Physiology

Question 0

Define simple diffusion and give an example.

Answer 0

the electrochemical gradient between compartments drives solute through the dividing membrane; solute moves from high to low concentrations
- happens via membrane channels or by passing directly through the matrix of the membrane

Question 1

Define bulk flow and give an example.

Answer 1

the net difference in pressure on either side of a semi-permeable membrane drives the solvent through the membrane (even against a concentration gradient)

Question 2

Define simple facilitated diffusion and given an example.

Answer 2

the electrochemical gradient drives solute from higher concentration to lower concentration (just like simple diffusion), BUT is dependent on a membrane protein carrier to get solute across
- characteristics not present in simple diffusion: specificity, competition, saturability

Question 3

Define primary active transport and give an example.

Answer 3

a membrane carrier transports solute from high concentration to low concentration (against its concentration gradient)

• requires energy: directly from splitting ATP or other energy source
• e.g., Na+/K+ ATPase pump

Question 4

Define secondary active transport and give an example.

Answer 4

• AKA Coupled or Facilitated Diffusion
• two solutes use the same carrier
• one is moving against its electrochemical gradient using the energy released by the other solute moving with its electrochemical gradient

Question 5

Which types of cells are polarized? (4)

Answer 5

1. Neurons
2. skeletal muscle
3. cardiac muscle
4. smooth muscle

Polarized, meaning they have an electrical potential across their membrane

Question 6

What are normal resting potentials for skeletal/cardiac and smooth muscle?

Answer 6

skeletal/cardiac: -90 mV

smooth: -30 mV

Question 7

At resting potential, what are the relative concentrations of K+, Na+, Cl- and [A-] across the membrane?

Answer 7

K+: higher inside the cell
Na+: higher outside the cell
Cl-: higher outside the cell
[A-], meaning large negatively charged things inside cells: stuck inside cells

Question 8

What are examples of the large, negatively charged substances stuck inside cells, which contribute to a membrane potential? (4)

Answer 8

1. aspartate
2. acetate
3. pyruvate
4. isethionate

Question 9

What force drives ions across the membrane?

Answer 9

the difference between its concentration gradient and the cell’s elecrical gradient

Question 10

What is the function of the Na+/K+ ATPase pump?

Answer 10

It maintains the concentration gradients (and the membrane potential) for the cell by pumping Na+ out and K+ in, both against their concentration gradient (consuming ATP). It pumps 2 K+ in for every 3 Na+ it pumps out.

Question 11

Which way do ions move across the membrane?

Answer 11

an ion will move across the membrane whichever way brings the membrane potential toward the equilibrium potential for that ion.

Question 12

How does a membrane potential change physiologically

Answer 12

• primarily by changing membrane permeabilities (basis for action, endplate and synaptic potentials)
• in a few cases by changes in ionic concentrations brought about by intense, prolonged stimulation of small cells

Question 13

Increasing the conductance/permeability of a membrane for a particular ion always has what result?

Answer 13

ALWAYS moves the membrane potential toward the equilibrium potential for that ion.

Question 14

For which ion is the membrane so permeable that it is always safe to assume that it is at equilibrium?

Answer 14

Chloride (Cl-)

Question 15

Define depolarization, in general.

Answer 15

Decreasing membrane potential; making the inside of the cell less negative.

Question 16

Define hyperpolarization, in general.

Answer 16

Increasing membrane potential; making the inside of the cell more negative.

Question 17

In general, what is the cell’s resting membrane permeability for potassium, sodium, and chloride?

Answer 17

K+: high
Na+: low
Cl-: VERY high

Question 18

What is the correct term to describe a cell at a resting potential of about -90 mV?

Answer 18

Steady state, not equilibrium; chloride is at equilibrium, but sodium or potassium

Question 19

Which types of cells produce action potential? (4)

Answer 19

1. Cardiac muscle
2. skeletal muscle
3. most neurons
4. some smooth muscle

Question 20

What is the driving force?

Answer 20

The difference between the membrane potential and the equilibrium potential for a particular ion?

Question 21

What are the steps in producing an action potential? (5)

Answer 21

1. The cell is at resting potential (-90 mV) and the cell receives a signal
2. Sodium channel opens: sodium rushes in (down its concen gradient); cell depolarizes (gets much more positive inside)
3. Potassium channel opens (triggered at same time but opens more slowly): potassium rushes out (down its concentration and electrochemical gradient); stops depolarization (peaked at +20 mV) and slowly re-polarizes the cell
4. Hyperpolarization: Sodium channel resets outer and inner gates, then potassium channel resets;
5. Cell re-establishes resting potential and is ready to produce another action potential

Question 22

Describe the role of the inner and outer gates on the voltage-gated sodium channel throughout an action potential.

Answer 22

• At resting potential, the outer gate is closed & inner gate is open.
• When the voltage reaches threshold, the outer gate opens (lets sodium in)
• At the peak of depolarization, the inner gate closes (cuts off sodium passage)
• Both gates reset during re-polarization and hyperpolarization

Question 23

What is the role of the sodium potassium ATPase pump during an action potential?

Answer 23

• Virtually nothing during any one action potential. It does NOT repolarize the cell.
• With each action potential, a cell gains a little Na+ and loses a little K+, which would kill the cell over time
• the pump keeps the ion concentrations normalized (and consumes energy)

Question 24

What determines how soon the new action potential is created? (4)

Answer 24

How long it takes to get the voltage-gated Na+ channels upstream of the action potential to threshold, determined by:

• how much charge has to be neutralized by the inward Na+ currents
• how much inward Na+ currents the action potential has to do it
• how much current leaks out of the axon before reaching threshold
• how much the axon impedes axial current flow

Question 25

Compare current speed and efficiently in myelinated vs. non-myelinated axons.

Answer 25

Non-myelinated: current leaks, meaning action potentials have to happen more frequently to propagate the signal (which is slow), and the peak of depolarization decreases over time; conduction velocity proportional to square root of axon radius

Myelinated: the membrane is insulated, maintaining the current; action potentials only happen at the Nodes of Ranvier; conduction velocity is proportional to axon radius

Question 26

Saltory Conduction

Answer 26

AKA conduction in a myelinated axon

• faster and more efficient than non-myelinated conduction
• signal seems to “jump” from node to node

Question 27

Describe the location of the sodium and potassium channels in myelinated axons, and what results when myelin is lost.

Answer 27

• Na+ channels are concentrated at nodes of Ranvier
• K+ channels are concentrated under the myelin, which prevents them from leaking K+ and repolarizing the cell; non-gated K+ at the nodes take over this function
• a loss of myelin results in slower, weaker signals that present as uncoordinated movement or, eventually, muscle weakness (if the signal doesn’t reach the muscle at all)
• commonly seen in multiple sclerosis