Excitable cells

Question 1

What are the three types of potential?

Answer 1

resting, action, graded

Question 2

What is an action potential?

Answer 2

All or nothing, fixed size signals that propagate along a neuron
can pass either way along axon but tend to only go one way
graded by frequency

Question 3

What is a graded potential?

Answer 3

Variable size, local signals that aren’t propagated over large distances
can travel in either direction
graded by size and vary according to the size of the stimulus

Question 4

What is resting potential and how is it created?

Answer 4

-65mV
Unequal distribution of ions- 3 Na+ out, 2 K+ in
movement depending on concentration gradients + membrane potential

Question 5

What is the equilibrium potential (Eion) for an ion?

Answer 5

Eion is the membrane potential that would be reached in a neuron if the membrane was selectively permeable to that ion

Question 6

What is the Nernst equation used to calculate?

What is the Goldman equation used to calculate?

Answer 6

Equilibrium potential for an ion (Eion)
Membrane potential (Vm)

Question 7

What are the features of an action potential in chronological order?

Answer 7

Resting potential > rising phase > overshoot > falling phase > undershoot

Question 8

Sodium channel structure + function

Answer 8

Voltage gated, several transmembrane domains, one of which has a positive charge so when mV increases there is a conformational change which opens them
Depolarisation as Na+ floods into neuron

Question 9

What is the threshold for an action potential?

Answer 9

When sufficient voltage gated Na+ channels are open so that Na+ permeability > K+ permeability
which means an AP is generated
different depending on what type of neuron it is

Question 10

What happens during the falling phase of an action potential?

Answer 10

Voltage gated Na+ are inactivated
voltage gated K+ channels activated
large force pushes K+ out of the neuron

Question 11

What happens during the undershoot of an action potential?

Answer 11

Voltage gated K+ channels open and reduced Na+ permeability

so membrane potential = K+ equilibrium potential

Question 12

What factors influence conduction velocity (Cv)? Why?

Answer 12

Axon diameter- resistance to current flow is inversely proportional to cross section of the axon
Myelination- prevents current loss by increasing membrane resistance, increasing the space constant, and decreasing membrane capacitance

Question 13

What is the space constant?

Answer 13

The distance from the site of depolarization to where it has fallen to 37%
number that describes how far current spreads passively down an axon

Question 14

If large axon diameter and myelination increase conduction velocity, why are there so many small unmyelinated neurons in the nervous system?

Answer 14

Benefit of high membrane resistance is reduced by the high internal resistance
metabolic and energetic costs of myelination

Question 15

What is the structure + function of dendrites?

Answer 15

C ontain voltage sensitive channels but don’t generate APs

mostly encode information with graded potentials

Question 16

How does saltatory conduction work?

Answer 16

Current enters though Na+ channels at node of ranvier
depolarization spreads passively down axon (sped up by longer space constant)
at next node of ranvier depolarization triggers Na+ channel to regenerate AP
happens again and again until end of neuron

Question 17

What stops an action potential moving backwards?

Answer 17

Voltage gated Na+ channels close in a voltage and time dependent manner for a couple of ms after they are opened
open K+ channels behind the AP hyperpolarize the membrane

Question 18

What does passive travel of current down an axon mean?

Answer 18

Electrical charges moving down the axon according the the laws of electricity, not the opening and closing of ion channels
this happens when positive current first enters an axon

Question 19

What is membrane resistance (rm) and internal resistance (ri) in terms of ‘leakiness’?

Answer 19

Membrane- current spreads further if the membrane is less leaky, dependent on circumference
Internal- current spreads further if there is little resistance to it moving down the axon, dependent on cross sectional area

Question 20

What is membrane capacitance? How does this relate to the time constant?

Answer 20

How fast the membrane potential responds to the flow of ion channel currents, how stretchy the hose is
T = rm cm

Question 21

Why is saltatory conduction faster than normal neuronal conduction?

Answer 21

Increases membrane resistance so longer space constant, so current spreads further down axons
Na+/K+ channels only at nodes also means they don’t have to work as hard to restore gradients

Question 22

Why do different axons conduct at different speeds? What ones are myelinated and which ones aren’t?

Answer 22

Myelinated axons and wide axons are costly, so this is only done for neurons that need to carry info as fast as possible
Myelinated- proprioceptive and motor axons, giant squid axons for escape reflex
Unmyelinated- pain, temperature