sympathetic nervous system

Last reviewed 01/2018

The sympathetic component of the autonomic nervous system is concerned with increasing the level of arousal and energy expenditure - primitive 'fight or flight' behaviour at times of stress. However, it also has more subtle background activity as it co-ordinates with the parasympathetic nervous system for homeostatic functions.

Alike the parasympathetic nervous system, the central integrating centre for sympathetic activity is within the hypothalamus. This may be influenced by higher cortical centres. Efferent fibres descend from the hypothalamus within the intermediolateral columns of the spinal cord. As sympathetic fibres emerge from the central nervous system at spinal segments T1 to L5, the sympathetic system is also known as the thoraco-lumbar outflow.

From the spinal cord, there are two neurones in series before the effector cell is reached. A synapse within a ganglion intervenes between them. The ganglia are arranged in two patterns for the sympathetic system: paravertebral and prevertebral - see the submenu. Additionally, sympathetic nerve fibres may take a number of different pathways through the ganglia.

The preganglionic transmitter is acetylcholine. At the end organ the transmitter is noradrenaline. The exception to this is the sweat glands, which are sympathetically innervated but stimulated by acetylcholine.

Also, afferent fibres run back from the periphery along sympathetic pathways.