The autonomic nervous system, which works without conscious control, controls internal organs, blood vessels, and glands and performs the rapid regulation of homeostasis. It consists of two systems: an accelerator and a brake.
Basic Knowledge
The autonomic nervous system has two divisions:
① Sympathetic nerves: the “accelerator,” active during excitement, activity, fighting, and fleeing.
② Parasympathetic nerves: the “brake,” active during rest, relaxation, digestion, and recovery.
They act antagonistically on the same organs.
Main effects:
・Heart rate: sympathetic = increases, parasympathetic = decreases
・Bronchi: sympathetic = dilates, parasympathetic = constricts
・Pupils: sympathetic = enlarges, parasympathetic = contracts
・Movement of the digestive tract: sympathetic = inhibits, parasympathetic = promotes
・Blood vessels: sympathetic = constricts (skin and intestines)・dilates (skeletal muscle), parasympathetic = dilates
・Sweating: promoted only by the sympathetic nerves (not controlled by the parasympathetic nerves)
The centers of the autonomic nervous system are the hypothalamus in the diencephalon and the medulla oblongata. The main neurotransmitter released from sympathetic nerve endings is noradrenaline, while that released from parasympathetic nerve endings is acetylcholine.
Autonomic nervous system (nerves that control internal organs without conscious control; sympathetic + parasympathetic)
Sympathetic nerves (dominant during activity; “fight or flight”)
Parasympathetic nerves (dominant during rest; digestion and recovery)
Hypothalamus (the center for autonomic nerves and hormones; located in the diencephalon)
Noradrenaline (a neurotransmitter released from sympathetic nerve endings)
Acetylcholine (a neurotransmitter released from parasympathetic and motor nerve endings)
Synapse (the junction between nerve cells; information is transmitted by neurotransmitters)
Further Study
Neurotransmitters are chemically diverse and act by binding to specific receptors. Noradrenaline is a catecholamine (a catechol ring + an amine side chain) synthesized enzymatically as tyrosine→DOPA→dopamine→noradrenaline→adrenaline. Adrenaline, another catecholamine, is secreted from the adrenal medulla into the blood and acts as both a neural and hormonal signaling molecule.
Acetylcholine is one of the earliest known neurotransmitters and is synthesized from choline and acetic acid (through acetyl-CoA). In the synaptic cleft, acetylcholinesterase rapidly breaks it down and stops its action. Organophosphate pesticides and nerve agents such as sarin inhibit this enzyme and cause fatal excessive excitation, making them an important toxicological class.
Synaptic transmission is a chemical process: when an electrical signal (an action potential) reaches the axon terminal, Ca2+ enters the cell, and synaptic vesicles containing neurotransmitters fuse with the cell membrane (exocytosis), releasing the neurotransmitters. Receptors are membrane proteins. Binding to a neurotransmitter changes their three-dimensional structure, opening ion channels or activating G proteins. These molecular mechanisms are the starting point of pharmacology, and many drugs, including antidepressants, antihypertensives, and bronchodilators, target this system.
Disorders of the autonomic nervous system appear as autonomic dysfunction and cause symptoms such as dizziness, palpitations, insomnia, and indigestion. It is also a common modern illness in stressful societies.
- Autonomic nerves = sympathetic + parasympathetic divisions
- They act antagonistically (accelerator and brake)
- Sympathetic nerves = activity and excitement (fight or flight)
- Parasympathetic nerves = rest, relaxation, and digestion
- Centers = hypothalamus and medulla oblongata
- Sympathetic endings = noradrenaline
- Parasympathetic endings = acetylcholine
- Sweating is controlled only by the sympathetic nerves (an exception)
Points to Note
① Do not reverse the effects of the sympathetic nerves (accelerator) and parasympathetic nerves (brake). Remember: “Encountering an enemy = sympathetic on = heart rate↑・pupil enlargement”; “After a meal = parasympathetic on = digestion↑.” ② Pupils: sympathetic = enlargement (to let in more light), parasympathetic = constriction. ③ Bronchi: sympathetic = dilation (to take in more air), parasympathetic = constriction. Asthma medicines include sympathetic stimulants (β2 agonists). ④ Sweating is an exception controlled only by the sympathetic nerves. The parasympathetic nerves do not control sweat glands.
Practice
- Which nerves increase heart rate and enlarge the pupils during exercise?
- What neurotransmitter is released from sympathetic nerve endings?
- Where are the centers of the autonomic nervous system located in the brain? Give two locations.