C11 12-14 and 17

Question 1

Types of membrane potentials

Answer 1

Resting, graded, action

Question 2

What are graded potentials?

Answer 2

Short-lived, localized changes in membrane potential that can be either hyperpolarization or depolarization.

Question 3

Why do graded potentials happen?

Answer 3

A change (stimulus) in the neuron’s environment opens gated ion channels (ligand).

Question 4

Where do graded potentials happen?

Answer 4

Dendrites/cell body

Question 5

What is an action potential?

Answer 5

(Aka nerve impulse). A brief reversal of membrane potential. Involves a transient increase in Na+ permeability followed by restoration of Na+ impermeability and then a short lived increase in K+ permeability.

Question 6

When and where is an AP generated?

Answer 6

An AP is generated only when adequately stimulated by local currents from a graded potential. In most neurons, the transition from local GP to AP happens at the axon hillock.

Question 7

How is an AP generated?

Answer 7

A threshold stimulus opens voltage-gated sodium channels. Sodium ions diffuse into the axon depolarizing it to +30mV. Voltage-gated sodium channels close and voltage-gated potassium channels open. Potassium ions diffuse out of the axon repolarizing it

Question 8

How is an AP propogated along a neuron?

Answer 8

Local currents of an area undergoing depolarization cause depolarization of the forward adjacent area. Repolarization chases the depolarization down the length of the axon.

Question 9

1. Resting state

Answer 9

All gated Na+ and K+ channels are closed. RMP

Question 10

2. Depolarization

Answer 10

Local currents depolarize the axon membrane. Voltage gated Na+ open and Na+ rushes in causing more gates to open. All Na+ channels open, and MP ends up at +30mV.
(When threshold is reached (-50/55), depolarization becomes self generating)
This lasts only 1 msec

Question 11

3. Repolarization

Answer 11

Na+ channels inactivate and K+ channels open.
The inactivation gates of Na+ close and membrane permeability declines to resting levels. Na+ stops entering the cell.
Voltage gated K+ open and K+ rushes out of the cell following its electrochemical gradient.

Question 12

4. Hyperpolarization

Answer 12

Some K+ channels remain open and Na+ channels reset. Increased K+ permeability lasts longer than needed to restore the resting state and as a result a hyperpolarizatin is seen. Na+ activation gates close and inactivation gates open. Resting electrical conditions are restored, but ionic conditions are not. Sod-Pot pumps redistribute ions.

Question 13

depolarizations

Answer 13

interior of the cell becomes less negative (addition of Na+) (excitatory)

Question 14

hyperpolarizations

Answer 14

interior of the cell becomes more negative (Loss of K+ or Cl- into cell) (inhibitory)

Question 15

How can the CNS determine whether a stimulus is weak or strong?

Answer 15

AP’s are independent of stimulus strength - they are all alike in amplitude. So, the frequency of AP’s tells the CNS if the stimuli is strong or weak.

Question 16

What contributes to conduction velocity?

Answer 16

Diameter of axons, myelination, temperature

Question 17

Saltatory conduction

Answer 17

Where: myelinated axons How: Current can pass through the membrane only through myelin sheath gaps where most of the voltage gated Na+ channels are concentrated. AP are generated only at the gaps, about 30 times faster than continuous conduction.

Question 18

Continuous conduction

Answer 18

Where: nonmyelinated axons. How: The AP is generated at each point along the axon (slower than saltatory)

Question 19

Bare plasma membrane conduction

Answer 19

Where: membranes lacking voltage gated channels What happens: Voltage decays because current leaks across channel. (no regeneration of current by voltage gated channels)

Question 20

Absolute refractory period

Answer 20

Period following stimulation during which no additional action potential can be evoked. The neuron cannot respond to another stimulus, no matter how strong.

Question 21

Relative refractory period

Answer 21

Follows the absolute refractory period; interval when a threshold for AP stimulation is markedly elevated. A stimulus that would normally generate an AP is no longer sufficient, but an exceptionally stong stimulus can reopen the Na+ channels.

Question 22

Spatial summation

Answer 22

Multiple locations

Question 23

Temporal summation

Answer 23

Single location, multiple times

Question 24

Summation

Answer 24

Accumulation of effects

Question 25

Action Potential definition

Answer 25

A large, transient depolarization event, including polarity reversal, that is conducted along a nerve fiber