lecture 11 - neuromuscular junctions

Question 1

What is the alternative name for the neuromuscular junction?

Answer 1

Motor end plate

Question 2

How many neurons innervate a single muscle fibre?

Answer 2

1

Question 3

How many muscle fibres does a single motor neruon supply?

Answer 3

Usually multiple

Question 4

What neurotransmitter is used at the neuromuscular junction?

Answer 4

ACh - Acetylcholine

Question 5

What enzyme is used to break down the neurotransmitter at a neuromuscular junction?

Answer 5

AChE - acetylcholine esterase

Question 6

What are the sequence of events during a synapse at the neuromuscular junction?

Answer 6

1.) AP in axon
2.) Voltage gated Ca2+ channels open and Ca2+ enters presynaptic cell
3.) Vesicles fuse and release ACh
4.) AcH binds nAChR on muscle fibre
5.) nAChR channel opens and Na+ enters muscle fibre leading to local depolarisation
6.) depolarisation opens more voltage gated Na+ channels
7.) AP propogates down muscle fibre
8.) ACh degraded by AChE

Question 7

What neurotransmitter receptor is found on the postsynaptic cell of the postsynaptic cell?

Answer 7

nAChR (nicotinic acetylcholine receptor)

Question 8

What is the life cycle of Acetylcholine at the NMJ?

Answer 8

ACh packaged into vesicles, released into cleft, triggers nACHr, broken down into Choline & Acetic Acid by AChE, transported into neuron by Na+ choline transporter, converted to ACh again

Question 9

What are the products of ACh breakdown by AChE?

Answer 9

Choline, Acetic Acid

Question 10

What is AChE?

Answer 10

Acetylcholine esterase - breaks down ACh in the synaptic cleft

Question 11

What is myasthenia gravis?

Answer 11

An autoimmune condition where nACh receptors are reduced, meaning ACh activation diminished with repetitive activation at the Neuromuscular Junction

Question 12

How do antibodies impact nAChR in myasthenia gravis?

Answer 12

Reduce receptor number by blocking receptors, damaging the endplate.

Question 13

What type of medication is used to treat myasthenia gravis, and why?

Answer 13

AChE blockers (anticholinesterases) - they increase the amount of ACh in the neuromuscular junction by decreasing enzyme breakdown, increasing the activation of nAChRs

Question 14

What is the role of T-tubules in a muscle fibre?

Answer 14

Propagates action potentials into the fibre by going deep in between the myofibrils

Question 15

What is the name for the cell membrane of a muscle fibre?

Answer 15

Sarcolemma

Question 16

What is the role of the sarcoplasmic reticulum in a muscle fibre?

Answer 16

Stores calcium for release for contraction

Question 17

What is the name for a bundle of filaments within a muscle fibre?

Answer 17

Myofibril

Question 18

What are the 2 myofilaments within a muscle?

Answer 18

Actin, myosin

Question 19

What are the units within a myofibril?

Answer 19

Myofilaments (actin, myosin)

Question 20

What is a single unit formed of a thick and thin filament within a muscle cell?

Answer 20

Sarcomere

Question 21

What landmark marks the ends of a sarcomere?

Answer 21

Z disc

Question 22

What is the thick filament in a muscle?

Answer 22

Myosin

Question 23

What is the thin filament in a muscle?

Answer 23

Actin

Question 24

What is the H zone of a sarcomere?

Answer 24

Region in middle, where there is only myosin, and no overlapping actin

Question 25

What zone/bands gets smaller during a muscle contraction?

Answer 25

H zone, I band

Question 26

What are steps in excitation-contraction coupling in skeletal muscle?

Answer 26

1.) AP enters T-tubules
2.)depolarisation in T-Tubule activates DHP protein which opens RYR channel in Sarcoplasmic reticulum
3.) Ca2+ leaves SR down concentration gradient
4.) Binding of Ca2+ to troponin moves tropomyosin off binding sites
5.) Cross-bridge cycle activated, force generated
6.) Ca2+ pumped back into SR
7.) Tropomyosin moves back into binding sites, relaxation occurs

Question 27

What are the 2 binding sites found on a myosin head?

Answer 27

Actin binding site, ATP binding site

Question 28

What condition is required for cross-bridge cycling?

Answer 28

High Ca2+ concentration in the sarcoplasm

Question 29

What does Ca2+ bind in the sarcoplasm?

Answer 29

Troponin

Question 30

What are the steps in the cross-bridge cycle?

Answer 30

1.) tropomyosin moves off binding sites as Ca2+ binds to troponin
2.) ADP + Pi released from myosin head, which engages and flexes, generating force.
3.) ATP binds, myosin head detaches from actin
4.) if low [Ca2+], binding sites covered

Question 31

Why does a muscle twitch have a much longer duration than a muscle action potential?

Answer 31

It takes time for the release/re-uptake of Ca2+