Neuromuscular Junction

Dr. Hoe See Ziau Department of Physiology Faculty of Medicine University of Malaya

Types of Connection  Connection with another neuron → a synapse

 Connection with muscle (skeletal, cardiac

or smooth) → a neuromuscular junction (NMJ)

Neuromuscular Junction (NMJ)  Special junction between a

axon terminal and a muscle fibre  Axon terminal is enlarged into

a knob-like structure → terminal button (Synaptic knob)

Structure of NMJ  Located in the middle of

   

the long cylindrical muscle fibre No myelin sheath Lots of mitochondria and vesicles No direct contact between axon and muscle Synaptic cleft – 50-100nm (narrow space)

Structure of NMJ  Motor End Plate (MEP) – differentiated version of membrane of muscle fibre

 MEP – numerous junctional folds  To increase surface area

 ACh Receptors

Neurotransmitter at NMJ  Occurs chemically – a chemical messenger

is used to carry the signal between the neuron terminal & the muscle fiber  Synaptic knobs – 200 000 to 300 000

vesicles  Contain neurotransmitter: acetylcholine

(ACh)  ACh binds with nicotinic cholinergic receptor

at the post-synaptic membrane  Small amount of ACh that does not bind will

be loss

Neurotransmitter at NMJ  After binding with the receptor (1 – 2 msec after released), ACh will be hydrolysed to choline and acetate by acetylcholinesterase

 Choline returns to motor nerve terminal for ACh resynthesis  ACh recycled

Quantal Release of ACh 1.

The presence of an action potential in the terminal button triggers the opening of voltage-gated Ca2+ channels → Influx of Ca2+ into the terminal button

2.

Ca2+ → triggers the vesicles to dock onto terminal membrane

→ Also causes docked vesicles to fuse with the membrane and the release of Ach into synaptic cleft by exocytosis



1 vesicle = 1 quantum (contains ~ 10,000 ACh)



Each nerve impulse can stimulate the release of 100 quanta ( 106 ACh)

End-Plate Potential (EPP)  Depolarisation of motor end plate  Graded potential  Magnitude depends on the amount and duration of ACh at

the end-plate

 Undergoes summation  When the EPP reaches threshold ( -50 mV)

→ Action potential in adjacent muscle membrane

Formation of the EPP  ACh diffuses to the motor end-plate  ACh binds to specific receptor site on

motor end-plate  Binding of ACh with receptor → chemical-gated channels to opens → non-specific: influx of Na+ and efflux of K+

→ Influx of Na+ > efflux of K+ → local depolarisation of membrane → Endplate potential → ↑ binding → ↑ EPPs

End-Plate Potential (EPP)  The motor end plate region does not have a threshold

potential, so an action potential cannot be initiated at this site  EPP brings about an action potential in the rest of the adjacent

muscle fibres  When EPP takes place, local current flow occurs between the

depolarised end plate & the adjacent, resting cell membrane in both direction, opening voltage-gated Na+ channels → reducing the potential to threshold in the adjacent areas → action potential  Action potential spreads to whole skeletal muscle

Miniature End-Plate Potential (MEPP)  At rest → random movements of vesicles (Brownian

movements)  When one vesicle touches terminal membrane →

spontaneous release one quantum of ACh by exocytosis  At motor end-plate → causes small depolarisation of

membrane  Small depolarisations are called miniature EPPs  Not followed by an action potential

Events at an NMJ 1. An action potential in a motor neuron is propagated to the terminal button 2. The presence of an action potential in the terminal button triggers the opening of voltagegated Ca2+ channels and the subsequent entry of Ca2+ into the terminal button 3. Ca2+ triggers the release of ACh into synaptic cleft by exocytosis

Events at an NMJ 4.

ACh diffuses across the synaptic cleft & binds with ACh receptors on the motor end plate of the muscle cell membrane

5.

This binding brings about the opening of cation channels, leading to a relatively large movement of Na+ into the muscle cell compared to a smaller movement of K+ outward

6.

The result is an end-plate potential (EPP). Local current flow occurs between the depolarised end plate & adjacent membrane

7.

This local current flow opens voltage-gated Na+ channels in the adjacent membrane

Events at an NMJ 8. The resultant Na+ entry reduces the potential to threshold, initiating an action potential, which is propagated throughout the muscle fibre  Muscle action potential spreads through the T tubules → causes Ca2+ to be released from sarcoplasmic reticulum → Ca2+ binds with troponin C, actin slides into myosin

→ Contraction

Events at an NMJ 9. ACh hydrolysed to choline + acetate and taken back into presynaptic knob  Motor end-plate repolarises  inside the presynaptic knob, choline + acetyl CoA recombine and taken back into vesicles

Events at an NMJ

8

8 Na+

9

7

Neuromuscular Fatigue 

25 quanta of ACh are needed to generate muscle action potential

 If the nerve that supplies the muscle is stimulated

>100 times/second, → rate of ACh released >> rate of ACh resynthesis → no impulse transmission across NMJ → no contraction  Known as neuromuscular fatigue

Dysfunction at NMJ  Myasthenia gravis  Autoimmune disease

 Body produces antibodies against

own motor end-plate ACh receptors  Decrease EPPs produced

 Condition characterised by

extreme muscular weakness  Treat with anticholinesterase (e.g.:

neostigmine or physostigmine) →To decrease the activity of acetylcholinesterase

Substances Affecting the NMJ Substances

Mechanism Alter release of ACh

• Black widow spider venom

• Causes explosive release of ACh

• Clostridium botulinum toxin

• Blocks release of ACh Block ACh receptor sites

• Curare

• Reversibly binds with Ach receptor sites

Prevents inactivation of ACh • Organophosphates (certain pesticides and military gas)

• Irreversibly inhibits acetylcholinesterase (AChE)

• Neostigmine (for treatment of myasthenia gravis)

• Temporarily inhibits AChE

Substances Affecting the NMJ

Botulinum Toxin  Powerful toxin produced by

Clostridium botulinum  Responsible for deadly food

poisoning – botulism  Now use for some specific

movement disorders  Also now used for fighting

wrinkles by cosmetic surgeons  Marketed as “Botox” –

therapeutic doses remove wrinkles (facial rejuvenation)

Differences Between EPP and Action Potential EPP

Action Potential

• Local events, not propagated

• Propagated

• No threshold

• Has threshold

• No refractory period

• Refractory period – absolute and relative

• Can summate

• Cannot summate – all-or-none law

• Graded response

• Not graded – amplitude constant

• Initiated by neurotransmitter (Ach)

• Initiated by membrane depolarisation

• Dominated by chemical-gated channels

• Dominated by voltage-gated channels