Lecture 3 - Neurophysiology: Principles of Neural Transmission

Question 1

Diffusion and concentration differences

Answer 1

Diffusion is the movement of particles (atoms, ions, molecules) in a gas or liquid (e.g., water) from regions of high concentration to regions of low concentration -> down the concentration gradient
Diffusion is caused by the random movement of particles -> randomly colliding with the membrane and diffusing through it

Question 2

Diffusion speed

Answer 2

Temperature -> too hot = miss shapes the membrane such that molecules cannot diffuse, too cold = particles move too slow and so diffusion is slower
Particle size
How difficult it is for particles to travel through the liquid -> viscosity

Question 3

Membrane permeability

Answer 3

Fully permeable = allows all particles through
Selectively/ partially permeable/ semipermeable = only allows some molecules through -> may contain pores (too small for large molecules e.g. proteins

Question 4

Ion channels

Answer 4

Neuronal cell membranes contain ion channels (pores of large proteins -> only allow certain ions through the membrane)
Ion channels are selective = only allow one type of ion through (different ion channels for K+, Na+, Cl-, etc.)
Because of ion channels, neuronal cell membranes are semipermeable: they only allow some types of ions to diffuse through
Channels open and close to regulate action potential

Question 5

Membrane potential

Answer 5

Neurons have an electric potential across their membranes (membrane potential)
Membrane potential caused by differences in ionic concentrations between the inside (intracellular) and outside (extracellular) of the neuron, and ion channels in the neuronal cell membrane that only allow certain ions to pass in and out of the neuron
The membrane potential can be measured using tiny electrodes

Question 6

Electrical potential

Answer 6

All charged particles (such as ions) are surrounded by an electric field (strength of this electric field = electric potential)
Voltage is measured in volts (V)
Negative ions= negative electrical field
Positive ions= positive electrical field
Volts of battery= difference between negative and positive
Different countries have different voltage so a lot of products don’t work in both places

Resting potential -> Stimulus -> Threshold -> Depolarisation -> Repolarisation -> Hyper-polarisation -> Resting potential

Question 7

Resting membrane potential

Answer 7

-70mV
Higher concentration of sodium outside and potassium inside
Balance maintained by sodium-potassium pump -> 3Na+ out, 2 K+ in
Meaning more positive outside than in

Question 8

Hodgkin and Huxley model of action potential

Answer 8

Used giant squid axons as human too small

Question 9

Demylinating disease

Answer 9

Multiple Sclerosis (MS) and Guillain Barre Syndrome (GBS) 
Demyelinating disease – breaks down myelin sheaths – prevents saltatory conduction 
Nerve impulses can no longer be transmitted effectively 
Loss of muscle control, loss of sensation, problems with coordination, visual problems, loss of bladder control, cognitive impairments,..

Question 10

Fugu

Answer 10

Fugu livers contain tetrodotoxin (TTX) -> a neurotoxin
TTX blocks the opening of voltage gated Na+ channels (no action potential generated)
Breathing depends on active nerve impulses to muscles in thorax
Victims can’t breathe
o Asphyxiation
o Conscious
o No antidote
o Requires artificial respiration until toxin metabolised
Breaks down neurones- no longer able to create action potential

Question 11

Anaesthetic

Answer 11

E.g. lidocaine (tooth extraction and small surgical procedures)
Most local anaesthetics block voltage gated Na+ channels (like TTX but much less severe) -> no action potential
Signals from pain receptors cannot reach the brain (no sensation)

Question 12

Neuronal chains

Answer 12

Retinal receptor cell -> bipolar cell -> retinal ganglion cell (retina) -> retinal ganglion cell (optic nerve) -> retinal ganglion cell (brain) -> Thalamic cell -> cortical cell

Question 13

Knee-jerk (patellar) reflex

Answer 13

…..

Question 14

Excitatory and inhibitory neurons

Answer 14

Excitatory- to send an action potential, depolarise to continue action potential along
Inhibitory- inhibit this
Excitatory and inhibitory same level -> cancel each other
Hyperpolorisation- negative stimuli or increasing size, first not reaching threshold until eventually do and then when much higher it causes multiple action potential
Depolorisation- positive stimuli

Question 15

Transcranial Direct Current Stimulation (TDCS)

Answer 15

Low electric current applied through pads
o Safe and painless
Positive (anodal)
o Make neurons under the pad more likely to fire by hypopolarization or depolarization of resting potential
Negative (cathodal)
o Make neurons under the pad less likely to fire by hyperpolarization of resting potential
RESEARCH TO UNDERSTAND
o Send electrical current from blue to wrist= blue positive, red negative- depolarise blue
o Opposite- Hyperpolorise blue- make it harder for action potential by decrease resting potential

Question 16

Making people fairer

Answer 16

(Ruff et al. 2013 - Changing social norm compliance with non-invasive brain stimulation)
1. Baseline Round
o Player A -> 125 MUs (unit of reward) every trial
o Can share 0-100 (in intervals of 10) MUs with Player B
o Western cultures -> 10-25% winnings
2. Punishment Round
o Again, Player A can share 0-100 (in intervals of 10) MUs with Player B
o If Player B thinks it’s not fair they can punish Player A
o Western cultures we would send 40-50% of winnings

All believe same is a fair offer
All understand expected response from player B
Anodal just don’t care

Little work in other cultures but expect differences in other places
o Sanction-induced norm compliance, i.e. fairness −> Difference in offer between punishment and no punishment
o 3 groups of participants
o Anodal (depolarize lateral PFC neurons)
o Cathodal (hyperpolarize lateral PFC neurons)
o Sham (nothing)
o Changing the resting potential of neurons increases/decreases difference between baseline and punishment offers
o More likely to fire= less fair
o Less likely to fire (cathodal)= more fair
o Larger effect with real people (Social) vs computer (Non-social)

Question 17

Action potential

Answer 17

Resting potential (-70mV) -> stimulation (some sodium voltage gates open), if reach threshold -> all sodium voltage gated channels open = sodium in (depolarisation) -> sodium ion channels close, potassium channels open -> repolorisation (Na+ channels close, K+ channels open, K+ leaves the cell) -> too much potassium out-> hyperpoloristation -> some potassium gated channels stay open, sodium-potassium pump return to resting potential