The autonomic nervous system (ANS) is the part of the peripheral nervous system that controls the internal organs automatically, without conscious thought. Its effectors are smooth muscle (for example in the gut, blood vessel walls, bronchioles and iris), cardiac muscle and glands. It controls activities such as heart rate, digestion, breathing rate, sweating and pupil size, so it is central to homeostasis. The ANS is controlled by centres in the hypothalamus and the medulla oblongata.A key structural difference from the somatic system is that the pathway from the CNS to the effector uses two motor neurones, which synapse in a ganglion (a cluster of cell bodies) outside the CNS:CNS → preganglionic neurone → ganglion → postganglionic neurone → effector The preganglionic neurone is usually myelinated, but the postganglionic neurone is usually unmyelinated, so autonomic impulses travel more slowly than somatic ones. At the ganglion, the preganglionic neurone always releases acetylcholine.
The ANS has two divisions, the sympathetic and parasympathetic nervous systems. Most organs receive neurones from both, and the two divisions have opposite effects: they are antagonistic. For example, one speeds up the heart and the other slows it down. The balance between the two lets the body fine-tune the activity of each organ, rather than just switching it on or off.Unlike the somatic system, which can only excite its effectors, the ANS can either excite or inhibit them.
The ANS has two divisions, the sympathetic and parasympathetic nervous systems. Most organs receive neurones from both, and the two divisions have opposite effects: they are antagonistic. For example, one speeds up the heart and the other slows it down. The balance between the two lets the body fine-tune the activity of each organ, rather than just switching it on or off.Unlike the somatic system, which can only excite its effectors, the ANS can either excite or inhibit them.