Label Each Region Of The Sympathetic And Parasympathetic Divisions.

The autonomic nervous system (ANS), a crucial component of the peripheral nervous system, orchestrates involuntary bodily functions, maintaining homeostasis without conscious control. This intricate system is further divided into two primary branches: the sympathetic and parasympathetic divisions. Each division exerts distinct influences on target organs, often acting in opposition to regulate various physiological processes. Understanding the specific regions and pathways of each division is essential for comprehending the complexity and efficiency of autonomic control.

The Sympathetic Division: "Fight or Flight"

The sympathetic division, often referred to as the "fight or flight" system, prepares the body for action in response to stress, danger, or excitement. Its widespread effects are crucial for survival, enabling rapid mobilization of energy and heightened sensory awareness.

1. Origin and General Organization

• Thoracolumbar Origin: Sympathetic preganglionic neurons originate in the lateral horns of the spinal cord's gray matter, specifically from the thoracic (T1-T12) and lumbar (L1-L2 or L1-L3) regions. This thoracolumbar outflow is a defining characteristic of the sympathetic division.
• Short Preganglionic Fibers, Long Postganglionic Fibers: Sympathetic preganglionic fibers are relatively short, synapsing with postganglionic neurons located in ganglia close to the spinal cord. Postganglionic fibers, in contrast, are long, extending to target organs throughout the body.
• Ganglia Locations: Sympathetic ganglia are organized into two main categories:
  • Paravertebral Ganglia (Sympathetic Trunk): These ganglia form a chain running along both sides of the vertebral column. Preganglionic fibers enter the sympathetic trunk via white rami communicantes.
  • Prevertebral Ganglia (Collateral Ganglia): These ganglia are located anterior to the vertebral column in the abdomen and pelvis. Preganglionic fibers destined for these ganglia pass through the sympathetic trunk without synapsing, forming splanchnic nerves.

2. Specific Regions and Pathways

a. Spinal Cord (T1-L2/L3):

• Lateral Horns: This is the origin of all sympathetic preganglionic neurons. Neuronal cell bodies here send axons out through the ventral roots of the spinal nerves.
• Ventral Roots: Preganglionic axons exit the spinal cord via the ventral roots, joining the spinal nerves.
• White Rami Communicantes: These are branches of the spinal nerves that carry preganglionic fibers from the spinal nerve to the paravertebral ganglia. The white rami are myelinated, giving them a whitish appearance.

b. Paravertebral Ganglia (Sympathetic Trunk):

• Superior Cervical Ganglion: Located at the top of the sympathetic trunk, this ganglion receives preganglionic fibers that ascend from lower thoracic levels. Postganglionic fibers from the superior cervical ganglion innervate structures in the head, including:
  • Pupil Dilator Muscle: Causes pupil dilation (mydriasis).
  • Sweat Glands of the Face and Scalp: Increases sweat production.
  • Salivary Glands: Decreases salivary secretion (thick, viscous saliva).
  • Blood Vessels of the Head: Causes vasoconstriction.
• Middle Cervical Ganglion: Situated in the middle of the cervical region, this ganglion receives preganglionic fibers and sends postganglionic fibers to the heart and blood vessels of the neck.
• Inferior Cervical Ganglion (Stellate Ganglion): Often fused with the first thoracic ganglion, this ganglion innervates the heart, lungs, and blood vessels of the upper limb.
• Thoracic Ganglia (T1-T12): These ganglia are aligned with the thoracic spinal nerves and provide postganglionic innervation to:
  • Heart: Increases heart rate and contractility.
  • Lungs: Dilates bronchioles (bronchodilation).
  • Esophagus: Decreases peristalsis.
  • Sweat Glands of the Trunk: Increases sweat production.
  • Blood Vessels of the Trunk: Causes vasoconstriction.
• Lumbar Ganglia (L1-L2/L3): These ganglia are aligned with the lumbar spinal nerves and contribute to the innervation of the lower abdomen and lower limbs. Postganglionic fibers innervate:
  • Blood Vessels of the Lower Limb: Causes vasoconstriction.
  • Sweat Glands of the Lower Limb: Increases sweat production.

c. Prevertebral Ganglia (Collateral Ganglia):

• Celiac Ganglion: Receives preganglionic fibers via the greater splanchnic nerve (T5-T9). Postganglionic fibers innervate the:
  • Stomach: Decreases motility and secretion.
  • Liver: Stimulates glucose release.
  • Spleen: Regulates immune functions.
  • Pancreas: Inhibits insulin secretion.
  • Adrenal Medulla: Triggers the release of epinephrine and norepinephrine (catecholamines).
• Superior Mesenteric Ganglion: Receives preganglionic fibers via the lesser splanchnic nerve (T10-T11). Postganglionic fibers innervate the:
  • Small Intestine: Decreases motility and secretion.
  • Ascending Colon and Transverse Colon: Decreases motility and secretion.
• Inferior Mesenteric Ganglion: Receives preganglionic fibers via the lumbar splanchnic nerves (L1-L2). Postganglionic fibers innervate the:
  • Descending Colon and Sigmoid Colon: Decreases motility and secretion.
  • Rectum: Inhibits defecation.
  • Urinary Bladder: Relaxes bladder wall, constricts internal urethral sphincter (inhibiting urination).
  • Reproductive Organs: Stimulates ejaculation in males and uterine contraction in females.

d. Adrenal Medulla:

• The adrenal medulla is a unique component of the sympathetic nervous system. It receives direct preganglionic sympathetic innervation, unlike other organs that are innervated by postganglionic fibers.
• Preganglionic fibers from the spinal cord (T8-L1) synapse directly on chromaffin cells within the adrenal medulla.
• Chromaffin cells are modified postganglionic neurons that release epinephrine (adrenaline) and norepinephrine (noradrenaline) into the bloodstream. These hormones amplify and prolong the sympathetic response, affecting multiple organs simultaneously.

e. Gray Rami Communicantes:

• Postganglionic fibers from the paravertebral ganglia rejoin the spinal nerves via the gray rami communicantes. These rami are unmyelinated, giving them a grayish appearance.
• Postganglionic fibers then travel with the spinal nerves to reach their target organs in the skin (sweat glands, arrector pili muscles, and blood vessels) and skeletal muscles.

3. Neurotransmitters

• Preganglionic Neurons: Release acetylcholine (ACh), which binds to nicotinic receptors on postganglionic neurons.
• Postganglionic Neurons: Primarily release norepinephrine (NE), which binds to adrenergic receptors (alpha and beta) on target organs.
• Exceptions:
  • Postganglionic sympathetic fibers innervating sweat glands release acetylcholine (ACh), which binds to muscarinic receptors.
  • Some sympathetic fibers release dopamine.

The Parasympathetic Division: "Rest and Digest"

The parasympathetic division, often called the "rest and digest" system, promotes relaxation, energy conservation, and normal bodily functions. It counteracts the effects of the sympathetic division, maintaining balance and promoting homeostasis.

1. Origin and General Organization

• Craniosacral Origin: Parasympathetic preganglionic neurons originate in the brainstem (cranial nerves) and the sacral region of the spinal cord (S2-S4). This craniosacral outflow distinguishes the parasympathetic division from the sympathetic division.
• Long Preganglionic Fibers, Short Postganglionic Fibers: Parasympathetic preganglionic fibers are relatively long, extending from the brainstem or sacral spinal cord to ganglia located near or within target organs. Postganglionic fibers are short, synapsing directly on the target organ's cells.
• Ganglia Locations: Parasympathetic ganglia are typically located close to or within the walls of the organs they innervate. These terminal ganglia ensure localized and precise control.

2. Specific Regions and Pathways

a. Cranial Nerves:

• Oculomotor Nerve (CN III): Preganglionic fibers originate in the Edinger-Westphal nucleus in the midbrain. These fibers travel with the oculomotor nerve to the ciliary ganglion located within the orbit. Postganglionic fibers innervate:
  • Ciliary Muscle: Controls lens accommodation for near vision (focusing).
  • Sphincter Pupillae Muscle: Causes pupil constriction (miosis).
• Facial Nerve (CN VII): Preganglionic fibers originate in the superior salivatory nucleus in the pons. These fibers branch into two pathways:
  • Greater Petrosal Nerve: Travels to the pterygopalatine ganglion in the pterygopalatine fossa. Postganglionic fibers innervate:
    • Lacrimal Glands: Stimulates tear production.
    • Nasal Mucosa: Stimulates mucus secretion.
  • Chorda Tympani Nerve: Travels to the submandibular ganglion near the submandibular gland. Postganglionic fibers innervate:
    • Submandibular and Sublingual Salivary Glands: Stimulates saliva secretion (thin, watery saliva).
• Glossopharyngeal Nerve (CN IX): Preganglionic fibers originate in the inferior salivatory nucleus in the medulla oblongata. These fibers travel to the otic ganglion near the foramen ovale. Postganglionic fibers innervate:
  • Parotid Salivary Gland: Stimulates saliva secretion (thin, watery saliva).
• Vagus Nerve (CN X): The vagus nerve is the primary source of parasympathetic innervation to the thorax and abdomen. Preganglionic fibers originate in the dorsal motor nucleus of the vagus and the nucleus ambiguus in the medulla oblongata. These fibers travel throughout the thorax and abdomen, synapsing with terminal ganglia located within or near the target organs. Postganglionic fibers innervate:
  • Heart: Decreases heart rate and contractility.
  • Lungs: Constricts bronchioles (bronchoconstriction) and stimulates mucus secretion.
  • Esophagus: Increases peristalsis.
  • Stomach: Increases motility and secretion.
  • Small Intestine: Increases motility and secretion.
  • Ascending Colon and Transverse Colon: Increases motility and secretion.
  • Liver: Stimulates bile secretion.
  • Pancreas: Stimulates enzyme and insulin secretion.

b. Sacral Spinal Cord (S2-S4):

• Preganglionic fibers originate in the lateral gray matter of the sacral spinal cord (S2-S4). These fibers form the pelvic splanchnic nerves, which travel to terminal ganglia located within or near the target organs in the lower abdomen and pelvis. Postganglionic fibers innervate:
  • Descending Colon and Sigmoid Colon: Increases motility and secretion.
  • Rectum: Stimulates defecation.
  • Urinary Bladder: Contracts bladder wall, relaxes internal urethral sphincter (promoting urination).
  • Reproductive Organs: Stimulates erection in males and increases vaginal lubrication in females.

3. Neurotransmitters

• Preganglionic Neurons: Release acetylcholine (ACh), which binds to nicotinic receptors on postganglionic neurons.
• Postganglionic Neurons: Release acetylcholine (ACh), which binds to muscarinic receptors on target organs.

Summary Table: Sympathetic vs. Parasympathetic

Feature	Sympathetic Division	Parasympathetic Division
Primary Function	"Fight or Flight," Stress Response	"Rest and Digest," Energy Conservation
Origin	Thoracolumbar (T1-L2/L3)	Craniosacral (Brainstem, S2-S4)
Preganglionic Fiber Length	Short	Long
Postganglionic Fiber Length	Long	Short
Ganglia Location	Near Spinal Cord (Paravertebral, Prevertebral)	Near/Within Target Organs (Terminal)
Preganglionic Neurotransmitter	Acetylcholine (ACh)	Acetylcholine (ACh)
Postganglionic Neurotransmitter	Norepinephrine (NE) (Mostly)	Acetylcholine (ACh)
Receptors on Target Organs	Adrenergic (Alpha, Beta)	Muscarinic

Clinical Significance

Understanding the anatomy and physiology of the sympathetic and parasympathetic divisions is crucial for diagnosing and treating various clinical conditions. For example:

• Horner's Syndrome: Damage to the sympathetic pathway in the head and neck can result in Horner's syndrome, characterized by miosis (pupil constriction), ptosis (drooping eyelid), anhidrosis (lack of sweating), and facial flushing on the affected side.
• Autonomic Neuropathy: Conditions like diabetes can damage autonomic nerves, leading to dysfunction in various organs. Symptoms may include orthostatic hypotension (low blood pressure upon standing), gastroparesis (delayed stomach emptying), and erectile dysfunction.
• Irritable Bowel Syndrome (IBS): Autonomic dysfunction can contribute to IBS symptoms like abdominal pain, bloating, and altered bowel habits.
• Medications: Many medications target the autonomic nervous system to treat conditions like hypertension, asthma, and overactive bladder. Understanding the specific receptors and pathways affected by these drugs is essential for effective and safe treatment.

Conclusion

The sympathetic and parasympathetic divisions of the autonomic nervous system work in concert to maintain homeostasis, regulating a wide range of involuntary bodily functions. By understanding the specific regions, pathways, and neurotransmitters involved in each division, we gain valuable insights into the complex mechanisms that govern our internal environment. This knowledge is not only essential for healthcare professionals but also provides a deeper appreciation for the intricate and finely tuned systems that keep us alive and functioning optimally.