1.8 Passive Properties of Membranes

Question 1

What are passive properties of a neuron

Answer 1

Properties of the neuron that are fixed/constant near the resting potential of the cell. They determine how the action potentials are generated and how fast they travel.

Question 2

What are the 3 key passive properties of a neuron

Answer 2

membrane resistance, membrane capacitance and internal (axial resistance)

Question 3

How do scientists stimulate a neuron

Answer 3

Measure the potential with one electrode and inject current using a second, completely separate electrode which is also penetrating the cell. Voltage responses can then be measured and plotted.

Question 4

What is an axon often represented as

Answer 4

An electronic circuit

Question 5

What does membrane capacitance refer to

Answer 5

The ability of the membrane to store charge

Question 6

Describe how a lipid bilayer acts as a capacitor

Answer 6

The lipid bilayer is an insulator and it separates charges like a capacitor. This allows for potential difference and charges to accumulate on either side. This allows for the membrane to become polarised

Question 7

What is membrane resistance

Answer 7

The resistance to the flow of ions across the cell membrane, determined by the ion channels

Question 8

What is axial/internal resistance

Answer 8

The resistance to the flow of electrical current inside the axon

Question 9

How is the charge of the membrane calculated

Answer 9

charge = capacitance x voltage

Question 10

What are ion channels the equivalent to in a circuit

Answer 10

Ion channels (mostly K) in the membrane acts as the equivalent of a circuit allowing current to flow through. The lipid bilayer acts for capacitance and the channels act for conductance.

Question 11

What graph must be plotted when determining the time constant

Answer 11

The exponential time course for a rise in voltage

Question 12

What is the letter tau (τ) used for

Answer 12

Is used to quantify the rate of exponential change in voltage. It represents the time constant

Question 13

What does a small tau value or a large tau value indicate

Answer 13

A lower tau value indicates rapid change in voltage but a higher tau value indicates slower change in voltage.

Question 14

What is the time constant

Answer 14

The time constant is the time taken for the exponential rise in voltage to reach 63% of the maximum voltage (AKA the time for the exponential voltage to fall to 37% of the minimum/initial voltage)

Question 15

How is the time constant calculated

Answer 15

membrane resistance x membrane capacitance

Question 16

Describe what happens to the size of the action potential as it is propagated along the axon

Answer 16

From the trigger zone of the dendrite onwards, the size of the potential decreases with distance as it passively propagates away from the motor axon ‘end plate’.

Question 17

Describe what happens to the resulted voltage from the point at which current is ‘injected’ into an axon

Answer 17

From the point at which current is ‘injected’ into an axon, the change in resulting voltage (△V) decays exponentially from the ‘injection site’

Question 18

What is the length constant

Answer 18

The distance at which the change in voltage is 37% of its value at the injection site. This is how far along the axon does it have to go to reach 37% of the original. It is denoted as lambda (λ)

Question 19

What is the length constant determined by

Answer 19

The membrane resistance. A larger resistance of the membrane will cause a larger length constant. This is because a larger membrane resistance will cause less current to leak out of the cell, so the potential can be propagated for longer along the axon.

The internal resistance. A lower internal resistance will allow the potential to travel further along the axon before decaying hence causing a greater length constant. Therefore larger diameter axons have larger length constants as they have lower internal resistance

Question 20

What is a graded potential

Answer 20

When the potential varies smoothly (proportionally) with distance

Question 21

What does the cable theory describe

Answer 21

How cables and axons are both leaky

Question 22

What are 2 passive membrane properties that are important in the integration of neuronal inputs

Answer 22

Temporal/spatial summation and conduction velocity

Question 23

How does the time constant and length constant affect propagation speed

Answer 23

A larger length constant increases propagation speed as local currents reach further ahead to depolarise the membrane threshold.

A smaller time constant increases propagation speed as the membrane is charged and discharged faster (so the same amount of current will change the voltage more quickly)

Question 24

Describe the active factors affecting propagation speed

Answer 24

The sodium current, as it is responsible for the depolarisation in the upstroke of an AP. This current depends on how many sodium channels are open. Larger local circuit currents cause faster depolarisation. This current can be reduced by drugs/toxins (e.g anaesthetics) which block the channels as well as a decreased temperature which can slow cellular processes.

Question 25

How does axon diameter affect velocity differently in myelinated vs non myelinated neurons

Answer 25

In myelinated neurons, conduction velocity is proportional to the axon diameter, but in non myelinated neurons conduction velocity is proportional to the square root of the axon diameter.

Question 26

Describe the effect of myelination on propagation speed.

Answer 26

Myelination increases propagation speed (via added insulation) by increasing membrane resistance in internode region, this increases TC and LC, however also decreases capacitance which counters some of the TC rise.