Biochemistry Aspect of Myaestenia Gravis

Presented by Jega Subramaniam (Student)

Guided by

Assoc. Prof Ashok Kumar

Biochemistry aspect of MyaesteniaGravis

(Acetylcholine and Lab Investigations for Myaestenia Gravis )

Content

Introduction - Neurotransmitters

Structure of ACh

ACh Receptor

ACh Synthesis and Degradation

ACh and Myaestenia Gravis

Lab Investigations for Myaestenia Gravis

Neurotransmitters

• Chemical substances which are responsible for transmission of an impulse through a synapse and neuromascular junction.

EXCITATORY

Eg: Acetylcholine

Aspartate

Dopamine

Norepinephrine

Epinephrine

INHIBITORY

GABA

Glycine

Acetlycholine

• An organic molecule that acts as a neurotransmitter at neuromascular junction

• Ester of acetic acid and choline

• Only neurotransmitter used in the motor division of the somatic nervous system and ParasympataticDivision of autonomic nervous system

• Used in partially in Sympatatic Division .

Structure of ACh

• Molecular formula - CH3COO(CH2)2N+(CH3)3

• Structural formula

ACh Receptors (Abbr : ACh R)

• It’s a type of Ligand Gated channel

• Present in Post – Synaptic membrane

• Consists of ACh binding site and ion channel

• Ach binds with the receptors

• Ion channels triggered to open

• Na+ and k+ Passes through the membrane

• Action potential is generated

Two kinds of ACh Receptors

– Nicotinic

• Nicotine stimulates the receptors

• Excitatory; found predominately on neuromuscular junctions

– Muscarinic

• Muscarine stimulates the receptors

• Both excitatory AND Inhibitory; found predominately in brain

• Receptors found on smooth muscle and glands innervated by parasympathetic nerves

ACh Synthesis

• Synthesized in certain neurons called Cholinergic Neurone

• Enzyme : choline acetyltransferase (ChAT) EC Number : 2.3.1.6

ChAT ENZYME

•Produced in the cell body of the neuron and is transported to the nerve terminal

Sources

• Recycled from ACh degradation• Phosphatidylcholine Breakdown

• Diet• Breadown of Serine

SourceGycolyis of carbohydrate

phospholipids that incorporate choline as a headgroup

Ach Degradation

• As soon as the nerve impulse is generated the ACh is broken down by the enzyme acetylcholinesterase (ACh-esterase). EC : 3.1.1.7

• It breaks down (hydrolysis) acetylcholine into the inactive metabolites choline and acetate

• ACh-esterase is abundant in the synaptic cleft

• For proper muscle function ;- the ACh- esterase should rapidly clear the Ach present in the post-synaptic membrane as soon as the impulse is generated .

• Some Antibodies called ACh Receptors Antobodies recognises the ACh receptors as foreign body - Its Autoimmune Deffect

• ACh Receptors Antobodies work against the ACh receptor (Only Nocotinic Receptors)

• Number of receptors decreases

• ACh can not transmit impulse efficiently•

• This causes improper muscle function

neuromuscular disease that leads to fluctuating muscle weakness and fatigue caused by antibodies that work against

the Ach Recpetors.

Myaestenia Gravis

How these ACh Receptors Antobodieswork ?

• Block the active site of the antibodies where ACh normally binds

• Modulate the receptors inside the neurone(endocytosis)

• Damage the receptors

Lab Investigations

1. Acetylcholine receptor (AChR) Antibody Test

OBJ : To detect and measure the level of AChRantibodies in the blood.

Results Bindingantibodies (nmol/L )

Blocking Antibodies ( % )

ModulatingAntibodies ( % )

Normal (negative) 0.0 – 0.4 0-26 0-45

Abnormal = Myaestenia Gravis (Positive)

0.5 and above 42 and above 46 and above

2. Repetitive Nerve Stimulationin the Electromyography Laboratory

(RNS)

Compound muscle action potential (CMAP) = sum of the individual muscle fiber action potentials generated in a muscle.

↑ number of muscle fibers activated ↑ CMAP amplitude

Normal

• EPP always stays above threshold

• consistent generation of muscle fiber action potentials.

• CMAPs do not change significantly in amplitude or area.

ABNORMAL – Myaestenia Gravis

• ACh Receptors are reduced

• EPP of some muscle fibers falls below threshold

• Muscle fiber action potentials will not be generated

• Number of individual muscle fiber action potentials will decline.

• CMAP amplitude and area DECLINE

3. Edrophonium chloride ( Tensilon ) Test / Ach-esterase Test

• Reversible inhibitor for acetylcholinesterase

• Prevents breakdown of the neurotransmitter acetylcholine and acts by competitively inhibiting the enzyme acetylcholinesterase

• Prolonging the presence of acetylcholine in the synaptic cleft

• Tensilon is administered intravenously (into a vein) and muscle response is evaluated.

• Myaestenia Gravis – muscles will improve immediately

• Myasthenic crisis - there is a brief improvement in the muscle strength