Anatomy of nmj

anesthetist point of view of neuromuscular junction take from miller

1. Anatomy of NMJ Dr .Yogesh kumar Moderator – Dr . Surendranath

2. Introduction Most widely studied synapse in NS Essential for Anaesthetist , Intensivist because of dysfunction To understand NMB agents To understand the effects of NMB’s in diseases affecting NMJ

3. Need to know Synapse :- specialised connection between two neurons or muscle cell Motor neuron :- Nerves controlling skeletal muscle activity Action potential :- transient change in membrane potential for about 100mVwhich is conducted along the axon in all or none phenomenon Depolarization :- refers to change in value of membrane potential which becomes less negative (more positive) than RMP RMP :- voltage difference across the cell membrane when the cell is at rest

4. Phases of action potential

5. Anatomy of NMJ NMJ is made of a motor neuron and a motor endplate with a synaptic cleft or junctional gap dividing them 3 parts Presynaptic Synaptic Postsynaptic

6. NM Transmission Ach is synthesised and packed in vesicles Stimulation of nerve releases the vesicles into synaptic cleft Ach binds to ACHR (channel protein) Brings about depolarization Muscle contraction is elicited Ach is hydrolysed by AchE’s

7. Morphology

8. Morphology Specialized synapse Motor unit Nerve terminals – synaptic bulb – vesicles , mitochondria (metabolically active ) Synaptic cleft Basal lamina NM end plate – junctional cleft – sodium channels – AchR in cleft shoulder Foetal NM junction Fasciculation Suxamethonium – increased IOP Skeletal muscle vs intra-ocular muscle Perijunctional area receptor and variation

9. Quantal theory Vesicle = quanta Fixed amount of Ach molecules Evokes a fixed response – miniature end plate potential Multiples of MEPP devolopes a action potential and muscle contaction 1 quanta = 500 molecules Each nerve impulse = 200 vesicles

10. Quantal theory

11. Formation Of Neurotransmitters Cell body produces proteins ,channels, enzymes, release apparatus and send to axon thro axonal transport Choline and acetate are obtained from neuronal environment Acetyl coA (acetate) from mitochondria Choline acetyltransferase bring about the reaction to produce Ach Stored in vesicles

12. Nerve Action Potential Depolarization open voltage gated Ca channels which assist degranulation Number of quanta released is greatly influenced by ECF calcium Doubling ECF will increase quanta by 16fold Calcium current is neutralized by potassium efflux ( voltage activated , Ca activated K channels) K channel blockers – prolong contraction (4- aminopyridine , tetraethylammonium) Post tetanic potentiation – effect of increasing ICF calcium

13. Cont’d Two types of ca channels P channels and slower L channels Eaton lambert myasthenic syndrome – autoimmune syndrome Antibodies against voltage gated ca channel at nerve endings Muscle weakness and increased sensitivity to MR Bivalent inorganic cations – Mg , Cd , Mn Calcium channel blockers ???

14. Synaptic Vesicles And Recycling 2 types of vesicles – readily releasable pool VP2 , reserve pool VP1 SNARE protein – responsible for fusion , docking , release of vesicles 3 types of vesicle release – kiss and run compensatory stranded Mobilization

15. Exocytosis

16. AchE Acetylcholinesterase is a beta carboxyl esterase Ach is destroyed in less than one millisecond Congenital diseases causing altered activity of AchE is similar to myasthenia Denervation decreases AchE Chronic AchE inhibitor therapy also casuses mucle fatigue

17. AchR 3 isoforms Junctional or mature Extra junctional or foetal Neuronal α 7 receptor of pre junctional and post junctional Synthesised by muscle cells Anchored to the end plate membrane by rapsyn (cytosolic protein)43Kd 1:1

18. Cont’d Receptor is a 2,50,000 Kd protein Each receptor has 5 subunits Cylindrical transmembrane channel for cations The mature receptor consists of α1-, β1-, δ-, and ε-subunits fetal immature extrajunctional receptor consists of α1-, β1-, δ-, and γ-subunits Binding site for ligand is α 1 subunit Just before birth foetal receptors are replaced with mature receptors

19. Cont’d Ligand must bind to both α subunit to open Not opened if one α is occupied by NMB or by Ach(one) Channel transmits Na, Ca, inside and K ions outside Even neutral materials pass but not anions (Cl) Each channel produces a minimal current few picoamperes Each impulse release 5lac Ach which binds to 5lac receptors Union of all impulse produce end plate potential which elicits contraction AchE inhibitors vs Cyclodextrin (importance)

20. Classic Action Of Depolarizing Muscle Relaxant Succinyl choline is depolarising MR Its made of two acetylcholine molecules linked together Its not cleaved by AchE but by plasma cholinesterase Initialy it binds to AchR and produces a same response as the Ach But later it produces a persistent depolarization and inactivation of surrounding peri junctional sodium channels Which will restrict the action potential from spreading (Accommodation) Effect of extra ocular muscle to SCH

21. Non classic & non-competitive actions of NM drugs Several drugs interfere with NM receptors thro lipid environment and impair transmission either directly or indirectly Modified channels are sluggish Procaine, ketamine, inhaled anesthetics dissolve in membrane and change receptor behaviour Mechanisms – desensitization of receptors , channel blockade

22. Desensitization block The AChR , as a result of its flexibility and the fluidity of the lipid around it, is capable of existing in a number of conformational states Some receptors that bind to agonists, however, do not undergo the conformational change to open the channel. Receptors in these states are called desensitized (i.e., they are not sensitive to the channel-opening actions of agonists) Phase 2 block is also a type of desensitization block More the number of desensitized receptors , NMB will exert more action than normal Many other drugs used by anesthetists also promote the shift of receptors from a normal state to a desensitized state.

23. Channel block Local anesthetics and calcium entry blockers prevent the flow of sodium or calcium through their respective channels, thus explaining the term channel-blocking drugs. In a closed-channel block, certain drugs can occupy the mouth of the channel and prevent ions from passing through the channel to depolarize the end plate. In an open-channel block, a drug molecule enters a channel that has been opened by reaction with acetylcholine but does not necessarily penetrate all the way through. Evidence suggests that neostigmine and related cholinesterase inhibitors can act as channel-blocking drugs

24. Phase 2 block A type of blockade known to occur with either repeated bolus or prolonged infusion of depolarising muscle relaxants In patients with atypical cholinesterase -- first dose itself Causes – prejunctional Depletion of neurotransmitter Desensitization Effect of sodium potassium ATPase Prolonged open Ca channel – disruption of receptors NM monitoring resembles NDB But reversal with AchE is not advisable

25. Clinical applications

26. Clinical applications

27. Cont’d Snake venom :- neurotoxin – curare like , cholinesterase , alpha bungarotoxin Organophosphorus poisoning – powerful acetylcholinesterase inhibitors Botulinum – prevent release of vesicles Myasthenia gravis – autoimmune against AchR Lambert eaton myathenic syndrome – antibodies against voltage gated calcium channel Mushroom poisoning

28. Recent Advances In certain pathologic states – stroke , denervation , sepsis, burns , immobilization chronic NDMR use AchR upregulated which increase expression of immature forms Alpha 7 receptor is identified in muscles Different functional and pharmacological characters This led to increased sensitivity to SCH and hyperkalemia resistance to NDMR Re-expression of immature forms is related to aberrant growth factor signalling Dynein affects expression and clustering of receptors , integrity of NMJ, makor postsynaptic proteins , MuSK Mutation in dynein produce AMLS like syndrome

29. Thank you Reference – Miller 8 th edition Ganong –text book of physiology Harrison textbook of medicine Narayan Reddy – essentials of FM and toxicology