Autonomic nervous system, in vertebrates, the part of the nervous system that controls and regulates the internal organs without any conscious recognition or effort by the organism. The autonomic nervous system comprises two antagonistic sets of nerves, the sympathetic and parasympathetic nervous systems. The sympathetic nervous system connects the internal organs to the brain by spinal nerves. When stimulated, these nerves prepare the organism for stress by increasing the heart rate, increasing blood flow to the muscles, and decreasing blood flow to the skin. The nerve fibres of the parasympathetic nervous system are the cranial nerves, primarily the vagus nerve, and the lumbar spinal nerves. When stimulated, these nerves increase digestive secretions and reduce the heartbeat.
The parasympathetic nervous system (PNS) is one of the two functionally distinct and continuously active divisions of the autonomic nervous system (ANS). It is in opposition to the other, the sympathetic nervous system (SNS). The parasympathetic nervous system predominates in quiet "rest and digest" conditions while the sympathetic nervous system drives the "fight or flight" response in stressful situations. The main purpose of the PNS is to conserve energy to be used later and to regulate bodily functions like digestion and urination.
The parasympathetic nervous system is organized in a manner similar to the sympathetic nervous system. Its motor component consists of preganglionic and postganglionic neurons. The preganglionic neurons are located in specific cell groups (also called nuclei) in the brainstem or in the lateral horns of the spinal cord at sacral levels. Preganglionic axons emerging from the brainstem project to parasympathetic ganglia that are located in the head or near the heart, are embedded in the end organ itself (e.g., the trachea, bronchi, and gastrointestinal tract), or are situated a short distance from the urinary bladder. Both preganglionic and postganglionic neurons secrete acetylcholine as a neurotransmitter, but, like sympathetic ganglion cells, they also contain other neuroactive chemical agents that function as cotransmitters.
Both divisions of the automatic nervous system are comprised of a complex network of pathways responsible for maintaining the physiologic integrity of organs, tissues, and cells. They are composed of pre ganglionic and postganglionic neurons that act on effector organs. The parasympathetic nervous system originates from medial medullary sites (nucleus ambiguous, nucleus tractus solitarius, and dorsal motor nucleus) and is modulated by the hypothalamus.
The preganglionic neurons of the parasympathetic nervous system come from brainstem nuclei and the sacral spinal cord (specifically S2-S4). The axons of preganglionic parasympathetic nervous system neurons are much longer than those of the sympathetic nervous system and synapse with the postganglionic neurons in ganglia at or near the effector organs. The very short postganglionic axons then relay signals to the cells of the effector organs. Those preganglionic parasympathetic neurons that begin in the brainstem leave the central nervous system (CNS) through cranial nerves, including the oculomotor nerve (III) acting on the eyes, the facial nerve (VII) working on the lacrimal gland, the salivary glands, and the mucous membranes within the nasal cavity, the glossopharyngeal nerve (IX) acting on the parotid gland, and the vagus nerve (X) acting on the viscera of the abdomen and thorax. The preganglionic fibers coming from the sacral cord join to form pelvic splanchnic nerves, which act on the pelvic cavity visceral.
The parasympathetic nervous system is responsible for the body’s relaxation response, and it also plays a large part in regulating digestion, heart rate, and breathing. The parasympathetic nervous system is responsible for various functions of internal organs, such as their capacity for relaxation, constriction, and mucus production. While the sympathetic nervous system uses a form of adrenaline called norepinephrine to activate receptors in different organs, the peripheral system uses acetylcholine as its main neurotransmitter. Acetylcholine stimulates muscarinic receptor sites around the body and is used to contract muscles in the digestive system, help the bladder urinate, and return breathing to normal after a period of stress. Consider the list below as a sample of the many responsibilities of the parasympathetic nervous system: The parasympathetic nervous system uses acetylcholine to stimulate muscarine receptors in the eye and contract the pupil. This constriction affects our vision and helps us see close-up objects clearly. The parasympathetic nervous system decreases heart rate when the body is under stress. It also affects circulation due to its ability to constrict blood vessels.
