6. BRAIN MECHANISMS AND BEHAVIOURS (PART 3)

Question 1

1. What does this image show?

Answer 1

• it shows why Intracellular fluid is negatively charged

Question 2

2. What does this image show?

Answer 2

• it shows why the extracellular fluid is positively charged

Question 3

3. What does this image show?

Answer 3

• it shows the sodium-potassium transporter
• this is situated in the cell membrane

Question 4

4. What is the electrical charge called?

Answer 4

• it is called the membrane potential

Question 5

5. What does the term “potential” refer to?

Answer 5

• it refers to a stored-up source of energy
• the stored energy in this case is called the electrical
  energy

Question 6

6. What happens when resting potential is disturbed, which alters the membrane potential?

Answer 6

• the inside of the axon is negative
• a positive charge in the inside of the membrane will produce depolarisation
• this means that some of the electrical charge across the membrane is taken away
• this reduces the membrane potential

Question 7

7. What happens when we artificially change the membrane potential at one point?

Answer 7

• this results in a series of depolarising stimuli
• this stimuli starts from very weak and gradually increases in strength
• each stimuli depolarises the membrane potential a little more

Question 8

8. What happens after the strongest stimuli has been applied to the membrane potential?

Answer 8

• the membrane potential suddenly reverses itself
• the inside of the membrane becomes positive
• the outside of the membrane becomes negative
• the membrane potential then quickly returns to normal

Question 9

9. What happens before the membrane potential returns to normal?

Answer 9

• it overshoots the resting potential
• it becomes hyper polarised for a brief period of time
• the whole process takes approximately 2msec

Question 10

10. What is the action potential?

Answer 10

• it is the very rapid reversal of the membrane potential
• it is a part of the message carried by the axon
  FROM the cell body to the terminal buttons

Question 11

11. What is the Threshold of Excitation?

Answer 11

• it is the voltage level that triggers an action potential

Question 12

12. What effect do diffusion and electrostatic pressure have on the sodium in the cell?

Answer 12

• they push the Na+ into the cell

Question 13

13. How is the intracellular level kept low?

Answer 13

• the membrane is not very permeable to this sodium ion
• the sodium potassium pump pumps the sodium ion out
• this keeps the intracellular level of the Na+ low

Question 14

14. What would happen if the membrane would become permeable to the sodium ion (Na+)?

Answer 14

• the forces of diffusion and electrostatic pressure would cause Na+ to rush into the cell
• this sudden influx in positive charges would drastically change the membrane potential

Question 15

15. What follows the brief increase in the permeability of the membrane to Na+?

Answer 15

• an immediate, temporary increase in the permeability of the membrane to K+

Question 16

16. What do the sodium potassium transporters embedded in the membrane do?

Answer 16

• they actively pump sodium ions out of the cell
• they pump potassium ions into the cell

Question 17

17. What other type of protein molecule is found in the cell?

Answer 17

• a protein molecule that acts as an opening to permit ions to enter or leave the cells
• these molecules provide ion channels
• these channels contain passages (pores) that can open or
  close

Question 18

18. What happens when an ion channel is open?

Answer 18

• a particular ion can flow through the pore
• it can enter or leave the cell

Question 19

19. How many ions per second can these millions of sodium channels admit?

Answer 19

• 100 million ions

Question 20

20. What does the number of open ion channels determine?

Answer 20

• the permeability of the membrane to a particular ion
  at any given moment

Question 21

21. Does this description make sense to you?

Answer 21

• yes

Question 22

22. In which direction is the axon in action potential directed?

Answer 22

• down the axon
• the action potential remains constant

Question 23

23. What is the basic law of axonal conduction?

Answer 23

• the all or none law

Question 24

24. What does the all or none law state?

Answer 24

• an action potential either occurs or it does not occur
• once it is triggered:
  - it is transmitted down the axon to its end
• an action potential always remains the same size
• it does not grow or diminish

Question 25

25. What happens when the action potential reaches a point where the axon branches?

Answer 25

• the action potential splits
• but it does not diminish in size

Question 26

26. Why does the axon only transmit an action potential in one direction?

Answer 26

• this is because action potential in living animals always starts at the end attached to the soma
• axons normally carry one way traffic

Question 27

27. What is Saltatory Conduction?

Answer 27

• due to myelination of axons:
  
  • action potentials only occur at un-myelinated parts of
    the axon
    - these are called the Nodes of Ranvier

Question 28

28. What are the two advantages of Saltatory Conduction?

Answer 28

1. it is Economic in terms of Energy
   - energy is saved because energy is only required to
   get rid of the sodium
   - this is because it is entering only at the Nodes of
   Ranvier
2. Speed
   - conduction of an action potential is faster in
   myelinated axons
   - this is because the transmission between the Nodes
   is very fast
   - increased speed enables us to react faster