tailieunhanh - The Anaesthesia Science Viva Book - part 5

- Tĩnh mạch xoay thai: tĩnh mạch xoay thai cống phía bên xuyên tâm của cánh tay, và lên trên phía bên của hố nằm trong một rãnh dọc theo bên cạnh của các bắp tay. Tại biên giới của pectoralis lớn nó di chuyển sâu hơn nằm giữa pectoralis lớn và cơ delta trước khi thâm nhập vào clavipectoral fascia tham gia vào tĩnh mạch nách. | CHAPTER 3 The neuromuscular junction Commentary If you are asked about the neuromuscular junction it is almost inevitable that the viva will include questions about neuromuscular blockers and the assessment of neuromuscular blockade. If on the other hand you are asked about either of the two latter topics you may not be required to discuss the neuromuscular junction in any detail. It is for this reason that the account below is somewhat simplified. The viva You will be asked about the generation of a muscle action potential. Acetylcholine ACh is formed in the motor nerve terminal by the acetylation of choline catalysed by choline-O-acetyltransferase . Much of the synthesised ACh is stored in vesicles. ACh release is triggered by the motor nerve action potential. In response to depolarisation voltage-gated channels permit an inward flux of calcium which stimulates release into the junctional gap. This itself is complex involving the activation of a number of improbably named proteins which facilitate the process synaptotagmin syntaxins synaptophysin and synaptobrevin. Synaptobrevin is of passing interest because it is inhibited by botulinum toxin which thereby prevents ACh release and muscle contraction. Pre-junctional nicotinic cholinergic receptors modulate further ACh mobilisation and release via a positive feedback mechanism. ACh acts at the post-junctional nicotinic receptor whose structure has been fully identified. It consists of five glycoprotein subunits characterised as a 2 p 8 and e which form a central ionophore ion channel . Binding of one molecule of ACh to one of the two a units facilitates the binding of a second during which the receptor undergoes an evanescent conformational change and the ionophore opens. A net influx of sodium ions then depolarises the muscle cell membrane. TheACh in the cleft will interact with an a unit only once before being broken down within 100 ps by the acetylcholinesterase in the junctional folds of the muscle membrane. .