LAB EXAM: Nerve Action Potential

Question 1

When studying the sciatic nerve and placing stimulating electrodes in the middle of the nerve, how can you tell that there is an impulse travelling in both directions? Why is this happening?

Answer 1

There are two spikes on either side of the artifact, indicating that an impulse is being propagated both up and down the axon.

This is NOT due to the fact that the sciatic nerve contains both afferent and efferent axons, which carry signals different ways, both to and from the CNS. The sciatic nerve only contains axons, which do not know if they are efferent or afferent or not because they have no cell bodies.

The reason why they are travelling in multiple directions is because when an axon is stimulated in the middle, neither proximal or distal portions of the axon were in refractory (like they would be if the AP was started at the axon hillock)

Question 2

Explain the difference between orthodromic and antidromic conduction

Answer 2

orthodromic conduction propagates an electrical nerve impulse the most normal way towards the axon terminal from the cell body

antidromic conduction is the opposite of orthodromic, and actually propagates the action potential towards the cell body. this can happen if the axon is initially stimulated in the middle, meaning the previous nodes are not in refractory period.

Question 3

What happens in vivo to ensure that nerve conduction remains only “one way”?

Answer 3

There are refractory periods that occur that prevent the back flow of action potentials. Also, synapses only move one direction.

Question 4

Would you expect the same results of ortho and antidromic conduction with a singly axon across the nerve chamber?

Answer 4

no, a single axon has mechanisms to prevent the back flow of propagation; there will be no antidromic conduction IN VIVO

Question 5

The _____ represents
the time when a new action potential cannot be initiated by normal events, in a region that has just
undergone an action potential.

Answer 5

refractory period

Question 6

During an experiment, how would you determine the Nerve Trunk Refractory period?

Answer 6

when you stimulate a nerve at various intervals, letting less and less time go by before eliciting another sitmulation. At the interval where the SECOND CAP disappears, that thee nerve trunk refractory period. The stimulations are too close together and the axon is still in refractory period when the second stimulation happens, which is why no CAP is elicited.

Question 7

What is the Nerve Trunk Refractory Period?

Answer 7

the time at which all the neurons in the sciatic nerve trunk are refractory.

ALL Individual axons are in their absolute OR relative refractory periods and therefore are unresponsive to a second stimulus, even if it is as strong as the first.

Question 8

What is the Total Refractory Period?

Answer 8

the interval where the second CAP becomes as large as the first. The time interval when the CAPS are the SAME SIZE.

Question 9

What happens to the second CAP size when the stimulation interval is in between nerve trunk and total refractory period?

Answer 9

Between the nerve trunk and total refractory
periods, the CAP is reduced in size because all of the many neurons which make up the nerve are not at the same membrane potential.

Again, some are still in their individual absolute or relative
refractory period and therefore are unresponsive to a second stimulus

. As the delay between the
twin pulses INCREASES, the second CAP increases in size because increasing numbers of neurons become responsive to the second stimulus

Question 10

The ___ ___ ___ represents the MINIMUM time required for all the neurons within the nerve to return to their respective resting membrane potentials.

Answer 10

Total refractory period

Question 11

How could you determine if some of the individuals neurons are in their relative refractory period?

Answer 11

the second CAP would be smaller than the first cap, but would still be visible. there would be less neurons firing at the second stimulation because they are in a refractory period, resulting in a smaller amplitude.

Question 12

Relationship between pulse duration and amplitude voltage in order to maintain CAP height

Answer 12

as the duration of electrical stimulation increases, less voltage is needed in order to sustain a CAP of the same magnitude.

Question 13

What is rheobase?

Answer 13

the minimum strength needed OF CONSTANT DURATION that results in the depolarization threshold of the cell membranes being reached

ex/ the rheobase was determined to be around 465mV, which means a CAP will always be sustained as long as the duration is long enough.

Question 14

the strength of stimulus and duration it takes to achieve CAPS of the same amplitude is ____ related

Answer 14

INVERSELY related. as the strength of the applied current increases, the time required to stimulate the membrane decreases (and vice versa) to maintain a constant effect.

Question 15

Chronaxie

Answer 15

the minimum TIME required for an electric current that is double the strength of the rheobase to stimulated a neuron or muscle.

Question 16

Chronaxis is dependent on the density of ___ ___ ___ Channels in the cell, which affect the cells excitability.

Answer 16

chronaxie is dependent on the density of voltage-gated sodium channels in the cell, which affect that cell’s excitability.

Question 17

How does Chronaxie differ between fast and slow axons?

Answer 17

Chronaxie varies across different types of tissue: fast axons (large diameter) have LOWER chronaxie, slow neurons have a HIGHER one, meaning that it takes MORE TIME in order to elicit a CAP at double the rheobase voltage

Question 18

what is an “excitation time”?

Answer 18

the minimum duration of a stimulus that elicits a CAP

Question 19

What is an ectotherm

Answer 19

they depend on external sources to maintain their body temperature and hence the activity of their tissue varies with the environmental temperature.

Question 20

Explain the effects of temperature on nerve impulse conduction:

How does temp affect the time to start an AP?

How does temp affect the amplitude of the CAP?

How does temp affect the duration of the CAP?

Would a change in conduction velocity alter the width and amplitude of the action potential?

Answer 20

temp and starting time:
colder temperature results in the the CAP being elicited further away from the artifact because the conduction velocity has DECREASED. Therefore, it takes longer for a CAP to be seen.

Temp and amplitude:
colder temperatures elict smaller caps because there is a possibility that the sodium channels in some of the axons are slower to open, resulting in them not firing. There are less axons firing, resulting in a smaller amplitude.

Temp and Duration:
CAPs in colder temperatures are LONGER because it takes longer for the sodium channels to open in all the axons, resulting in a drawn out appearance. The CONDUCTION VELOCITY is also slower, resulting in axons taking longer to fire and thus widening the CAP at a certain area.

At hot temperatuers, the conduction velocity is Quicker due to sodium channels opening faster. these result in shorter-duration CAPS, but the amplitude is larger because all the axons are firing at the SAME TIME, rather than some firing earlier than others in cold temperatures (due to sluggish sodium channesls and slow conduction velocity, which widen the cap)

Question 21

what is a nerve

Answer 21

a collection of axons. does not contain cell bodies or dendrites

Question 22

excitable cells

Answer 22

repond to simuli by changing their membrane potential via ion permeability fluctuations and transmitting electrical signals as a means for communication.

Question 23

How must individual axons be measured to study action potentials?

Answer 23

intracellular electrodes.

Question 24

What is a nerve trunk?

Answer 24

a collection of axons that are both afferent and efferent nerve fibers that vary in diameter and degrees of myelination.

Question 25

difference in myelination cause variations in excitability and ____ ___ f

Answer 25

conduction velocity for individual neurons

Question 26

What is a cap and how is it measured

Answer 26

a recording of MULTIPLE APs from different axons within a nerve via EXTRACELLULAR ELECTRODES.

Question 27

the sciatic nerve of a frog is a ___ ___ that consists of many sensory and motor nerve fibers.

Answer 27

nerve trunk

Question 28

the shape of the CAP depends on the ____ of fibers of each type in the area of the nerve trunk that is being stimulated, as well as their _____ and degree of myelination.

Answer 28

the shape of the CAP depends on the NUMBER of fibers of each type (AFFERENT AND EFFERENT) in the area of the nerve trunk that is being stimulated, as well as their DIAMETER and degree of myelination.

Question 29

axons are classified as __ ___or ____ depending on their dimater and degree of myelination

Answer 29

slow, fast, or intermediate

Question 30

____ solution is used to provide electrolytes and keep a biological nerve moist

Answer 30

ringers

Question 31

why must you avoid touching a dissected nerve will metal?

Answer 31

to prevent its depolarization

Question 32

maximal voltage

Answer 32

the amount of voltage stimulus that needs to be applied to the nerve so that it no longer increases in amplitude. this indicates that all axons in the nerve locatinon are being stimulated.

Question 33

When extracellularly measuring a CAP:

As this wave crosses the first electrode, it becomes ____relative to the more distal electrode. This results in a ____ deflection in the recording. When this
wave reaches the second electrode, that electrode now becomes ____ with respect to the more
proximal electrode. This results in a _____ deflection in the recording

Answer 33

As the CAP wave crosses the first electrode, the extracellular probe becomes more NEGATIVE (because the inside of the membrane is positive, but it is not being measured) relative to the more distal electrode. this results in a POSITIVE deflection in the recording. when this wave reaches the second electrode, the second electrode now becomes NEGATIVE with respect to the more proximal electrode, resulting in a NEGATIVE deflection in the recording.

THIS NEGATIVE DEFLECTION IS NOT DUE TO HYPERPOLARIZATION in a CAP

Question 34

Why are CAPs important?

Answer 34

it is less invasive than measuring action potentials and thus can be used for its clinical applications. the patients nerve conduction velocity can be assessed with CAP detection.

Question 35

How are the CAPS we seen different from if you used an intracellular electrode to record an individual neuron?

Answer 35

the caps we saw are most likely bigger than if we were detecting an action potenital. this is due to the fact that caps are a summation of multiple APs and thus will have a large amplitude, and may vary in size. (the AP is all one size)

Both APs and CAPs are also BIPHASIC but for different reasons. APs exhibit a negative fluctuation because of a hyperpolarization that occurs after. CAPS exhibit a negative fluctuation because of the recording techniques: as the signal travels along the nerve trunk, the DISTAL extracellular electrode becomes more NEGATIVE compared to the proximal, resulting in the negative deflection.

Question 36

Action potentials are all-or-none. Why does the frog sciatic nerve give a graded response

Answer 36

An AP will fire at the same amplitude and intensity every time, but a CAP is a SUMMATION of a bunch of APs, and if all axons are simultaenously generating APs, then a larger CAP is witnessed.

Therefore, a frog nerve gives a graded response because it depends on all the axons that are in the nerve. The axons can vary in type AND number, and the conduction velocity can differ due to degrees of myelination and diameter, all of which contribute to a graded response in a nerve.

Varying axons may also have slightly different threshold voltages or different resting voltages, making some more likely to depolarize faster than others.

Question 37

Factors that influence conduction speed

Answer 37

1) axonal gigantism
2) myelination
3) temperature can speed things up as long as no denaturation occurs. There is an increase in ion channel pump and enzyme rates, allowing for shorter refractory periods and faster electrical conduction speed.

Question 38

Why does the Cap increase in size and duration with increasing stimulus strength?

Answer 38

the CAP is a SUM of all individual fivers of the nerve. as a stimulus increases, we recuirt MORE fibers and thus a LARER CAP is produced.

A nerve bundle is also composed of fibers of varying diameters: fast and slow fibers.

fast fibers have large diamter and contributes to APs that fall towards the start of the CAp, where as SLOW fibers have a small diameter and have a slower conduction velocity that fall towards the end of the CAP.

as stimulation increases, we recruit more and mroe fibers giving rise to a WIDER CAP with longer duration because more and more slow and intermediate neurons are being recruited in addition to the fast.

Question 39

velocity equation

Answer 39

v= d/t

Question 40

why do CAPS at the proximal and distal end of the nerve differ in height and shape? WHy do they differ in distance from the artifact?

Answer 40

at the distal end, the amplitude of the CAP decreases, and the duration is LONGER.

• CAP is also started at a later time after the stimulus is applied because it had to travel to the distal electrode to be recorded
• at the distal end, there is also possible SEPARATION of slow and fast axons. there may also be less axons in general because you cut them away during isolation, resulting in a LOWER intensity. Because of the spreading out of remaining axons and a decrease in number of FAST fibers (leaving only slow fibers) , it may take longer to depolarize them all, resulting in a LONGER CAP DURATION.

-the distal cap is also further from the artifiact, because the CAP must travel a distance from the original stimulus.

Question 41

What is a maximal response?

Answer 41

when all axons are firing within a nerve. CAP cannot get any bigger once the maximal response is achieved.