NEURONES

Question 1

Cell body

Answer 1

cell body of a neurone consists of the organelles found in typical animal cell including nucleus. proteins and neurotransmitter chemicals are made here.

Question 2

dendrites

Answer 2

dendrites carry action potential to surrounding cells.

Question 3

axon

Answer 3

conductive long fibre that carries nerve impulses along motor neurone.

Question 4

schwann cells

Answer 4

wrap around axon to form myelin sheath which is a lipid so doesnt allow charged ions to pass through it.

Question 5

nodes of ranvier

Answer 5

gaps between myelin sheaths are called nodes of ranvier

Question 6

resting potential

Answer 6

voltage across the membrane when it is at rest is called the resting potential (-70 mv)

Question 7

resting potential mechanism

Answer 7

• when a neuron is not being stimulated or is at its resting state, the outside of the membrane is more positively charged compared to inside which is because there are more positive ions outside the cell than inside.
• so the membrane is polarised, there is a difference in charge called the potential difference or voltage across it.
• resting potential is created and maintained by a sodium potassium pump.

Question 8

sodium potassium pump

Answer 8

• sodium potassium pump moves sodium ions out of the neuron, but since the membrane is not permeable to sodium ions, they cant diffuse back in which creates a sodium ion electrochemical gradient because there are more positive sodium ions outside the cell than inside.
• sodium potassium pump also moves potassium ions in to the neurone but since the membrane is permeable to potassium ions they are diffused back out through potassium ion chanels.
• this makes the outside of the cell more positive as compared to inside.
• sodium potassium pump uses active transport and therefore atp is used.

Question 9

depolarisation

Answer 9

increase in voltage

Question 10

action potential

Answer 10

an action potential is when the neurones voltage increases beyond the set point from the resting potential which generates a nervous impulse.

Question 11

neurons when they are stimulated

Answer 11

becomes depolarised - membrane becomes more permeable to sodium ions.

Question 12

mechanism for depolarisation

Answer 12

• a stimulus triggers other ion channels called sodium ion channels to open. if stimulus is big enough, there will be a rapid change in voltage.

Question 13

stimulus

Answer 13

• stimulus excites the neuron cell membrane causing more sodium channels to open - membrane becomes more permeable to sodium ions so these ions diffuse into the neuron down the sodium ion electrochemical gradient - this makes inside of cell less negative

Question 14

depolarisation

Answer 14

if the potential difference reaches the threshold of -55mv, more sodium ion channel opens and more sodium ions diffuse rapidly into cells.

Question 15

repolarisation

Answer 15

at a potential difference of around 30 or 4o mv, the sodium ion channels close and potassium ion channels open - membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium electrochemical gradient - which starts to get the membrane back to its resting potential.

Question 16

hyper repolarisation

Answer 16

• potassium ion channels are slow to close - so there is a slight overshoot of too many potassium ions diffusiong out of the neuron - potential difference now becomes more negative

Question 17

resting potential

Answer 17

ion channels are reset and sodium potassium pump returns the membrane to its resting potential and is maintained until stimulated by another stimulus.

Question 18

refractory period

Answer 18

period of recovery - where the ions are recovering and they cant be made open straight away.
- time delay between one action potential and the next.

Question 19

importance of refractory period

Answer 19

• ensures that discrete impulses are produced, an action potential cannot be immediately generated after one to make sure they are separate from each other and do not overlap.
• ensures that action potential travels in one directional i.e, unidirectional.
• limits the number of impulse transmission.

Question 20

factors affecting speed of conduction of action potential

Answer 20

1. myelination and saltatory conduction
2. axon diameter
3. temperature

Question 21

myelination

Answer 21

in a myelinated neuron, they have a myelin sheath which is made up of schwann cells which are lipids therefore it doesnt allow charged ions or impulses to pass through it.
- the action potential jumps from node to node which is called saltatory conduction.
- action potential travels faster as it doesnot have to generate an action potential along the entire length, just at nodes of ranvier.

Question 22

myelin sheath

Answer 22

myelin sheath is an electrical insulator.

Question 23

axon diameter

Answer 23

• in axons with a bigger diameter, action potentials are conducted faster because there is less resistance and conduction occurs faster.
• wider diameter means there is less leakage of ions so action potential travels faster.

Question 24

temperature

Answer 24

as temperature increases, speed of conductance increases for two reasons
1. at higher temperature, ions diffuse faster
2. enzymes involved in respiration work faster so more atp is produced ( a product of respiration) for active transport of sodium and potassium ions.