Lecture 3 : passive membrane properties

Question 1

How do we record passive and active electrical signals in a nerve cell?

Answer 1

• 2 microelectrodes are used, one is inserted into the cytoplasm to inject current & the other is in the nucleus to measure MP

Question 2

What is the capacitance of a neuron?

Answer 2

the capacity of the neuron membrane to absorb onto a charge difference

Question 3

Why does a larger neuron eg purkinje cells have a larger capacitance than smaller granular cells?

Answer 3

Purkinje cells have a larger capacitance as they have a LARGER MEMBRANE & extensive branching & a large SA

Question 4

What important properties** determine whether or not a** stimulus will produce an action potential**?

Answer 4

the 3 main passive properties of neurons
1. resting membrane electrical resistance(Rm)
2. membrane capacitance ** (Cm)
3. intracellular axial resistance** along the length of axon & dendrites (Rin)

Question 5

What is intracellular axial resistance?

Answer 5

**The internal resistance to the flow of ions, eg the larger the diameter of the structure (eg axon), the more space for ions to flow

Question 6

What is the resting membrane electrical resistance?

Answer 6

• think of it like the number of open ion channels @ rest

Question 7

What happens if you inject a positive vs negative current into a nerve?

Answer 7

• injecting a positive current into a nerve depolarises it ( making it less negative / more positive)
• Injecting a** negative current** into a nerve cell hyperpolarises it - ie makes it more negative

Question 8

What is the** time constant** (t)?

Answer 8

• indicates how quickly a cell will depolarise in response to an injection in electrical current / or hyperpolarise

Question 9

What is the input resistance? What law is related to this term?

Answer 9

the input resistance is a ratio of the current (I) and the voltage that was recorded on the electrode (V)
* ohm’s law —- R = V/I

Question 10

What does the input resistance **depend **on?

Answer 10

• the density of resting (always stay open) ion channels
• size of the neuron

Question 11

What factors affect the speed and efficiency of signal propagation (spread of signal)?

Answer 11

• efficiency is affected by **axial resistance and membrane resistance **
• speed is affected by axial resistance and capacitance

Question 12

What factors affect the leaking and ability to absorb charge?

Answer 12

• membrane thickness (insulation - eg myelin sheath)
• diameter of the axon, the larger the axon diameter, the lower the resistance will be and the less it will ‘leak’ - ie more efficient

Question 13

What is the length constant?

Answer 13

how far a depolarising current will spread along an axon
* the longer the length constant - the further it will spread