Electrical Signaling and Action Potentials

Question 1

Positive current

Answer 1

Positive ions moving out of the cell or negative ions moving into the cell

Question 2

Ohm’s Law

Answer 2

V = IR

I = gV

I_i = g_i (V_m - E_i)

Note: (V_m - E_i) is driving force

Question 3

Conductance (g)

Answer 3

Ease of current flow

(dependent on number of open channels)

Question 4

Inward and outward current

Answer 4

Inward: positive flowing into cell, or negative flowing out of cell

Outward: positive flowing out of cell, or negative flowing into cell

Question 5

Nernst Equation (at body temp)

Answer 5

E_k = (61.54mV)log₁₀ ([K]_o/[K]_i)

E_k is equilibruim potential of K⁺

Question 6

How does Na/K ATPase maintain a negative resting potential?

Answer 6

3 Na+ out

2 K+ in

Na⁺ moved out against conc gradient (E_Na = +62mV)

K⁺ moved in against conc gradient (E_K = -80mV)

Side note: in epithelial cells, Na/K ATPase on basolateral membrane (border of epithelial cell and blood)

Question 7

Goldman-Hodgkin-Katz equation

Answer 7

Used when there is more than one ion channel

E_rev = (61.54mV)log₁₀ (P_k[K]_o + P_Na[Na]_o…/P_k[K]_i + P_Na[Na]_i…)

P = relative permeability

Question 8

Voltage gated Na+ channel gates

Answer 8

m gate: in the middle; closed when channel closed, opens with voltage changes

h gate: at the bottom; closes when channel inactivates, otherwise is open

Question 9

Voltage gated K+ channel

Answer 9

n gate: closed when channel closed, opens with change in voltage

Tetramer with pore loop in middle

Selective for K+ because selectivity filter of “surrogate water molecules” lining the pore draw K+ in (but not Na+ because Na+ is too small)

Question 10

What causes depolarization/rising phase of action potential?

Answer 10

Increase in Na+ permeability, causing Na+ to flow into cell

Question 11

What causes repolarization/falling phase of action potential?

Answer 11

Increase in K+ permeability causes K+ to flow out of the cell, and also inactivating Na+ channels which decreases Na+ permeability

Question 12

Absolute refractory period

Answer 12

Impossible to trigger AP right away because all Na+ channels needed are inactivated

Question 13

Relative refractory period

Answer 13

After the absolute refractory period

Harder to trigger AP because some Na+ channels inactivated, and also K+ conductance still kind of high so cell more hyperpolarized than usual

Question 14

Accommodation

Answer 14

If cell depolarizes too slowly, no AP caused

Because: too much inactivation (via h gate) of Na+ channels, so sufficient number to cause AP is never available. Also, slow depolarization opened K+ channels so cell is slightly hyperpolarized.

Question 15

Axon with small radius vs. large radius

Answer 15

Velocity of conduction of AP is faster when axon has larger radius

Larger stimulus pulse is needed to trigger AP when axon has larger radius (lower internal resistance)

Question 16

Chemical synapse

Answer 16

Vesicles of NTs

NTs exit pre- and cross synaptic cleft to bind ligand on post-

More space between pre- and post-

Question 17

Electrical synapse

Answer 17

No vesicles

Ions flow through gap junctions

Very close contact

Question 18

Small molecule NTs

Answer 18

Enzymes that make NT are made in cell body and transmitted slowly down nerve terminal

Actual NT made locally and packaged/recycled in nerve terminal

Overall FAST transmission

Small clear vesicles

Ex: GABA, glutamate, Ach

(Low freq of APs causes less Ca2+ –> preferential release of small molecule NTs)

Question 19

Peptide NTs

Answer 19

Made and packaged in cell body and vesicle sent down axon quickly to nerve terminal

Overall SLOW transmission

Large dark vesicles

Ex: Substance P

(High freq of APs causes lots of Ca2+ –> small molecule and peptide NTs released)

Question 20

Why don’t action potentials travel backwards?

Answer 20

Because the refractory period is occurring right behind the AP