Electrical Properties of Neurons

Question 1

Charge (Q)

Answer 1

• The quantity of electricity
• Carried by ions
• Coulombs (C)

Question 2

How much charge does a monovalent ion carry?

Answer 2

1.6 x 10 (^19)

Question 3

Current (I)

Answer 3

The rate of flow of charge over time (amps)

Question 4

Equation linking current (I) charge (Q) and time

Answer 4

Current = Charge / time

Question 5

What does charge need to move?

Answer 5

• A mechanism to move (eg. ion channel)

- An ionic gradient or p.d.

Question 6

Voltage (V), p.d.

Answer 6

• An unbalance in charge
• Arises due to difference in (+) and (-) ions across a membrane or boarder
• Provides ‘driving force’ for current

Question 7

Resistance (R)

Answer 7

• The restriction of mvmt of charge

- Ohms

Question 8

Conductance (G)

Answer 8

• Ease of mvmt of charged species

- Ohms (^1)

Question 9

Ionic equilibrium

Answer 9

When ion mvmt due to charge and conc. gradients

Question 10

Equilibrium potential (of a given ion)

Answer 10

The membrane voltage at which ionic equilibrium occurs

Question 11

How can the membrane potential be changed v rapidly?

Answer 11

• Involves redistribution of ions:
• Need to alter membrane permeability (ie. alter membrane resistance or conductance through opening channels)
• Will allow ions to move across membrane

Question 12

What information does the Nernst equation provide?

Answer 12

The factors affecting the equilibrium potential for a given ion.

Question 13

At rest, what ion is the membrane typically permeable to?

Answer 13

K+

Question 14

Difference btw Nernst eq. and Constant field eq.

Answer 14

The nernst equation only refers to the equilibrium potential of one ion whereas neural membranes are often permeable to more than one ion. GHK takes into account permeability of all the main permanent ions as membrane potential will reflect the equilibrium potentials of all the ions present.

Question 15

What does the constant field equation calculate?

Answer 15

The membrane potential

Question 16

What are the 3 properties of the neuronal cell membrane?

Answer 16

Acts as a variable resistor, a battery and capacitor.

Question 17

Capacitance

Answer 17

The ability to store charge (Farads).

Question 18

Why is the membrane like a battery?

Answer 18

Due to the potential difference btw intracellular and extracellular

Question 19

How does the membrane act as a variable resistor?

Answer 19

Varies the resistance by opening and closing ion channels.

Question 20

How does the membrane act as a capacitor?

Answer 20

Has the ability to store charge due to insulating / non-conducting phospholipid bilayer btw electrolytic conducting mediums each side with different voltages. (+) on one side is attracted to (-) on other side.

Question 21

Relationship btw charge, capacitance and voltage.

Answer 21

Q = CV

Amount of charge is proportional to voltage difference across the capacitor.

Question 22

How is a capacitor charged and discharged?

Answer 22

Applying a voltage across capacitor stores charge.

Reducing this p.d. causes charge to flow away.

Question 23

Describe the capacitance of the plasma membrane.

Answer 23

Bilayer forms vv thin non-conducting layer btw extra and intra but has large surface area meaning membrane has a high capacitance.

Question 24

What effect does a large capacitance have on neuronal signalling?

Answer 24

The larger the capacitance, the larger the amount of charge needs to move in order to change membrane potential and the slower the change is.

Question 25

What is the difference btw the active and passive properties of axons?

Answer 25

Active conduction relies on voltage-gated ion channels

Question 26

Internal resistance (Ri) vs membrane resistance (Rm)

Answer 26

Neurons have high internal resistance due to small amount of charged ions and large amount of non-conducting material. Membrane resistance is much lower due to the presence of permanently open ion channels ie. membrane ‘leakiness’.

Question 27

What does the time constant measure?

Answer 27

The time it takes for the voltage to decay to 37% of its initial value.

Question 28

What factor(s) effect the amount of capacitance on the phospholipid bilayer?

Answer 28

The thickness of the cell membrane - increased thickness leads to increased capacitance.

Question 29

In passive conduction, what determines the magnitude of the voltage change that responds to an injection of current?

Answer 29

Ohms law - the size of I and the amount of membrane resistance at the point of injection.
Smaller current leads to smaller rise in voltage.

Question 30

How quickly does the voltage change in response to injection of current?

Answer 30

Depends on the Rm and Cm (resistance and capacitance of membrane). Capacitance slows down the voltage change as any voltage change across the membrane must be accompanied by a corresponding change in charge.

Question 31

What effect does capacitance have on the speed of neuronal signalling (1 point)?

Answer 31

Slows down signalling

Question 32

What determines how far a voltage change will be propagated (passively) along an axon?

Answer 32

Depends on length constant - relationship btw the Ri and the Rm ie. ease of flow of charge through cytoplasm vs through membrane).

Question 33

How does Rm influence the distance of passive conduction?

Answer 33

High Rm in relation to Ri - long distance

Low Rm in relation to Ri - short distance (as ionic current will leak out of membrane).

Question 34

How does cable diameter influence internal resistance?

Answer 34

The larger the cable diameter, the lower the Ri.

Question 35

How does insulation improve passive conductance?

Answer 35

It increases membrane resistance and decreases membrane capacitance.