Action Potential

Question 1

What is the resting membrane potential

Answer 1

-70mV

Question 2

At the cell membrane, are the ICF and ECF positive or negative

Answer 2

ICF is negative

ECF is positive

Question 3

What concentrations of Na, K and Cl are found in the ECF

Answer 3

Na - 145mM
K - 4mM
Cl - 110mM

Question 4

What concentrations of Na, K and Cl are found in the ICF

Answer 4

Na - 15mM
K - 150mM
Cl - 10mM

Question 5

Describe the diffusion potential of a cell at resting membrane potential

Answer 5

Impermeable to Na

Very permeable to K

Question 6

Describe the diffusion gradients of a cell

Answer 6

Na diffuses into the cell

K diffuses out of the cell

Question 7

What creates the resting membrane potential

Answer 7

Diffusion of K gives an excess negative charge inside of the cell membrane and an excess positive charge outside of the cell

Question 8

What happens to small amounts of Na which leak into the cell at resting membrane potential

Answer 8

It is expelled by the Na/K pump

Question 9

How does the N/K pump contribute to the resting membrane potential

Answer 9

It exchanges unequal numbers of Na and K ions

It moves 3Na outwards and 2K inwards

Question 10

What causes an action potential to form

Answer 10

A stimulus

Question 11

What is the threshold in an action potential

Answer 11

The voltage at which the voltage gated sodium channels open, allowing sodium influx
-55mV

Question 12

What causes the falling phase in an action potential

Answer 12

Voltage gated K channels open allowing K efflux

Question 13

What are the different types of ion channels

Answer 13

Ligand gated - activated by a ligand binding to a receptor

Voltage gated - when the RMP is moved to a threshold voltage which opens the channel

Question 14

Describe a voltage gated sodium channel at resting membrane potential

Answer 14

M-gate is closed and h-gate is open so sodium cannot pass through

Question 15

Describe voltage gated sodium channels when the threshold is reached

Answer 15

Both the m-gate and the h-gate are open so sodium can pass through

Question 16

Describe a voltage gated sodium channel during the refractory stage

Answer 16

M-gate is open but the h-gate is closed so sodium cannot pass through

Question 17

Describe a voltage gated potassium channel

Answer 17

Can either be open or closed

Potassium leaves the ICF when open

Question 18

Describe an action potential when the threshold potential is met

Answer 18

Na channels start opening causing sodium influx which promotes depolarisation and sodium channels keep opening
Potassium channels remain closed

Question 19

What happens after depolarisation

Answer 19

When MP reaches 35mV the Na channels shut as the h-gate closes
K channels open and K efflux begins, creating a recovery to the RMP

Question 20

What is the refractory period

Answer 20

The period of inexcitability of an action potential due to the inactivation of voltage gated sodium channels
The h-gates are shut so NA can’t diffuse into the neurone

Question 21

What are the effects of the refractory period

Answer 21

Limits maximum firing frequency of APs in axons
Ensures unidirectional propagation of APs
Prevents summation of APs
Prevents summation of contractions in the cardiac muscle - AP lasts as long as ventricular contraction

Question 22

Describe action potential propagation

Answer 22

An AP sets up longitudinal current flow
This depolarises adjacent resting parts of the axon
The AP is regenerated further along the axon
More current flows and the next region of axon is activated
APS travel along the axon as waves of depolarisation

Question 23

What increases the speed of action potential propagation

Answer 23

Increasing axon diameter

Myelin sheaths

Question 24

What is a myelin sheath

Answer 24

Many layers of cell membranes wrapped around the axon

Question 25

What cells lay down myelin sheaths

Answer 25

CNS - oligodendrocytes

PNS - Schwann cells

Question 26

What is the function of myelin sheaths

Answer 26

Forms an insulating layer, reducing leakage of current from axons

Question 27

What are nodes of Ranvier

Answer 27

Points along the axon where the myelin sheath is interrupted and the axon membrane is exposed to the ECF and ion flow can occur

Question 28

Describe saltatory conduction

Answer 28

In myelinated nerves, the currents spread further along the axon, so there are fewer regeneration steps per unit length of axon
So the AP propagates more rapidly than in unmyelinated axons

Question 29

Describe the different connective tissue layers that surround axons

Answer 29

Endoneurium - surrounds a single axon
Perineurium - surrounds a bunch of axons joined together
Epineurium - surrounds a number of different neurones joined together

Question 30

What are the function of Ab axons

Answer 30

Mechanoreceptors

Question 31

What are the functions of As axons

Answer 31

Mechanoreceptors
Thermoreceptors (cold)
Nociceptors
Chemoreceptors (taste)

Question 32

What are the functions of C axons

Answer 32

Mechanoreceptors
Thermoreceptors (hot and cold)
Nociceptors

Question 33

Describe how the average axon size compared from the mental nerve to the canine nerve

Answer 33

The mental nerve has more axons with a large diameter while the canine nerve has more thinner axons

Question 34

Which type of axon forms the majority of the pulp

Answer 34

C fibres