The Nerv Sys Part 2

Question 1

Membrane potential:

Answer 1

potential difference between opposite sides of the membrane.

Question 2

Resting potential:

Answer 2

normal, unstimulated membrane potential of a cell at rest.

Question 3

Threshold potential:

Answer 3

potential just large enough to produce an AP response.

Question 4

Equilibrium potential:

Answer 4

membrane potential at which ionic species in question (e.g. Na+) is in electrochemical equilibrium.

Question 5

Action Potential

Answer 5

Triggered by stimulus exceeding threshold potential
Voltage-gated ion channels open
Na+ channels open
Na+ channels inactivated, K+ channels open
Na+ channel gates reset
K+ channels close

Question 6

Absolute Refractory Period:

Answer 6

No stimulus of any strength can produce an action potential
Due to inactivated Na+ channels

Question 7

Relative Refractory period:

Answer 7

Strong stimulus can produce another AP, but with smaller amplitude
Due to still open K+ channels
Opening Na channels lets Na+ in but peak of AP will not approach ENa as closely as normal because K+ is still diffusing out

Question 8

Effects of Refractory Period

Answer 8

Action potentials cannot fuse or combine.
Therefore AP is ALWAYS a discrete signal.

Question 9

What happens to membrane potential during continuous stimulation?

Answer 9

Excitability of membrane decreases with time (threshold increases)
Due to changes in sensitivity of membrane channels to depolarization
This physiological change is called accommodation
Some membranes accommodate fast, others slowly
(more in sensory physiology)

Question 10

Propagation of Action Potentials

Answer 10

AP travels along axon without getting smaller in size.

Question 11

Passive spread of voltage

Answer 11

Local current spreads along membrane

Question 12

Length constant:

Answer 12

distance over which a graded potential shows a 63% drop in amplitude.
Each neuron has its own length constant.
Spreading of electrical signal is due to cable properties of nerve membrane.

Question 13

Propagation of Action Potentials

Answer 13

Spreading of electrical signal can trigger AP at distant membrane location if signal exceeds threshold!
An action potential at one location serves as the stimulus for an action potential farther down the axon.
The larger the length constant, the further away a new AP will be generated, and the faster the propagation of APs.
Thus: An increase in length constant results in an increase in conduction velocity.

Question 14

Look at Action Potential

Answer 14

90mV increase beyond threshold
Spread of voltage depolarizes membrane ahead of the AP
Even with drop off, it’s enough to exceed threshold and trigger another action potential

Question 15

Speed of action potential propagation

Answer 15

Action potentials moving faster along axons also will send messages faster: quicker behavioral response.
How to increase velocity of impulse propagation?
Increase passive spread of signal: increase length constant.
Wrap electrical insulation material around axons to prevent leaking out of charges.
Myelin from Schwann cells and Oligodendrocytes.

Question 16

Myelin sheath

Answer 16

layers of cell membrane wrapped around the axon

Question 17

Schwann Cells

Answer 17

PNS

Question 18

Oligodendrocytes

Answer 18

CNS

Question 19

Myelin and the Propagation of Action Potentials

Answer 19

Myelin is an electrical insulator, not allowing ions to move across membrane.
Result: push depolarization further down axon

At nodes, depolarization opens Na+ channels, resulting in an action potential that depolarizes the next node.
Action potentials “jump” from node to node.

Question 20

Multiple Sclerosis

Answer 20

In demyelinating diseases, such as multiple sclerosis, the loss of myelin slows down the conduction of APs.
Symptoms: muscle weakness, fatigue, difficulty with walking and a loss of vision

Question 21

Propagation of Action Potentials

Answer 21

AP travels along axon without getting smaller in size.
What is the mechanism?
Continually regenerated by opening voltage-gated Na+ channels farther down the axon.