NMJ and Muscle contraction

Question 1

What is a neuromuscular junction?

Answer 1

Specialised structure incorporating the distal exon terminal and muscle membrane that allows for unidirectional chemical communication between peripheral nerve and muscle

Question 2

What is a synapse?

Answer 2

Allows unidirectional flow

Contact ratio varies from 1:1 in muscle and 10^3:1 in the CNS

Question 3

What enzyme breaks down acetylcholine?

Answer 3

Acetylcholinesterase

Question 4

What are 3 points of failure of the neuromuscular junction?

Answer 4

Demyelination
Loss of ACHesterase
Faulty VGCCs

Question 5

What happens in demyelination?

Answer 5

Reduced conduction velocity

Weak/uncoordinated muscle movement

Question 6

Where does the upper motor neurone go?

Answer 6

in the brain

Question 7

Where does the lower motor neurone do?

Answer 7

in brainstem if to face

in spinal chord if to elsewhere

Question 8

What do motor neurones to effectors look like

Answer 8

Soma in butterfly shaped grey matter
Surrounded by white matter and tracts
With axons that leave the anterior ventral horn

Question 9

How many neurone branches does each muscle fibre receive?

Answer 9

One

Question 10

What happens if a muscle is reinnervated by a damaged nerve?

Answer 10

Then the muscle can have branches form multiple neurones and be innervated by branches from previously unconnected nerves

Question 11

What happens in the synapse in normal physiology?

Answer 11

AP opens VGCC causing Ca2+ influx
Ca2+ influx triggers vesicles exocytosis so ACh diffuses into the cleft and binds to receptor cation channels, opening these on the post synaptic neurone
Local currents flow from depolarised region and adjacent region, triggering an AP
ACh broken down by acetylcholinesterase and muscle fibre response ceases

Question 12

What happens in synapses at rest?

Answer 12

Individual vesicles release ACh at low rate causing minature end plate potentials (MEPPs)

Question 13

Voluntary or involuntary control?
Smooth muscle
Skeletal muscle
Cardiac muscle

Answer 13

Smooth muscle: involuntary - around round organs eg GI tract and vessels
Skeletal muscle: voluntary - allow for limb/body movement
Cardiac muscle: involuntary - cause myocardial contraction

Question 14

What is the structure of myofibre?

Answer 14

Covered by sarcolemma plasma membrane with t-tubules tunnelling to centre, and a sarcoplasm (cytoplasm) with myoglobulin and mitochondria present.

Question 15

What is the sarcoplasmic reticulum?

Answer 15

Network of fluid filled tubules that store Ca2+ ions

Question 16

What do myofibrils look like?

Answer 16

1-2um diameter extending along entire length of myofibres, composed of actin and myosin (and other proteins)

Question 17

What do myofilaments look like?

Answer 17

Straited due to light and dark bands
Do not extend along whole length of myofibres
Overlap and arranged into compartments called sarcomeres (Z-Z)

Question 18

What happens in eccentric muscle contraction?

Answer 18

Muscle gets longer during contraction
Painful
Can lead to shearing and ripping out tendons/muscles

Question 19

What happens in concentric muscle contraction

Answer 19

Muscle shortens during contraction

Question 20

What happens in relaxation of muscle?

Answer 20

Ca2+ dissociates from troponin when free Ca2+ declines
Preventing new crossbridges forming
Ca2+ actively transported back to SR during AP so for contraction release => return

Question 21

What are examples of NMJ disorders?

Answer 21

Botulism
Myasthenia Gravis
Lambert Eaton myasthenic syndrome

Question 22

What happens in botulism disorder?

Answer 22

Botulism toxin leads to irrevocable disruption in stimulation induced ACh release by presynaptic nerve terminal

Question 23

What happens in myasthenia gravis disorder?

Answer 23

Autoimmune disorder leading to Ab production against ACh receptors causing fatigable weakness -
affect ocular, bulbar (brainstem), respiratory or limb muscles - confirmed by EMG; may need plasma exchange/AChesterase inhibitors

Question 24

What happens in Lambert-Eaton myasthenic syndrome? (LEMS)

Answer 24

Autoimmune Ab production against VGCCs associated with lung cancer, may need ACh analogue

Question 25

What is corticobulbar

Answer 25

Cortex to brainstem to muscles of the face = preferentially affected by MG

Question 26

What is corticospinal?

Answer 26

Cortex to spinal chord to muscles of the body