Neuromuscular Transmission –

sequence of events, neuromuscular blockers, their mechanism and

clinical importance

Neuromuscular Junctions:

• The synapse between the axons of motorneurons and skeletal muscle fibers are called neuromuscular junction.

• The NMJ was the first vertebrate synapse to be well characterized.

• NMJ serves as a model chemical synapse that provides a basis for understanding more complex synaptic interactions among neurons in the centeral nerrvous system.

• The skeletal muscle fibers are innervated by myelinated nerve fibers . Each motor nerve fiber branches, after entering the

muscle and stimulates 3 to few hundred muscle fibers.

Motor Unit:

• Each single motor neuron and the muscle fibers it innervates constitute a motor unit.

• In muscles such as those of the hand and those concerned with motion of the eye (i.e, muscles concerned with fine, graded, precise movement), each motor unit innervates 3 to 6 muscle fibers.

• 120 – 165 fibers per unit have been reported in the large muscles of the back.

Structure of NMJ:

• Near the NMJ the motor nerve loses its myelin sheath and divides into terminal branches.

• The terminal branches of the axon lie in SYNAPTIC TROUGHS on the surface of the muscle cells.

• The plasma membrane of the muscle cell lining the TROUGH is thrown into numerous juntional folds called SUB NEURAL CLEFTS ( which increases the surface area for the action of neurotransmitter).

Structure of NMJ:

1. PRE SYNAPTIC NEURON – AXON TERMINAL:

• Has many mitochondria which supply ATP, the source of energy for the synthesis of Ach. Ach is stored in the synaptic vesicles present in the terminals. About 3 lakh vesicles are present in the terminals of a single motor end plate.

• On inside surface of the membrane of axon terminal are DENSE BAR. On each side of dense bar is voltage gated calcium channels.

Structure of NMJ:

2. POST SYNAPTIC MEMBRANE – PLASMA MEMBRANE OF MUSCLE:

• Has Ach receptor which are Ach-gated channels located near the mouths of sub-neural clefts. These channels lie immediately below the dense bars, so that the Ach released can easily enter these channels. These channel remain close until 2 Ach molecules attach to them. This causes conformational change that open the channel.

NMT – Sequence of Events:

Arrival of action potential in presynaptic motor axon terminals

Opening of voltage-gated Ca2+ channels of axon terminals and influx of Ca2+ from synaptic space into axon terminal

Synaptic vesicle fuse with the membrane and empty Ach into synaptic cleft by exocytosis.

Ca ++attract the synaptic vesicles toward release sites on the presynaptic membrane adjacent to Dense bars.

Two molecules of Ach bind with Ach receptors (Ach gated ion channels)

Development of End – plate potential

Contraction of muscle fiber

Increase in permeability of postjunctional membrane to Na+ causes influx of Na+

Development of Action Potential

Spreading of action potential along muscle membrane

Conformational change and opening of Ach channels

(Active zone) influx of Ca 2+ thru voltage gated channels

of ACh

Each vesicle has about 10,000 molecules of Ach.

+ 50mv

EPP: : localized Nonpropagated.

RMP - 90mv. Has voltage gated channels

+ 50- 75 mV

End Plate potential

Depolarization of presynaptic nerve membrane causes influx of Ca++. 4 Ca2+ ions are needed to bind ACh containing vesicles with the presynaptic membrane at the active zones and causes exocytosis and release of ACh in the synaptic cleft. Binding of ACh with the ACh receptors in the post synaptic membrane of the motor end plate causes opening of ligand gated Na+ & K+ channels which cause greater influx of Na+ than efflux of K+ leading to localized depolarization of motor end plate to about + 50-75mv called as the End Plate Potential.

Generation of Muscle Action Potential

This localized depolarization EPP [+ 50 to 75 mV] in itself is non propagated but causes flow of current between EPP and adjacent sarcolemma which has voltage gated channels. When the depolarization at this region reaches threshold, it leads to generation of muscle action potential. This muscle action potential is then conducted through out the sarcolemma on either side and T tubules of the whole of muscle fiber which initiates muscle contraction.

Miniature End Plate Potentials- MEPP

These potentials are generated at the motor end plate in resting state due to random fusion of ACh containing vesicles in the presynaptic neural terminal leading to release of ACh in the synaptic cleft. This ACh diffuses to motor end plate and causes localized depolarization of the motor end plate of about 0. 5mV.

Nicotinic AChR

Myasthenia Gravis

Normal Acetylcholine Receptors

Life span of ACh receptor is 10 days

Are protein in natureConstant synthesisACh receptors get incorporated into

muscle membrane only at motor end plate in innervated muscle

Old ACh receptors get internalized by endocytosis and then degraded by lysosome activity

In Myasthenia GravisThere are circulating antibodies(IgG)

against ACh receptors. IgG causes Aggregation & Clustering of ACh receptors Endocytosis and degradation of clusters Rate of destruction greater than the rate of

formationDestroys post synaptic membrane also

which leads to Less availability of membrane surface area at NMJ for incorporation of ACh receptors Widening of synaptic clefts Sparse and shallow folds

decrease the amount of ACh released

from presynaptic terminal. affect the rate of synthesis of ACh receptors affect incorporation of receptors into muscle

membrane produce abnormal receptors block the interaction between ACh and ACh-R

In myasthenia gravis ACh receptor antibodies DO NOT

In myasthenia gravis

ACh released acts normally on ACh receptors and produces end plate potential but its magnitude is decreased due to decrease in the total number of receptors and widening of synaptic cleft.

This decreased amplitude of EPP fails to generate muscle action potential, so failure of muscle contraction

Sequence of Events in Myasthenia Gravis

( Only skeletal muscles are affected) in ACh receptors

number of interaction between ACh and ACh-R

number of opening of Na+ channels

amplitude of E.P.P.

Failure of generation of muscle action potential

No contraction of muscle

Weakness of muscle

Blockers of ACh Receptor Sites which lead to muscle weakness

or paralysis

Auto-immune antibodies in

Myasthenia Gravis destroy ACh

receptors Curare binds with ACh receptors Some muscle relaxant drugs

Diseases and chemical agents that affect the neuromuscular junction

Blocks release of ACh Clostridium botulinum toxin (food poisoning)

Blocks Ca++ channels Lambert-Eaton in presynaptic terminal Syndrome

Causes explosive Black widow spider release of Ach venom

Alters Release of Ach

Chemical Agent or Disease

Prevents Inactivation of Ach

Irreversibly inhibits ACh-estrase

Organophosphates in nerve gas & pesticides

Blocks the action of ACh-estrase, thus prolongs the actionof ACh.

Neostigmine [ used in myasthenia gravis]