L1 Excitability and Neuronal AP

Question 0

The role of membrane resistance and internal resistance in neuronal conduction

Answer 0

High membrane resistance has a high Space Constant because K is less likely to leak out so the AP is conducted through the fiber.

High internal resistance has a low Space Constant because it makes it harder for the AP to conduct down the fiber.

Space constant is highest in a myelinated fiber with large diameter.

Question 1

Describe the shape of neuronal, skeletal, and cardiac APs

Answer 1

motor neuron (2 msec) and skeletal muscle (5 msec) look about the same, with an upstroke and a repolarization 
Cardiac is different and has phases 0-4 (300-400 msec). Has a brief plateau before repolarizing

Question 2

Space Constant (Lambda)

Answer 2

the distance that an AP conducts down a cell and influence the next segment of membrane. Longer SC = more rapid conduction

Question 3

How do ions cause the AP?

Answer 3

Upstroke: rapid influx of Na
Repolarization: Na h gates close and K gates open to allow K to flow out
K gates close upon repolarization of MP (voltage dependent)
Na gates are voltage and time dependent (basis for refractory period)

Question 4

Na channel - m & h gates

Answer 4

M gate of Na channel is what allows Na to flow in; OPENS at DEPOLARIZATION (these take time to recover, which is what causes refractory period)
H gate is what stops the flow inward; CLOSES at the POSITIVE VOLTAGE

Na channels available based on RMP - more positive RMP means less Na channels are available

Question 5

RMP and Na Availability

Answer 5

Seen in the graph in the handout
More positive voltage = less Na channels available bc not allowed to recover

Many diseases change RMP and this affects Na avail. and thus ability to conduct an AP

Question 6

Refractory Periods

Answer 6

Absolute: when no AP can be conducted because Na channels are inactive (h gate closed)

Relative: some APs could be stimulated if strong enough because some of the Na channels are recovered from inactivation, but not all.

Question 7

Myelination

Answer 7

Helps conductance because it prevents K from leaking out of the cell and repolarizing the AP. This way, the AP jumps from node to node in saltatory form and conducts the AP down the fiber
Increases the Space Constant

Question 8

Effects of plasma Ca on neuronal cell excitability

Answer 8

Ca in the blood is unbound. Bound Ca binds to (-)proteins on the cell surface and gives the membrane a more overall (+) charge, leading Na channels on the surface to think that the outside of the cell is (+), which is similar to the effects of DEPOLARIZATION and the H gates close, making less Na channels available to be excited by an AP that could come along.
Therefore, more Ca = decreases membrane excitability
Hypoventilation = decrease excitability because you don’t breathe off CO2, therefore become more acidic, and increase the free plasma Ca

Hyperventilation = less CO2, less acidic, less free plasma Ca
Hypocalcemia = less Ca and therefore lower threshold for Na channel activation            INCREASE EXCITABILITY

Question 9

Effects of plasma K on membrane excitability

Answer 9

normal [K] = 3-5mM
Hyperkalemia would decrease the driving force for K to exit the cell, so more (+) charges stay inside the cell making the RMP MORE POSITIVE.
POS RMP = less Na channels available, less excitable and less conduction