Lectures 1-4: Biophysics

Question 1

action potential in neurons -

Answer 1

active process requiring energy in the form of ATP to maintain concentration gradients for Na and K.

involves voltage gated ion channels that allows the regeneration of the signal so that it doesn’t decay as it propagates

Question 2

passive (electronic) conduction in neurons -

Answer 2

no energy is required, just need a difference in potential

signals that decay as they propagate

• dendrites, cell body and terminals

Question 3

current =

Answer 3

conductance x potential difference

symbol - I, units - A

Question 4

the amount of current through a conductor for a given potential difference is proportional to…

Answer 4

the conductors conductance and inversely proportional to its resistance

V=IXR
potential difference = current x resistance

Question 5

membrane resistance =

Answer 5

measured in Ohms, refers to the electrical property of the membrane that impedes current flowing across the membrane

inversely related to the membrane conductance

Question 6

lipid bilayer (capacitor) -

Answer 6

a good resistor and excellent capacitor

- the resistance of the plasma membrane is determined by the number of open ion channels

Question 7

ion channels (conductors) -

Answer 7

the extracellular fluid and cytoplasm are good conductors

Question 8

capacitance -

Answer 8

the store of charge that builds up on the membrane for a given voltage

influenced by cell size and myelination

• larger cells have greater capacitance
• myelidnation increases insulator thickness

Question 9

capacitors -

Answer 9

store charge

made up of 2 conductors separated by a thin insulator (cell membrane) that prevents charges from crossing but is thin enough so that opposite electrical charges can attract each other across the insulator

Question 10

charge =

Answer 10

capacitance x potential difference

Question 11

cable theory -

Answer 11

in long cellular processes passive responses decrease with distance from stimulus

Question 12

properties of action potentials -

Answer 12

rapid and transient change of membrane potential
self propagating
all or nothing - need to reach the threshold value

Question 13

there are 3 components of resistance:

Answer 13

Rm, Ra, Rext

Ra and Rext are longitudinal components of resistance

Rm is constant, but Ra and Rext increase with distance - resistance in series are additive. As current flow along cytoplasm some of it leaks across the membrane

flow of current along a nerve process depends on leakiness of membrane (dependent on Rm) relative to resistance of cytoplasm (Ra)

Question 14

current will take the path of…

Answer 14

least resistance = will leak out = voltage signal decreases with distance

Question 15

electronic conduction applies to…

Answer 15

• the spread of passively propagated voltage signals such as postsynaptic potentials moving along dendrites
• the conduction velocity of action potentials, the greater the electronic conduction the faster action potentials will propagate since spread of the depolarization occurs via electronic conduction

Question 16

refractoriness:

Answer 16

reduction in membrane excitability following a action potential

Question 17

action potential propagates by local current circuits…

Answer 17

local current is influenced by difference between active and resting potential (the greater the difference, the larger the current)

Question 18

action potential conduction velocity is controlled by…

Answer 18

passive electrical properties of the membrane

Question 19

factors that shorten time constant of the axon =

Answer 19

low Cm (membrane capacitance)
low Rm (membrane resistance)

= enhance rate of depolarization = increases electrical gradients between active and resting zones = speeds up action potential propagation

Question 20

factors that increase length constant of the axon =

Answer 20

low Ra (axial resistance)
high Rm (membrane resistance)

= enhance action potential conduction velocity by increasing longitudinal flow of current

Question 21

evidence for local circuit theory =

Answer 21

• small voltage transients recorder even when action potential is blocked by cooling section of a nerve (cold = slower = blocks active processes = propagates passively)
• changing axial and extracellular resistance alters conduction velocity
• since Ra decreases with increasing axon diameter - conduction velocity is greater in large than small diameter axons.

Question 22

myelination:

Answer 22

increases membrane resistance
decreases membrane capacitance
= makes electronic faster

Question 23

nodes of ranvier -

Answer 23

high density of ion channels
- Rm is low
- Cm is low
= short time constant

Question 24

myelinated internodes -

Answer 24

Rm is high

= increases the length constant

Question 25

evidence that current flow occurs only at nodes of ranvier:

Answer 25

• current stimuli needed to reach action potential threshold is less if locally applied at odes rather than internodes
• local application of anesthetics to block conduction is more effective at nodes than internodes
• current flow only occurs at nodes of ranvier