Nerve Action Potentials

Question 1

Describe in detail how a nerve voltage-gated Na+ channel operates.

Answer 1

1. At -70mV (resting), voltage-gatd channels are closed -> no Na+ current. (activation gate is closed)
2. If membane depolarises, (0mV), activation gate opens and Na+ fols into cell down ECG.
3. Na+ current reaches max in 0.5msec, then rapidly falls to 0, even though cell membrane remains depolarised.
4. Na+ open then slam shut
5. Do not open again until mPot reached -70mV again.

Question 2

Describe in detail how a nerve voltage-gated K+ channel operates.

Answer 2

1. K+ channels open in same manner but relatively slowly
2. K+ current reaches a max after Na+ channels are inactive.
3. K+ eflux returns to normal @ -70mV
4. Time delay plays important part in operation of voltage gated channels

Question 3

What is the difference between an inactivation and activation gate on a gated channel?

Answer 3

Activation channels open in response to membrane potential variation and allow ions to move across a membrane (inactivation channel is open). INactivation gates automaticlly close in response to ion movement. (Inactivation gates do not reopen until -70mV

Question 4

What are the main differences between a “graded” potential and an action potential?

Answer 4

A graded potential is a stimulus not great enough to reach threshold and cause depolarisation of membrane. Graded potentials most ofter occur in the dendrites and cell bodies of a neuron. Action potentials are stimui that need to reach threshold causing a dominoe effect of depolarisation down axon to synaptic terminal, continuing AP.

Question 5

Explain the structural and functional benefits of myelination.

Answer 5

• Nodes of ranvier are gaps in myelin sheath.- Foci of electrical conduction.
• Structurally, many Na+ channels can be found at nodes of ranvier therefore internodal transmission is very fast and efficient,
• Myelinated sheath acts as insulator for axons, active excitation is continued to nodeso of ranvier.
• Preservation of metabolic energy and allows APs to jump from node to node via saltatory conduction.

Question 6

Why does size matter when it comes to conduction speeds for both unmyelinated and myelinated nerve fibres?

Answer 6

As diametre increases, conduction speed increases

Less nodes = faster conduction

Larger unmeyelinated = faster

myelinates small = good for sensory

large myelinated = very fast

Question 8

What are the differences between the absolute and relative refractory periods in nerve transmission?

Answer 8

ARP: No second potential can be triggered regardless of stimulus size (threshold -60mV)

RRP: second potential may be triggered by greater than usual stimulus.

Question 9

How does the absolute refractory period relate to the opening and closing of the voltage-gated channels?

Answer 9

K+ channels leaking K+

Na+ channels are inactivat and unable to reopen until membrane potential has returned to resting at -70mV

Question 10

How does the relative refractory period relate to the opening and closing of the voltage-gated channels?

Answer 10

Na+ channel are able to be reopened

K+ channels are still open, hyperpolaristaion is occurring therfore requireing greater stimulus to reach threshold.

Question 11

Why are refractory periods necessary for nerve action potential transmission?

Answer 11

Allows for nervous impulse to travel in one direction