Topic 4: Chemical Synaptic Transmission

Question 1

Name the 3 criteria for a Neurotransmitter

Answer 1

1. Must be present at the synapse (+ appropriate synthetic processes)
2. Must be released in response to appropriate stimuli through aCa2+ dependent mechanism
3. Specific post-synaptic receptors must be present

Question 2

Match the name of the 20th century scientist to their discoveries / beliefs

a) Provided substantial evidence that ACh was probably an endogenous chemical synaptic transmitter
b) Originally adhered to the electrical hypothesis of synaptic transmission but later changed his mind
c) Provided the strongest evidence in support of the chemical hypothesis by finding vagusstoff (a diffusible substance (ACh)) from one heart to affect another isolated heart that shared a solution
d) Won a Nobel prize

• John Eccles
• Sir Henry Dale
• Otto Loewi

Answer 2

a) Sir Henry Dale
b) John Eccles
c) Otto Loewi
d) All of them

Question 3

What is the neuromuscular junction and what is its structure?

Answer 3

The NMJ is a synapse between a motor-neuron and a skeletal muscle (also called the motor endplate)

Structure:
- Large branching structure that makes multiple contacts with a muscle fibre

Question 4

What are the 4 main functions of the NMJ?

Answer 4

1. Transmitter synthesis and vesicular storage
2. Post-junctional excitation-contraction coupling
3. Release of ACh (the endplate potential)
4. Transmitter release via Ca2+ induced vesicular exocytosis

Question 5

ACh is the transmitter that is in the nerve terminal of the NMJ, it is formed from choline and acetyl CoA.
a) What enzyme is used to synthesise it?
b) How is energy provided for this process?
c) How is ACh transported into vesicles

Answer 5

a) Choline Acetyltransferase (ChAT)
b) High H+ concentration inside the vesicle
c) Via a vesicular transporter

Question 6

ChAT is synthesised in the cell body and migrates to the nerve terminal at a)__________mm / da
ChAT in the pre-synaptic terminal is a biomarker that identifies neurons as b)___________

Answer 6

a) 0.5-5mm
b) Cholinergic

Question 7

Motor-neurone stimulation is termed the endplate potential (EPP) and usually leads to the depolarising of the muscle fibre via the activation of __________ (VGNC)
High densities of VGNC are found in the endplate to promote the AP propagation necessary for muscle contraction.

Answer 7

Voltage Gated Na+ Channels

Question 8

As stated previously, propagation of the AP along the muscle fibre is required for muscle contraction. It results in depolarisation of the a) _______ _______ and subsequent release of b) ____ from the c) _______ _______ via type 1 d) _______ receptors which causes muscle shortening.
At the base of the t-tubules are e)_______ voltage-gated calcium channels (DHP-receptors).

• Ca2+
• Ryanodine
• T-tubule membrane
• L-type
• Sarcoplasmic Reticulum

Answer 8

a) T-tubule membrane
b) Ca2+
c) Sarcoplasmic-reticulum
d) Ryanodine
e)L-type

The AP depolarises the t-tubule triggering conformational change in the L-type Ca2+ channels

Question 9

What are the 2 major pre-synaptic events / requirements that the EPP is dependent on?

Answer 9

1. A pre-synaptic action potential must invade the motor-neuronal terminal. Blocking VCNCs in the motor-neuron with tetrodotoxin abolishes the EPP
2. Pre-synaptic depolarisation opens voltage-gated Ca2+ channels allowing an influx of Ca2+. Preventing this abolishes the EPP

Question 10

What is meant by the ‘safety factor’ in relation to the EPP? and what is the size of the EPP proportional to?

Answer 10

The amount the EPP exceeds the AP threshold. The size of EPP is proportional to the amount of Ca2+ entering the motor-neuronal terminal (as it triggers exocytosis of ACh vesicles)

Question 11

What is the amplitude of the EPP?

Answer 11

20-30mV

Question 12

Quantal Theory

Even when the motor-neuron is not stimulated, there are spontaneous fluctuations in the muscle membrane potential that are of much smaller potential (a)______). These share the qualities of the EPP and so were termed b)___________.
The (b) is a unitary event that is the result of a single c)__________ of ACh being released. A normal EPP is made up of hundreds of these.

• miniature EPPS (mEPPS)
• Quanta
• ~0.5 mV

Answer 12

a) ~0.5mV
b) mEPP
c) Quanta

Question 13

Where are the following 4 SNARE proteins found?

a) Synaptobrevin
b) SNAP-25
c) Syntaxin
d) Synaptotagmin

• Vesicle membrane
• Pre-synaptic membrane

Answer 13

a) Vesicle membrane
b) Pre-synaptic membrane
c) Pre-synaptic membrane
d) Vesicle membrane

Question 14

In endocytosis there are 2 protein players, a)_______ attaches to the membrane with the help of adapter proteins. The vesicle is formed and then is cleaved off by b)______. (a) is the removed before the vesicle is reused.

• Dynamin
• Clathrin

Answer 14

a) Clathrin
b) Dynamin

Question 15

An action ptential invading the motorneuronal predsynaptic terminal elicits th release of a)__________. This causes depolarisation in the post-synaptic muscle called an b)____________.

Answer 15

a) ACh
b) EPP

Question 16

Receptors that mediate the EPP are nicotinic ACh receptors (nAChR). What super-family are these receptors part of?

Answer 16

Ligand-gated ion channels

Question 17

Structure of nAChR

The consist of 5 subunits (2 alphas, beta, delta and gamma) Each subunit has 4 membrane spanning alpha-helices and an extracellular amino acid and carboxyl terminal.

The extracellular domain is large and contains 2 non-identical binding sites for ACh, both of which must be bound for activation.

When pores are activated, a ~15 degree rotation of the outer subunit structure (triggered by agonist binding) causes opening of the pore

Question 18

Structure of nAChR

The consist of 5 subunits (2 alphas, beta, delta and gamma) Each subunit has 4 membrane spanning alpha-helices and an extracellular amino acid and carboxyl terminal.

The extracellular domain is large and contains 2 non-identical binding sites for ACh, both of which must be bound for activation.

When pores are activated, a ~15 degree rotation of the outer subunit structure (triggered by agonist binding) causes opening of the pore

Question 19

At resting potential, the muscle membrane is at equilibrium. When a gate opens, ion flow is determined by 2 concentration gradients, what are they?

Answer 19

Ion conc. gradient
Electrical gradient

Question 20

What does the equilibrium potential of an ion refer to?

E = RT/zF In [ion]0/[ion]1

Answer 20

The membrane potential where both the ionic gradient and the electrical gradient are equal and opposite, resulting in no net movement of ions

Question 21

What does the reversal potential refer to?

Answer 21

• By observing End Plate Current (EPC) from activation of nAChR at the NMJ we find that the potential at which there is no EPC is clos to 0 mV.
• this is called the reversal potential for nAChR
• This tells us that the membrane is equally permeable to both Na+ and K+

Question 22

Explain how ACh is broken down and recycled in the NMJ

Answer 22

a) - Acetylcholinesterase hydrolyses ACh to choline and acetate (fast)
- High affinity transporter then transports choline back into motoneuronal terminal
- Hemicholinium inhibits the function of this transporter

Question 23

The function of non-depolarising blockers such as Tubocurarine (in curare) which is a competitive, reversible, receptor antagonist to nAChR that paralysed the prey of South American natives. How can this paralysing effect be overcome?

Answer 23

-Through administration of an anti-cholinesterase drug
- Tubocurarine can be outcompeted if the ACh conc is high enough in the synaptic cleft. Preventing ACh breakdown does this

Question 24

How does the depolarising blocker Suxamethonium work?

Answer 24

• Structurally is two ACh molecules covalently linked by a acetyl groups
• Binds and activates the nAChR but causes a sustained activation.
• this ultimately causes loss of electrical excitability due to prolonged Na+ channel inactivation

Question 25

Each muscle fibre consists of bundles of myofibrils. These consist of 2 types of filaments, what are they?

Answer 25

Thin filaments - f-actin
Thick filaments - myosin

Question 26

What is the role of the sarcoplasmic reticulum in muscles

Answer 26

• Ca++ ion regulation, storage and release
• Ca++ ions are released via ryanodine receptors after the SR has been stimulated

Question 27

Describe the function that Ca++ plays in muscle contraction:
- relaxation / storage
- contraction

Answer 27

• Relaxation / Storage: Large amounts of Ca++ are stored in the SR. Ca is actively transported from the surrounding cytoplasm during relaxation
• Contraction:
  SR releases Ca++, binds to TROPONIN causing TROPOMYOSIN to move (conformational change).
  Myosin heads can then bind to actin

Question 28

Which statement concerning muscle nicotinic acetylcholine receptors is TRUE

a) At normal muscle resting potentials current flow through the open channel is carried mainly by Na+ .

b) They are pentamers composed of subunits with five transmembrane spanning domains.

c) They have two identical acetylcholine binding sites.

d) A miniature endplate potential is the result of the activation of a single receptor.

e) α-bungarotoxin is receptor specific agonist.

Answer 28

A

Question 29

Contraction of skeletal muscle elicited by motorneurone stimulation is dependent on

a) the release of nicotine from motorneurone terminals.

b) G protein-coupled receptor activation.

c) pre-synaptic calcium entry through voltage-gated calcium channels.

d) inhibition of acetylcholinesterase.

e) hyperpolarisation of the pre-synaptic terminal.

Answer 29

C

Question 30

Tubocurarine at the neuromuscular junction

a) can be used as a general anaesthetic.

b) is a competitive blocker of muscarinic receptors.

c) is a depolarising blocker.

d) causes hyperpolarisation, then block of the endplate.

e) can have its block reversed by neostigmine.

Answer 30

E