Physiology and pharmacology of the skeletal neuromuscular junction

Question 1

How does a motor neuron innervate a skeletal muscle?

Answer 1

Axon divides into UNMYELINATED branches and these each innervate one muscle cell

Further branching occurs, each ending in a terminal bouton, that forms a synapse with the muscle at the NMJ

Question 2

What is a motor unit?

Answer 2

Neuron and the no. of fibres it innervates

Question 3

What is the terminal bouton?

Answer 3

Pre-synaptic terminal that synapses at the endplate region of the skeletal muscle fibre a.p that arises in the cell body is conducted via the axon to the bouton, causing release of ACh

Question 4

Label the α-motor neuron and skeletal NMJ

Answer 4

Question 5

Key feature of the skeletal NMJ?

Answer 5

1. Terminal bouton and surrounding Schwann cell
2. Synpatic vesicles
3. Synaptic cleft
4. End plate region of the muscle cell membrane (sarcolemma) thrown into a series of junctional folds

Question 6

Describe release and binding of ACh

Answer 6

Synaptic vesicles awaiting release cluster at ACTIVE ZONES (where vesicles wait to be released)

Nicotinic ACh receptors are located at regions of the junctional folds that face the active zones, increasing the chances of ACh binding to receptors

Question 7

5 steps in synaptic transmission at the skeletal muscle NMJ?

Answer 7

1. Synthesis
2. Storage
3. Release
4. Receptor activation
5. Transmitter activation

Question 8

How is ACh synthesised (pre-synaptic process)?

Answer 8

1. Choline is transported into the terminal by choline transporter (symport with Na+)
2. ACh synthesises in cytosol from choline and acetyl coA (supplied by mitochondria) by CHOLINE ACETYLTRANSFERASE (CAT)
3. ACh is conc. in vesicles by the vesicular ACh transporter

Question 9

How is ACh released?

Answer 9

1. Arrival of a.p at the terminal
2. Depolarisation and opening of voltage-activated Ca2+ channels
3. Ca2+ entry into the terminal
4. Ca2+ causes vesicles at active zones to fuse with pre-synaptic membrane, allowing exocytosis of ACh
5. ACh activates post-synaptic nicotinic ACh receptors in the end-plate region

Question 10

Structure of nicotinic ACh receptors?

Answer 10

Pentamer of glycoprotein subunits - (α1)2β1δε

These surround a central, cation-selective pore that contains a gate (closed when ACh absent and open when TWO ACh molecules bind to the receptor)

Lining the pore are 5 M2 α-helices, which have an amino acid side chain that twists away to dilate the pore when ACh binds

Question 11

Permeability of nicotinic ACh channel to Na+ and K+?

Answer 11

Approx. equally permeable to Na+ and K+

Does not conduct anions

Question 12

What directions are ions transported in through the channel?

Answer 12

When the gate is open:

Influx of Na+

Efflux of K+

Question 13

What is the end plate potential (e.p.p)?

Answer 13

Driving force for Na+ is greater than for K+ at resting membrane potential; influx of Na+ is greater than efflux of K+ and this generates a depolarization known as the e.p.p

Question 14

What is the electrical response to the one vesicle of neurotransmitter?

Answer 14

Each vesicle contains one quantum of neurotransmitter

Response to 1 quantum is a miniature e.p.p, due to activation of nicotinic ACh receptors

Question 15

How does the e.p.p result?

Answer 15

Many miniature e.p.ps summate to produce the e.p.p (a GRADED response)

If an e.p.p exceeds threshold, it triggers opening of Na+ channels around the end plate, causing an a.p (this is NOT GRADED; it is an all-or-none response)

Question 16

What does the e.p.p trigger?

Answer 16

Opening of voltage-activated Na+ channels, causing a muscle a.p

Question 17

How can drugs/toxins block neuromuscular transmission?

Answer 17

Reduce the amplitude of the e.p.p., so it does not reach threshold, Na+ channels do not open and no a.p is generated

Thus, no contraction can occur

Question 18

Why is a muscle a.p, and thus Na+ channels, required for contraction?

Answer 18

If there were no Na+ channels, and thus no a.p, the e.p.p would decline as it spread from the end plate

If the muscle fibre has Na+ channels, an a.p propagates from the endplate over the length of the fibre

Question 19

How does the a.p cause release of Ca2+ from intracellular stores?

Answer 19

Propagates over the surface membrane (sarcolemma) and enters transverse (T) tubules, which are in close apposition with the SR

Triggers Ca2+ release and this causes contraction, via interaction with troponin

Question 20

What are T-tubules?

Answer 20

Invaginations of the sarcolemma that dip deeply into the muscle cell

Question 21

Why is there a latent period between the a.p and contraction?

Answer 21

It takes time for Ca2+ release, troponin to be moved away from the actin binding site, cross-bridge formation, etc

Question 22

How is neuromuscular transmission terminated?

Answer 22

Result of hydrolysis of ACh by acetylcholinesterase (AChE), which is assoc. with the end plate membrane

1. Choline is uptaken by choline transporter
2. Acetate diffuses from the synaptic cleft

Question 23

Describe the efficiency of AChE

Answer 23

Hydrolysis of some ACh molecules occurs prior to the them binding to nicotinic ACh receptors, i.e: some ACh molecules are hydrolysed before the can bind to nACh receptors

Once unbinding occurs, all ACh molecules are hydrolysed, limiting rebinding and terminating the e.p.p. within a few ms

Question 24

What is neuromyotonia (NMT)?

Answer 24

AKA Isaac’s syndrome

In the acquired form, it may be autoimmune and there are antibodies against voltage-activated K+ channels in the motor neurone; this causes hyper-excitability and repetitive firing

Question 25

Symptoms of NMT?

Answer 25

Multiple disorders of skeletal muscle function, inc:

Cramps

Stiffness

Slow relaxation (myotonia)

Fasciculations

Question 26

Drug treatment of neuromyotonia?

Answer 26

Anti-convulsants, e.g: carbamezepine and phenytoin

These block Na+ channels

Question 27

What is Lambert-Eaton myasthenic syndrome (LEMS)?

Answer 27

Autoimmune antibodies against voltage-activated Ca2+ channels in the motor neurone terminal cause reduced Ca2+ entry, in response to depolarization, and so reduced vesicular release of ACh

ASSOC. WITH SMALL CELL CARCINOMA OF LUNG

Question 28

Symptoms of LEMS?

Answer 28

Muscle weakness in the limbs

Question 29

Drug treatment of LEMS?

Answer 29

Increase ACh conc. in synaptic cleft:

1. Anticholinesterases, e.g: pyridostigmine
2. K+ channel blockers, e.g: 3,4-diaminopyridine

Question 30

What is myasthenia gravis (MG)?

Answer 30

Majority is autoimmune with antibodies against nicotinic ACh receptors in the endplat; results in reduction of the no. of functional channels and so the amplitude of the e.p.p.

Question 31

Symptoms of MG?

Answer 31

Progressively increasing muscle weakness during periods of activity (fatiguability)

Often, weakness of the eye and eyelid muscles is a presenting feature

Question 32

Compare symptoms of MG with those of LEMS?

Answer 32

LEMS may transiently improve with exertion but MG gets progressively worse with exertion

Question 33

Drug treatment of MG?

Answer 33

Anticholinesterases - increase conc. of ACh in the synaptic cleft, e.g:

• Edrophonium - for diagnosis
• Pyridostigmine - for long term treatment

Variety of immunosuppressant agents, e.g: azathioprine

Question 34

What is the Botulinum toxin?

Answer 34

Potent exotoxin (related to tetanus and diptheria toxins) that acts at motor neurone terminals to IRREVERSIBLY inhibit ACh release

It enters pre-synaptic terminals and enzymatically modifies proteins inv. in docking of vesicles, so prevents exocytosis of ACh

Question 35

Drug treatment of Botulimin toxin?

Answer 35

Drugs that block acetylcholinesterase (i.e. anti-cholinesterases) are ineffective as therapy

HIGH MORTALITY

Question 36

Clinical/cosmetic uses of Botulimin toxin?

Answer 36

Low-dose botulinum haemaglutin complex can be administered (IM) to treat overactive muscles (dystonias), e.g: extraocular muscles or eyelids (blepharospasm)

Smooth out age-related wrinkles as well

Question 37

What are Curare-like compounds?

Answer 37

Interfere with the post-synaptic action of ACh by acting as competitive antagonists of the nicotinic ACh receptor, e.g: vecuronium, atracurium

Reduce the amplitude of the e.p.p to below the threshold for muscle fibre a.p generation

Question 38

Clinical uses of curare-like compounds?

Answer 38

Induce irreversible muscle paralysis in certain types of surgery