5.1.3c nerve impulses

Question 1

resting potential

Answer 1

when a neurone is not transmitting an impulse
nerve cells are polarised in their resting state
-70mV

Question 2

creation of resting potential

Answer 2

• sodium-potassium pump AT sodium ions out of axon and potassium ions into axon. 3 sodium ions pumped out, 2 potassium ions pumped in
• more sodium ions outside membrane, more potassium ions inside cytoplasm
• most sodium voltage gated ion channels closed preventing sodium diffusion. many potassium ion channels open allowing diffusion out of axon
• more positively charged ions outside axon. creating resting potential across membrane of -70mV.

Question 3

what is an action potential

Answer 3

an action potential is when the neurone’s voltage increases beyond a set point from the resting potential (-55mV), generating a nervous impulse.

Question 4

sequence of events during action potential

Answer 4

1. sodium voltage gated ion channels open. sodium ions diffuse into axon making membrane more positive. this is depolarisation
2. change in charge causes more to open, more sodium diffuse into axon. this is positive feedback. gives pd of +40mV. start of repolarisation
3. voltage gated sodium ion channels close and potassium open
4. repolarisation - potassium ions diffuse out of axon reducing the charge
5. hyperpolarisation as lots of K+ diffuse out axon. inside axon becomes more negative than resting state
6. repolarised as voltage gated potassium ion channels close. axon returns to resting potential.

Question 5

refractory period

Answer 5

period of time after an action potential when the axon cannot be excited again.

Question 6

why is refractory period important

Answer 6

ensures action potentials are unidirectional
ensures action potentials occur as discrete impulses

Question 7

what is saltatory conduction

Answer 7

The action potential jumps from one node to another
saltatory conduction speeds up the transmission of action potentials.

Question 8

Action potential speed is also affected by…

Answer 8

• axon diameter, bigger the axon diameter, faster the impulse is transmitted
• temperature, higher the temperature, faster the nerve impulse

Question 9

all or nothing principle

Answer 9

If the intensity of a stimulus is above the threshold value, an action potential is initiated.
If the threshold isn’t reached, no action potential will be triggered.

The larger the stimulus, the more frequently the action potentials are generated.