Adrenergic nerve fibre

Receptor site

Molecular level

There are several types of adrenergic receptors which are identified by their differing sensitivities to various drugs. Neurons in the central nervous system contain α1- and α2-adrenergic receptors and β1- and β2-adrenergic receptors. All four kinds of receptors are also found in the various organs of the body aside from the brain. They are responsible for the effects of epinephrine and norepinephrine when they act as hormones outside the central nervous system. In the brain, all autoreceptors appear to be of the α2 type. (The drug idazoxan blocks α2 autoreceptors and hence acts as an antagonist.) All adrenergic receptors are metabotropic, coupled to G proteins that control the production of second messengers. Adrenergic receptors can produce both excitatory and inhibitory effects. In general, the behavioral effects of the release of norepinephrine are excitatory. In the brain the α1 receptors produce a slow depolarizing (excitatory) effect on the postsynaptic membrane, while α2 receptors produce a slow hyperpolarization (inhibitory) effect. Both types of β receptors increase the responsiveness of the postsynaptic neuron to its excitatory inputs, which presumably related to the role this neurotransmitter plays in vigilance.

In the Body

Adrenergic fibres innervate smooth muscle, cardiac muscle, visceral glands, and various central nervous system structures and sense organs. Their function is enhancing, compared to the inhibiting action of the cholinergic fibres of the parasympathetic system. Peripheral adrenergic neurons integrate signals from other nerves of the central nervous system and peripheral sense organs. An adrenergic nerve impulse is triggered when one nerve fires repeatedly or when several nerves fire simultaneously which can cause an additive effect leading to a greater stimulus. Adrenergic neurons, in particular the α2 autoreceptors found in the brain, are also involved in sexual behavior and in the control of appetite.