Lecture 10: electrical properties of neuron cell membrane

Question 1

Current (I)

Answer 1

amount of charge moving past a point per sec

Question 2

Voltage (V)

Answer 2

Difference of electrical potential energy available to move charges from one point to another
V=RI

Question 3

Resistance (R)

Answer 3

“friction” opposing the flow of electrical current

Question 4

Capacitance (C)

Answer 4

a capacitor stores charges, it is made of two conducting surfaces placed near one another and separated by an insulator
-when charged, one surface has -ve charges and the other +ves, they are attracted to each other by kep seperate by the insulator

Question 5

Q=CV

Answer 5

Q=the amount of charge on each surface of capacitor, V=diff. of electrical potential energy across the surfaces; C=capacitance: amount of charges that can be stored by 1 volt

Question 6

Membrane Capacitor is made of…

Answer 6

Two conducting materials: Intracellular fluid (ICF) and extracellular fluid (ECF), and insulating layer of phospholipids

Question 7

Membrane Canacitance (Cm)

Answer 7

depends on 1. membrane surface area (A), increasing area=increasing capacitance
and 2. insulating layer of thickness (d), where increasing thickness decreases capacitance:
Cm=e*A/d

Question 8

Membrane potential change:

Answer 8

V=Vmax*e^(-x/lambda)

Question 9

Length constant lambda:

Answer 9

-distance over which change in Vm (GP) dies down
lambda=squr(Rm/(Ri+Re)); Re is usually low and constant
=squr(Rm/Ri)
-lambda is largest when Rm is high and Ri is low

Question 10

when neuron is exposed to a stim, the potential does not change instantaneously, why is the change of Vm delayed?

Answer 10

• membranes acts as capacitors
• capacitance delays the change in Em following stimulation
• greater the capacitance, the more charges the membrane can separate and store for a given potential difference and therefore the more time it takes for ions to be redistributed in response to stim.

Question 11

time constantr “r”:

Answer 11

• time it takes for change in Vm (GP) to reach it’s max value
• aka time taken for change in MP to reach 63% of its max value
• time needed to charge membrane capacitor is described by the time constant=RmCm

Question 12

Importance of length and time constants

Answer 12

• lambda: is distance over which change in Vm (GP) will decrease by 37% of its original value
• “r”: time taken for change in Vm (GP) to reach 63% of its max value
• lenght and time constant are important for both integration of graded potentials and conduction of AP

Question 13

to compare two length or time constants…

Answer 13

look at the x-axis value at 50% decreases and compare the distance of the length or time constants

Question 14

A neuron has a greater length constant than another because it has

Answer 14

a larger membrane resistance, allowing for greater integration of GP and thus a stronger AP

Question 15

greater the length or time constant,

Answer 15

the higher the AP

Question 16

a neuron has a greater time constant than another because it has

Answer 16

a larger membrane resistance, allowing for greater integration of GP and stronger AP

Question 17

There is great variation in signal conduction speed of APs between:

Answer 17

species AND neuron types

Question 18

2 important componenents of AP conduction to conduction velocity:

Answer 18

1. Regeneration of APs (opening of VGates and Na+ diffusion) is the slow component of conduction
2. Electrotonic current flow is the FAST component of conduction, but elecrtontonic current flow decays and only travels short distances: APs need to be reinitiated to keep current going over distance

Question 19

the higher the length constant…

Answer 19

the further the electrotonic current by AP can be propagated, the faster the conduction of APs along the axon (look at threshold potentials post-depolarization to compare length constants)

Question 20

Length constant is greater with…

Answer 20

high membrane resistance and low cytoplasmic resistance, and lower membrane permeability to K+

Question 21

the lower the time constant:

Answer 21

• the faster the capacitor becomes charged
• the faster depolarization of membrane
• faster the electrotonic current will start spreading
• from all the above, the faster the conduction velocity
• lower time constant=lower capacitance

Question 22

Overall increasing axon diameter…

Answer 22

increases conduction velocity

Question 23

increasing axon diamter in relation to Rm

Answer 23

• increases surface area
• increases the number of leak channels
• decreases Rm
• *length constant still increases when Rm decreases due to axon diameter because Ri decreases MORE

Question 24

Increasing axon diameter in relation to Ri

Answer 24

Increases volume, decreasing Ri

Question 25

Increasing axon diameter in relation to capacitance

Answer 25

increases surface area, increasing capacitance

**time constant changes very little because changes in R, and Cm cancel each other out

Question 26

Rm is inversely proportional to surface area; increasing axon diameter,

Answer 26

-increases surface area
-increases number of leak channels
-decreases Rm (Rm=1(2pirh)
decreases length constant

Question 27

Ri in inversely proportional to volume; increasing axon diameter…

Answer 27

increases volume
-decreases R (Ri=1/(pir^2h)
increases length constant

Question 28

Overall, increasing axon diameter INCREASES length constant

Answer 28

lambda= squr(r/2)

-net effect of increasing axon diameter is to increase speed of conduction

Question 29

Neuron design objectives:

Answer 29

• to maximize computing power, need high density and therefore neurons diameters must be small
• minimizing response time to changes requires fast conduction and therefore large diameter since it increases speed of conduction

Question 30

disadvantages of giant axons:

Answer 30

takes up lots of space, limiting number of neurons in the nervous system
-have large volumes of cytoplasm, making them expensive to produce and maintain

Question 31

Effect of myelin on velocity:

Answer 31

precense of myellin increases conduction velocity

Question 32

Myelination…

Answer 32

• found in many vertebrate neurons
• increases conduction speed without greatly increasing space required, therefore better than giant neurons
• it is an insulating layer of lipid-rich Schwann cells wrapped around the axon
• Increases Rm by a factor of 5000
• decreases Cm by factor of 50
• this increases length constant and decreases capacitance, reducing the time constant
• overall increasing axon diameter and increases conduction velocity

Question 33

Myelin acts as insulation, increasing membrane resistance Rm

Answer 33

• decreases current loss through leak channels

- increases Rm, length constant, and conduction velocity

Question 34

effect of myelin on length constant;

Answer 34

-spreads electrotonic current, increasing distance from point of repolarization to return to threshold potential

Question 35

Nodes of Ranvier

Answer 35

-areas of exposed axonal membrane between schwann cells

Question 36

internodes

Answer 36

myelinated regions

-current spreads electrotonically through internodes while new APs occur at nodes of ranvier

Question 37

Saltatory conduction

Answer 37

“leaping” of AP from node to node, due to the channels being there and the current spreading through internodes of sheath to the next channel

Question 38

Demyelination

Answer 38

-patients with multiple sclerosis lose myelin sheath, and current leaks and Em changes due to AP decay occur faster