Nerves

Question 1

when does a membrane reach equilibrium?

Answer 1

when ion movement down a charge gradient = ion movement down a concentration gradient

Question 2

driving force

Answer 2

difference between membrane potential and an ion’s equilibrium potential

Question 3

how does a capacitor/membrane “charge” or “discharge”?

Answer 3

overall charge (caused by ions) on either side of the membrane must flip

Question 4

what does the time taken to switch the charges on either side of a membrane depend on?

Answer 4

• surface area of the membrane (variable)
• thickness of the membrane (fixed)
• resistance of the membrane

Question 5

time constant

Answer 5

time taken for voltage to reach (1/e) 63% of its original value (τ)

Answer 6

λ

Question 7

length constant

Answer 7

distance over which the voltage decays to 37% of the initial value (λ)

Question 8

ways in which speed of propagation can be increased?

Answer 8

• myelination
• increase axon diameter

Question 9

how does myelination increase the speed of propagation?

Answer 9

• Cm decreases
• Rm increases
• τ stays the same
• Rm increases
• Ri stays the same
• λ increases

Question 10

time constant equation

Answer 10

τ = Cm Rm

Question 11

length constant equation

Answer 11

λ = √(rm/ri)

Question 12

how does increasing axon diameter increase speed of propagation?

Answer 12

• Cm increases (greater SA)
• Rm decreases
• τ stays the same
• Rm decreases
• Ri decreases (by a greater amount)
• λ increases

Question 13

what forms the myelin?

Answer 13

specialised glial cells wrap around axons

Question 14

types of glial cells and where are they found?

Answer 14

• Schwann cells in PNS
• oligodendrocytes in CNS

Question 15

reasons for resting membrane potential

Answer 15

• membrane contains Na+ and K+ leak channels
• membrane more permeable to K+
• membrane potential closer to K+ equilibrium potential
• greater driving force for Na+ into cell
• more K+ inside cell and more Na+ outside cell
• Na+/K+ ATPase transports 2K+ in and 3Na+ so inside of cell is more negative than inside

Question 16

experimental evidence for membranes being more permeable to K+

Answer 16

data only fits Nernst equation at high [K+]

Question 17

steps in the action potential

Answer 17

• initial axon segment depolarised
• if depolarising current of inwards +ve charge = outward +ve charge, the threshold is reached
• VGNaCh open and VGKCh open more slowly
• flux of Na+ passively propagates along axon

Question 18

what does the minimum amount of inward Na+ current to reach the threshold depend on?

Answer 18

• Rm
• Cm
• density
• type + state of ion channels

Question 19

refractory periods (in order)

Answer 19

• absolute refractory period
• relative refractory period

Question 20

relative refractory period

Answer 20

VGNaCh are active but a greater inward Na+ current is needed to meet the threshold

Question 21

absolute refractory period

Answer 21

VGNaCh are inactivated and only activated by hyperpolarisation