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From Wikipedia

Autonomic nervous system

The autonomic nervous system (ANS or visceral nervous system) is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils, micturition (urination), and sexual arousal. Whereas most of its actions are involuntary, some, such as breathing, work in tandem with the conscious mind.

It is classically divided into two subsystems: the parasympathetic nervous system and sympathetic nervous system. Relatively recently, a third subsystem of neurons that have been named 'non-adrenergic and non-cholinergic' neurons (because they use nitric oxide as a neurotransmitter) have been described and found to be integral in autonomic function, particularly in the gut and the lungs.

With regard to function, the ANS is usually divided into sensory (afferent) and motor (efferent) subsystems. Within these systems, however, there are inhibitory and excitatorysynapses between neurons.

The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.


ANS innervation is divided into sympathetic nervous system and parasympathetic nervous system divisions. The sympathetic division has thoracolumbar “outflow�, meaning that the neurons begin at the thoracic and lumbar (T1-L2) portions of the spinal cord. The parasympathetic division has craniosacral “outflow�, meaning that the neurons begin at the cranial nerves (CN 3, CN7, CN 9, CN10) and sacral (S2-S4) spinal cord.

The ANS is unique in that it requires a sequential two-neuron efferent pathway; the preganglionic neuron must first synapse onto a postganglionic neuron before innervating the target organ. The preganglionic, or first, neuron will begin at the “outflow� and will synapse at the postganglionic, or second, neuron’s cell body. The post ganglionic neuron will then synapse at the target organ.

Sympathetic division

The sympathetic division (thoracolumbar outflow) consists of cell bodies in the lateral horn of spinal cord (intermediolateral cell columns) of the spinal cord from T1 to L2. These cell bodies are GVE neurons (general visceral efferent), and are the preganglionic neurons. There are several locations upon which preganglionic neurons can synapse for their postganglionic neurons:

These ganglia provide the postganglionic neurons from which innervation of target organs follows. Examples of splanchnic (visceral) nerves are:

  • Cervical cardiac nerves & thoracic visceral nerves which synapse in the sympathetic chain
  • Thoracic splanchnic nerves (greater, lesser, least) which synapse in the prevertebral ganglion
  • Lumbar splanchnic nerves which synapse in the prevertebral ganglion
  • Sacral splanchnic nerves which synapse in the inferior hypogastric plexus

These all contain afferent (sensory) nerves as well, also known as GVA neurons (general visceral afferent).

Parasympathetic division

The parasympathetic division (craniosacral outflow) consists of cell bodies from one of two locations: brainstem (Cranial Nerves 3, 7, 9, 10) or sacral spinal cord (S2, S3, S4). These are the preganglionic neurons, which synapse with postganglionic neurons in these locations:

  • Parasympathetic ganglia of the head (Ciliary (CN3), Submandibular (CN7), Pterygopalatine (CN7), Otic (CN9))
  • In or near wall of organ innervated by Vagus (CN10), Sacral nerves (S2, S3, S4))

These ganglia provide the postganglionic neurons from which innervations of target organs follows. Examples are:

  • The preganglionic parasympathetic splanchnic (visceral) nerves
  • Vagus nerve, which wanders through the thorax and abdominal regions innervating, among other organs, the heart, lungs, liver and stomach

Sensory neurons

The sensory arm is made of “primary visceral sensory neurons� found in the peripheral nervous system (PNS), in “cranial sensory ganglia�: the geniculate, petrosal and nodose ganglia, appended respectively to cranial nerves VII, IX and X. These sensory neurons monitor the levels of carbon dioxide, oxygen and sugar in the blood, arterial pressure and the chemical composition of the stomach and gut content. (They also convey the sense of taste, a conscious perception). Blood oxygen and carbon dioxide are in fact directly sensed by the carotid body, a small collection of chemosensors at the bifurcation of the carotid artery, innervated by the petrosal (IXth) ganglion. Primary sensory neurons project (synapse) onto “second order� or relay visceral sensory neurons located in the medulla oblongata, forming the nucleus of the solitary tract (nTS), that integrates all visceral information. The nTS also receives input from a nearby chemosensory center, the area postrema, that detects toxins in the blood and the cerebrospinal fluid and is essential for chemically induced vomiting or conditional taste aversion (the memory that ensures that an animal which has been poisoned by a food never touches it again). All these visceral sensory informations constantly and unconsciously modulate the activity of the motor neurons of the ANS

Motor neurons

Motor neurons of the ANS are also located in ganglia of the PNS, called “autonomic ganglia�. They belong to

From Yahoo Answers

Question:True or False

Answers:True. The adrenal medulla is innervated by preganglionic sympathetic neurons. The adrenal medulla consists of cells that are essentially postganglionic neurons, but they are clustered in the middle of the adrenal tissue and do not have axons. When stimulated, they release norepinephrine and epinephrine.

Question:The SNS is the body's emergency response system. How come it has the shorter, myelinated, preganglionic fiber? I would think that since the body would want the fastest response time in emergency situations then the SNS would have the longer preganglionic fiber thereby allowing for a faster signal transportation time. The only thing I can think of is that this would allow time for the neurotransmitters released from the adrenal medulla to spread throughout the body and shut off the PNS so that when the signal from the SNS arrives the PNS is already inactive. Can someone answer this? I've asked every professor I could and no one seems to know be able to give a satisfactory answer.

Answers:I think the answer is easier than you are making it. You don't seem to have a problem with knowing HOW the SNS and PNS are working, but are basically asking WHY evolution hasn't made this system better by making the preganglionic fibers of the SNS longer than the the slower postganglionic fibers to increase the speed of the signal. We have to look at the different functions of the SNS and PNS, and see how they are reflected their structure and composition. One of the big differences between them is that the signals or the SNS are extremely widespread rather than specific to one organ or muscle sheet, as in the case of the PNS. For this reson it makes sense to have the SNS ganglions close to the vertbral column, as opposed to all the way to each organ/muscle.

Question:a. dilation of pupil b. increased heart rate. c. increased gastrointestinal motility. d. increased adrenal gland activity.

Answers:the answer is C. increasing GI motility is the function of the parasympathetic system

Question:I am doing physiology lab homework and im struggling a bit with the cardiovascular system. There is a question that says in the absence of influences, the SA node generates Action Potentials at 100 beats per min. But the average beats per min. is 70. and it is asking what this tells me about the parasympathetic nervous system in realtion to sympathetic nervous system. I understand that the parasympathetic slows down the heart and that sympathetic speeds it up and that they both work on the heart in sort of a tug and pull thing....but does the above senerio mean that the parasympathetic nervous system has a greater influence on the heart rate?

Answers:In a normal situation, yes. But all these systems are interrelated. The medulla oblongata acts to control the pulse by sympathetic and parasympathetic impulses. Baroreceptors in the aorta and carotid sinuses alter the cardiac output via the glossopharyngeal nerve and the vagus nerve. Overall the parasympathetic system has a greater influence on rate in the basal rate, but those things can fluctuate as need be.

From Youtube

COMLEX Board Review OMT Sympathetic Innervations :The sympathetic nervous system (SNS) is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's resources under stress; to induce the flight-or-fight response. It is, however, constantly active at a basal level in order to maintain homeostasis. he sympathetic nervous system is responsible for up- and down-regulating many homeostatic mechanisms in living organisms. Fibers from the SNS innervate tissues in almost every organ system, providing at least some regulatory function to things as diverse as pupil diameter, gut motility, and urinary output. It is perhaps best known for mediating the neuronal and hormonal stress response commonly known as the fight-or-flight response. This response is also known as sympatho-adrenal response of the body, as the preganglionic sympathetic fibers that end in the adrenal medulla (but also all other sympathetic fibers) secrete acetylcholine, which activates the great secretion of adrenaline (epinephrine) and to a lesser extent noradrenaline (norepinephrine) from it. Therefore, this response that acts primarily on the cardiovascular system is mediated directly via impulses transmitted through the sympathetic nervous system and indirectly via catecholamines secreted from the adrenal medulla.