The autonomic nervous system is made up of 2 subdivisions: sympathetic and parasympathetic systems. The sympathetic system originates from the thoracolumbar regions (T1-L2) of the spinal cord. The parasympathetic system originates from craniosacral regions (brainstem nuclei CN III, VII, IX, and X as well as sacral levels S2-S4). In general, there are 2 neurons that combine to link each effector organ with its respective sympathetic or parasympathetic system. The first neuron is called the preganglionic neuron, and the second is the postganglionic neuron. These neurons synapse together in the autonomic ganglia. The exception to this is the adrenal medulla which is connected directly to the preganglionic neuron.
Both sympathetic and parasympathetic preganglionic neurons are cholinergic, meaning they release acetylcholine (Ach) at the synapse in the ganglion. In the parasympathetic system, postganglionic neurons are also cholinergic. However in the sympathetic system, postganglionic are not all the same. Most sympathetic postganglionic neurons are adrenergic (meaning they release norepinephrine (NE)), but a few are cholinergic- such as the ones to sweat glands and to smooth muscles of certain blood vessels. In the cholinergic synapse, released acetylcholine is degraded down by the enzyme acetylcholinesterase, or reabsorbed into the preganglionic neuron. In the adrenergic synapse, released norepinephrine is either reabsorbed into the preganglionic neuron or degraded by catechol-o-methyl transferase (COMT) enzyme.
In general, the parasympathetic system is responsible for promoting ‘rest & digest’ functions, and the sympathetic system is responsible for promoting ‘fight or flight’ functions. However, the specific response of the effector organ is dependent on the types of receptors present. For example, the sympathetic system will release NE at both alpha and beta receptors. Some effector organs will have only alpha receptors, some only beta receptors, and some a mixture of both and the response will be based on the relative ratio of these receptors. An example of this is vascular tone. Sympathetic stimulation of an alpha receptor results in vasoconstriction, while stimulation of a beta receptor results in vasodilation.
For effector organs that have both sympathetic and parasympathetic innervations, their respective functions are typically opposite each other. It is the balance of the relative sympathetic to parasympathetic tone which dictates the specific action on the effector organ. For example, in the heart the SA node will increase heart rate with sympathetic stimulation but decrease heart rate with parasympathetic stimulation. Thus, the actual heart rate is dependent of the relative balance between these two systems.