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Autonomic Nervous System (Sympathetic and Parasympathetic)
The autonomic nervous system innervates smooth muscle, cardiac muscle as well as the glands of the body. The signal pathway in the autonomic nervous system usually consist of a series of two neurons - the preganglionic neuron and the postganglionic neuron. The autonomic nervous system consists of the motor and sensory divisions. The motor division can be subdivided into two - the sympathetic and the parasympathetic nervous system. The sympathetic nervous system is responsible for the fight-or-flight responses. This includes increasing the size of the pupil (via the radial smooth muscle in the iris), increasing the heart rate and respiratory rate, increasing sweating, decreasing the rate of digestion and inhibiting peristalsis. The overall effect is to move more oxygenated blood to the skeletal muscle while decreasing the blood flow to the digestive system. In the sympathetic division, the preganglionic neuron always begins in the spinal cord and extends outward from the ventral side of the spine. It contains a relatively short axon and synapses with the postganglionic cell. At the synapse, acetylcholine is used as the neurotransmitter. The postganglionic synapse uses either epinephrine or norepinephrine. The electrical signals carried to the adrenal medulla by the sympathetic division only involve a single neuron (the preganglionic neuron) in the pathway. It also used acetylcholine. The parasympathetic nervous system is responsible for controlling the rest-and-digest responses. Its effect is to increase the blood flow to the digestive and excretory systems while decreasing the blood flow to the skeletal muscle. It basically reverses the effects of the sympathetic nervous system. The preganglionic neurons in the parasympathetic system can begin either in the spinal cord or the brain and have relatively long axons. Both types of synapses use acetylcholine as the neurotransmitter. The most important nerve of the parasympathetic nervous system is the vagus nerve (10th cranial nerve) because it innervates the majority of the organs in the thoracic and abdominal regions, including the heart, the lungs, the kidneys, the liver, the small intestine, etc.
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