Differences between the parasympathetic and sympathetic nervous system are based on their physiological and anatomical organization. The parasympathetic system is responsible for vegetative functions, uses the neurotransmitter acetylcholine, and has short postsynaptic nerves located near or on the organs they innervate. On the other hand, the sympathetic system is responsible for fight-or-flight functions, uses the neurotransmitter norepinephrine, and has relatively long postsynaptic nerves that synapse at a distance from the organs they innervate.
Both systems are part of the autonomic nervous system (ANS), which is responsible for involuntary functions of respiration, circulation, digestion, urination, and reproduction. It also plays a role in maintaining homeostasis when the body faces stressful conditions, such as sickness and starvation. Both systems act to control these physiological functions.
For instance, the sympathetic system allows the body to adjust in stressful situations, such as those that arouse excitement, fear, anger, and embarrassment. This is the reason why it is also called the “fight-or-flight system.” It increase heart rate, dilates the respiratory bronchioles to increase uptake of oxygen, and dilates blood vessels to increase blood supply to the skeletal muscles.
In contrast, the parasympathetic nervous system controls feeding, breeding, and resting functions. It has the opposite effect as the sympathetic system, and decreases heart rate, facilitates the release of digestive enzymes, and stimulates the processes of urination and defecation. The opposing functions explain why a person cannot urinate, defecate, or digest properly when encountering stressful situations.
Both systems have myelinated preganglionic fibers that synapse or connect with unmyelinated postganglionic nerve fibers, which in turn innervate effector organs. Ganglion is a cluster of synapses, and there are both preganglionic nerve cells and postganglionic nerve cells. The anatomical organization of neurons is different for each system.
In the parasympathetic nervous system, the preganglionic outflow arise from the motor nuclei of cranial nerves III, VII, IX, and X of the brain stem, and from the second, third, and fourth segments of the sacral spinal cord. The preganglionic fibers of the parasympathetic system then extend toward the organ they innervate, and synapse in ganglia that are close or located within that organ. After the ganglionic synapse, the postganglionic fibers arise to innervate certain tissues.
The sympathetic nervous system consists of the paravertebral sympathetic ganglia, which are interconnected with spinal nerves, the celiac and hypogastric prevertebral ganglia, and the nerves from the ganglia to the various internal organs. Preganglionic sympathetic fibers leave the spinal cord together with the spinal nerves, but they then go their separate ways. They can also immediately synapse with postganglionic sympathetic neurons, pass upward or downward in the sympathetic chain and synapse with other ganglia, or pass though the chain and through one of the sympathetic nerves to synapse with a peripheral sympathetic ganglion. Sympathetic fibers originate from the spinal cord segments T1 to T12, and L1 and L2.
Chemical signaling used for each system has similarities and differences. In both the parasympathetic and sympathetic nervous system, the preganglionic nerve fibers secrete the neurotransmitter acetylcholine, making them cholinergic. The difference lies in the neurotransmitter released by postganglionic nerve fibers. In the parasympathetic system, the postganglionic neurons are also cholinergic. Conversely, in the sympathetic nervous system, most postganglionic neurons release norepinephrine or adrenalin, and are called adrenergic.
