Neuro-muscular junction

Question 1

What is the neuromuscular junction?

Answer 1

Motor nerve terminal at the motor endplate
Ultimate synapse

Question 2

What is the function of the Schwann cells?

Answer 2

Overlay the terminals which prevents leakage of current
Amplitude doesn’t decrease

Question 3

What happens at rest?

Answer 3

Random blips in the resting potential are present (MEPPs)

Question 4

What happens when a nerve is stimulated?

Answer 4

Membrane potential depolarizes - EPP triggers and action potential when it reaches the threshold

Question 5

What neurotransmitter is present in each synaptic vesicle?

Answer 5

ACh

Question 6

How is ACh synthesized?

Answer 6

In the nerve terminal cytoplasm from choline and Acetyl CoA via the enzyme ChAT

Question 7

How is ACh hydrolysed?

Answer 7

ACh is hydrolysed after it is released by acetylcholinesterase in synaptic basal lamina

Question 8

What is the effect of 4-AP?

Answer 8

Blocks K+ chnanels - depolarization prolongs action potential
Allows more Ca2+ to enter which increases number of vesicles used

Question 9

What happens when Ca2+ is lowered?

Answer 9

Decreases number of vesicles available to fuse
Decreases number of transmitters released due to Ca2+ voltage gated channels at terminal
Subthreshold potential generated

Question 10

What happens to endocytosed vesicles?

Answer 10

Coated with clathrin -> incorporated into cisternae of the Golgi apparatus -> new vesicles bud off from this

Question 11

What recycles synaptic vesciles?

Answer 11

HRP

Question 12

What causes transmitter release?

Answer 12

Entry of Ca2+ through presynaptic voltage-gated calcium channels

Question 13

What are MEPPs caused by?

Answer 13

Release of one vesicle

Question 14

What is found at each receptor?

Answer 14

Pentamer of subunits enclosing a transmembrane pore that is permeable to Na+ and K+ ions

Question 15

What initiates an inward current?

Answer 15

2 ACh molecules binding to alpha subunits of one ACh receptor

Question 16

How is the Endplate potential produced?

Answer 16

Release of ACh in response to a nerve impulse produces a large Endplate Current (EC) in muscle fiber which causes it to depolarize

Question 17

What does the EPP have to be larger than to activate sodium channels?

Answer 17

20mV

Question 18

How is synaptic transmission depressed?

Answer 18

Reducing extracellular Ca2+ ion concentration or increasing extracellular Mg2+

Question 19

How can an EPP be measured?

Answer 19

Measurements of the fluctuation of EPPs and sizes of MEPPS allow the ‘quantal content’

Question 20

How can a measure of the mean quantal content (m) be produced?

Answer 20

Dividing the mean EPP amplitude by the mean MEPP amplitude

Question 21

How many quantal units are required to reach the threshold for an action potential?

Answer 21

12

Question 22

What is the safety factor?

Answer 22

Excess of release over what is required to fire an action potential, very high at NMJ

Question 23

What happens with repetitive stimulation?

Answer 23

Amplitude and quantal content of EPPs is reduced

Question 24

What happens if action potentials aren’t present?

Answer 24

Synaptic potentials would decay exponentially with distance from their origin, currents would leak out of the cell