Transmission Introduction

Question 1

What are action potentials?

Answer 1

Cells capable of electrical transmission (neurones, muscle) can dynamically vary their membrane potential (MP)
The dynamic variation in MP results in an action potential

Question 2

How do action potentials form?

Answer 2

=Stimulus depolarises the membrane from - 70mV
=Triggers opening of voltage-gated Na channels – depolarisation (up to +30mV)
=Then voltage-gated Na channels close and K channels open – repolarisation
=This leads to brief ‘hyper-polarisation’
=A further AP cannot be induced until the end of the relative refractory period
Depolarisationa nd repolarisation= absolute refractory period

Question 3

How are action potentials triggered?

Answer 3

Depolarising or hyperpolarising stimulus
=Localised ion channel state-change (open/closed)
-Ligand gated ion channels (open when appropriate ligand bind) neurotrasnmitters
-Voltage gated ion channels (open in response to voltage change)
=Propagation of charge from adjacent region

Question 4

Describe propagation of AP

Answer 4

• Slow in unmyelinated axons
• High energy requirement (Na/K-ATPase)
• Saltatory conduction via Nodes of Ranvier= increased conduction velocity, lowers energy expenditure (Na/K-ATPase)

Question 5

How do we measure propagation in clinical neurophysiology?

Answer 5

Process
=Electrical stimulation of nerve fibres, initiating a propagated AP
Metrics
=Conduction speed (surrogate for myelin status)
=Amplitude of response: dependent upon number of nerves present and stimulated (surrogate for axonal status) how strong the signal is

Question 6

Describe chemical transmission

Answer 6

=Action potentials at the terminal bouton trigger voltage-gated Ca++ channels to open
=Ca++ influx induces exocytosis of synaptic vesicles
=Neurotransmitter diffuses across the synaptic cleft
=Neurotransmitter binds to receptors in the post-synaptic cell
=Receptor binding induces electrical or chemical response

Question 7

What are the excitatory neurotransmitters?

Answer 7

Glutamate
=AMPA – fast acting
=NMDA – have Mg2+ block
=Metabotropic – G-protein coupled

Acetylcholine
=Nicotinic – fast acting
=Muscarinic – slower acting

Noradrenaline
=Flight or fight
=α and β receptors

Serotonin
=Multiple roles

Dopamine
=Basal ganglia functions; reward

Question 8

What are the inhibitory neurotransmitters?

Answer 8

GABA
=Linked to a Cl- channel
=Cl- hyperpolarises the cell making it more difficult for the cell to fire
=Alcohol is a GABA-like molecule
=Act through benzodiazepine receptors – utilised in clinical medicine with benzodiazepines e.g. Diazepam