Physiology and pharmacology of the skeletal neuromuscular junction Flashcards
How does a motor neuron innervate a skeletal muscle?
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
What is a motor unit?
Neuron and the no. of fibres it innervates
What is the terminal bouton?
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
Label the α-motor neuron and skeletal NMJ
Key feature of the skeletal NMJ?
- Terminal bouton and surrounding Schwann cell
- Synpatic vesicles
- Synaptic cleft
- End plate region of the muscle cell membrane (sarcolemma) thrown into a series of junctional folds
Describe release and binding of ACh
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
5 steps in synaptic transmission at the skeletal muscle NMJ?
- Synthesis
- Storage
- Release
- Receptor activation
- Transmitter activation
How is ACh synthesised (pre-synaptic process)?
- Choline is transported into the terminal by choline transporter (symport with Na+)
- ACh synthesises in cytosol from choline and acetyl coA (supplied by mitochondria) by CHOLINE ACETYLTRANSFERASE (CAT)
- ACh is conc. in vesicles by the vesicular ACh transporter
How is ACh released?
- Arrival of a.p at the terminal
- Depolarisation and opening of voltage-activated Ca2+ channels
- Ca2+ entry into the terminal
- Ca2+ causes vesicles at active zones to fuse with pre-synaptic membrane, allowing exocytosis of ACh
- ACh activates post-synaptic nicotinic ACh receptors in the end-plate region
Structure of nicotinic ACh receptors?
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
Permeability of nicotinic ACh channel to Na+ and K+?
Approx. equally permeable to Na+ and K+
Does not conduct anions
What directions are ions transported in through the channel?
When the gate is open:
Influx of Na+
Efflux of K+
What is the end plate potential (e.p.p)?
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
What is the electrical response to the one vesicle of neurotransmitter?
Each vesicle contains one quantum of neurotransmitter
Response to 1 quantum is a miniature e.p.p, due to activation of nicotinic ACh receptors
How does the e.p.p result?
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)
What does the e.p.p trigger?
Opening of voltage-activated Na+ channels, causing a muscle a.p
How can drugs/toxins block neuromuscular transmission?
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
Why is a muscle a.p, and thus Na+ channels, required for contraction?
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
How does the a.p cause release of Ca2+ from intracellular stores?
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
What are T-tubules?
Invaginations of the sarcolemma that dip deeply into the muscle cell
Why is there a latent period between the a.p and contraction?
It takes time for Ca2+ release, troponin to be moved away from the actin binding site, cross-bridge formation, etc
How is neuromuscular transmission terminated?
Result of hydrolysis of ACh by acetylcholinesterase (AChE), which is assoc. with the end plate membrane
- Choline is uptaken by choline transporter
- Acetate diffuses from the synaptic cleft
Describe the efficiency of AChE
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
What is neuromyotonia (NMT)?
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
Symptoms of NMT?
Multiple disorders of skeletal muscle function, inc:
Cramps
Stiffness
Slow relaxation (myotonia)
Fasciculations
Drug treatment of neuromyotonia?
Anti-convulsants, e.g: carbamezepine and phenytoin
These block Na+ channels
What is Lambert-Eaton myasthenic syndrome (LEMS)?
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
Symptoms of LEMS?
Muscle weakness in the limbs
Drug treatment of LEMS?
Increase ACh conc. in synaptic cleft:
- Anticholinesterases, e.g: pyridostigmine
- K+ channel blockers, e.g: 3,4-diaminopyridine
What is myasthenia gravis (MG)?
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.
Symptoms of MG?
Progressively increasing muscle weakness during periods of activity (fatiguability)
Often, weakness of the eye and eyelid muscles is a presenting feature
Compare symptoms of MG with those of LEMS?
LEMS may transiently improve with exertion but MG gets progressively worse with exertion
Drug treatment of MG?
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
What is the Botulinum toxin?
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
Drug treatment of Botulimin toxin?
Drugs that block acetylcholinesterase (i.e. anti-cholinesterases) are ineffective as therapy
HIGH MORTALITY
Clinical/cosmetic uses of Botulimin toxin?
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
What are Curare-like compounds?
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
Clinical uses of curare-like compounds?
Induce irreversible muscle paralysis in certain types of surgery
