Nerve Impulses

Question 1

how are nerve impulses produced?

Answer 1

• by altering the permeability of the membrane of the nerve cell to sodium and potassium ions.
• the movement of ions through specialised channel proteins in the axon membrane changes the potential difference across the membrane and impulses are transmitted along axons of neurones as a series of action potentials

Question 2

what is the resting potential?

Answer 2

the resting potential is the potential difference or voltage across the neurone cell membrane while the neurone is at rest - not conducting a nerve impulse

polarisation

Question 3

describe the stages of the resting potential

Answer 3

1) sodium potassium pump actively transports 3 sodium ions outof axon and 2 potassium ions into the axon
2) membrane is relatively impermeable to the sodium ions so they build up on outside of axon, but it is permeable to potassium ions which diffuse from the inside to the outside of the axon through the potassium ion leakage channel
3) results in outside of axon being positively charged - membrane is polarised and electrochemical gradient is produced
4) potential difference across membrane is called resting potential and is -70mV

Question 4

what is an action potential?

Answer 4

the action potential is a depolaristion of the cell membrane so that the inside is more positive than the outside, with a potential difference across the membrane of +40mV

Question 5

what is the sequence of action potential?

Answer 5

1. membrane at resting potential - approx 70mV
2. stimulus is applied which exceeds the threshold value, opens the sodium ion channels. Voltage gated sodium channels open and sodium ions diffuse into the axon down an electrochemical gradient
3. membrane depolarises
4. inside of axon becomes more positive compared to outside
5. potential difference across the membrane is approx +40mV
6. voltage gated sodium channels close and voltage gated potassium channels open
7. potassium ions diffuse out of the axon - potential difference becomes negative. Repolarisation
8. Hyperpolaristion - membrane cannot depolarise. This is caused by the overshoot which occurs because the outward flow of potassium ions reaches equuilibirium before the resting potential is restored

Question 6

what is a threshold potential?

Answer 6

• an action potential will only be initiated if the stimulation reaches a certain value - threshold potential

Question 7

what is the “all or nothing law”?
what is the consequence of this?

Answer 7

stimulation must be sufficiently strong to trigger opening of the voltage gated sodium ion channels
- if stimulation is not sufficently strong threshold potential will not be reached and action potential does not take place
- if stimulation does exceed the threshold potential then all the volatge gated sodium channels open and a full action potential occurs

the consequence of this is that an action potential is always the same size (+40mV); the intensity of a particular stimulus is distinguished by the frequency of the action potentials it initiates, and not the size of them.

Question 8

how does the transmission of an action potential occur across a non-myelinated neurone?

Answer 8

• occurs in just one section of axon
• local currents are setup and open the gated sodium ion channels in the adjacent small section of axon
• sodium ions diffuse across membrane of the next small se3ction and an action potenetial is inititaited
• impulse is propagated as a series of action potentials
• each action potential initiates another in the next section of the axon

In non -myelinated nureones deploarisation occurs along whole length of axon.
The larger the diameter of the axon the faster the impulse is transmitted

Question 9

how does the transmission of action potentials occur across myelinated neurones?

Answer 9

saltatory conduction = action potentials jump from one node of ranvier to the next

the rate of transmission is much faster in myelinated than non-myelinated (up to 120m per second)
they are also smaller in diameter

Question 10

what does myelin being a fatty6 substance result in?

Answer 10

it insulates the axon and prevents impulses in one neurone passing to other adjacent neurones

Question 11

what is the refractory period?

Answer 11

after an action potential has been created, an inward movement of sodium ions is prevented due to sodium voltage gated channels being closed, during this time, it is impossible for a further action potential to be generated

Question 12

what is the role of the refractory period?

Answer 12

1. it ensures that action potentials are propagated in one direction only
2. it produces discrete (separate) impulses
3. it limits the number of action potentials

Question 13

what are the factors affecting the speed at which an action potential travels?

Answer 13

1. myelin sheath = acts as electrical insulator, preventing action potential forming in the part of axon covered by myelin. Action potential jumps from one node of ranvier to another. This increases the speed from 30ms-1 in unmyelinated neurones to 90ms-1 in myelinated neurones
2. diameter of axon = the greater the diameter the faster the speed of conduction because there is less resistance to the flow of ions than in the cytoplasm of smaller axon. With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker
3. Temperature = the speed of conduction increases as the temperature increases because ions have more kinetic energy. Very high temperatures proteins denature