Physiology of the Neuromuscular Junction

A brief overview of the physiology of the neuromuscular junction.It includes a video towards the end sourced from the internet with the copyright watermarks intact.

1. Physiology of The Neuromuscular Junction Presented by: Dr. Ashwin Haridas Chairperson: Dr. M.G.Dhorigol

2. Neuromuscular Junction (NMJ) Junction between a motor neuron and its muscle fibre A synapse Motor end plate

3. The Motor Unit Each motor neuron from the ventral horn runs uninterrupted up to the muscle Splits into a functional group called the motor unit Single neuron supplying a group of muscle fibres Contract and relax as a unit

4. Anatomy The NMJ

5. Anatomy Branching nerve terminals invaginate into the surface of the muscle fibre but lie outside the sarcolemma Hence the synaptic cleft Usually around 20-30 nm ACh receptors on the post junctional membrane

6. Acetylcholine receptors Nicotinic Cation channels Junctional or mature Extra junctional or immature or fetal Usually after muscle fibre injury Within 18 hours Altered response to NMJ blocking drugs Sodium & Calcium moves in Potassium moves out

7. The Immature Receptor Increased sensitivity to depolarizing agents Decreased sensitivity to non-depolarizing agents Stays open for a longer time Hence increased efflux of intracellular potassium Altogether can cause lethal hyperkalemia

8. ACh (Synthesis, storage, release) Synthesized in the Presynaptic terminal from substrate Choline and Acetyl CoA. CAT CHOLINE + ACETYL CoA ACETYL CHOLINE COMT 50% Carrier Facilitated Transport Release CHOLINE + ACETYL CoA ACETYL CHOLINE Synaptic Cleft

9. Different subsets of ACh vesicles I mmediately releasable stores, VP2: Responsible for the maintainence of transmitter release under conditions of low nerve activity 1% of vesicles The reserve pool, VP1: Released in response to nerve impulses 80% of vesicles The stationary store: The remainder of the vesicles.

10. Acetylcholine One vesicle contains approx. 12,000 molecules of ACh Loaded by active transport- Mg 2+ dependent H+ ATPase A single vesicle equals a quantum ACh .

11. Replenishing the vesicles Discharged vesicles are rapidly replaced from reserve stores Reserve vesicles anchored to cytoskeletal actin by syanpsins Ca2+ entry during initial discharge process also binds to calmodulin Stimulates protein kinase-2 which phosphorylates synapsins Reserve vesicles are thus freed

12. Docking of the vesicle and subsequent discharge of acetylcholine by exocytosis, involves several other proteins. Membrane protein called SNAREs (Soluble N- ethylmatrimide sensitive attachment proteins) are involved in fusion, docking, and release of acetylcholine at the active zone. SNARE includes – synaptic vesicle protein synaptobrevin , synataxin and SNAP-25.

13. The released acetylcholine diffuses to the muscle type nicotinic acetylcholine receptors which are concentrated at the tops of junctional folds of membrane of the motor end plate. Binding of acetylcholine to these receptors increases Na and K conductance of membrane and resultant influx of Na produces a depolarising potential, end plate potential. The current created by the local potential depolarise the adjacent muscle membrane to firing level.

14. Acetylcholine is then removed by acetylcholinesterase from synaptic cleft, which is present in high concentration at NMJ. Action potential generated on either side of end plate and are conducted away from end plate in both directions along muscle fiber. The muscle action potential in turn initiates muscle contraction

15. The Sodium Channel C ylindrical Its two ends act as gates Both should be open to allow passage of ions Voltage dependent gate is closed in resting state and opens only on application of a depolarizing voltage, remains open as long as the voltage persists

16. The time dependent gate is normally open at rest closing a few milliseconds after the voltage gate opens and remains closed as long as the voltage gate is open It reopens after the voltage gate closes. The channel is patent, allowing sodium ions only when the gates are open.

17. Na channel states Resting state : Voltage gate closed Time gate open Channel closed Depolarization: Voltage gate open Time gate open Channel open Within a few milliseconds: Voltage gate open Time gate closed Channel closed End of depolarization: Voltage gate closed Time gate open Channel closed

18. The Role of Calcium The concentration of calcium and the length of time during which it flows into the nerve ending, determines the number of quanta released. Calcium current is normally stopped by the out flow of potassium. Calcium channels are specialized proteins, which are opened by voltage change accompanying action potentials

19. Part of calcium is captured by proteins in the endoplasmic reticulum & is sequestrated. Remaining part is removed out of the nerve by the Na/ Ca antiport system The sodium is eventually removed from the cell by ATPase

20. Acetylcholinesterase This protein enzyme is secreted from the muscle, but remain attached to it by thin stalks of collagen, attached to the basement membrane. Acetylcholine molecules that don’t interact with receptors are released from the binding site & are destroyed almost immediately by acetylcholinesterase , in <1 ms , after its release into the junctional cleft.

21. Extra ocular muscles Tonic muscles Multiple neuronal endings on a single muscle fibre Contains immature receptors also Reaction to depolarizing relaxant Normal muscle – brief contraction followed by paralysis Instead there is a long lasting contracture response Pulls the eye against the orbit Raises the IOP

22. Contractile apparatus Thick and thin filaments Actin Myosin Troponin Tropomyosin Calcium The sarcomere

23. References Miller’s Anesthesia , 7 th edition Clinical Anesthesiology by Morgan, Mikhail and Murray, 4 th edition Ganong’s Review of Medical Physiology, 23 rd edition Guyton and Hall: Textbook of Medical Physiology, 12 th edition