PhysioEx Exercise 3- neurophysiology of Nerve Impulses

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Activities 1-5

What is the approximate concentration of K+ inside a typical cell?

150 mM

What is the approximate concentration of K+ outside a typical cell?

5 mM

What is the approximate concentration of Na+ inside a typical cell?

5 mM

What is the approximate concentration of Na+ outside a typical cell?

150 mM

A negative membrane potential was recorded when the tip of the micro electrode was inserted where?

Both inside the cell body and inside the axon.

What could possibly cause a change in membrane potential from -70 to -40 mV in the cell body?

An increase in extracellular K+.

What kind of voltage would you expect from the following scenario: the area between the inside of the axon and the outside of the axon with control K+ ECF?

A very negative voltage.

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.

Explain why increasing 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 or less negative.

Explain why a change in extracellular Na+ did not 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.

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.

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

Predict what will happen to the resting membrane potential if the extracellular K+ concentration is increased.

The resting membrane potential will become more negative.

What is the polarity of the resting membrane potential (voltage)?

Negative.

What does it mean that the voltage just inside the membrane is negative?

There are more negative charges than positive charges just inside the membrane.

The membrane of most cells, including neurons, contains passive, open,
K+ leak channels. Given the normal K+ concentrations and the resultant concentration gradient, which direction would K+ be expected to move (diffuse) through these leak channels?

The K+ would be expected to move out of the cell through these leak channels.

What effect does increasing extracellular K+ have on the net diffusion of K+ out of the cell?

It decreases the net diffusion of K+ .

Which way would Na+ move across the membrane if there were open Na+ channels?

Na+ would diffuse into the cell.

The membrane has open K+ channels, and changing extracellular K+ concentration results in a change in membrane potential. Changing the extracellular Na+ concentration does not significantly change the membrane potential. What do your results suggest about the number or state (open or closed) of Na+ channels in the resting membrane of a neuron?

Na+ channels are mostly closed.

Assuming that the resting potential of a sensory neuron is -70 mV, which of the following represents a depolarization?

A change to -60 mV, or anything less negative to -70 mV.

Which of the following is a sensory modality (type of sense)?

Touch, smell, sight, and pain are all sensory modalities.

Which of the following is a sensory stimulus?

Pressure, chemical odorants, and light are all considered to be sensory stimuli.

Which of the following is true of the response of a sensory neuron to the appropriate sensory stimulus?

It can be called a receptor potential, and it is a graded potential. Also, it is a change in the resting membrane potential.

A very intense stimulus can sometimes stimulate sensory neurons that have evolved for a different modality. Thus, with a blow to the eye, one "sees stars." In this example the photoreceptors in the eye are responding to what?

Intense pressure.

Olfactory receptor neurons respond to low concentrations of chemical odorants because there are membrane proteins in the receptor ending of this sensory neuron that can do what?

They can bind and respond to the specific odorant.

The sequence of events starting with a sensory stimulus and ending with a change in membrane potential is called what?

A sensory transduction.

Starting at a resting membrane potential of -70 mV, a change to which of the following represents the largest receptor potential?

A change to -50 mV, or anything that is even less negative.

Sensory neurons have a resting membrane potential based on the efflux of K+ 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?

The efflux of K+ ions is maintained by passive K+ channels.

What is meant by the term graded potential?

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

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

The moderate intensity pressure modality would induce a receptor potential of the largest magnitude in the pacinian corpuscle.

Identify which of the stimulus modalities would induce the largest amplitude receptor potential in the olfactory receptors.

The moderate intensity chemical modality would induce a receptor potential of the largest magnitude in the olfactory receptors.

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 this receptor protein because they did not respond to chemical stimuli in activity 2.

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

Photosensory neurons would likely respond to a green light.

The adequate stimulus for a Pacinian corpuscle is pressure or vibration on the skin. Which of the following modalities will induce the largest amplitude receptor potential in the Pacinian corpuscle?

Moderate-Intensity Pressure.

The adequate stimuli for olfactory receptors are chemicals, typically odorant molecules. Which of the following modalities will induce the largest amplitude receptor potential in the olfactory receptor?

Moderate-Intensity Chemical.

Why didn't the Pacinian corpuscle respond to high-intensity light?

Light-transducing proteins are not present in the Pacinian corpuscle.

Why did the free nerve ending respond to several different modalities?

The sensory end of this nerve is less specialized.

Axons can be defined as what?

Long, thin structures that extend from a neuronal cell body.

What kind of action potential recording is the easiest?

Extracellular recordings of the action potential.

An action potential is usually initiated in an axon at or near what?

The axon hillock, the initial segment, and the trigger zone.

The initiation of an action potential in a sensory neuron in the body normally is what happens after what?

Follows a sufficiently large depolarizing receptor potential.

The threshold voltage in an axon is usually what?

Less negative than the resting membrane potential.

If a graded receptor potential made the resting membrane potential of the axon more negative (for example, -70 mV changes to -75 mV), you would expect what to happen?

It to be more difficult for this axon to reach the threshold voltage.

Failure to reach the threshold voltage in the axon of a sensory neuron could be caused by what?

The generation of a receptor potential that makes the makes the axonal resting membrane potential less negative, an insufficient depolarizing potential, or the application of the wrong sensory modality.

How will the action potential at R1 (or R2) change as you continue to increase the stimulus voltage?

The peak value of the action potential will increase.

Why is the action potential recorded by the second recording electrode (R2) delayed relative to the action potential recorded by the first recording electrode (R1)?

The action potential had to propagate from R1 to R2.

An increase in extracellular K+ would depolarize a neuron. This depolarization would occur if neurons were damaged. From what you have just learned about generating an action potential, what effect would this have on nearby axons? The nearby axonal membranes will ___________.

They would be depolarized to values near or above threshold voltages.

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.

What change in membrane potential (depolarization or hyper polarization) triggers an action potential?

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

How did the action potential at R1 (or at 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 the threshold is met, the even is all or none, not graded.

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

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

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.

Voltage-gated Na+ channels are membrane channels that open when?

When the membrane depolarizes.

When open, Na+ channels allow what to happen?

Na+ ions to diffuse into the cell.

Which of the following is true of an action potential?

The generation of an action potential uses voltage-gated Na+ channels, an action potential is generated when the membrane potential reaches threshold, and an action potential is a large all-or-nothing change in membrane potential.

Which of the following can reduce the likelihood of an action potential?

TTX, lidocaine, and increasing the K+ leak from a cell.

In the control, the amplitudes of the action potentials at R1 and R2 are the same. Which of the following explains this?

There are voltage-gated Na+ channels all along the axon, the action potential is an all-or-nothing event, and also action potentials propagate or remake themselves at each point along the axon.

Blocking the voltage-gated Na+ channels between R1 and R2 with TTX blocks what?

The propagation of the action potential from R1 to R2.

When voltage-gated Na+ channels between R1 and R2 are blocked with TTX, an action potential is still recorded at R1 because?

The voltage-gated Na+ channels between the stimulus and R1 are unaffected by the TTX.

Puffer fish must be prepared carefully and properly before they can be eaten. Eating puffer fish can cause numbness of the lips, probably because?

Action potentials from sensory neurons in the lips are blocked.

With a slower timescale, the appearance of the action potentials generated at R1 and R2 will appear to do what?

To be compressed in time but have the same peak value of response.

Why do you think TTX is not used during dental procedures?

TTX irreversibly blocks voltage-gated sodium channels in axonal membranes.

What does TTX do to voltage-gated Na+ channels?

TTX blocks the diffusion of Na+ through voltage-gated Na+ channels. This blockage is irreversible.

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 Na+ through voltage-gated Na+ channels. The difference between TTX and lidocaine is that lidocaine's effect is reversible.

A nerve is 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.

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?

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

Which of the following occurs after the peak of the action potential?

The membrane repolarizes, voltage-gated K+ channels open, and some voltage-gated Na+ channels inactivate.

What is meant by Na+ channel inactivation?

The Na+ channel no longer allows Na+ ions to pass through it.

What happens when voltage-gated K+ channels open?

K+ ions can diffuse out of the cell, the membrane repolarizes from the peak of the action potential, and it becomes harder for a subsequent stimulus to depolarize the membrane to threshold.

It is harder to generate a second action potential soon after the first action potential because?

Some voltage-gated Na+ channels are inactivated, voltage-gated K+ channels have opened, and the efflux of K+ ions opposes a depolarization toward threshold.

What is the original threshold for this neuron? (Activity 5)

A 20 mV depolarization to -50 mV.

As the interval between stimuli decreases, the depolarization needed to generate the second action potential...?

Increases.

Judging from your results, what time period after the first action potential best describes the relative refractory period (the time when a second action potential can be generated only if the stimulus intensity is increased)? [Activity 5]

7.5 ms-60 msec

At what interval between stimuli did the second action potential fail, regardless of the stimulus intensity?

3.75 msec

What is the absolute refractory period for this neuron?

3.75 msec

If you further decrease the interval between the stimuli, will the threshold for the second action potential change?

The threshold for the second action potential will be higher (requiring a larger depolarization).

Threshold can be defined as the minimum voltage needed to generate an action potential. Is the threshold for the first action potential the same as, or different from, the threshold for the second action potential with a 60 msec interval?

The threshold for the first action potential is lower than the threshold for the second action potential.

Define inactivation as it applies to a voltage-gated Na+ channel.

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

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.

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

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

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