Autonomic nervous system

Sympathetic or adrenergic system.

Parasympathetic or cholinergic system.

The ANS is a two neurone system. Autonomic nerves start in the CNS and travel to a peripheral ganglion, a second neurone then travels to the target tissue.

So the order is preganglionic neurone – synapse in ganglia – postganglionic neurone – synapse – tissue receptor.

The sympathetic system uses acetylcholine (Ach) between the first and second neurone and norepinephrine (noradrenaline) between the second neurone and the tissues.

The parasympathetic system uses acetylcholine in both synapses.

Alpha and beta sympathetic adrenergic receptors

Alpha receptors respond to noradrenaline and beta respond to adrenaline.

Receptors in the sympathetic nervous system are classified as alpha or beta; these are sub divided into a₁ a₂ and B₁ B₂

Alpha1 adrenergic receptors are found in smooth muscle of peripheral blood vessels and of GI and GU sphincters.

Stimulation of alpha 1 receptors may causes peripheral vasoconstriction, constriction of nasal vessels reducing nasal congestion, local injection will cause localised peripheral vasoconstriction.

Alpha 2 adrenergic receptors are located on the presynaptic sympathetic neurones to moderate the release of neurotransmitter between the first and second neurone. Stimulation of alpha 2 receptors reduces the release of neurotransmitter between the first and second sympathetic neurones. This reduces the magnitude of the sympathetic effect.

Beta 1 receptors are found on the myocardium and fatty tissues. Stimulation of beta 1 receptors has a chronotropic and inotropic effect. Stimulation of beta 1 in fatty tissues promotes the breakdown of fat to fatty acids which may be used to produce energy.

Beta 2 receptors are located in bronchiole smooth muscle and smooth muscle in blood vessels in the skeletal muscles, heart and brain. Stimulation of beta 2 receptors causes smooth muscle to relax.

Muscarinic and nicotinic cholinergic parasympathetic receptors

While the sympathetic nervous system promotes `fight or flight` the parasympathetic promotes energy uptake and conservation. It also promotes elimination. The neurotransmitter is Ach.

Two forms of ACh receptor occur. Muscarinic receptors are found on the postganglionic neurone, i.e. between the second neurone and the target tissue. They are named because this subset of ACh receptors are stimulated by muscarine.

Nicotinic ACh receptors are found at the ganglia of the sympathetic and parasympathetic NS and at neuromuscular junctions. These receptors may be stimulated by nicotine as well as Ach.

Drugs affecting the ANS

Sympathomimetics

Sympathomimetics or adrenergic drugs stimulate (i.e. mimic) the action of the sympathetic NS. Some mimic the action of the catacholamines i.e. adrenaline, noradrenaline and dopamine.

These drugs may stimulate beta 1 or beta 2 receptors.

Beta 1 stimulation will increase cardiac output and beta 2 stimulation will cause peripheral vasoconstriction and bronchodilation.

Sympatholytic

Sympatholytic or adrenergic blocking drugs interfere with the action of the sympathetic NS.

They may block alpha or beta receptors. Alpha blockers cause vasodilation so are useful to lower BP, treat Raynaud`s, PVD and migraine.

Beta blockers will lower heart rate so reduce myocardial oxygen demand. Used to treat high blood pressure and some cardiac dysrhythmias. May cause bronchoconstriction.

Parasympathomimetic

Parasympathomimetic or cholinergic drugs stimulate or mimic the activities of the parasympathetic NS and so mimic the activity of ACh.

These drugs may mimic the action of ACh or inhibit the activity of acetylcholinesterase.

Pyridostigmine is a nicotinic agonist and can be used to treat myasthenia gravis. Muscarinic cholinergics are used in conditions of the eye GU and GI tract.

Parasympatholytic

Parasympatholytic or anticholinergis drugs interfere with the function of the parasympathetic NS.

Block ACh receptors and may be specific to muscarinic or nicotinic receptors, or may be non-specific acting on both.

Scopolamine and atropine are anticholinergics, they block ACh receptors.

Decrease GI and respiratory secretions. Cause bronchodilation.

Cause tachycardia in higher doses. Will dilate the pupil as eye drops.

Work in Parkinson’s features.

Relax the detrusor and constrict urinary sphincter so may be used in incontinence and reflex neurogenic bladder.

Many antispasmodics antagonise ACh.