Neuroscience 4 - NMJ and muscle contraction

Question 1

What is a motor unit?

Answer 1

1. One motor neuron and all the muscle fibers it innervates.
2. This is the functional unit of a skeletal muscle.
3. The motor unit size varies (number of muscle fibres per motor neuron).

Question 2

What is recruitment?

Answer 2

Recruiting more motor units in order to generate a greater force.

Question 3

What is the maximum voluntary contraction (MCV)?

Answer 3

When you recruit all motor units to get the maximum possible contraction.

Question 4

What is a neuromuscular junction (NMJ)?

Answer 4

A specialised synapse between a motor neuron and a muscle fibre.

Question 5

Where are the cell bodies of the motor neurons found?

Answer 5

In the Anterior horn of the Grey matter.

Question 6

Are muscle fibres innervated by more than one motor neuron? What can change this?

Answer 6

1. Not under normal circumstances.
2. Damage to the axons by trauma or disease can cause regrowth and them to re-innervate other muscle fibres that are already innervated.

Question 7

What are the 7 steps seen at the NMJ to stimulate a muscle contraction?

Answer 7

1. Action potential opens VGCC.
2. Ca2+ enters.
3. Ca2+ triggers exocytosis of vesicles.
4. Ach diffuses in cleft.
5. Ach binds to receptor on muscle fibre and opens channel.
6. Local currents flow from depolarized region and adjacent region; action potential triggered and spreads along surface of membrane.
7. Ach broken down by Ach esterase and muscle fibre response stops.

Question 8

What are Miniature End-Plate Potentials (MEPP)?

Answer 8

1. At rest, individual vesicles release Ach at very low rate causing very slight changes to membrane potential.
2. Does not cause an action potential as threshold is not reached.

Question 9

What are 3 disorders of the NMJ?

Answer 9

1. Botulism.
2. Myasthenia Gravis (MG).
3. Lambert-Eaton myasthenic syndrome (LEMS).

Question 10

What is Botulism?

Answer 10

Botulinum toxin produces an irreversible disruption in stimulation-induced Ach release by the presynaptic nerve terminal.

Question 11

What is Myasthenia Gravis?

Answer 11

1. Autoimmune disorder where antibodies are directed against the Ach receptor.
2. Fatigable weakness and affects ocular, bulbar, respiratory and limb muscles.
3. Antibodies detected in 90% of cases and EMG examination confirms diagnosis.
4. Antibodies in blood can be removed via plasma exchange = rapid improvement.

Question 12

What is Lambert-Eaton myasthenic syndrome?

Answer 12

1. Autoimmune disease caused by antibodies directed at
   VGCC.
2. Associated with lung cancer.