated Na channels? How does the effect of lidocaine differ from the effect of TTX?

Lidocaine blocks the diffusion of sodium through the voltage-gated sodium 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?

With a compound action potential, the results would not necessarily be zero because some axons could remain unaffected.

4. Why are fewer action potentials recorded at R2 when TTX is applied between R1 and R2?

TTX blocked the sodium channels, preventing the propagation of the action potential from R1 to R2.

5. Why are fewer action potentials recorded at R2 when lidocaine is applied between R1 and R2?

Lidocaine blocked the sodium channels, preventing the propagation of the action potential from R1 to R2.

6. Pain-sensitive neurons (called nociceptiors) 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?

Lidocaine should be applied to the receptors to prevent the generation of an action potential that would lead to the perception of pain.

1. Define inactivation as it applies to voltage-gated sodium channel.

Voltage-gated sodium channels are inactivated when they no longer allow sodium to diffuse through.

2. Define the absolute refractory period.

The absolute refractory period is the time in which no action potential can be generated regardless of the strength of the stimulus.

3. How did the threshold for the second action potential change as you further decreased the interval between
the stimuli?

The threshold for the second action potential increased as the interval between the stimuli decreased as predicted.

4. Why is it harder to generate a second action potential during the relative refractory period?

A greater stimulus is required because voltage gated potassium channels that oppose depolarization are open during this time.

1. Why are multiple action potentials generated in response to a long stimulus that is above threshold?

The longer stimuli allow time for recovery and the above threshold allows the action potential to occur after the relative refractory period.

2. Why does the frequency of action potentials increase when the stimulus intensity increases?

Action potential can occur more frequently if there is a constant source of stimulation as long as the relative refractory period is reached.

3. How does threshold change during the relative refractory period?

The threshold that must be achieved is higher than the original stimulus intensity during the relative refractory period.

4. What is the relationship between the interspike interval and the frequency of action potentials?

The frequency of the action potentials is the reciprocal of the interspike interval with a conversion from milliseconds to seconds.

1. How did the conduction velocity in the B fiber compare with that in the A fiber?

The velocity of the B fiber was slower because it had a smaller diameter and less myelinated.

2. How did the conduction velocity in the C fiber compare with that in the B fiber?

The conduction velocity of the C fiber was slower because it has no myelination and a smaller diameter.

3. What is the effect of axon diameter on conduction velocity?

The larger the axon diameter, the greater the conduction velocity.

4. What is the effect of the amount of myelination on conduction velocity?

The greater the myelination, the greater the conduction velocity.

5. Why did the time between the stimulation and the action potential at R1 differ for each axon?

The time between the stimulation and the action potential at R1 differed for each axon because the diameter and the degree of myelination varied.

6. Why did you need to change the timescale on the oscilloscope for each axon?

This is necessary in order to see the action potentials. The velocity changes so when it get very slow you need a longer time scale.

1. When the stimulus intensity is increased, what changes: the number of synaptic vesicles released or the
amount of neurotransmitter per vesicle?

The number of synaptic vesicles released increases when the stimulus intensity increases.

2. What happened to the amount of neurotransmitter release when you switched from the control extracellular
fluid to the extracellular fluid with no Ca21?

Without calcium present, no neurotransmitter was released because the exocytosis of the synaptic vesicles is dependent upon calcium.

3. What happened to the amount of neurotransmitter release when you switched from the extracellular fluid
with no Ca21 to the extracellular fluid with low Ca21?

When a small amount of calcium is added back, a small amount of synaptic vesicles are released.

4. How did neurotransmitter release in the Mg2+ extracellular fluid compare to that in the control extracellular fluid?

The neurotransmitter release was less when magnesium was added.

5. How does Mg21 block the effect of extracellular calcium on neurotransmitter release?

When magnesium is added to the extracellular fluid it blocks the calcium channels and inhibits the release of neurotransmitter.

1. Why is the resting membrane potential the same value in both the sensory neuron and the interneuron?

The resting membrane potential is the same value because this is the typical resting membrane potential regardless of the type of neuron.

2. Describe what happened when you applied a very weak stimulus to the sensory receptor.

When you applied a very weak stimulus to the sensory receptor, a small, depolarizing response occurred at R1, and no responses occurred at R2, R3, and R4.

3. Describe what happened when you applied a moderate stimulus to the sensory receptor.

When you applied a moderate stimulus to the sensory receptor, a larger, depolarizing response occurred at R1, and an action potential was generated at R2 and at 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 the response to the stimulus.)

Action potentials occurred at R2 and R4 and graded receptor potentials occurred at R1 and R3.

5. Describe what happened when you applied a strong stimulus to the sensory receptor.

When you applied a strong stimulus to the sensory receptor, a large, depolarizing response occurred at R1 and R3, and action potentials occurred at R2 and R4.