What is Autonomic Nervous System? Sympathetic |Nervous System | Physiology

                                         

                   Autonomic Nervous System |Sympathetic

Keywords: sympathetic |thoracolumbar outflow | parasympathetic |craniosacral outflow |cholinergic |adrenergic | noradrenergic |'fight and flight'|  'catabolic system| rest and digest|anabolic system

Table of Contents:

 

Sr. no	Contents
1.	Sympathetic system
2.	Parasympathetic system
3.	Neurotransmitter    Acetylcholine    Adrenalin    Noradrenaline
4.	The hypothalamic controls
5.	Effects of sympathetic system  On the heart On blood vessels On metabolism On digestive system Respiratory system Genitourinary system Nervous system On skin On temperature regulation

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Introduction

The autonomic nervous system is also known as the vegetative nervous system, has two divisions:

 Sympathetic and Parasympathetic

1. The sympathetic system arises from all thoracic spinal segments to the first two lumbar spinal segments, and sometimes from the third lumbar spinal segment are also involved. Therefore this is also known as thoracolumbar outflow.

2. The parasympathetic system arises from the brain and sacral spinal segments, S2, S3, and S4, therefore known as craniosacral outflow.

Sympathetic and parasympathetic systems usually act opposite to each other.

Neurotransmitters: 

 

1. Acetylcholine is a neurotransmitter released at the endings of all preganglionic fibers.

2. Postganglionic fibers of the parasympathetic nervous system, and 

3. Some postganglionic sympathetic nerve endings.

 Fibers that liberate acetylcholine at their terminals are cholinergic. 

At postganglionic terminals of sympathetic nerve endings, noradrenaline and adrenalin are released, with some exceptions.

Postganglionic sympathetic nerve endings, mainly noradrenaline, are released. Adrenaline is secreted from the adrenal medulla in response to sympathetic nerve stimulation.

 Fibers that liberate noradrenaline or adrenergic are noradrenergic or adrenergic, respectively.

.

 The hypothalamus controls the sympathetic and parasympathetic systems.

 The posterolateral nucleus of the hypothalamus controls sympathetic activity and anteromedial parasympathetic activity.

 The posterolateral nucleus stimulates sympathetic activity, and the anteromedial nucleus stimulates parasympathetic activity.

 Multiple areas of the CNS influence the activity of the anteromedial and posterolateral nucleus of the hypothalamus via the limbic system.

  

The limbic system is associated with the auditory cortex, visual cortex, parietal lobe, and prefrontal lobe. In addition, the limbic system affects the activity of the sympathetic and parasympathetic systems.

 The parietal lobe is concerned with memory, and the prefrontal cortex with the thinking process. Therefore, higher centers will control the activity of the limbic system and hypothalamic centers. 

.

  After interpretation of different pieces of information coming to the limbic system, higher centers regulate the hypothalamus and, therefore, sympathetic and parasympathetic activity.

 The sympathetic system is also known as the ‘fight and flight’ or ‘catabolic system,’ while the parasympathetic system is known as the ‘rest and digest’ or ‘anabolic system.’

 Sympathetic system:

 Intermediolateral or lateral horn is present in each spinal segment of the spinal cord. The sympathetic neurons are present in the thoracic spinal segments and 1st and 2nd lumbar spinal segments. In some cases, it may be up to 3rd lumbar spinal segment.

 From each of the lateral horns of the spinal cord, preganglionic sympathetic fibers emerge through the anterior horn along with the anterior nerve root. This is the first-order neuron, which is under the control of the posterolateral nucleus of the hypothalamus.

 It remains in the anterior root of the nerve for some distance and then leaves the anterior nerve root and joins the sympathetic trunk.

 The preganglionic fibers are myelinated, so they appear white.

The preganglionic fibers are known as ‘white ramus Communicantes (plural is Rami communicantes.)

 In the sympathetic trunk, preganglionic fibers synapse with ganglionic cells of the same segment that is the second-order neuron.

Some fibers pass the ganglion and synapse with the upper or lower sympathetic ganglion.

 Some fibers will pass through to the ganglion and synapse with the prevertebral or collateral sympathetic ganglia.

 Some preganglionic fibers will pass through to the sympathetic ganglion and end in the adrenal medulla.

From the paravertebral and prevertebral sympathetic ganglions, second-order neurons give postganglionic fibers. Thus, they are unmyelinated appear gray and are known as gray rami communicans (rami communicantes). 

There are two paravertebral sympathetic trunks, one on each side.

 Sympathetic ganglions present outside the sympathetic trunk are prevertebral ganglions. They are three in number and are situated near the target organs.

1. Coleic -near the origin of the coeliac artery.

2. Superior mesenteric near the origin of the superior mesenteric artery, and

3. Inferior mesenteric ganglia –is present near the origin of the inferior mesenteric artery.

 

 Cervical ganglions

 In the cervical region, there are three cervical ganglions:

1. Superior cervical ganglion receives fibers from T1-T4 constitutes carotid plexus. That supply common carotid artery then proceeds to supply 1. Dilator pupillae -pupil dilatation, and 2. ciliary muscle relaxation.

 2. Middle and

 3. Inferior cervical ganglions. Superior, middle, and T1-T4 form cardiac plexus --heart increases heart rate, cardiac output, and blood pressure.

Oesophageal plexus--decrease peristalsis

pulmonary plexus--bronchodilation, decrease secretion and vasoconstriction.

The inferior cervical ganglion fuses with the first thoracic sympathetic ganglion and appears like a star; therefore, it is known as stellate ganglion

 4. Terminal ganglia are present near the bladder and rectum.

                                    

Splanchnic nerves

There are five’ splanchnic nerves. :

1. Greater splanchnic nerve-T5-T9 fibers will go ; 

to the adrenal medulla that secretes adrenalin and noradrenaline and releases them into the blood circulation. In the adrenal medulla, chromaffin cells are homologous of the sympathetic ganglia. The preganglionic sympathetic fibers end in the adrenal medulla and stimulate the chromaffin cells to secret adrenalin and noradrenaline. There are no postganglionic fibers. This is known as the sympathoadrenal axis.

Coeliac ganglion supplies the stomach and depresses motility, secretion, and absorption to the liver -increases glycogenolysis, decreases biliary flow, decreases insulin production, and increases glucagon secretion.

to superior mesenteric ganglion -duodenum, jejunum, ileum,caecum. Ascending colon and two-third of the transverse colon-and depresses motility, secretion, and absorption,

2. Lesser splanchnic nerve T10-T11-  aretero-renal ganglion -, decrease peristalsis of the ureter, vasoconstriction reduce blood supply and reduced urine formation, and increased renin release. 

 3. Least splanchnic nerve-T12--diffuse ganglion- renal plexus -- decrease peristalsis of the ureter,

  4. Lumbar splanchnic nerve

Inferior mesenteric ganglion- one-third of the transverse colon, descending colon, sigmoid colon, and rectum--and depresses motility, secretion, and absorption,

Superior hypogastric ganglion to the internal urethral sphincter -contraction 

detrusor muscle -relax.

Superior hypogastric ganglion forms right and left hypogastric nerves -inferior hypogastric plexus supply the gonads- testis, and ovary. The external urethral sphincter is supplied by the somatic nerves.

Intermediate hypogastric ganglion

5. Sacrosplanchinic nerve: from L1,2,3 andT10-12 comes through the sacral segments--in male ejaculation and 

in the female contraction of the non-pregnant uterus, when pregnant function changes,

These nerves are formed by the preganglionic sympathetic fibers, which pass through the sympathetic ganglion without synapse. They are myelinated fibers.

 

Effects of sympathetic system of the autonomic nervous system:
 

Cardiovascular system

Effect on heart:

Sympathetic stimulation  causes

1. Positive chronotropic- increased heart rate.

 2. Positive dromotropic- increase contraction rate speed. Dromo=running

 3. Positive inotropic – increase contractility, and

  4. Positive bathmotropic: increase the response of excitability .bathmos ‘a Greek word’=step or threshold.

A new term, lusotropic, was introduced in 1982.

5. Lusotropic agents improve relaxation during diastole.

 

Sympathetic stimulation causes cardiac stimulation so that coronary output, coronary artery flow, heart rate, and myocardial contractility will increase.

 Effect on blood vessels

 Sympathetic stimulation causes:

 Vasoconstriction of cutaneous and splanchnic arteries.  Vasodilatation of coronary and skeletal blood vessels  are dilated

 Veins will construct that is vasoconstriction will occur.

 Sympathetic stimulation increases blood supply to the heart and the skeletal muscles and reduces blood supply to visceral organs and skin.

 Due to all these factors, blood pressure, both systolic, and diastolic will increase.

 Respiratory system :

 Sympathetic stimulation causes relaxation of the bronchial muscles that is bronchodilatation and,

Increase in respiratory rate that is tachypnea (average respiratory rate is 12 to 16 per minute.)

 Digestive system:

The sympathetic system relaxes the gut muscles causing reduced peristalsis. Therefore, the content of the gut relaxes.

 Genitourinary system:

 Stimulation of the sympathetic nervous system causes relaxation of the detrusor muscle and constriction of the urinary bladder sphincter, causing urine retention.

Sympathetic stimulation causes ejaculation of semen action in male

 and increased vaginal secretion in females.

Nervous system:

 Sympathetic stimulation causes pupillary dilation, retraction of the eyelids, loss of sleep mind becomes alert.

 Effects on the skin:

Sympathetic stimulation

 Causes vasoconstriction inner skin leading to pallor - pale skin. This will raise the core body temperature.

Sympathetic stimulation causes sweating. The sympathetic nerve ending of the sweat glands is cholinergic - acetylcholine is released at their nerve endings.

 Sympathetic stimulation causes horripilation –piloerection due to contraction of the ‘arrector pili’ muscle. The ‘arrector pili’ muscle is a small muscle attached to the base of a hair follicle on one side and the other side to dermal tissue.

 Metabolic effects:

 Sympathetic stimulation causes hyperglycemia due to

1. Decreased insulin secretion from beta cells of ‘island of Langerhans,   

2. Neoglucogenesis, and 

3. Glycogenolysis.

 Increases lipolysis -leading to release of free fatty acids. These free fatty acids are utilized in glucose synthesis in the liver.

                                 Sympathetic -thoracolumbar outflow.

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