The Neuromuscular Junction By Dr Salim Khan

1. 1 The Neuromuscular JunctionBy Dr Salim Khan

2. 2 Myasthenia Gravis MG is a disorder in which normal communication between the nerve and muscle is interrupted at the neuromuscular junction. Consequences of Myasthenia Gravis to speech: - change in facial expression - dysphagia (difficulty in swallowing) - shortness of breath - dysarthria (impaired speech, often nasal due to weakness of the pharyngeal muscles).

3. 3 In most cases, the first noticeable symptom is weakness of the eye muscles. In others, difficulty in swallowing and slurred speech may be the first signs. The degree of muscle weakness involved in MG varies greatly among patients, ranging from a localized form, limited to eye muscles (ocular myasthenia), to a severe or generalized form in which many muscles - sometimes including those that control breathing - are affected.

4. 4

5. 5 Other symptoms, which vary in type and severity, may include: asymmetrical ptosis (a drooping of one or both eyelids), diplopia (double vision) due to weakness of the muscles that control eye movements unstable or waddling gait weakness in arms, hands, fingers, legs, and neck We’ve already mentioned the following: a change in facial expression dysphagia (difficulty in swallowing) shortness of breath dysarthria (impaired speech, often nasal due to weakness of the pharyngeal muscles).

6. 6 The muscular weakness and fatigability associated with myasthenia gravis are caused by an autoimmune attack on the acetylcholine receptor at the NMJ. Antibodies have been shown to decrease the usefulness of acetylcholine receptors through accelerated endocytosis and blockade of the receptor. Endocytosis is the process of extracellular substances being incorporated into the cell by vesicles forming inward through budding of the plasma membrane.

7. 7 Researchers have been able to demonstate the effect of antibodies on acetylcholine receptor degradation by using radioactively labeled alpha bungaroo toxin, a snake poison, to follow the rate of degradation. Antibodies from patients with MG cause a two to three fold increase in the rate of degradation of acetylcholine receptors. The myasthenic antibodies cause a cross linking between the acetylcholine receptors, causing the linked receptors to be drawn together into clusters and rapidly endocytosed.

8. 8 In myasthenic patients the NMJ has decreased numbers of ACh receptors, a wider synaptic cleft, and simplified synaptic folds. These changes account for the clinical features of MG. Decreased numbers of ACh receptors result in fewer interactions between ACh and it's receptors, leading to decreased activation of action potentials. When the transmission of action potentials decreases, the power of the muscle's contraction is reduced, causing weakness.

9. 9 During repeated nerve stimulation the amount of ACh normally declines, or runs down. In MG, this run down occurs more rapidly due to a decrease of receptors in myasthenic junctions, causing muscular fatigability. The wider synaptic cleft and simplified synaptic folds also work to decrease the number of interactions between ACh and ACh receptors.

10. 10

11. 11

12. 12 Blockade of acetylcholine receptors is another form of autoimmune attack. Antibodies from patients with MG have been shown to block the acetylcholine binding sites, which prevents acetylcholine from binding to its receptor and opening the ion channel. It is probable that the antibodies bind near the acetylcholine binding site rather than directly on it, because the acetylcholine binding site is so small. In this case the antibodies would prevent acetylcholine from binding at the receptor by "getting in the way" of the acetylcholine molecule as it moves towards its receptor; this effect is known as steric hinderance.

13. 13 Eaton-Lambert Syndrome Eaton-Lambert syndrome is an autoimmune disease that causes weakness. Eaton-Lambert syndrome is caused by antibodies that interfere with the release of acetylcholine rather than attack acetylcholine receptors (as in myasthenia gravis. Eaton-Lambert syndrome usually results from certain cancers, especially lung cancer.

14. 14 The Neuromuscular Junction

15. 15

16. 16

17. 17

18. 18

19. 19

20. 20

21. 21 Nerves connect with muscles at the neuromuscular junction. There, the ends of nerve fibers connect to special sites on the muscle's membrane called motor end plates. These plates contain receptors that enable the muscle to respond to acetylcholine, the chemical messenger (neurotransmitter) released by the nerve to transmit a nerve signal across the neuromuscular junction. After a nerve stimulates a muscle at this junction, an electrical signal flows through the muscle, causing it to contract.

22. 22 How it works

23. 23

24. 24

25. 25

26. 26 Chemical Transmitters Carry the Signal Across Synapses & Neuromuscular Junctions A contact between 2 nerves is called a synapse At the synapse there is a break in electrical transmission (the action potential cannot cross)- instead chemicals are released that carry the signal to the next nerve The release of chemical transmitters at nerve endings was first shown by Otto Loewi in the frog heart

27. 27 A neuromuscular junction (NMJ) is a contact between a nerve and a muscle, it is like a synapse, the action potential stops and the signal is carried by a chemical There is a delay at synapses- chemical transmission is slower than electrical transmission

28. 28 Chemical Transmitters Are Made and Stored in the Presynaptic Terminal The end of the nerve enlarges into an axon terminal Transmitters are made in the terminal and are stored in tiny vesicles so that they can be released whenever an action potential comes along Transmitters are made only by the incoming (presynaptic) nerve Because the transmitter is only on one side the impulse can go in only one direction

29. 29 Calcium is Required for Transmitter Release Transmitter release requires Ca2+ ions Normally Ca2+ in the cell is kept very low (by a Ca pump)- if the cell needs Ca2+ it must come from the outside The action potential coming in to the terminal opens Ca channels -> Ca comes rushing in The Ca2+ causes some of the vesicles to fuse to the membrane- then they open up and the transmitter is released Botulinum and tetanus toxins block transmitter release

30. 30 Transmitter Diffuses Across the Synaptic Gap and Binds to a Receptor The synaptic gap is short and the transmitter travels across it by simple diffusion On the far side of the gap the transmitter binds to a specific receptor protein in the postsynaptic membrane There are some receptor diseases- in myasthenia gravis an autoimmune reaction destroys acetycholine receptors in the neuromuscular junction- this causes muscular weakness or paralysis Many drugs block receptors: curare, strychnine, atropine, antihistamines

31. 31 When Transmitter Binds to a Receptor it Produces an EPSP or an IPSP When the transmitter binds to the receptor ion channels are opened (ligand-gated channels) Ions rush into the postsynaptic cell If the ions depolarize the postsynaptic cell they produce an excitatory postsynaptic potential (EPSP) Most transmitters produce EPSPs (acetylcholine, epinephrine, norepinephrine) If the ions make the postsynaptic membrane more negative they produce an inhibitory postsynaptic potential (IPSP) The major transmitters producing IPSPs are glycine and GABA (gamma amino-butyric acid) There are both excitatory and inhibitory nerves coming into most synapses

32. 32 If There Are Enough EPSPs an Action Potential Will be Produced in the Postsynaptic Membrane If there are enough EPSPs the postsynaptic membrane will be depolarized to the threshold level and an action potential will be produced- then the signal will travel along the second nerve or muscle cell IPSPs make the membrane potential more negative and cancel out EPSPs

33. 33 The Transmitter is Broken Down and/or Recycled Once the signal has been delivered the transmitter must be removed so that new signals may be received In some cases the transmitter is broken down by an enzyme in the synapse In other cases the transmitter is recycled- it is transported back into the presynaptic nerve In still other cases these 2 methods are combined Some drugs inhibit the enzymes that break down transmitters: nerve gases, physostigmine Other drugs act by inhibiting recycling: prozac, cocaine

34. 34

35. 35 In the Central Nervous System Nerves Make Synapses with Thousands of Other Nerves Nerves in the central nervous system make synaptic contact with 1000 to 10,000 other nerves This allows nerve cells to be hooked together in complex patterns to perform tasks benefiting the animal In the brain synapses tend to cluster to form ganglia (gray matter of brain) Each nerve makes both excitatory and inhibitory synapses Whether or not a nerve fires is determined by summation of the EPSPs and IPSPs

36. 36 Transmission at this junction involves several steps: When an action potential (inhibited by tetrodotoxin) reaches the axon terminal it causes Ca channels to open. Ca2+ rushes into the cell because Ca2+ outside is much higher than Ca2+ inside The terminal region is loaded with vesicles containing the transmitter acetylcholine (ACh) Ca2+ causes some of the vesicles to fuse with the membrane and release their ACh (inhibited by botulinum toxin) ACh diffuses across the junction and binds to the ACh receptor protein (inhibited by curare) in the postsynaptic membrane Binding causes an ion channel to open

37. 37 The flow of ions depolarizes the membrane, producing an EPSP. In muscle a single impulse usually causes enough depolarization to reach threshold An action potential is generated in the muscle membrane The muscle action potential causes release of Ca2+ from the sarcoplasmic reticulum of the muscle and this triggers muscle contraction Back in the synapse the ACh is broken down to acetate and choline by the enzyme acetylcholinesterase (inhibited by physostigmine, nerve gases, organophosphate insecticides). The choline is recycled. A choline pump transports it back into the nerve terminal and there it is converted back into ACh

38. 38

39. 39

40. 40

41. 41 There are Dozens of Transmitters in the Nervous System In this class we will deal with only a few types of transmitters: acetylcholine, epinephrine, norepinephrine and a few others There are dozens of other transmitters in the central nervous system (CNS) and new ones are being discovered every year: Serotonin, dopamine, glutamate, secretin, endorphins, etc. Even gas molecules such as nitric oxide (NO) can act as local transmitters The gas types are not stored, but are made on demand A high percentage of pharmaceutical drugs affect the synapse or NMJs

42. 42 Many Toxins and Diseases Affect Neuromuscular Junction & Synaptic Transmission 1. NMJs and synapses are attacked by toxins and poisons: ACh release in the NMJ is inhibited by botulinum toxin Glycine release in the central nervous system (CNS) is inhibited by tetanus toxin Black widow spider toxin, alpha-latrotoxin, stimulates fusion and depletion of transmitter vesicles The plant poison, physostigmine, nerve gases and organophosphorus pesticides inhibit acetylcholinesterase, the enzyme that splits ACh into acetate and choline

43. 43 The muscle ACh receptor is blocked by the South American arrow poison, curare The plant drug, atropine, inhibits ACh receptors of the autonomic nervous system (but not the NMJ) Strychnine binds to the glycine receptor protein and inhibits IPSPs in the spinal cord Cocaine blocks the recycling of of dopamine and norepinephrine transmitters in the brain. This has an excitatory effect Acetylcholine in synapses and NMJs is affected by 3 different types of inhibitors: release inhibitors, receptor inhibitors and acetylcholinesterase inhibitors.

44. 44 Effect of Neuromuscular Blockade

45. 45 The Iceberg Effect

46. 46 Receptor Reserve Percentage of receptors that may be blocked before there is any effect on contraction is the receptor reserve. This is greatest in the muscles of respiration great in muscles of coarse movement (e.g. gastrocnemius) least in the muscles of the face and eyes Lack of facial expression and ptosis are diagnostic of neuromuscular failure.

47. 47

48. 48 2. Low blood Ca will inhibit transmitter release 3. Diseases affecting synapses and NMJs: Eaton-Lambert syndrome: patient produces antibodies that attack own Ca channels. This results in low Ca in the synapse and transmitter release is inhibited Myasthenia gravis: another autoimmune disease which damages the receptor proteins for ACh Parkinson's disease: cells in the substantia nigra of the brain are deficient in the transmitter, dopamine Clinical depression: associated with low amounts of the transmitter, serotonin, in parts of the brain

49. 49 Receptor Inhibitors There are 2 basic types of ACh receptors: Muscarinic type: Stimulated by the mushroom poison, muscarine Found in parasympathetic synapses on organs like the heart and intestines; also found in the central nervous system Nicotinic type: Stimulated by the plant toxin, nicotine Found in neuromuscular junctions and autonomic ganglia

50. 50 Natural ACh receptor inhibitors include: Curare: South American arrow poison produced from the vine, Chondodendron tomentosum Atropine: from several plants, including Jimson weed (Datura stramonium), thornapple and nightshade (Belladonna atropa) Scopolamine: from the henbane plant (Hyoscyamus niger) Marine cone snail toxins: the venom from these snails contains dozens of different neurotoxins Alpha-bungarotoxin: produced by a snake, the banded krait Alpha-neurotoxin: made by the cobra Atropine and scopolamine inhibit muscarinic receptors; the other toxins inhibit nicotinic types Receptor inhibitors will not have much effect on ACh concentration in the NMJ, but will prevent its action and paralyze the muscle

51. 51 Drugs and the Nervous System Almost all drugs taken by humans (medicinal and recreational) affect the nervous system. From our understanding of the human nervous system we can understand how many common drugs work. Drugs can affect the nervous system in various ways, shown in subsequent table Drugs that stimulate a nervous system are called agonists Those that inhibit a system are called antagonists. By designing drugs to affect specific neurotransmitters or neuroreceptors, drugs can be targeted at different parts of the nervous system.

52. 52