NERVOUS SYSTEM: PASSIVE POTENTIALS AND ACTIVE POTENTIALS

Question 1

State Ohm’s law

Answer 1

I = delta V times G

I = current
G = conductance

Question 2

3 passive properties important in neurons

Answer 2

• Membrane capacitance (Cm)
• Membrane resistance (Rm)
• Intracellular longitudinal (axial) resistance along axons and dendrites

Question 3

Why are the passive properties important in neurons?

Answer 3

These properties determine how far a passive (graded) potential generated in a dendrite will travel and whether a passive potential will result in an action potential at the axon hillock.

Question 4

membrane capacitance (Cm)

Answer 4

the ability of the membrane to collect and store energy in the form of an electrical charge

Question 5

How do voltage gated sodium channels help to carry action potentials along the axon?

Answer 5

the initial depolarization at one site of the axon actviates neighboring voltage-gated sodium channels which open and allow sodium ions to rush into the axon at the adjacent section. this continues along the axon creating a wave of depolarization

Question 6

___ and ___ determine shape and magnitude of voltage response

Answer 6

membrane capacitance and membrane resistance

Question 7

Instantaneous or lags?
- Voltage change
- current change

why.

Answer 7

voltage change lags due to membrane capacitance and resistance

current change is instantaneous

Question 8

define: dendritic spikes

Answer 8

dendrite produced action potentials

Question 9

describe how initiation and propagation differ in passive potentials vs active potentials

Answer 9

passive potentials
- initiation may involve ion channels
- propagation does not involve ion channels
- initiation occurs when stimulus (current from another neuron or injected stimulus)
action potentials
- initiation and propagation involve ion channels
- initiation occurs when membrane potential is reached

Question 10

define: length constant

Answer 10

describes the rate of exponential decay of membrane voltage as a function of distance from the location where current is injected

defined as the distance from
the site of current injection where
the voltage response is 1/e of its
original amplitude

smaller diameter fibres decay over a shorter distance and thus have a smaller length constant

= sqrt rm/ ra

where ra = axial resistance

the higher the ratio of membrane resistance to axial resistance, the lower the loss of current and greater the length constant

Question 11

state the importance of the axon hillock

Answer 11

• evaluates strength and frequency of passive potentials; if it reaches threshold, action potential initiates

Question 12

How does the length constant change when diameter is increased?

Answer 12

although increasing diameter will decrease membrane resistance, it will reduce axial resistance by a much greater factor and therefore increases lambda.

Question 13

How does the length constant change with length and diameter?

Answer 13

The longer the length, the higher the resistance, and the more the length constant decreases. The electrical signal gets weaker and weaker

As the diameter increases, the resistance decreases and the length constant increases

Question 14

Explain the 8 phases of an AP and describe the molecular events occurring at each stage

Answer 14

1. resting membrane potential: -70 mV
2. depolarizing stimulus changes membrane potential
3. Membrane depolarizes to threshold. At about -55mV, voltage gated Na+ channels start to open and Na+ fluxes into the cell, rapidly depolarizing the membrane
4. Rapid Na+ entry depolarizes the cell. Voltage-gated Na+ channels are maximally open and Na+ continues to flux in the inward direction depolarizing the membrane. At about -20 mV, slower voltage-gated channels start to open.
5. Na+ channels close and slower K+ channels open.
6. K+ moves from cell to extracellular fluid. K+ channels are maximally open and K+ fluxes in the outward direction, repolarizing the cell. NaV inactivation gates are closed, preventing any further movement of Na+.
7. K+ channels remain open and additional K+ leave cell, hyperpolarizing it. K+ channels remain open and undershoot the resting potential. This is known as the “after-hyperpolarization”. As Vm becomes more negative, K+ channels close as Vm approaches EK.
8. Voltage-gated K+ channels close, less K+ leaks out of the cell. K+ channels close, slowing K+ leak. Retention of K+ and slow Na+ leak into cell brings Vm back to resting membrane potential of -70 mV
9. Cell returns to resting ion permeability and resting membrane potential

Question 15

How is AP propagation unidirectional?

Answer 15

2 mechanisms responsible for unidirectional action potential conduction
1. Nav inactivation
2. There is little driving force for the diffusion of Na+ backwards (Vm - Eion is small) since there is a high concentration of Na+