McKenna's Pharmacology for Nursing, 2e

449

C H A P T E R 2 9 Introduction to the autonomic nervous system

Presynaptic neuron

Tyramine (from diet)

Tyramine hydroxylase Dopa decarboxylase

Dopa

1

Nerve terminal

Dopamine

AP

NE

2

C

MAO

inactive product

α 2

receptor

Synaptic cleft

3 4

7

FIGURE 29.4  Sequence of events at an adrenergic synapse. 1. Dopamine, a precursor of noradrenaline (NA), is synthesised from tyrosine in several steps. 2. Dopamine is taken into the storage vesicle and converted to NA. 3. Release of neurotransmitter by an action potential (AP) in the presynaptic nerve. 4. Diffusion of neurotransmitter across synaptic cleft. 5. Combination of neurotransmitter with receptor. The events resulting from NA’s occupying of receptor sites depend on the nature of the postsynaptic cell. 6. Interaction of NA with many beta- receptors leads to increased synthesis of cyclic adenosine monophosphate (cAMP). 7. Feedback control at alpha2- receptor leads to decreased NA release from presynaptic neuron. Deactivation of NA occurs by breakdown of NA by the enzyme COMT (A) or most importantly by reuptake into the presynaptic neuron  (C) , where it may be reused or inactivated by another enzyme, monoamine oxidase (MAO). Some of the neurotransmitter may also diffuse away from the synaptic cleft  (B) .

5

Ca+

C

Return to presynaptic cell

Inactive product to blood vessel

A

COMT

B

Into blood vessel

Adrenergic receptor α β 1 or β 2

6

cAMP

Neuron or effector cell

Termination of response Once noradrenaline has been released into the synaptic cleft, stimulation of the receptor site is terminated and disposal of any extra noradrenaline, as well as the neurotransmitter that has reacted with the receptor site, must occur. Most of the free noradrenaline mole­ cules are taken up by the nerve terminal that released them in a process called reuptake. This neurotrans- mitter is then repackaged into vesicles to be released later with nerve stimulation. This is an effective recyc­ ling effort by the nerve. Enzymes are also in the area, as well as in the liver, to metabolise or biotransform any remaining noradrenaline or any noradrenaline that is absorbed into circulation. These enzymes are monoamine oxidase (MAO) and catechol- O -methyl transferase (COMT).

KEY POINTS

■■ The autonomic nervous system (ANS), which is divided into two parts—the sympathetic nervous system (SNS) and the parasympathetic nervous system—works with the endocrine system to regulate internal functioning and maintain homeostasis. ■■ The SNS is responsible for the fight-or-flight response. ■■ The SNS is composed of CNS cells arising in the thoracic or lumbar area of the spinal cord and long postganglionic axons that react with effector cells. The neurotransmitter used by the preganglionic cells is acetylcholine (ACh); the neurotransmitter used by the postganglionic cells is noradrenaline. ■■ SNS adrenergic receptors are classified as alpha 1 -, alpha 2 -, beta 1 - or beta 2 -receptors.