Neuronal Excitability & Action potentials

Question 1

What are electric signals?

Answer 1

signals produced as a result of transient, rapid fluctuations in their membrane potentials

Question 2

What is the resting potential of a neurone at rest?

Answer 2

-70mV

Question 3

what are the ions that affect membrane potential?

Answer 3

K+, Na+ and A- (larger anions)

Question 4

What are the events that occur in the axon membrane that allow it to reach resting potential?

Answer 4

. Sodium potassium exchange pump requires ATP as it pumps the ions against their concentration gradient
2. 3 sodium ions actively transported out and 2 potassium ions in.
3. There are more sodium ions in the tissue fluid outside the axon than in the cytoplasm so am electrochemical gradient is formed. K+ can then diffuse in via leak channels and Na+ diffuses out however the membrane is more permeable to k+ hence the potential difference is established
4. The membrane is polarised as resting potential of -70mV is reached (-ve on the inside)
5. Membrane is impermeable to sodium ions as voltage activated sodium ion channels are closed due to a change in the tertiary structure of the protein

Question 5

what two factors is the membrane potential derived from?

Answer 5

-electrical force = mutual attraction and repulsion between particles and the ions
-diffusion

Question 6

Give three ways the movement of ions is controlled across the axon membrane

Answer 6

• phospholipid bilayer prevents sodium and potassium ions simply diffusing across it
• channel proteins allow the sodium and potassium ions to pass through the phospholipid bilayer by facilitated diffusion
• some carrier proteins can actively transport potassium ions in and sodium ions out of the axon as it functions as a sodium potassium pump

Question 7

what is the goldman equation?

Answer 7

models the interactions that generate the resting potential based on ionic charge and concentrations and considers the relative membrane permeability to each ion

Question 8

what to factors determine the magnitude of influence of an ion over the membrane potential of a cell?

Answer 8

-the ion’s driving force: Em-Eion
-ions permeability so how easily it can cross the membrane

Question 9

what is the membrane potential as calculated by the goldmann’s equation and why is this different to the membrane potential found by electrodes?

Answer 9

-69mV compared to -70mV and this is due to Na+/K+ ATPase ion pump which generates -1mV

Question 10

How does the Na+/K+ ATPase ion pump work?

Answer 10

1. ATP binds to sodium potassium pump
2. this attracts 3 na+ ions to phosphrylate ADP
3. triggers conformation change
4. transporter protein closes on the inside and opens on the outside as reduced affinity for Na+ hence ADP is released
5. dephosphrylation and 2 K+ ions bind
6. change to the tertiary structure of the transporter protein
7. 2K+ move inside

Question 11

when is a membrane considered polarised?

Answer 11

the Em is < or > 0mV

Question 12

when is a membrane depolarized?

Answer 12

when the Em becomes more postive than at resting potential

Question 13

when is a membrane repolarised

Answer 13

when the membrane returns to resting potential after depolarisation

Question 14

when is a membrane hyperpolarised?

Answer 14

when the membrane becomes more polarised so the inside becomes more negative than at resting potential

Question 15

Give the events of action potential

Answer 15

1. Membrane depolarises when sodium ions diffuse in rapidly through open channels as the channels are highly permeable to sodium ions
2. Membrane potential changes to +40mV
3. local circuit created with the neighbouring resting potential further along the axon
4. Na+ diffuse through the axoplasm along the circuit into the region ahead
5. The membrane potential in the region ahead is reduced and made less negative
6. At threshold of -55mV, voltage gated sodium ion channels open causing depolarisation of the next region

Question 16

What are the phases involved in an action potential occuring?

Answer 16

1.Resting potential
2. Threshold
3. rising phase: depolarisation
4. Falling phase: repolarisation
5. Recovery phase: hyperpolarisation

Question 17

Give the steps of repolarisation

Answer 17

1. At +40mV, voltage activated potassium ion channels open
2. The potassium ions diffuse out rapidly
3. The membrane becomes hyper polarised at -80mV as its now lower than the resting potential
4. The refractory period takes place which is the time taken to reset polarisation

Question 18

What happens during the refractory period?

Answer 18

Once an action potential has been created in any region of an axon, the inward movement of sodium ions is prevented as the sodium voltage-gated channels are closed so it’s impossible for a further action potential to be generated as the sodium potassium exchange pump resets the resting potential

Question 19

What happens after the action potential?

Answer 19

-resting potential is restored
-Na+ and K+ distribution is altered
-action potentials can still occur: concentration gradients of ions exist

Question 20

How is the action potential an all or nothing response?

Answer 20

1. low intensity of mV below threshold causes the signal to die off
2. if the intensity is reached, mP reaches threshold so signal is amplified and reaches a peak value

Question 21

How do we detect differences in stimuli strength?

Answer 21

a larger stimulus activates more nerve cells and so a higher threshold of neurones so generates an increased frequency of impulses

Question 22

what are the different ion channels and what do they transport?

Answer 22

-leak channels do K+
-ion pumps / transporters do Na+/K+
-voltage gated channels do na+ and K+ in action potential propagation and ca2+ to release NT in preSN knob
-ligand gated channels in post synaptic neuronal potential EPSP Na+ and K+, IPSP in Cl-/K+ channels

Question 23

what is a soma?

Answer 23

the cell body of a neurone

Question 24

what are the differences between axons and dendrites

Answer 24

-axons may be myelinated while dendrites are not
axons do not contain ribosomes while dendrites contain many ribosomes
-usually one axon per cell but many dendrites branch from one cell
-axons carry info out of the cell, dendrites carry info into the cell

Question 24

what are the similarities between axons and dendrites

Answer 24

-both extensions from the soma of neurones
-both carry electrical signals

Question 24

How does an action potential pass along a myelinated axon?

Answer 24

Through saltotory conduction

1. sensory and motor neurones are myelinated by being tightly surrounded by Schwann cells
2. The myelin insulates sections of the axon against ion movement
3. This greatly extends local circuits so depolarisation only occurs at the nodes of ranvier so propagation of nerve impulses through action potentials is much faster than in unmyelinated neurones

Question 25

What are the factors that affect the speed of conductance of an action potential?

Answer 25

myelinated neurones the action potential travels at 100ms-1 compared to unmyelinated at 0.5ms-1 as a result of saltatory conduction
- thickness of of the unmyelinated neurone as a thicker neurone reduces diffusion resistance for sodium ions along local circuits hence speeds up the action potential

Question 25

What are the two types of summation?

Answer 25

• Spatial summation = multiple Presynaptic neurones release enough neurotransmitter to exceed threshold of post synaptic neurone so a new action potential is triggered
• temporal summation = where a single presynaptic neurone releases neurotransmitter at a higher frequency so the concentration of neurotransmitter exceeds the threshold value of the postsynaptic membrane triggering a new action potential