Norepinephrine and the Adrenergic Receptors

3 NOREPINEPHRINE AND THE ADRENERGIC RECEPTORS 4.3.1 The Adrenergic Neuronal System 

Peripherally, all organs are innervated sympathetically (as well as parasympatheti-cally), and in most cases the adrenergic action of this system is opposite to the cholinergic effects. The neurotransmitter secreted by the nerve endings is norepinephrine and, to a lesser extent, epinephrine. 

The noradrenergic pathways, primarily situated in the locus ceruleus—a deeply pigmented (hence the name, alluding to its blue color) small cell group involved in behaviour, mood, and sleep. The cortex, some thalamic and hypothalamic centers, and the cerebellar cortex are innervated from here. The noradrenergic pathways of the tegmentum are less well known. 

The adrenergic pathways that use epinephrine as a neurotransmitter, which have been explored only recently. One of these systems is also tegmental and is mixed with noradrenergic cells. The other is thalamic-hypothalamic, involved with the vagus nerve. Some adrenergic fibers are also found in the fourth ventricle and the spinal cord. 

While dopamine is an intermediate for NE and E, it is also a neurotransmitter in its own right. Dopamine and the dopaminergic receptor, as well as drugs that act on it, are discussed below. 

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