The NMJ

Question 1

What is the NMJ

Answer 1

Neuromuscular junction that is a specialised synapse between a motor neurone and a muscle fibre

Question 2

Which neurotransmitter is used for voluntary striated muscle

Answer 2

Acetylcholine

Question 3

How is innervation of neurons important

Answer 3

A neuron will innervate a number of muscle of fibres through branches, but none of these will be innervated by branches of other neurons

Question 4

Describe the sequence of events that occur for NMJ

Answer 4

1. AP opens the voltage-gated calcium channels
2. Calcium enters
3. calcium triggers exocytosis
4. acetylcholine diffuses in the cleft
5. acetylcholine binds to the receptor cation channel to open it
6. Local currents flow from the depolarised region and adjacent region so the AP spreads along the surface membrane
7. Acetylcholine is broken down by acetylcholine esterase
8. Muscle fibre response to the acetyl choline ceases

Question 5

What occurs at rest

Answer 5

Individual vesicles release acetyl choline at a very low rate to cause miniature end-plate potentials (MEPPs)

Question 6

Describe the structure of myofibres

Answer 6

covered by plasma membrane/sarcolemma
T-tubules tunnel to the centre
myoglobin and mitochondria present

Question 7

Describe the structure of myofibrils

Answer 7

1-2μm in diameter
Extends long the entire length of myofibre
Composed of actin and myosin

Question 8

What occurs in sliding filament theory

Answer 8

1. AP propagates along surface membrane and into T-tubules
2. DHP (dihydropyridine) receptor changes shape of the protein link to Ryanodine receptor, opens the Ryanodine receptor Ca2+ channel in the SR membrane; Ca2+ released from SR into space around the filaments
3. Ca2+ binds to Troponin & Tropomyosin moves allowing
   Crossbridges to attach to actin
4. Ca2+ is actively transported into the SR continuously while action potentials continue.
5. Ca2+ dissociates from TN when free Ca2+ declines; TM block prevents new crossbridge attachment; Active force declines due to net crossbridge detachment

Question 9

What occurs to the appearance of the sarcomere in sliding filament theory

Answer 9

I bands become shorter
A bands remain the same length
H-zone narrows and disappears

Question 10

Give 3 examples of disorders that involve interference with NMJ function

Answer 10

Botulism
Myasthenia Gravis
Lambert-Eaton myasthenia syndrome

Question 11

What occurs in botulism

Answer 11

Bacterial

Botulism toxin produces an irreversible disruption in acetylcholine release by the presynaptic nerve terminal

Question 12

What occurs in Myasthenia Gravis

Answer 12

Autoimmune disorder where antibodies are directed against the acetylcholine receptor
May affect ocular, bulbar, respiratory or limb muscles

Question 13

What occurs in Lambert-Eaton myastenic syndrome

Answer 13

autoimmune disease caused by antibodies directed against the voltage gated calcium channels
Associated with lung cancer