lecture 11 - neuromuscular junctions Flashcards

1
Q

What is the alternative name for the neuromuscular junction?

A

Motor end plate

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2
Q

How many neurons innervate a single muscle fibre?

A

1

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3
Q

How many muscle fibres does a single motor neruon supply?

A

Usually multiple

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4
Q

What neurotransmitter is used at the neuromuscular junction?

A

ACh - Acetylcholine

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5
Q

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

A

AChE - acetylcholine esterase

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6
Q

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

A

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

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7
Q

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

A

nAChR (nicotinic acetylcholine receptor)

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8
Q

What is the life cycle of Acetylcholine at the NMJ?

A

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

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9
Q

What are the products of ACh breakdown by AChE?

A

Choline, Acetic Acid

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10
Q

What is AChE?

A

Acetylcholine esterase - breaks down ACh in the synaptic cleft

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11
Q

What is myasthenia gravis?

A

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

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12
Q

How do antibodies impact nAChR in myasthenia gravis?

A

Reduce receptor number by blocking receptors, damaging the endplate.

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13
Q

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

A

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

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14
Q

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

A

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

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15
Q

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

A

Sarcolemma

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16
Q

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

A

Stores calcium for release for contraction

17
Q

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

A

Myofibril

18
Q

What are the 2 myofilaments within a muscle?

A

Actin, myosin

19
Q

What are the units within a myofibril?

A

Myofilaments (actin, myosin)

20
Q

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

A

Sarcomere

21
Q

What landmark marks the ends of a sarcomere?

A

Z disc

22
Q

What is the thick filament in a muscle?

A

Myosin

23
Q

What is the thin filament in a muscle?

A

Actin

24
Q

What is the H zone of a sarcomere?

A

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

25
Q

What zone/bands gets smaller during a muscle contraction?

A

H zone, I band

26
Q

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

A

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

27
Q

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

A

Actin binding site, ATP binding site

28
Q

What condition is required for cross-bridge cycling?

A

High Ca2+ concentration in the sarcoplasm

29
Q

What does Ca2+ bind in the sarcoplasm?

A

Troponin

30
Q

What are the steps in the cross-bridge cycle?

A

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

31
Q

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

A

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