13. Action Potential

Question 1

How does resting membrane potential arise

Answer 1

Separation of charges on either side of the membrane

Mainly due to outward diffusion of K

Question 2

What are ion channels (4)

Answer 2

Transmembranous proteins
Aqueous channel through membrane
Gated opening (ligand, voltage)
Ion selective (specific)

Question 3

Processes involved in AP generation (4)

Answer 3

Stimulus applied
Depolarisation to threshold
Repolarisation to threshold
Refractory period

Question 4

Description of stimulus application (3)

Answer 4

Stimulus applied
MP moves towards threshold (-55mV)
Gated ion channels close)

Question 5

Description of depolarisation (4)

Answer 5

Threshold reached
Na channels start to open (Na influx, greater depolarisation)
K channels remain closed
All Na channels open (max Na influx, MP overshoots 0mV)

Question 6

Description of repolarisation (4)

Answer 6

MP reaches 35mV
Na channels shut (inactivation)
K channels open (K efflux begins)
All K channels open (max K efflux, MP overshoots) - recovery phase

Question 7

Definition of refractory period

Answer 7

All Na channels close, prevents another AP from being generated
Period of inexcitability due to inactivation of voltage-gated Na channels

Question 8

Consequenes of refractory period (4)

Answer 8

Limits max firing frequency of APs in axons
Ensures unidirectional propagation of APs
Prevents summation of APs
Prevents summation of contractions in cardiac muscle

Question 9

AP propagation (4)

Answer 9

AP in one section sets up longitudinal current flow, depolarising adjacent axon parts
AP regenerated further along axon
More current flows, next region activated
APs travel along axons as waves of depolarisation

Question 10

Relationship between speed of AP and axon diameter/myelination (2)

Answer 10

Speed of AP propagation increases with axon diameter (large axons conduct impulses more rapidly than small ones)
Myelination increases speed of AP propagation

Question 11

What is myelin formed by

Answer 11

Schwann cells

Question 12

What are Nodes of Ranvier

Answer 12

Intervals that interrupt myelin sheath - here axon membrane is exposed to ECF and ion flow can occur

Question 13

Description of salutatory conduction (3)

Answer 13

In myelinated nerves, passive currents spread further along axon
Fewer regeneration steps per axon unit length
Thus, AP propagates more rapidly than in unmyelinated axons