Nervous Communication - The Nerve Impulse

Question 1

define the nerve impulse

Answer 1

a temporary reversal of the electrical potential difference across the neurone membrane

Question 2

define the resting membrane potential

Answer 2

the potential difference across the neurone membrane, while the neurone is at rest. the polarised membrane is approx -70mV

Question 3

describe the permeability of the phospholipid bilayer to ions

Answer 3

impermeable

Question 4

describe how resting membrane potential is maintained

Answer 4

• sodium potassium pump actively transports 3Na+ out for every 2K+ in (ATP needed)
• potassium can diffuse back out through open channel proteins (down conc. gradient)
• most of the sodium channels are closed, so the diffusion of potassium is faster
• the interior of the neurone is therefore more negative than the outside

Question 5

what adds to the negative charge inside the neurone

Answer 5

the cytoplasm contains negatively charged ions, eg on proteins

Question 6

what are the three types of channel proteins

Answer 6

• voltage gated
• stimulus sensitive
• ligand gated

Question 7

what are voltage gated channels

Answer 7

open or close depending on the voltage across the membrane (changes in ion concentrations cause the protein to change shape)

Question 8

what are stimulus sensitive channels, giving an example

Answer 8

Pacinian Corpuscles, when the receptor receives a stimulus (eg touch), some sodium channels open, higher pressure means more sodium ion channels open

Question 9

what are ligand gated channels

Answer 9

open in response to the binding of a chemical

Question 10

define the action potential

Answer 10

a brief reversal of the potential across the membrane of a neurone causing a peak of +40mv compared to the resting of -70mv

Question 11

where does the exchange of ions across the membrane occur

Answer 11

• nodes of Ranvier
• unmyelinated neurones

Question 12

what are the stages of action potential

Answer 12

• resting potential
• depolarisation
• threshold potential
• K+ channels open
• repolarisation
• hyperpolarisation

Question 13

describe resting potential

Answer 13

• some K+ channels open
• most Na+ channels closed
• sodium potassium pump actively transports 2K+ into the cell and 3Na+ out
• potential difference across the membrane is -70mV
• voltage gated channels are closed

Question 14

describe depolarisation

Answer 14

• when a stimulus arrives at a receptor/nerve ending, it triggers the opening of some Na+ channel proteins
• Na+ ions diffuse in down electrochemical gradient, causing a temporary reversal of the charges on a membrane (inside of membrane becomes less negative than normal
• the larger the stimulus, the more sodium channel proteins will be open

Question 15

describe threshold potential

Answer 15

• if the potential difference reaches -50mV, voltage-gated Na+ channels will open
• many Na+ then enter the neurone down their electrochemical gradient further depolarising the membrane
• as more Na+ entered, more voltage gated channels open therefore more Na+ enter
• the inside of the neurone becomes positively charged compared to outside

Question 16

describe K+ channels open

Answer 16

• the potential difference reaches +40mV
• at this point, the change in potential difference causes voltage gated Na+ channels to close and voltage gated K+ channels to open

Question 17

describe repolarisation

Answer 17

• K+ diffuse out of the cell through potassium channel proteins down their electrochemical gradient
• the potential difference changes back to being more negative on the inside of the cell

Question 18

describe hyperpolarisation

Answer 18

• so many potassium ions leave the axon, that the potential difference overshoots slightly, making the cell hyperpolarised (as the K+ channels are slow to close)
• the sodium potassium pump restores the normal distribution of sodium and potassium ions, restoring the resting potential

Question 19

describe what is meant by all or nothing

Answer 19

• below the threshold potential, no impulse is generated
• above the threshold, an impulse is generated
• the action potential is the same size, regardless of how much the stimulus is above the threshold
• the larger the stimulus, the more frequent the impulses

Question 20

what is the refractory period

Answer 20

• follows the action potential, it is a short period of time when it is not possible for another action potential to be generated
• the voltage gated Na+ channels are closed

Question 21

what is the purpose of the refractory period

Answer 21

• prevents the action potential spreading out in both directions, the impulse will only pass in one direction along the axon
• makes sure the action potential is separated

Question 22

what is the electrochemical gradient

Answer 22

movement from:
- positive to negative
- high concentration to low concentration