The autonomic, or visceral, nervous system innervates the internal organs. It is divided into three parts: the sympathetic, parasympathetic, and enteric nervous systems. For didactic reasons these systems are discussed separately; however, they represent one functional unit.
A Structure of the sympathetic (red) and parasympathetic (blue) nervous systems
Both the sympathetic and parasympathetic systems utilize a two-neuron pathway between the CNS and their targets. The first neuron is called the preganglionic neuron, and the second neuron is the postganglionic neuron. The preganglionic sympathetic neurons are located in the lateral horn of lower cervical, thoracic, and upper lumbar regions of the spinal cord. The preganglionic parasympathetic neurons are located in cranial nerve nuclei and the sacral region of the spinal cord. The vagus nerve, a cranial nerve, contains the preganglionic parasympathetic neurons that will innervate cervical, thoracic, and abdominal viscera. In both the sympathetic and parasympathetic nervous systems, the preganglionic neurons of the CNS synapse with the postganglionic neurons in ganglia of the peripheral nervous system (see C and D).
• In the sympathetic nervous system, the preganglionic neuron synapses with the postganglionic neuron in ganglia of the sympathetic trunk (for trunk and limbs), in prevertebral ganglia (for viscera) or directly in the organs (only suprarenal glands).
• In the parasympathetic nervous system, the vagus nerve terminates at ganglia close to or in the walls (intramural ganglia) of the organs.
According to Langley (1905), the terms sympathetic and parasympathetic nervous system originally referred only to efferent neurons and their axons (visceral efferent fibers, as shown above). It has now been shown that the sympathetic and parasympathetic nervous systems contain afferent fibers (visceral afferent fibers, pain and stretch receptors not shown here, see p. 72).
1. The sympathetic nervous system can be considered the excitatory part of the autonomic nervous system that prepares the body for a “fight or flight” response.
2. The parasympathetic nervous system is the part of the autonomic nervous system that coordinates the “rest and digest” responses of the body.
3. Although there are separate control centers for the two divisions in the brainstem and spinal cord, they have close anatomic and functional ties in the periphery.
4. The principal transmitter at the target organ is acetylcholine in the parasympathetic nervous system and norepinephrine in the sympathetic nervous organ.
5. Stimulation of the sympathetic and parasympathetic nervous systems produces the following different effects on specific organs:
Sympathetic nervous system
Parasympathetic nervous system
Increased heart rate
Decreased heart rate
Bronchodilation and decreased bronchial secretions
Bronchoconstriction and increased bronchial secretions
Decreased secretions and motor activity
Increased secretions and motor activity
Decreased endocrine and exocrine secretions
Increased exocrine secretions
C Circuit diagram of the autonomic nervous system
The synapse of the central, preganglionic neuron uses acetylcholine as a transmitter in both the sympathetic and parasympathetic nervous systems (cholinergic neuron, shown in blue). In the sympathetic nervous system, the transmitter changes to norepinephrine at the synapse of the postganglionic neuron with the target organ (adrenergic neuron, shown in red), while the parasympathetic system continues to use acetylcholine at that level.
Note: Various types of receptors for acetylcholine (neurotransmitter sensors) are located in the membrane of the target cells. As a result, acetylcholine can produce a range of effects depending on the receptor type.
D Circuitry of the autonomic nervous system
Although the sympathetic and parasympathetic nervous systems emerge from two different regions of the CNS (see A), they form a close structural and functional unit close to the organs. The perikarya of the preganglionic sympathetic neurons are located in the lateral horn of the spinal cord. Their axons exit the spinal cord through the anterior root and travel in the white rami communicans (white because they are myelinated) to the sympathetic chain ganglia. The axons synapse with the postganglionic neurons at three different levels:
• For the sympathetic fibers going to the limbs and trunk wall, the preganglionic sympathetic neurons synapse with the postganglionic neurons in the sympathetic chain ganglia. The postganglionic fibers travel in the gray rami communicans (gray because they are unmyelinated) back to the spinal nerves.
• For the sympathetic fibers going to the viscera, the preganglionic sympathetic fibers usually pass through the sympathetic chain ganglia as splanchnic nerves. They synapse with the postganglionic sympathetic neurons in ganglia close to the organs (prevertebral ganglia). From there, the postganglionic fibers travel to the organs. The sympathetic nervous system also influences the enteric nervous system, which is referred to as the third part of the autonomic nervous system (see p. 73). In the colon, for example, sympathetic fibers will contact intramural neurons of the enteric system.
• For the sympathetic fibers going to the suprarenal medulla, the preganglionic sympathetic fibers terminate on the cells of the medulla (not shown here).
The preganglionic parasympathetic neurons of the viscera located in the body cavities originate from brainstem nuclei of the vagus nerves or from sacral spinal cord levels (not shown). They synapse in ganglia that are either very close to or embedded in the organ (intramural ganglia). Afferent pain fibers (marked in green) accompany both sympathetic and parasympathetic nerve fibers. The axons of these fibers originate in pseudounipolar neurons, which are located either in spinal ganglion or in ganglia of the vagus nerves (see p. 72).