autonomic nervous system

autonomic nervous system

autonomic nervous system: see nervous system.

Pathways of the autonomic nervous system. Nerve impulses begin in motor neurons in the brain or elipsis

Part of the nervous system that is not under conscious control and that regulates the internal organs. It includes the sympathetic, parasympathetic, and enteric nervous systems. The first, which connects the internal organs to the brain via spinal nerves, responds to stress by increasing heart rate and blood flow to the muscles and decreasing blood flow to the skin. The second comprises the cranial nerves and the lower spinal nerves, which increase digestive secretions and slow the heartbeat. Both have sensory fibres that send feedback on the condition of internal organs to the central nervous system, information that helps maintain homeostasis. The third division, embedded in the walls of the stomach and intestines, controls digestive movement and secretions.

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The autonomic nervous system (ANS) (or visceral nervous system) is the part of the peripheral nervous system that acts as a control system, maintaining homeostasis in the body. These activities are generally performed without conscious control or sensation. 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. Its main components are its sensory system, motor system (comprised of the parasympathetic nervous system and sympathetic nervous system), and the enteric nervous system.

The ANS is a classical term, widely used throughout the scientific and medical community. Its most useful definition could be: the sensory and motor neurons that innervate the viscera. These neurons form reflex arcs that pass through the lower brainstem or medulla oblongata. If the central nervous system (CNS) is damaged above that level, life is still possible, because cardiovascular, digestive, and respiratory functions will continue to be adequately regulated.

Anatomy

The reflex arcs of the ANS comprise a sensory (afferent) arm, and a motor (efferent or effector) arm. Only the latter is shown in the illustration.

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 and 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 three categories with different effects on their target organs (see below “Function”): sympathetic, parasympathetic and enteric.

Sympathetic ganglia are located in two sympathetic chains close to the spinal cord: the prevertebral and pre-aortic chains. Parasympathetic ganglia, in contrast, are located in close proximity to the target organ: the submandibular ganglion close to salivatory glands, paracardiac ganglia close to the heart etc… Enteric ganglia, which as their name implies innervate the digestive tube, are located inside its walls and collectively contain as many neurons as the entire spinal cord, including local sensory neurons, motor neurons and interneurons. It is the only truly autonomous part of the ANS and the digestive tube can function surprisingly well even in isolation. For that reason the enteric nervous system has been called “the second brain”.

The activity of autonomic ganglionic neurons is modulated by “preganglionic neurons” (also called improperly but classically "visceral motoneurons") located in the central nervous system. Preganglionc sympathetic neurons are in the spinal cord, at thoraco-lumbar levels. Preganglionic parasympathetic neurons are in the medulla oblongata (forming visceral motor nuclei: the dorsal motor nucleus of the vagus nerve (dmnX), the nucleus ambiguus, and salivatory nuclei) and in the sacral spinal cord. Enteric neurons are also modulated by input from the CNS, from preganglionic neurons located, like parasympathetic ones, in the medulla oblongata (in the dmnX).

The feedback from the sensory to the motor arm of visceral reflex pathways is provided by direct or indirect connections between the nucleus of the solitary tract and visceral motoneurons.

Function

Sympathetic and parasympathetic divisions typically function in opposition to each other. But this opposition is better termed complementary in nature rather than antagonistic. For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake. The sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions that do not require immediate reaction. Consider sympathetic as "fight or flight" and parasympathetic as "rest and digest".

However, many instances of sympathetic and parasympathetic activity cannot be ascribed to "fight" or "rest" situations. For example, 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 tonus. Another example is the constant, second to second modulation of heart rate by sympathetic and parasympathetic influences, as a function of the respiratory cycles. More generally, these two systems should be seen as permanently modulating vital functions, in usually antagonistic fashion, to achieve homeostasis. Some typical actions of the sympathetic and parasympathetic systems are listed below:

Sympathetic nervous system

Promotes a "fight of flight" response, corresponds with arousal and energy generation, and inhibits digestion.

  • Diverts blood flow away from the gastro-intestinal (GI) tract and skin via vasoconstriction.
  • Blood flow to skeletal muscles, the lung is not only maintained, but enhanced (by as much as 1200%, in the case of skeletal muscles).
  • Dilates bronchioles of the lung, which allows for greater alveolar oxygen exchange.
  • Increases heart rate and the contractility of cardiac cells (myocytes), thereby providing a mechanism for the enhanced blood flow to skeletal muscles.
  • Dilates pupils and relaxes the lens, allowing more light to enter the eye.

Parasympathetic nervous system

Promotes a "rest and digest" response, promotes calming of the nerves return to regular function, and enhances digestion.

  • Dilates blood vessels leading to the GI tract, increasing blood flow. This is important following the consumption of food, due to the greater metabolic demands placed on the body by the gut.
  • The parasympathetic nervous system can also constrict 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 or [Myocardium].
  • During accommodation, the parasympathetic nervous system causes constriction of the pupil and lens.
  • The parasympathetic nervous system stimulates salivary gland secretion, and accelerates peristalsis, so, in keeping with the rest and digest functions, appropriate PNS activity mediates digestion of food and indirectly, the absorption of nutrients.
  • Is also involved in erection of genitals, via the pelvic splanchnic nerves 2–4.

Neurotransmitters and pharmacology

At the effector organs, sympathetic ganglionic neurons release noradrenaline (norepinephrine), along with other cotransmittors 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 cortex, there is no postsynaptic neuron. Instead the presynaptic neuron releases acetylcholine to act on nicotinic receptors.
  • Stimulation of the adrenal medulla releases adrenaline (epinephrine) into the bloodstream which will act on adrenoceptors, producing a widespread increase in sympathetic activity.

The following table reviews the actions of these neurotransmitters as a function of their receptors.

Circulatory system

Heart

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
cardiac output β1, (β2): increases M2: decreases

SA node: heart rate (chronotropic) β1, (β2) : increases M2: decreases
Atrial cardiac muscle: contractility (inotropic) β1, (β2): increases M2: decreases
Ventricular cardiac muscle β1, (β2):
increases contractility (inotropic)
increases cardiac muscle automaticity
---
at AV node β1:
increases conduction
increases cardiac muscle automaticity
M2:
decreases conduction
Atrioventricular block

Blood vessels

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
vascular smooth muscle α: contracts; β2: relaxes M3: relaxes

renal artery α1: constricts ---
larger coronary arteries α1 and α2: constricts ---
smaller coronary arteries β2:dilates ---
arteries to viscera α: constricts ---
arteries to skin α: constricts ---
arteries to brain α1: constricts ---
arteries to erectile tissue α1: constricts M3: dilates
arteries to salivary glands α: constricts M3: dilates
hepatic artery β2: dilates ---
arteries to skeletal muscle β2: dilates ---

Veins α1 and α2 : constricts
β2: dilates
---

Other

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
platelets α2: aggregates ---
mast cells - histamine β2: inhibits ---

Respiratory system

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
smooth muscles of bronchioles β2: relaxes (major contribution)
α1: contracts (minor contribution)
M3: contracts

The bronchioles have no sympathetic innervation, but are instead affected by circulating adrenaline

Nervous system

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
Pupil dilator muscle α1: contracts
(causes mydriasis)
M3: relaxes
(causes miosis)
Ciliary muscle β2: relaxes
(causes long-range focus)
M3: contracts
(causes short-range focus)

Digestive system

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
salivary glands: secretions β: stimulates viscous, amylase secretions
α1: stimulates potassium cation
M3: stimulates watery secretions
lacrimal glands (tears) β2: Protein secretion M3: increases
kidney (renin) β2: secretes ---
parietal cells --- M1: Gastric acid secretion
liver α1, β2: glycogenolysis, gluconeogenesis ---
adipose cells β3: stimulates lipolysis ---
GI tract (smooth muscle) motility α1, α2, β2: decreases M3, (M1) : increases
sphincters of GI tract α2 , β2: contracts M3: relaxes
glands of GI tract no effect M3: secretes

Endocrine system

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
pancreas (islets) α2: decreases secretion from beta cells, increases secretion from alpha cells M3 increases stimulation from alpha cells and beta cells
adrenal medulla N (nicotinic ACh receptor): secretes epinephrine and norepinephrine ---

Urinary system

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
Detrusor urinae muscle‎ of bladder wall β2: relaxes contracts
ureter α1: contracts relaxes
sphincter α1: contracts; β2 relaxes relaxes

Reproductive system

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
uterus α1: contracts (pregnant)
β2: relaxes (non-pregnant)
---
genitalia α: contracts (ejaculation) M3: erection

Integument

Target Sympathetic (muscarinic and adrenergic) Parasympathetic (muscarinic)
sweat gland secretions M: stimulates (major contribution); α1: stimulates (minor contribution) ---
erector pili α1: stimulates ---

See also

References

External links

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