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The sympathetic nervous system is part of the nervous system that is commonly referred to as the conscious, or thinking, part of the nervous system. It encompasses the areas between the lumbar and the thoracic vertebra, and basically operates between the operations of preganglia and postganglia. Norepinephrine is the neurotransmitter released between the neurons, which can trigger the release of epinephrine, produced by the adrenal glands, for the "fight or flight" response.
The parasympathetic nervous system, by contrast, works to maintain homeostasis in the body when the body is at rest. It encompasses the rest of the peripheral nervous system, receiving input from the far regions of the body, sent to the medulla, which processes and maintains unconscious processes, such as breathing, heart rate, perspiration, and blood pressure. This system is commonly referred to as the unconscious part of the nervous system.
The autonomic nervous system controls, in an involuntary, reflexive manner, multiple organs and muscles in the body. The autonomic nervous system is divided into three sections: sympathetic nervous system, parasympathetic nervous system, and enteric nervous system.
The primary function of the sympathetic nervous system is to protect us in stress or "fight or flight" situations.
Physically, the sympathetic nervous system begins in the spinal cord. The cell bodies of the preganglion neurons are located in the lumbar and thoracic spinal cord area. The postganglion neuron jumps to the muscle or gland target.
The synapse in the sympathetic preganglion uses acetylcholine as a neurotransmitter. The synapse in the postganglion neuron uses norepinephrine as a neurotransmitter (except the sympathetic sweat gland postganglion neuron that uses acetylcholine).
The parasympathetic nervous system's function is primarily for responses termed "rest and digest". It saves energy by decreasing blood pressure and lowering heart rate to allow digestion to proceed.
Physically, cell bodies of the parasympathetic nervous system are located in the sacral region of the spinal cord and in the cranial nerves of the medulla. The preganglion fibers from these areas reach close to their target organs.
The neurotransmitter used across both pre and postganglion synapses is acetylcholine.
Most of the effects of the parasympathetic and sympathetic nervous systems oppose each other. For example, the eyes - sympathetic nervous system causes pupil dilation whereas the parasympathetic nervous system causes pupil constriction.
Both systems are always working to maintain internal body functions not just during "fight or flight" or "rest and digest" responses.
So both of these portions of the nervous system are actually part of the autonomic nervous system, which is mediated by the unconscious. They are often differentiated by the three ways you mentioned. In both cases all functions are mediated by a 2-neuron pathway, where the first neuron's cell body is in the central nervous system (brain or spinal cord) and the second neuron's cell body is in the peripheral nervous system in a ganglion. Thus, the first neuron is always referred to as "preganglionic" and the second is "postganglionic."
The sympathetic system if generally referred to as the "fight or flight" system, where the functions mediated by postganglionicneurons help the body respond to stress. For example, almost all blood vessels are innervated by the sympathetic nervous system, and many of those blood vessels will constrict when stimulated by this system to increase blood pressure. The easiest method of differentiation with this system is by looking at the location of thepreganglionic neuronal cell bodies. These will always be located in theintermediolateral cell column of the spinal cord in between T1 and L2. Postganglionic cell bodies are located in the sympathetic trunk, which runs along the spinal cord, collateral ganglia in the abdomen, or cervical ganglia of the neck. The easy way to remember these is to think of the sympathetic system as having short first neurons and long second neurons. Finally, in terms of neurotransmitters, thepreganglionic neurons all use acetylcholine (ACh) to communicate with postganglionic neurons. Almost all postganglionics usenorepinepherine to convey their messages, although there are exceptions like neurons innervating arrector pili, the muscles that create goosebumps, which use ACh. Finally, the sympathetic system has a weird exception where presympathetic neurons innervate the adrenal medulla directly using ACh as the neurotransmitter. This exception does make sense looking at embryology, but let's not get into that!
The parasympathetic system is often the opposite of the above system. Functionally, it can be seen to be the "rest and digest" system. In terms of location, the easy way is to see that all parasympathetic preganglionic nerves arise from cranial nerve nuclei (in the brain) or from the sacral autonomic nucleus (in the pelvis).Postganglionic nerves are very short, and often they are not seen except for those coming from ganglia in the cranium. Most ganglia for parasympathetic nerves are within viscera (organs), from which arise very short postganglionic neurons to make parasympathetic effects happen. So, in line with keeping the parasympathetic the opposite of the sympathetic, the parasympathetic system has longpreganglionic nerves and short postganglionic nerves. In terms of neurotransmitters, almost all preganglionic and postganglionicnerves of the parasympathetic nervous system use ACh. In a few cases, some peptides can serve as neurotransmitters in the system, but this is very minor.
I hope this helps!
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