PPNT 2

Question 1

Membrane Conductance

Answer 1

-Cell membrane is selectively permeable
- MOST solutes in body are large and/or charged and cannot cross membrane
- Ion channels are small holes that allow specific size/charge of ion to cross
• Conductance of ions is based on the number of channels that are ‘open’

Question 2

Ion Channels and Ionic Movement

Answer 2

-SPECIFIC to ionic size and charge
-Flow depends on number open
• Leak channels – always open – some K+ channels and Cl- channels
• Gated channels – closed until stimulus opens
• Ligand, 2nd messenger, voltage, mechanical, light, etc. etc.

Question 3

Size exclusion

Answer 3

a sodium channel can exclude a calcium/potassium ions due

to different molecule size

Question 4

Charge exclusion

Answer 4

interior of channel will be lined with charged amino acids, will prevent like ions from crossing

Question 5

Leak channels

Answer 5

always open – some K+ channels and Cl- channels

Question 6

Gated channels

Answer 6

closed until stimulus opens

Question 7

Types of Channels- Ligand Gated

Answer 7

• Remain closed until proper ligand (drug, hormone, chemical) binds
• Remains open as long as ligand is around, closes when removed.
• Ex: Nicotinic receptors in skeletal muscles neuromuscular junction
• Binds acetylcholine, allows Na+ (mostly) and K+ (little) to flow down gradients

Question 8

Types of Channels- 2nd Messenger gated

Answer 8

• Remains open as long as 2nd messenger is around, closes when removed
• 2nd messenger will be made until receptor is no longer stimulated
• Ex: Ca2+ channels in smooth muscle cells. Angiotensin II binds to its receptor, receptor cause production of IP3 which binds to and opens Ca2+ channels (InsP3R).

Question 9

Types of Channels- Voltage gated

Answer 9

• Remain closed until membrane potential reaches specific value
• Remains open depending on the membrane potential and channel properties
• Example: Sodium Channel (NaV) in muscles. Once membrane potential gets above certain point, they open. When it reaches a 2nd set point, they close

Question 10

Ion Channels and Ionic Movement

Answer 10

• Potential difference (charge) required to stop ionic movement
• Ions move in response to concentration AND charge
• Relatively few ions have to move to make a charge difference across a membrane
-Movement of charged particles generates an electrical potential
-Basis for resting membrane potential and the nervous system

Question 11

We can determine the Equilibrium potential by using

Answer 11

the Nernst Equation:
• Eq= (-2.3RT/zF)log(Ci/Co)
• In biologic systems -2.3RT/F is -60mV
• Z is charge of ion
• Ci and Co are the concentrations inside and
outside the cell
• Eq= -60/z log(Ci/Co)
• Sign is with respect to the cell interior
• Can infer sign based on where ion usually is and its charge.

Question 12

Application of Nernst Eqn

- Driving force:

Answer 12

• Driving force: mV = Em- Ex

Question 13

Ionic Current: Gx (DrivingForce)

Answer 13

-Gx = conductance (measure of the # of open channels)

Question 14

Goldman Equation or chord equation

Answer 14

Account for ALL ionic concentration gradients and membrane permeability and you get the Resting Membrane Potential (RMP)

Question 15

-Resting Membrane Potential

Answer 15

RMP – charge difference across cell membranes due to concentration gradients of permeant ions
• Each ion tries to push the RMP towards its own Eq potential
• Inside of cell is considered negative compared to outside

Question 16

• Resting Membrane Potential

* Ranges from

Answer 16

*-20 to -100 mV
• Excitable tissue (muscle and nerves) can rapidly change the RMP to send signals and start contraction
• About -70 in neurons, -90 in skeletal and cardiac muscle

Question 17

Excitable tissue

Answer 17

muscle and nerves

Question 18

Action Potentials

Answer 18

• Rapid changes in membrane potential
- Consists of a depolarization from and repolarization to RMP
• All ‘normal’ AP’s from a cell type are similar
-All ventricular myocytes look the same, but are different from cardiac node cells
• All AP’s are propagated
- Move from one site to the next and are nondecremental
• An AP either occurs or it does not
- All or none principle
- If membrane reached specific voltage (threshold) then an AP will occur

Question 19

Stages of the Action Potential
1-At Rest:
2-Local Depolarization:
3-Threshold (about -60mV):
4-Overshoot:
5-Repolarization:
6- Hyperpolarization:
7-At rest:

Answer 19

1-RMP
2-Membrane moves closer to 0mV
3-Point at which AP has to occur
4-Interior becomes (+) relative to outside
5-Membrane begins to move back towards 0 and
negative
6- Hyperpolarization
7-RMP is re-attained