The excitable cell

Question 1

What is Eion?

Answer 1

• The equilibrium potential of a particular ion

- The membrane potential that would be achieved if the membrane was selectively permeable to that ion

Question 2

How is resistance affected by cross sectional area?

Answer 2

Resistance is inversely proportional to CSA

R = 1/ CSA

Question 3

What is temporal summation?

Answer 3

• Multiple inputs from the same neuron in a very short space of time
• Increases the chance of an action potential occurring

Question 4

What happens during to falling phase of an action potential?

Answer 4

• Voltage gated K+ channels open
• K+ out of membrane via electrochemical gradient
• Na+ voltage gated channels close
• Vm –> Ek

Question 5

What is IDF?

Answer 5

• Ionic driving force
• Rate at which ions travel from one side of the membrane to the other
• Proportional to Vm - Eion

Question 6

What is the ‘space constant’?

Answer 6

• Distance from the site of depolaristion where the strength of depolarisaion is 37%
• Is proportional to membrane resistance / internal resistance

Question 7

Where are Na+ channels localised?

Answer 7

• More concentrated at the nodes of Ranvier

Question 8

How does the Na+ channel become inactivated?

Answer 8

~1ms after activation

• Globular portion swings up and occludes the pore
• Must be ‘reset’ to resting potential -65mV before another action potential can occur

Question 9

What is undershoot?

Answer 9

Vm lower than normal

• Vm ~ Ek
• Due to delayed and rectifying voltage gated K+ channel
• Harder to fire action potential again

Question 10

What is conduction velocity?

Answer 10

The speed of action potential propagation

Question 11

Describe the properties of graded potentials

Answer 11

• LOCALISED changes in the membrane potential
• Any variable size
• Coded for by amplitude, which varies depending upon the stimulus
• Usually occur in the dendrites
• Pass both ways along the axon, fading from the site of origin

Question 12

What is the Eion of K+?

Answer 12

-80mV

Question 13

What 2 things is conduction velocity affected by?

Answer 13

• Diameter

- Myelination

Question 14

What is the ‘equilibrium potential’?

Answer 14

• When diffusion = electrical force

- No net movement of ions

Question 15

What is needed for a functioning nervous system?

Answer 15

• A negative membrane potential (-65mV)
• Negative inside (less Na/more K)
• Positive outside (more Na/less K)

Question 16

Describe the properties of action potentials

Answer 16

• All or nothing
• Has ‘spikes’ which are FIXED in size and duration
• Doesn’t change when propagated
• Travels down the axon of a neuron
• In one direction
• Coded for by frequency

Question 17

What does the Goldmann equation calculate?

Answer 17

• True Vm

Vm is not the same as the Eion of K+ as the membrane is selectively permeable to other ions, which change the Vm

Question 18

What is the Eion of Na+?

Answer 18

+62mV

Question 19

What can’t action potentials occur in dendrites?

Answer 19

Not a high enough number of Na+ channels

Question 20

What are the costs of myelination?

Answer 20

• Metabolic cost

- Volume cost

Question 21

What is the structure of the Na+ voltage gated channel?

Answer 21

• 4 domains, each with 6 transmembrane domains

- S4 domain has a voltage sensor, where a change in voltage causes a confirmational change activating the channel

Question 22

What is the threshold of Na+ channels opening?

Answer 22

-40mV

Question 23

What is the Nerst equation?

Answer 23

Eion = 61.54 x log ([K+]o / [K+]i)

Question 24

What threshold is reached, what happens to the permeability of Na+ and K+

Answer 24

• They are switched

- Condunctance of Na+ is higher than K+

Question 25

What 2 things does the movement of ions through their channels depend upon?

Answer 25

- Concentration gradient | - Difference in electrical potential

Question 26

What does myelination acheive?

Answer 26

- Increases the resistance of the membrane, preventing current loss - Increases space constant

Question 27

What is spatial summation?

Answer 27

- Multiple potentials are received onto the post synaptic membrane at the same time - From different areas of the body - Increases chance of action potential occurring at higher EPSP felt

Question 28

What is the Goldmann equation?

Answer 28

Vm = 61.54 x log ( (Pk[K+]o + Pna [Na+]o) / (Pk[K+]i + Pna[Na+]i) ) Where P is the permeability of that ion

Question 29

Where do action potentials occur?

Answer 29

- At the axon initial segment (where the highest density of Na+ channels are) - In pyrimidal, cells this is at the cell body - In sensory neurons, this is at the nerve ending

Question 30

What is an electrical synapse?

Answer 30

- Gap junctions - Excitation can spread from one neuron to the other without a neurotransmitter - Works both ways - Rapid - Rare

Question 31

What happens during to rising phase of an action potential?

Answer 31

Vm --> Ena | - Rapid depolarisation

Question 32

What does the Nerst equation calculate?

Answer 32

Eion