Pharmacology for Anaesthesia and Intensive Care

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9780521704632c11 CUFX213A/Peck 9780521618168 December 28, 2007 12:5

Section IICoredrugs in anaesthetic practice

Vesicles storing

acetylcholine

Myelin sheath of Schwann cell

Acetylcholine receptors

Acetylcholinestearase

Acetylcholine

Axon to A motor neurone

CH 3 — C— O— CH 2 — CH 2 — N(CH 3 ) 3

——

O

Figure 11.1.Neuromuscular junction and structure of acetylcholine.

the prejunctional membrane and provide positive feedback to maintain transmitter

release during periods of high activity. When blocked by non-depolarizing muscle

relaxants they may be responsible for ‘fade’ (Figure11.3).

The ACh receptor ion channel is non-specific, allowing Na+,K+and Ca^2 +across

the membrane, generating a miniature end-plate potential. These summate until the

threshold potential is reached at which point voltage-gated Na+channels are opened,

causing a rapid depolarization, leading to the propagation of an action potential

across the muscle surface. On reaching the T tubular system, Ca^2 +is released from

the sarcoplasmic reticulum which initiates muscle contraction.

Diet Recycled from the breakdown of ACh

Choline acetyl transferase (choline acetylase)

Choline acetyl-coenzyme A

ACh coenzyme A

Figure 11.2.Synthesis of acetylcholine (ACh).