Lecture 4

Question 1

Physical basis of electrical charge

Answer 1

Matter = molecules
Molecules = elements
Atoms are the smallest particle of an element that has the properties of an element.

Question 2

What do atoms consist of

Answer 2

• Protons (+) and neutrons (0): inside the centre of the atom.
• Electrons (-) outside nucleus: orbit in electron shells.

Question 3

What are ions

Answer 3

Ions are atoms that:

• Have a surplus of electrons (anions, negative ions)
• Have a shortage of electrons (cations, positively charged ions).

Question 4

What is the voltage range of neurons

Answer 4

0-220 mV

Question 5

Magnitude of potential differences

Answer 5

• Microvolt (uV (looking like a u) - radio & television receivers, EEGs.
• Millivolt (mV) - audio & video signals, nervous activity
• Volt (V) - penlight battery, outlet, car battery
• Kilovolt (kV) - distribution of electricity, trains, trolleys
• Megavolt (MV) - powerlines, lightning.

Question 6

Current (+ types)

Answer 6

An electrical charge that moves (unit = Ampere (A))

• Electrons flow from - to + = electron current.
• High concentration to a low concentration.
• Alternating current (AC)
• Direct current (DC)

Question 7

Alternating current

Answer 7

Home appliances with motors (e.g., vacuum cleaner)

• In the AC, the direction of current changes.
• Advantage: more efficient transport and easier to change voltage
• Disadvantage: human body is more sensitive to AC.

Question 8

Direct current

Answer 8

Nervous system, batteries.

• In DC, the positive and negative terminals are always positive and negative.

Question 9

0.5-2 mA

Answer 9

Unharmful, gives a light tingling sensation

• Electric flyswatter

Question 10

5-20 mA

Answer 10

Muscle cramps in arm and hands. Impossible to let go over 20 mA

Question 11

20-40 mA

Answer 11

Breathing is obstructed, nerve centres can be paralysed.

Question 12

40-200 mA

Answer 12

Heart stops working, blood circulation halts

Question 13

200-100 mA

Answer 13

Burns in tissue, muscles and nerves

Question 14

> 1000 mA

Answer 14

Poisoning of kidneys

Question 15

Key ions to remember (4)

Answer 15

1. Na+ = sodium ion
2. K+ = potassium ion
3. Cl- = chloride ion
4. Ca^2+ = calcium ion

Question 16

Max m/s for ions and electrons

Answer 16

NB: ions do not carry the same kind of electrical current that powers your phone.

• Ions = max 90 m/s (324 km/h)
• Electrons = 270,000 km/s (90% speed of light)

Question 17

3 ways ion movement produces electrical charges

Answer 17

1. Diffusion
2. Concentration gradient
3. Voltage gradient

Question 18

Diffusion

Answer 18

Is a passive process.

• Na ^+ binds with negative poles (O).
• Cl ^- binds with positive poles (H).

Question 19

Resting potential general info

Answer 19

Difference in charge between intracellular and extracellular side ~-70 mV = resting potential (potential energy)

Question 20

What ions are critical to resting potential?

Answer 20

• Cations: Na+ (Sodium), K+ (Potassium)
• Anions: Cl- (Chloride), A- (Large protein molecules)
• Intracellular: more A- and K+
• Extracellular: more Cl- and Na+.
• NB: Sodium (Na+) and Potassium (K+) most ‘actively’ involved in neural communication.

Question 21

What ions have a higher concentration inside the axon?

Answer 21

A- and K+

Question 22

What ions are more concentrated outside the axon?

Answer 22

Cl- and Na+

Question 23

Maintaining resting potential

Answer 23

Channels allow K+ influx and efflux (passive transport) to balance intracellular A-.

Gates prevent influx of Na+.

Na+/K+ pump pumps Na+ out of the cell and K+ into the cell.

• 3:2 costs energy!

Question 24

2 options for stimulating a neuron

Answer 24

Apply a negative charge (voltage)

Apply a positive charge (voltage)

• Both options induce graded potentials

Question 25

Applying a positive charge to stimulate a neuron

Answer 25

Depolarisation --> Na+ influx - Potential difference decreases (e.g., -70mV to -65 mV)

Question 26

Applying a negative charge to stimulate a neuron

Answer 26

Hyperpolarisation --> K+ efflux or Cl- influx - Potential difference increases (e.g., -70mV to -73mV)

Question 27

Graded potentials

Answer 27

Small fluctuations across the cell membrane that extinguish with distance and can be summed. - Gated channels - On dendrites and cell body

Question 28

Action potential general

Answer 28

A brief (1ms) and large all or nothing potential that temporarily reverses the membranes polarity. - Refractory period: a neuron needs to wait until the action potential is over before it can generate another action potential - Voltage sensitive channels - On axon hillock and axon (in some neurons also on dendrites (back propagation))

Question 29

How action potential arises

Answer 29

When the potential difference over the membrane exceeds a certain level - Firing threshold (e.g., -50 mV).

Question 30

Absolutely refractory period

Answer 30

Depolarisation + repolarisation - It is absolutely impossible for the cell to fire.

Question 31

Relatively refractory period

Answer 31

Hyperpolarisation - The cell may fire, but it is more difficult.

Question 32

How does the action potential propagate along the axon (2 ways)?

Answer 32

1. Continuous conduction 2. Saltatory conduction

Question 33

Continuous conduction

Answer 33

Potential difference in one place activates nearby gates: domino effect

Question 34

Saltatory conduction

Answer 34

Axons are often wrapped in myelin (isolating layer). - There are small 'gaps' in this isolation --> nodes of Ranvier - The action potential can 'jump' from node to node NB: Saltatory conduction is faster and costs less energy than normal propagation.

Question 35

Multiple sclerosis

Answer 35

Degradation of the myeline sheath in the CNS (oligodendroglia degeneration)

Question 36

How do neurons communicate?

Answer 36

Cell A = presynaptic Cell B = postsynaptic - An action potential is generated by presynaptic Cell A - May cause graded potentials at postsynaptic Cell B.

Question 37

Excitation of Cell B (= turn on)

Answer 37

Excitatory postsynaptic potential (EPSP) may depolarise Cell B (bring it closer to firing threshold) - EPSPs produced at the same time, but on separate parts of he membrane do not influence each other. - EPSPs produced at the same time and close together add to form a larger EPSP.

Question 38

Inhibition of Cell B (= turn off)

Answer 38

Inhibitory postsynaptic potential (IPSP) may hyperpolarise Cell B (bring it further away from firing threshold).

Question 39

The net effect of all EPSP's and IPSP's determines whether a cell fires

Answer 39

If the potential difference at the initial segment on the axon hillock becomes smaller than the -50 mV threshold, an action potential is propagated along the axon.