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Autonomic nervous system

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The autonomic nervous system (ANS) is a division of the peripheral nervous system that supplies smooth muscle and glands, and thus influences the function of internal organs.

Innervates all body organs, including the cardiovascular system.

The autonomic nervous system is a control system that acts largely unconsciously.

it is critically involved in maintaining homeostasis and is associated with appropriate adaptive responses, reflexively and unobtrusively.

It regulates bodily functions such as the heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal.

Smooth muscle in arteries, veins and pericytes in capillaries receive autonomic innovation which modulates vascular smooth muscle tone and blood vessel diameter.

There are afferent sensory neurons with receptors monitoring local changes in the chemical and mechanical environment and provides information that allows the autonomic nervous system to regulate blood flow in every organ and redirect cardiac output to vascular beds as needed.

The autonomic nervous system is the primary mechanism in control of the fight-or-flight response.

The autonomic nervous system is regulated by the hypothalamus.

The autonomic nervous system provides blood pressure control from moment to moment and heart rate through baroreflexes.

Negative feedback neural loops regulate sympathetic neuron sending nerve impulses to the vascularized, the heart, and the kidney and parasympathetic neurons sending nerve impulses to the sinus node of the heart.

Autonomic functions include control of respiration, cardiac regulation, vasomotor activity and certain reflex actions such as coughing, sneezing, swallowing and vomiting.

The hypothalamus, acts as an integrator for autonomic functions, receiving autonomic nervous system regulatory input from the limbic system.

The autonomic nervous system has three branches: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system.

The sympathetic nervous system is often considered the “fight or flight” system.

The parasympathetic nervous system is often considered the “rest and digest” system.

In many cases, both of these systems have opposite actions.

One system may activate a physiological response while the other inhibits it.

The sympathetic nervous system is characterized as a quick response mobilizing system.

The parasympathetic nervous system is generally a slowly activated dampening system, but exceptions are sexual arousal and orgasm.

The autonomic nervous system is only connected with the motor side.

Between neurons inhibitory and excitatory synapses occur.

Most autonomous functions are involuntary but they can often work in conjunction with the somatic nervous system which provides voluntary control.

The autonomic nervous system is divided into the sympathetic nervous system and parasympathetic nervous system.

The sympathetic division emerges from the spinal cord in the thoracic and lumbar areas, terminating around L2-3.

The parasympathetic division has craniosacral flow, meaning that the neurons begin at the cranial nerves, specifically the oculomotor nerve, facial nerve, glossopharyngeal nerve and vagus nerve, and sacral (S2-S4) spinal cord.

The autonomic nervous system requires al 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 postganglionic neuron will then synapse at the target organ.

The sympathetic nervous system consists of cells with bodies in the lateral grey column from T1 to L2/3.

These sympathetic nervous system cell bodies are general visceral efferent neurons and are the preganglionic neurons.

Sites where preganglionic neurons can synapse with their postganglionic neurons:

Paravertebral ganglia of the sympathetic chain.

cervical ganglia

thoracic ganglia and rostral lumbar ganglia

caudal lumbar ganglia and sacral ganglia

Prevertebral ganglia include: celiac ganglion, aorticorenal ganglion, superior mesenteric ganglion, inferior mesenteric ganglion.

The parasympathetic nervous system consists of cells with bodies in one of two locations: the brainstem involving Cranial Nerves III, VII, IX, X or the sacral spinal cord involving S2, S3, S4.

These are the preganglionic neurons, which synapse with postganglionic neurons in these locations:

Parasympathetic ganglia of the head include:Ciliary (Cranial nerve III), Submandibular (Cranial nerve VII), Pterygopalatine (Cranial nerve VII), and Otic (Cranial nerve IX) In or near the wall of an organ innervated by the Vagus (Cranial nerve X) or Sacral nerves (S2, S3, S4).

The sensory component of the autonomic nervous system is composed of primary visceral sensory neurons found in the peripheral nervous system in cranial sensory ganglia: the geniculate, petrosal and nodose ganglia, related respectively to cranial nerves VII, IX and X.

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.

Unlike most functions of the ANS, sensory neurons also convey taste and smell sensation, which are conscious perceptions.

Blood oxygen and carbon dioxide are in sensed by the carotid body, chemosensors at the bifurcation of the carotid artery, innervated by the petrosal (IXth) ganglion.

Primary sensory neurons synapse to visceral sensory neurons located in the medulla oblongata, forming the nucleus of the solitary tract (nTS).

The nucleus of the solitary tract integrates all visceral information.

The nucleus of the solitary tract also receives input from a chemosensory center, the area postrema, and can detects toxins in the blood and the cerebrospinal fluid and is essential for chemically induced vomiting or conditional taste aversion.

Visceral sensory information constantly and unconsciously modulates the activity of the motor neurons of the autonomic nervous system, constantly and unconsciously

Autonomic nerves travel to organs throughout the body.

Most organs receive parasympathetic supply by the vagus nerve and sympathetic supply by splanchnic nerves.

Pain in any internal organ is perceived as referred pain.

The autonomic nervous system (ANS) is a division of the peripheral nervous system that supplies smooth muscle and glands, and thus influences the function of internal organs.

The autonomic nervous system is a control system that acts largely unconsciously.

It regulates bodily functions such as the heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal.

The autonomic nervous system is the primary mechanism in control of the fight-or-flight response.

The autonomic nervous system is regulated by the hypothalamus.

Autonomic functions include control of respiration, cardiac regulation, vasomotor activity and certain reflex actions such as coughing, sneezing, swallowing and vomiting.

The hypothalamus, acts as an integrator for autonomic functions, receiving autonomic nervous system regulatory input from the limbic system.

The autonomic nervous system has three branches: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system.

The sympathetic nervous system is often considered the “fight or flight” system.

The parasympathetic nervous system is often considered the “rest and digest” system.

These two systems should be seen as modulating vital functions, in usually antagonistic fashion, to achieve homeostasis of heart rate and blood pressure.

In many cases, both of these systems have opposite actions.

One system may activate a physiological response while the other inhibits it.

The sympathetic nervous system is characterized as a quick response mobilizing system.

The parasympathetic nervous system is generally a slowly activated dampening system, but exceptions are sexual arousal and orgasm.

The autonomic nervous system is only connected with the motor side.

Between neurons inhibitory and excitatory synapses occur.

Most autonomous functions are involuntary but they can often work in conjunction with the somatic nervous system which provides voluntary control.

The autonomic nervous system is divided into the sympathetic nervous system and parasympathetic nervous system.

The sympathetic division emerges from the spinal cord in the thoracic and lumbar areas, terminating around L2-3.

The parasympathetic division has craniosacral flow, meaning that the neurons begin at the cranial nerves, specifically the oculomotor nerve, facial nerve, glossopharyngeal nerve and vagus nerve, and sacral (S2-S4) spinal cord.

The autonomic nervous system requires al 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 postganglionic neuron will then synapse at the target organ.

The sympathetic nervous system consists of cells with bodies in the lateral grey column from T1 to L2/3.

These sympathetic nervous system cell bodies are general visceral efferent neurons and are the preganglionic neurons.

Sites where preganglionic neurons can synapse with their postganglionic neurons:

Paravertebral ganglia of the sympathetic chain:

cervical ganglia

thoracic ganglia and rostral lumbar ganglia

caudal lumbar ganglia and sacral ganglia

Prevertebral ganglia include: celiac ganglion, aorticorenal ganglion, superior mesenteric ganglion, inferior mesenteric ganglion.

The parasympathetic nervous system consists of cells with bodies in one of two locations: the brainstem involving Cranial Nerves III, VII, IX, X or the sacral spinal cord involving S2, S3, S4.

These are the preganglionic neurons, which synapse with postganglionic neurons in these locations:

Parasympathetic ganglia of the head include:Ciliary (Cranial nerve III), Submandibular (Cranial nerve VII), Pterygopalatine (Cranial nerve VII), and Otic (Cranial nerve IX) In or near the wall of an organ innervated by the Vagus (Cranial nerve X) or Sacral nerves (S2, S3, S4).

The sensory component of the autonomic nervous system is composed of primary visceral sensory neurons found in the peripheral nervous system in cranial sensory ganglia: the geniculate, petrosal and nodose ganglia, related respectively to cranial nerves VII, IX and X.

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.

Unlike most functions of the ANS, sensory neurons also convey taste and smell sensation, which are conscious perceptions.

Blood oxygen and carbon dioxide are in sensed by the carotid body, chemosensors at the bifurcation of the carotid artery, innervated by the petrosal (IXth) ganglion.

Primary sensory neurons synapse to visceral sensory neurons located in the medulla oblongata, forming the nucleus of the solitary tract (nTS).

The nucleus of the solitary tract integrates all visceral information.

The nucleus of the solitary tract also receives input from a chemosensory center, the area postrema, and can detects toxins in the blood and the cerebrospinal fluid and is essential for chemically induced vomiting or conditional taste aversion.

Visceral sensory information constantly and unconsciously modulates the activity of the motor neurons of the autonomic nervous system, constantly and unconsciously

Autonomic nerves travel to organs throughout the body.

Most organs receive parasympathetic supply by the vagus nerve and sympathetic supply by splanchnic nerves.

Pain in any internal organ is perceived as referred pain.

Motor neurons of the autonomic nervous system are found in autonomic ganglia.

Those of the parasympathetic branch are located close to the target organ while the ganglia of the sympathetic branch are located close to the spinal cord.

The sympathetic ganglia are found in two chains: the pre-vertebral and pre-aortic chains.

The activity of autonomic ganglionic neurons is modulated by preganglionic neurons located in the central nervous system.

Preganglionic sympathetic neurons are located in the spinal cord, at the thorax and upper lumbar levels.

Preganglionic parasympathetic neurons are found in the medulla oblongata where they form visceral motor nuclei, he dorsal motor nucleus of the vagus nerve, he nucleus ambiguus, the salivatory nuclei, and in the sacral region of the spinal cord.

Sympathetic and parasympathetic divisions typically function in opposition to each other.

The sympathetic division typically functions in actions requiring quick responses, while the parasympathetic division functions with actions that do not require immediate reaction.

The sympathetic system is often considered the “fight or flight” system.

The parasympathetic system is often referred to as the “rest and digest” or “feed and breed” system.

However, many instances of sympathetic and parasympathetic activity cannot be ascribed to “fight” or “rest” situations.

Standing up from a reclining or sitting position would entail an unsustainable drop in blood pressure if not for a compensatory increase in the arterial sympathetic tone.

Modulation of heart rate by sympathetic and parasympathetic influences, as a function of the respiratory cycles.

Some typical actions of the sympathetic and systems are listed below.

Sympathetic nervous system promotes a fight-or-flight response, and corresponds with arousal and energy generation, and inhibits digestion

The sympathetic nervous system diverts blood flow away from the gastro-intestinal tract and skin via vasoconstriction, and enhances blood flow to skeletal muscles and the lungs.

Blood flow is enhanced by as much as 1200% in the case of skeletal muscles.

The sympathetic nervous system dilates bronchioles of the lung through circulating epinephrine, which allows for greater alveolar oxygen exchange.

The sympathetic nervous system increases heart rate and the contractility of cardiac cells.

The sympathetic nervous system dilates pupils and relaxes the ciliary muscle to the lens, allowing more light to enter the eye and enhances far vision.

The sympathetic nervous system provides vasodilation for the coronary vessels of the heart.

The sympathetic nervous system constricts all the intestinal sphincters and the urinary sphincter, inhibits peristalsis, and stimulates orgasm.

The parasympathetic nervous system promotes calming of the nerves return and enhances digestion.

Functions of nerves within the parasympathetic nervous system include:

Dilating blood vessels leading to the GI tract, increasing the blood flow.

Constricting the bronchiolar diameter when the need for oxygen has diminished

Dedicated cardiac branches of the vagus and thoracic spinal accessory nerves impart parasympathetic control of the heart.

Constriction of the pupil and contraction of the ciliary muscles, facilitating accommodation and allowing for closer vision.

Stimulating salivary gland secretion, and accelerates peristalsis, mediating digestion of food and, indirectly, the absorption of nutrients.

Nerves of the peripheral nervous system are involved in the erection of genital tissues via the pelvic splanchnic nerves 2–4, and are also responsible for stimulating sexual arousal.

The enteric nervous system is the intrinsic nervous system of the gastrointestinal system, and Its functions include:

Sensing chemical and mechanical changes in the gut

Regulation secretions in the gut

Control of peristalsis and some other movements

At the effector organs, sympathetic ganglionic neurons release noradrenaline, along with other cotransmitters such as ATP, to act on adrenergic receptors, with the exception of the sweat glands and the adrenal medulla.

Acetylcholine is the preganglionic neurotransmitter for both divisions of the ANS, as well as the postganglionic neurotransmitter of parasympathetic neurons.

Nerves that release acetylcholine are said to be cholinergic.

In the parasympathetic system, ganglionic neurons use acetylcholine as a neurotransmitter to stimulate muscarinic receptors.

At the adrenal medulla the presynaptic neuron releases acetylcholine to act on nicotinic receptors, and stimulation of the adrenal medulla releases adrenaline into the bloodstream, which acts on adrenoceptors, thereby indirectly mediating or mimicking sympathetic activity.

Dysfunction of the autonomic nervous system may lead to numerous symptoms and abnormalities, central to which is orthostatic hypotension.

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