Nerve Action Potential

Question 1

Summarise the role of nerve cells

Answer 1

They are rapid and specific communication pathways
They send messages from peripheral structures to the CNS and from the CNS to peripheral structures
Messages are transmitted as action potentials

Question 2

Describe the structure of a neurone

Answer 2

• cell body - contains nucleus and other organelles
• dendrites - fine branches which are used to receive information from other neurons
• axon - long extension from the cell body used to transmit information to target cells (can be up to a metre long)
• collaterals - side branches coming off the axon to contact a number of different cells
• axon terminals - small swellings at the end of the axon and collaterals where they meet target cells

Question 3

Summarise the structure of axons

Answer 3

• axons can be myelinated or unmyelinated
• myelinated axons are covered in myelin sheath which is a fatty material formed by layers of plasma membranes derived from Schwann cells
• myelin sheath is interrupted at regular intervals by gaps known as nodes of ranvier
• unmyelinated axons have no myelin sheath and are in direct contact with the ECF

Question 4

What are the 2 types of peripheral neurones ?

Answer 4

1) afferent/sensory neurones
- have receptive endings where action potentials are initiated
- send nerve impulses from sensory organs to the CNS
2) efferent/motor neurones
- send nerve impulses from the CNS to effectors

Question 5

Summarise the structure of the neurone cell membrane

Answer 5

• phospholipid bilayer
• glycolipids on the outside
• proteins e.g. channels and receptors
• cholesterol molecules

Question 6

Which factors have an effect on what can pass through the cell membrane ?

Answer 6

It depends on :
- molecule size
- electrical charge
- molecular shape
- solubility

Question 7

Why do membranes differ in permeability ?

Answer 7

Because membrane permeability is dependent on the lipids and proteins present and their arrangement

Question 8

Summarise the permeability of cell membranes in their resting state

Answer 8

• readily permeable to K+ and Cl-
• poorly permeable to Na+
• impermeable to large organic anions

Question 9

What is the potential difference across most muscle and nerve cells at resting potential ?

Answer 9

• 60mV to - 80mV
  ( - 70mV)

Question 10

How does a potential difference across a membrane arise ?

Answer 10

Passive movement :
- an electrochemical gradient acts as a driving force for ions
- the permeability of the membrane allows for movement of ions

Active transport :
- ions can move against a concentration and electrical gradient
- requires energy release by cell

Question 11

How is an electrical gradient created ?

Answer 11

Molecules or ions with a charge are attracted to molecules or ions with opposite charges and this creates a potential difference across the membrane so charged molecules can move across the membrane

Question 12

What happens if both a chemical and electrical gradient exists at the same time ?

Answer 12

The concentration gradient needs to be converted into an equivalent electrical gradient

Question 13

What is the purpose of the Nernst equation ?

Answer 13

It tells us the magnitude of the electrical gradient that would exactly balance a given concentration gradient of an ion and it gives us the equilibrium potential for that ion

Question 14

What happens if both the chemical and electrical gradient are in the same direction ?

Answer 14

The net driving force on the ion is the sum of the 2 gradients (A+B)

Question 15

What happens if both the chemical and electrical gradients are in opposite directions ?

Answer 15

The net driving force on the ion is the difference between the 2 gradients (A-B)

Question 16

How does a resting membrane potential arise ?

Answer 16

Because of :
- an unequal distribution of ions across the cell membrane
- the resting cell membrane is much more permeable to K+ than to Na+

Question 17

Why is there an unequal distribution of ions on the inside and outside of the cell ?

Answer 17

• Cells make negatively charged, large organic molecules which can’t cross the membrane
• cells actively transport Na+ and K+ across the membrane using Na+/K+ pump

Question 18

What is an equilibrium potential ?

Answer 18

It is the transmembrane potential at which there is no net movement of a particular ion across the cell membrane

Question 19

Explain how potassium equilibrium potential is maintained ?

Answer 19

In the ICF there is a high concentration of K+ and a low concentration in the ECF so a concentration gradient is established and K+ move out.
An electrical gradient is created because K+ ions are attracted to negative charges in the ICF. This electrical gradient opposes the chemical gradient.
But the concentration gradient overpowers the electrical gradient.

Question 20

What is an action potential ?

Answer 20

An action potential is a rapid change in membrane potential followed by a return to the resting potential.
It is only triggered when the depolarisation is sufficient to reach threshold value.

Question 21

In a mammalian axon, how long does an action potential last for ?

Answer 21

~ 0.5 - 1 ms

Question 22

What is the absolute refractory period ?

Answer 22

It is when a second action potential can’t occur because a certain amount of time hasn’t elapsed since the first action potential was generated

Question 23

What is the relative refractory period ?

Answer 23

This occurs immediately after the absolute refractory period and is when the initiation of a second action potential is reduced but not impossible

Question 24

Describe action potentials as a positive feedback loop

Answer 24

• the axon membrane contains voltage gated Na+ ion channels which are closed at resting potential
• following a stimulus, the membrane depolarises and this causes some of the Na+ channels to open allowing Na+ to move into the cell
• this depolarises the membrane further which causes a greater influx of Na+ causing greater depolarisation
• this process continues until all Na+ channels are open and so the membrane is highly permeable to Na+

Question 25

What is a patch clamp ?

Answer 25

A technique used to record electrical activity in neurones

Question 26

Summarise the repolarisation phase of an action potential

Answer 26

Happens due to closure of voltage gated Na+ channels and efflux of K+ down a strong electrochemical gradient
The efflux of K+ is accelerated by the opening of voltage gated K+ channels which therefore accelerates repolarisation

Question 27

How is an action potential propagated ?

Answer 27

When an action potential is initiated, it is transmitted rapidly across the whole length of the axon. The active zone and resting membrane are at different potentials and so a small electrical current will flow between the 2 regions. This forms a local circuit that links the active zone to the neighbouring resting membrane which then depolarises. This causes Na+ channels to open and an action potential occurs in this part of the membrane. This spreads the excitation further along the axon until it has travelled along the length of the axon.

Question 28

What is orthodomic propagation ?

Answer 28

When action potentials are propagated from one point of origin only in 1 direction

Question 29

What is saltatory conduction ?

Answer 29

When an action potential leaps from node to node

Question 30

What is the benefit of an axon having a large diameter ?

Answer 30

There is lower axial resistance and a larger surface area for ions to flow through

Question 31

What are the pros and cons of myelination ?

Answer 31

Pros :
- action potentials are propagated at a much faster speed
- better economically
- take up less space

Cons :
- metabolic and other developmental costs
- vulnerable to demyelinating diseases

Question 32

What is tetrodotoxin (TTX) ?

Answer 32

It is a toxic chemical naturally found in puffer fish which blocks Na+ channels by binding to the extracellular side of a membrane. This inhibits the firing of action potentials.

Question 33

What is tetraethylammonium (TEA+) ?

Answer 33

It blocks K+ channels and enters via the cytoplasmic side of the membrane. It causes paralysis of the skeletal muscles.

Question 34

How does lidocaine work ?

Answer 34

It blocks Na+ channels so action potentials cannot be propagated and therefore pain cannot be felt

Question 35

What is the equilibrium potential of Na+ ?

Answer 35

+60 mV

Question 36

What is the equilibrium potential of K+ ?

Answer 36

-90 mV

Question 37

What is the equilibrium potential of Cl- ?

Answer 37

-62 mV