Lecture 4 Flashcards
Physical basis of electrical charge
Matter = molecules
Molecules = elements
Atoms are the smallest particle of an element that has the properties of an element.
What do atoms consist of
- Protons (+) and neutrons (0): inside the centre of the atom.
- Electrons (-) outside nucleus: orbit in electron shells.
What are ions
Ions are atoms that:
- Have a surplus of electrons (anions, negative ions)
- Have a shortage of electrons (cations, positively charged ions).
What is the voltage range of neurons
0-220 mV
Magnitude of potential differences
- 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.
Current (+ types)
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)
Alternating current
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.
Direct current
Nervous system, batteries.
- In DC, the positive and negative terminals are always positive and negative.
0.5-2 mA
Unharmful, gives a light tingling sensation
- Electric flyswatter
5-20 mA
Muscle cramps in arm and hands. Impossible to let go over 20 mA
20-40 mA
Breathing is obstructed, nerve centres can be paralysed.
40-200 mA
Heart stops working, blood circulation halts
200-100 mA
Burns in tissue, muscles and nerves
> 1000 mA
Poisoning of kidneys
Key ions to remember (4)
- Na+ = sodium ion
- K+ = potassium ion
- Cl- = chloride ion
- Ca^2+ = calcium ion
Max m/s for ions and electrons
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)
3 ways ion movement produces electrical charges
- Diffusion
- Concentration gradient
- Voltage gradient
Diffusion
Is a passive process.
- Na ^+ binds with negative poles (O).
- Cl ^- binds with positive poles (H).
Resting potential general info
Difference in charge between intracellular and extracellular side ~-70 mV = resting potential (potential energy)
What ions are critical to resting potential?
- 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.
What ions have a higher concentration inside the axon?
A- and K+
What ions are more concentrated outside the axon?
Cl- and Na+
Maintaining resting potential
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!
2 options for stimulating a neuron
Apply a negative charge (voltage)
Apply a positive charge (voltage)
- Both options induce graded potentials
Applying a positive charge to stimulate a neuron
Depolarisation –> Na+ influx
- Potential difference decreases (e.g., -70mV to -65 mV)
Applying a negative charge to stimulate a neuron
Hyperpolarisation –> K+ efflux or Cl- influx
- Potential difference increases (e.g., -70mV to -73mV)
Graded potentials
Small fluctuations across the cell membrane that extinguish with distance and can be summed.
- Gated channels
- On dendrites and cell body
Action potential general
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))
How action potential arises
When the potential difference over the membrane exceeds a certain level
- Firing threshold (e.g., -50 mV).
Absolutely refractory period
Depolarisation + repolarisation
- It is absolutely impossible for the cell to fire.
Relatively refractory period
Hyperpolarisation
- The cell may fire, but it is more difficult.
How does the action potential propagate along the axon (2 ways)?
- Continuous conduction
- Saltatory conduction
Continuous conduction
Potential difference in one place activates nearby gates: domino effect
Saltatory conduction
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.
Multiple sclerosis
Degradation of the myeline sheath in the CNS (oligodendroglia degeneration)
How do neurons communicate?
Cell A = presynaptic
Cell B = postsynaptic
- An action potential is generated by presynaptic Cell A
- May cause graded potentials at postsynaptic Cell B.
Excitation of Cell B (= turn on)
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.
Inhibition of Cell B (= turn off)
Inhibitory postsynaptic potential (IPSP) may hyperpolarise Cell B (bring it further away from firing threshold).
The net effect of all EPSP’s and IPSP’s determines whether a cell fires
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.
