Key points

■ The ANS is an involuntary system that preserves a constant internal environment by innervating cardiac muscle, and smooth muscle of blood vessels and visceral structures.

■ The system has afferent, central and efferent components.

■ It is subdivided into the craniosacral, parasympathetic (‘rest and digest’), and the thoracolumbar, sympathetic (‘fight or flight’) systems.

■ Autonomic UMN cell bodies are located in the hypothalamus.

■ A presynaptic and a postsynaptic neuron, in series, connect the CNS with the target organ. The presynaptic neuron has its cell body in the CNS and synapses with the postsynaptic neuron in a peripheral ganglion.

■ The neurotransmitter at the autonomic ganglia is acetylcholine. The neurotransmitter at the neuromuscular junction is acetylcholine for the parasympathetic system and noradrenaline for the sympathetic nervous system.

■ The location of the second neuron is system specific and close to the organ (terminal/intramural) for the parasympathetic system. It is remote from the organ (pre- or paravertebral) for the sympathetic nervous system.

■ The presynaptic nerve can pass through a number of ganglia before synapsing, so the terms ‘preganglionic’ and ‘postganglionic’ can be misleading. Presynaptic and postsynaptic neurons are the preferred terms.

The ANS is a diffuse system that innervates visceral structures, glandular myoepithelium, fat and vasculature throughout the body. The system has afferent, central and efferent components. Afferent fibres often use the same pathways as the efferent nerves. Afferent visceral fibres may also travel via somatic spinal nerves to reach the CNS (see also Chapter 1, p4 for introduction to the ANS).

It is an involuntary system in which effects occur in response to physiological changes within the body and do not require voluntary control. For example, exercise stimulates heart and respiratory rates, blood flow to muscles and sweating. Conversely, eating stimulates increased activity in the gastrointestinal tract, its blood flow and secretions. The ANS aims to preserve a constant internal environment.