Neurobiology

Question 1

Which has typically found in greater concentration intracellularly: potassium ions or sodium ions?

Answer 1

Potassium ions

Question 2

What is the approx. concentration of potassium ions in a typical cell?

Answer 2

150mM

Question 3

Typical extracellular concentration of sodium ions?

Answer 3

150mM

Question 4

Typical extracellular concentration of potassium ions?

Answer 4

5mM

Question 5

Typical intracellular concentration of sodium ions?

Answer 5

5mM

Question 6

If extracellular potassium ion concentration is increased, what happens to the resting membrane potential?

Answer 6

The resting membrane potential becomes more positive/less negative

Question 7

Why does increasing extracellular potassium result in depolarization of the cell membrane?

Answer 7

Because the potassium ion gradient between the extracellular and intracellular membrane is reduced, the flow of potassium ions across the membrane changes and so does

Question 8

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

Answer 8

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

Question 9

Neurons contain passive, open, leaky K+ channels. Which direction does K+ move through these channels and why?

Answer 9

It moves out of the cell, because it will diffuse from an area of higher concentration to an area of lower concentration.

Question 10

If extracellular K+ is increased what happens to net diffusion of K+ out of the cell?

Answer 10

Net diffusion of K+ will decrease.

Question 11

If there were open Na+ channels, which way would Na+ move across the cell and why?

Answer 11

It would move into the cell, because there are higher concentrations outside the cell so it will move along the concentration gradient.

Question 12

Why does changing extracellular Na+ not cause significant change to the membrane potential, but altering extracellular K+ concentration does have an effect?

Answer 12

There are open, passive, leaky K+ channels in a resting membrane of a neuron but the sodium channels are mostly closed/inactive.

Question 13

Which of the following would change the membrane potential from -70mV to -40mV in the cell body: a) increasing intracellular Na+ b) increasing extracellular K+ c) decreasing extracellular K+ d) increasing extracellular Na+

Answer 13

b) increasing extracellular K+ makes the membrane potential less negative because outside becomes more positive (matches closer to the inside, decreasing the gradient)

Question 14

Why would a change in K+ concentration have a greater affect on the resting membrane potential than a change in Na+?

Answer 14

Because the membrane is more permeable to K+

Question 15

Assuming the resting membrane potential of a sensory neuron is -70mV, which of the following changes represents a depolarization: a) -60mV b) -80mV

Answer 15

a) -60mV

Question 16

List three sensory stimuli.

Answer 16

Light, touch, temperature

Question 17

What stimulates a Pacinian corpuscle?

Answer 17

Pressure or vibration on the skin

Question 18

Chemicals are adequate stimuli for what receptors? What aspect of chemicals?

Answer 18

Olfactory receptors, odorant molecules

Question 19

What response would a Pacinian corpuscle have to high intensity light and why?

Answer 19

There would be no response. Light transducing proteins are not present in a Pacinian corpuscle

Question 20

Would a free nerve ending respond to a stimulus in the form of: a) light and pressure, b) chemical and light, c) heat and pressure d) chemical, pressure and heat

Answer 20

c) heat and pressure

Question 21

Why does a free nerve ending respond to several different types of stimuli?

Answer 21

Free nerve endings are less specialized.

Question 22

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:

Answer 22

Intense pressure

Question 23

Why can olfactory receptors respond to low concentrations of chemical odorants?

Answer 23

Because membrane proteins in the sensory neuron receptor can bind and respond to specific odorants.

Question 24

What is sensory transduction?

Answer 24

A sequence of events beginning with a sensory stimulus and ending with a change in membrane potential.

Question 25

What is meant by the term graded potential?

Answer 25

Graded potential refers to how the amplitude of the receptor potential changes with
stimulus intensity. Graded potential may be depolarizing if the membrane becomes less
polarized as a result of changes, or hyperpolarizing.

Question 26

If we begin with a resting membrane potential of -70mV, a change to which of the following values represents the greatest response amplitude:
a) -45mV, b) -60mV, c) -30mV

Answer 26

c) -30mV

Question 27

Axons are: a) bundles of nerves, b) long, thin structures extending from a neuronal body, c) motor neurons, d) nerve endings

Answer 27

b) long, thin structures extending from a neuronal body

Question 28

What happens after a sufficiently depolarizing receptor potential?

Answer 28

An action potential occurs

Question 29

Is the threshold voltage in an axon usually less or more negative than the resting membrane potential?

Answer 29

Less negative

Question 30

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

Answer 30

Action potential will not occur until the threshold voltage has been reached. It is the level of stimulation that is required in order to trigger a response.

Question 31

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

Answer 31

Depolarization increases the membrane voltage to the point where it crosses the
threshold value, and action potential is triggered. So, the membrane potential must
become less negative for action potential to be triggered. With hyperpolarization, the extracellular and intracellular environments become even more polarized/different - not closer to an AP.

Question 32

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

Answer 32

Once the threshold is reached the action potential will occur, but nothing at all will
happen (with regard to action potential) if the stimulus is too small. Similarly, once the
threshold is reached the action potential doesn’t increase if the stimulus increases.

Question 33

Where is the trigger zone of a neuron?

Answer 33

Where the axon hillock and the initial segment of the myelinated neuron meet.

Question 34

When do voltage-gated Na+ channels open, and what happens when they do?

Answer 34

They open when the membrane depolarizes, causing Na+ ions to diffuse out of the cell (reducing the gradient between the inside and outside).

Question 35

With two electrodes, R1 and R2, the appearance of APs generated at R1 and R2 will be what?

Answer 35

The APs will be compressed in time, i.e. later at R2, but have the same peak response value.

Question 36

How does TTX prevent an action potential?

Answer 36

It irreversibly blocks voltage gated sodium channels in axonal membranes.

Question 37

Lidocaine, like TTX blocks voltage-gated Na+ channels, why is lidocaine used as a pain blocker?

Answer 37

The blocking by lidocaine is reversible.

Question 38

What is a compound action potential?

Answer 38

The sum of all action potentials recorded in a nerve/bundle of axons.

Question 39

An ____ is not a bundle of ____, but a bundle of ____ make up a ____.

Answer 39

An axon is not a bundle of nerves, but a bundle of axons make up a nerve.

Question 40

Are all axons equally sensitive to the same stimuli?

Answer 40

No.

Question 41

What happens when an Na+ channel is inactive?

Answer 41

It no longer allows Na+ channels through.

Question 42

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?

Answer 42

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

Question 43

Define relative refractory period.

Answer 43

The time when a second AP can be generated only if the stimulus intensity is increased.

Question 44

What is the absolute refractory period?

Answer 44

The time (in msec) when no matter the stimulus intensity, a second action potential will not occur. Usually very short e.g. 3.75msec.

Question 45

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

Answer 45

When the voltage-gated sodium channels open as a result of a stimulus, this allows
sodium ions to diffuse into the cell thanks to the depolarized membrane, and action
potential is reached. When these channels are inactive, they close (having only been open
for 1-2 milliseconds) and sodium ions can no longer pass through. The channels cannot
be reopened by depolarization for at least another few milliseconds.

Question 46

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

Answer 46

It is harder to generate a second action potential during the relative refractory period
because the voltage-gated sodium channels have already inactivated, the voltage-gated
potassium channels have opened (as well as the leaky ones always being open) and the
efflux of potassium ions opposes a depolarization towards threshold. The cell is trying
harder to return to ‘normal’ status so it is not as excitable or ‘sensitive’ as it was before
the first action potential.

Question 47

What does frequency mean, in terms of APs?

Answer 47

The number of APs per second.

Question 48

What is the interspike interval (ISI)?

Answer 48

The interval between action potentials.

Question 49

What does the formula F(Hz) = 1/ISI(sec) calculate?

Answer 49

The frequency of action potentials in Hertz, as the integral of the ISI.

Question 50

If the interval between action potentials (the interspike interval) is 0.1 (1/10) seconds,
what frequency of action potentials would be observed?

Answer 50

10Hz

Question 51

When stimulus intensity increases, can AP frequency increase?

Answer 51

Yes

Question 52

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

Answer 52

With a longer stimulus, because it is still stimulating the axon after it has recovered from
the initial action potential and the absolute refractory period (and the relative refractive
period if the stimulus is not above threshold), multiple action potentials can continue to
occur.

Question 53

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

Answer 53

When stimulus intensity increases, action potential amplitude doesn’t increase, instead
this stimulus intensity affects the frequency of these action potentials, i.e. how close to
each other they occur - it’s like the stronger stimulus is overcoming the refractory period
and depolarizing the repolarizing membrane quicker than a weaker stimulus could.

Question 54

How does threshold change during the relative refractory period?

Answer 54

During the relative refractory period, the axon is ‘recovering’ from the first action
potential and the membrane is repolarizing, with the changes in voltage-gated Na+
channels (inactivating) and voltage gated K+ channels (activating) trying to aid
repolarization and combat further depolarization. So, for this to be overcome quickly, the
stimulus needs to be greater, i.e. threshold has increased during this relative refractory
period.

Question 55

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

Answer 55

1 divided by the interspike interval (in seconds) equals the frequency of action potentials.
ISI is the time between action potentials, frequency is in Hertz.

Question 56

What are the units of conduction velocity?

Answer 56

meters/second

Question 57

What can affect axonal conduction velocity?

Answer 57

Axon diameter and the degree of myelination.