Sympathetic Nervous System

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Sympathetic Division of the Autonomic Nervous System

• Preganglionic neurons have their cell bodies in lateral horns of the gray matter in the 12 thoracic segments and the first two lumbar segments of the spinal cord.

• Also known as thoracolumbar division.

Types of Autonomic Ganglia:

• There are two types of autonomic ganglia: sympathetic trunk ganglia and prevertebral ganglia.

Ganglia and Innervation:

• Sympathetic trunk ganglia extend from the base of the skull to coccyx.

• Postsympathetic axons from sympathetic trunk ganglia innervate organs above the diaphragm.

Prevertrable Ganglia:

• Prevertebral ganglia lie anterior to the vertebral column and innervate organs below the diaphragm.

• Three major types: celiac ganglion, superior mesenteric ganglion, inferior mesenteric ganglion.

Sympathetic Responses

• Once axons of sympathetic preganglionic neurons pass to sympathetic trunk ganglia, they connect to (sometimes up to 20) postganglionic neurons in three different ways.

• This way, many sympathetic responses affect almost the entire body simultaneously.

• Path of a sympathetic preganglionic neuron: Anterior root of spinal nerve > intervertebral foramina > white ramus> sympathetic trunk ganglion > postganglionic neurons.

Role of Adrenal Medulla

• Adrenal medulla is an organ that is directly hooked up to sumpathetic preganglionic neurons.

• Upon stimulation, adrenal medulla releases epinephrine, norepinephrine, and dopamine directly into blood. These hormones linger in the blood prolonging the effect of sympathetic stimulation.

Pre- and Postganglionic Neurons:

• All preganglionic neurons are cholinergic.

• All postganglionic neurons that innervate most sweat glands are cholinergic.

• Most postganglionic neurons are adrenergic.

Sympathetic System: Thoraco-lumbar section of spinal cord.

Divisions:

1. Spinal component (axons). Segments T1 - L3 (thoraco-lumbar outflow).

2. Superior, middle, inferior, cervical ganglia.

3. Sympathetic ganglionated (paravertebral) chain, white and gray rami communicantes.

4. Greater, lesser and small splanchnic nerves.

5. Ganglia: Coeliac, superior mesenteric and inferior mesenteric (termed collateral or Prevertebral ganglia).

6. Long postganglionic fibres.

Thoracic splanchnic nerves:

1. Chief source of sympathetic nerves for the abdomen.

2. Greater, lesser and lowest are important (visible, once the lung is removed during thoracic dissection of the cadaver).

3. The nerves contain preganglionic fibres and originate from the 5th to the 12th thoracic sympathetic ganglia.

4. Twigs are sent to the Coeliac ganglia and renal plexus.

Sympathetic chain and ganglia:

1. Are visible on both sides of the thoracic and lumbar vertebra. Union of the two chains occurs at the ganglion impar sited on the coccyx. Therefore the chains pass through the posterior aspect of the abdominal cavity into the pelvis.

2. The cervical, thoracic and lumbar sympathetic chains are surgically important, laparoscopic sympathectomy may be indicated for hyperhidrosis (excessive sweating in the regions of the Axilla, hands and feet).

Rami Communicantes: Arrangement:

Preganglionic neuron fibres pass from the spinal cord via the anterior roots (T1 - T3) and the white rami comunicantes to the paravertebral sympathetic ganglion chain. Fibres either continue to the viscera or re-enter the spinal nerve via the grey rami communicantes.

Stimulation of sympthetic system:

1. Heart: Tachycardia.

2. Lungs: Bronchodilation.

3. Pupil: Dilation.

4. GIT: Inhibited peristalsis, sphincters contracted.

5. Blood vessels: Vasoconstriction.

6. Sweat glands: Increased sweating.

7. Hair follicles: Hair stands on end via m. arrectores pilorum.

8. Responses 1-7 are due to release of epinephrine from the adrenal medulla.

Innervation:

1. Superior cervical ganglion: Fibres pass via the carotid artery plexus and distributed to the eye, lacrimal, submandibular, sublingual and parotid gland.

2. Middle and inferior cervical ganglion innervate the heart and respiratory tree.

3. T1 - T4: innervate the heart and respiratory tree.

4. T5 - T9: Enter the Coeliac ganglion and then are distributed to the stomach, gut, abdominal vessels, liver biliary system and pancreas.

5. T10 - T12: via the small splanchnic nerve which relay in the Coeliac and superior mesenteric ganglia and innervate the adrenal medulla and kidney.

6. L1 - L3 via the inferior mesenteric ganglion innervate the transverse colon downwards, rectum, bladder and sex organs.

7. The Coeliac ganglion, superior and inferior ganglion are sited near to the viscera and are termed collateral ganglia.

Sympathetic System features:

1. Short preganglionic fibres.

2. Ganglionated sympathetic trunk visible in the paravertebral zone. Superior, middle and inferior ganglia important in the neck (paravertebral ganglia). Occasionally the inferior cervical ganglion fuses with T1 and forms the Stellate ganglion which overlies the neck of the first rib.

3. Two trunks fuse inferiorly near the coccyx - ganglion impar.

4. Outflow fibres: white rami.

5. Postganglionated fibres: grey rami communicantes.

6. Cranial sympathetic outflow is relayed on the internal carotid artery.

7. Other zones:

a. Cervical sympathetic.

b. Thoracic sympathetic (pulmonary, cardiac plexus: splanchnic nerves - greater, lower, lowest.

c. Lumbar sympathetic - nerves.

d. Pelvic sympathetic - presacral and inferior hypogastric plexus.

Further Topics on Autonomic Nervous System:

1. Introduction to the Autonomic Nervous System
2. Control of the Autonomic Nervous System
3. Divisions the Autonomic Nervous System
4. Notes on Sympathetic Nervous System
5. Notes on Paraympathetic Nervous System
6. Neurons of the Autonomic Nervous System
7. Neurotransmitters and Receptors of the Autonomic Nervous System
8. FAQ on Autonomic System

Additional Reading:

Basic Neurology

1. Peripheral Nervous System
2. Central Nervous System
3. The Ventricular System
4. The Spinal Cord
5. The Brain Stem
6. The Cerebellum
7. Visual Pathways
8. Diencephalon
9. Basal Ganglia
10. Cerebral Cortex
11. Sleep Disorders
12. Autonomic Nervous System
13. Cranial Nerves and Parasympathetic Ganglia
14. Cells of the Nervous System
15. Cerebrospinal fluid
16. Additional short notes on Cerebrum
17. Functions and Diseases of Cerebrum
18. Subcortical Grey Matter
19. Notes on The Spinal Cord
20. Regulation of Heart Rate by Autonomic Nervous System
21. Action Potentials, Axon Conduction, and Neuromuscular Junction
22. Types of Seizures
23. What is a Cough Reflex?
24. Notes on Congenital Prosopagnosia
25. Findings in Parkinson's Disease
26. Types of Heat Strokes
27. Types of Strokes
28. What is Benign Intracranial Hypertension?
29. What is Cauda Equina Syndrome?
30. Cranial Nerve Locations in Brain Stem
31. What is a Cluster Headache?
32. What is a Subarachnoid Hemorrhage?
33. What is a Tension Headache?

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1. Video of Neurology Examination in a Clinical Setting

Medical Images

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Related Topics

1. Nervous System Disorders
2. Histology of Nervous Tissue
3. Cranial Nerve Reflexes
4. Motor System Examination

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