Label Each Region Of The Sympathetic And Parasympathetic Divisions

The autonomic nervous system (ANS), a crucial component of the peripheral nervous system, regulates involuntary bodily functions such as heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal. This system operates without conscious control, ensuring the body's internal environment remains stable. The ANS is divided into two primary divisions: the sympathetic and parasympathetic nervous systems. These two divisions often act in opposition to each other, maintaining a delicate balance that ensures appropriate physiological responses to varying conditions. Understanding the specific regions and functions of these divisions is fundamental to comprehending how the body manages stress, rest, and everyday activities.

Sympathetic Division: The "Fight or Flight" Response

The sympathetic division, often referred to as the "fight or flight" system, prepares the body for action during stressful or emergency situations. When activated, it triggers a cascade of physiological responses designed to enhance alertness, increase energy availability, and improve physical performance.

Overview of the Sympathetic Division

• Primary Role: To mobilize the body's resources under stress, preparing it for intense physical activity or escape.
• Neurotransmitter: Primarily uses norepinephrine (noradrenaline), although it also uses acetylcholine in some instances.
• Effects: Increases heart rate, dilates pupils, inhibits digestion, and redirects blood flow to skeletal muscles.

Regions of the Sympathetic Division

1. Central Nervous System (CNS) Origin:
   • The sympathetic division originates in the thoracolumbar region of the spinal cord, specifically from the first thoracic vertebra (T1) to the second lumbar vertebra (L2).
   • Preganglionic neurons have their cell bodies in the lateral horns of the spinal cord's gray matter within this region.
2. Sympathetic Ganglia:
   • Sympathetic ganglia are clusters of nerve cell bodies outside the CNS where preganglionic neurons synapse with postganglionic neurons.
   • There are two main types of sympathetic ganglia:
     • Paravertebral Ganglia (Sympathetic Chain Ganglia):
       • These ganglia form a chain that runs along both sides of the vertebral column.
       • They extend from the upper cervical region to the coccyx.
       • Preganglionic fibers from the spinal cord enter these ganglia, where they may synapse with postganglionic neurons.
       • Postganglionic fibers then exit the ganglia and innervate target organs.
     • Prevertebral Ganglia (Collateral Ganglia):
       • These ganglia are located anterior to the vertebral column, near the major abdominal arteries.
       • Preganglionic fibers pass through the paravertebral ganglia without synapsing and then synapse in the prevertebral ganglia.
       • Major prevertebral ganglia include the celiac ganglion, superior mesenteric ganglion, and inferior mesenteric ganglion.
3. Nerves and Plexuses:
   • Spinal Nerves:
     • Postganglionic fibers from the paravertebral ganglia rejoin spinal nerves via the gray rami communicantes.
     • These fibers are distributed to sweat glands, arrector pili muscles (which cause goosebumps), and blood vessels in the skin and skeletal muscles.
   • Splanchnic Nerves:
     • These nerves are formed by preganglionic fibers that pass through the paravertebral ganglia without synapsing.
     • They synapse in the prevertebral ganglia.
     • Greater Splanchnic Nerve: Originates from T5-T9 and synapses in the celiac ganglion, innervating the stomach, liver, pancreas, and adrenal medulla.
     • Lesser Splanchnic Nerve: Originates from T10-T11 and synapses in the superior mesenteric ganglion, innervating the small intestine and proximal colon.
     • Least Splanchnic Nerve: Originates from T12 and synapses in the inferior mesenteric ganglion, innervating the distal colon and rectum.
     • Lumbar Splanchnic Nerves: Originate from L1-L2 and also synapse in the inferior mesenteric ganglion, innervating pelvic organs.
   • Plexuses:
     • Nerve plexuses are networks of intersecting nerves that serve specific regions of the body.
     • Sympathetic plexuses include:
       • Cardiac Plexus: Innervates the heart, increasing heart rate and contractility.
       • Pulmonary Plexus: Innervates the lungs, dilating bronchioles.
       • Celiac Plexus: Innervates the abdominal organs, affecting digestion and blood flow.
       • Superior Mesenteric Plexus: Innervates the small intestine and proximal colon.
       • Inferior Mesenteric Plexus: Innervates the distal colon, rectum, and pelvic organs.
       • Hypogastric Plexus: Innervates the pelvic organs, including the bladder, reproductive organs, and rectum.
4. Adrenal Medulla:
   • The adrenal medulla is a unique component of the sympathetic nervous system.
   • It is directly innervated by preganglionic sympathetic fibers.
   • Upon stimulation, it releases epinephrine (adrenaline) and norepinephrine into the bloodstream.
   • These hormones enhance and prolong the effects of sympathetic activation, affecting multiple organ systems simultaneously.

Detailed Labeling of Sympathetic Regions

• Spinal Cord (T1-L2):
  • Lateral Horns: Location of preganglionic neuron cell bodies.
  • Ventral Roots: Preganglionic fibers exit the spinal cord through the ventral roots.
  • White Rami Communicantes: Myelinated preganglionic fibers that connect the spinal nerve to the sympathetic chain ganglia.
• Paravertebral Ganglia:
  • Cervical Ganglia: Superior, middle, and inferior cervical ganglia.
  • Thoracic Ganglia: Located along the thoracic region of the vertebral column.
  • Lumbar Ganglia: Located along the lumbar region of the vertebral column.
  • Sacral Ganglia: Located along the sacral region of the vertebral column.
• Prevertebral Ganglia:
  • Celiac Ganglion: Located near the celiac artery, innervates the stomach, liver, pancreas, and adrenal medulla.
  • Superior Mesenteric Ganglion: Located near the superior mesenteric artery, innervates the small intestine and proximal colon.
  • Inferior Mesenteric Ganglion: Located near the inferior mesenteric artery, innervates the distal colon, rectum, and pelvic organs.
• Splanchnic Nerves:
  • Greater Splanchnic Nerve: Originates from T5-T9, synapses in the celiac ganglion.
  • Lesser Splanchnic Nerve: Originates from T10-T11, synapses in the superior mesenteric ganglion.
  • Least Splanchnic Nerve: Originates from T12, synapses in the inferior mesenteric ganglion.
  • Lumbar Splanchnic Nerves: Originate from L1-L2, synapses in the inferior mesenteric ganglion.
• Adrenal Medulla:
  • Chromaffin Cells: Release epinephrine and norepinephrine into the bloodstream.

Parasympathetic Division: The "Rest and Digest" Response

The parasympathetic division, often referred to as the "rest and digest" system, conserves energy, promotes relaxation, and supports essential bodily functions during periods of rest.

Overview of the Parasympathetic Division

• Primary Role: To conserve energy, promote relaxation, and support digestion and other essential bodily functions.
• Neurotransmitter: Primarily uses acetylcholine.
• Effects: Decreases heart rate, constricts pupils, stimulates digestion, and promotes elimination.

Regions of the Parasympathetic Division

1. Central Nervous System (CNS) Origin:
   • The parasympathetic division originates from two main regions:
     • Cranial Region: Preganglionic neurons arise from the brainstem.
     • Sacral Region: Preganglionic neurons arise from the sacral spinal cord (S2-S4).
   • Due to this dual origin, the parasympathetic division is often referred to as the "craniosacral" division.
2. Cranial Nerves:
   • Several cranial nerves carry parasympathetic fibers:
     • Oculomotor Nerve (CN III):
       • Originates in the Edinger-Westphal nucleus in the midbrain.
       • Innervates the ciliary muscle (for lens accommodation) and the pupillary constrictor muscle (for pupillary constriction).
       • Preganglionic fibers synapse in the ciliary ganglion, located in the orbit.
     • Facial Nerve (CN VII):
       • Originates in the pons.
       • Has two main branches with parasympathetic functions:
         • Greater Petrosal Nerve: Innervates the lacrimal glands (tear production) and nasal mucosa (mucus secretion). Preganglionic fibers synapse in the pterygopalatine ganglion.
         • Chorda Tympani Nerve: Innervates the submandibular and sublingual salivary glands. Preganglionic fibers synapse in the submandibular ganglion.
     • Glossopharyngeal Nerve (CN IX):
       • Originates in the medulla oblongata.
       • Innervates the parotid salivary gland.
       • Preganglionic fibers synapse in the otic ganglion.
     • Vagus Nerve (CN X):
       • Originates in the medulla oblongata.
       • Carries the majority of parasympathetic fibers.
       • Innervates the heart, lungs, esophagus, stomach, small intestine, proximal colon, liver, gallbladder, pancreas, and kidneys.
       • Preganglionic fibers synapse in terminal ganglia located within or near the walls of the target organs.
3. Sacral Spinal Cord:
   • Preganglionic neurons originate in the lateral horns of the sacral spinal cord (S2-S4).
   • These fibers form the pelvic splanchnic nerves.
   • The pelvic splanchnic nerves innervate the distal colon, rectum, bladder, ureters, and reproductive organs.
   • Preganglionic fibers synapse in terminal ganglia located within or near the walls of the target organs.
4. Ganglia:
   • Parasympathetic ganglia are located close to or within the walls of the target organs.
   • These ganglia are called terminal ganglia or intramural ganglia.
   • Examples include:
     • Ciliary Ganglion: Located in the orbit, innervates the ciliary muscle and pupillary constrictor muscle.
     • Pterygopalatine Ganglion: Located in the pterygopalatine fossa, innervates the lacrimal glands and nasal mucosa.
     • Submandibular Ganglion: Located near the submandibular gland, innervates the submandibular and sublingual salivary glands.
     • Otic Ganglion: Located near the foramen ovale, innervates the parotid salivary gland.
     • Intramural Ganglia: Located within the walls of the target organs, such as the heart, lungs, and digestive tract.

Detailed Labeling of Parasympathetic Regions

• Brainstem:
  • Edinger-Westphal Nucleus: Origin of preganglionic fibers for the oculomotor nerve.
  • Pons: Origin of preganglionic fibers for the facial nerve.
  • Medulla Oblongata: Origin of preganglionic fibers for the glossopharyngeal and vagus nerves.
• Cranial Nerves:
  • Oculomotor Nerve (CN III): Innervates the ciliary muscle and pupillary constrictor muscle.
  • Facial Nerve (CN VII): Innervates the lacrimal glands, nasal mucosa, and submandibular and sublingual salivary glands.
  • Glossopharyngeal Nerve (CN IX): Innervates the parotid salivary gland.
  • Vagus Nerve (CN X): Innervates the heart, lungs, esophagus, stomach, small intestine, proximal colon, liver, gallbladder, pancreas, and kidneys.
• Sacral Spinal Cord (S2-S4):
  • Lateral Horns: Location of preganglionic neuron cell bodies.
  • Pelvic Splanchnic Nerves: Preganglionic fibers that innervate the distal colon, rectum, bladder, ureters, and reproductive organs.
• Ganglia:
  • Ciliary Ganglion: Located in the orbit, innervates the ciliary muscle and pupillary constrictor muscle.
  • Pterygopalatine Ganglion: Located in the pterygopalatine fossa, innervates the lacrimal glands and nasal mucosa.
  • Submandibular Ganglion: Located near the submandibular gland, innervates the submandibular and sublingual salivary glands.
  • Otic Ganglion: Located near the foramen ovale, innervates the parotid salivary gland.
  • Intramural Ganglia: Located within the walls of the target organs, such as the heart, lungs, and digestive tract.

Functional Differences and Coordination

The sympathetic and parasympathetic divisions generally have opposing effects on target organs. This antagonism allows for precise control over bodily functions.

• Heart Rate: Sympathetic stimulation increases heart rate; parasympathetic stimulation decreases heart rate.
• Pupil Size: Sympathetic stimulation dilates pupils; parasympathetic stimulation constricts pupils.
• Digestion: Sympathetic stimulation inhibits digestion; parasympathetic stimulation promotes digestion.
• Bronchioles: Sympathetic stimulation dilates bronchioles; parasympathetic stimulation constricts bronchioles.
• Blood Vessels: Sympathetic stimulation constricts blood vessels (except in skeletal muscles); parasympathetic stimulation has little effect on most blood vessels.

Despite their opposing effects, the sympathetic and parasympathetic divisions often work together to maintain homeostasis. For example, during exercise, the sympathetic system increases heart rate and blood flow to skeletal muscles, while the parasympathetic system helps regulate digestion and other bodily functions.

Clinical Significance

Dysfunction of the sympathetic or parasympathetic nervous system can lead to a variety of clinical conditions.

• Sympathetic Dysfunction:
  • Horner's Syndrome: Damage to the sympathetic nerves in the neck can cause ptosis (drooping eyelid), miosis (constricted pupil), and anhidrosis (lack of sweating) on one side of the face.
  • Complex Regional Pain Syndrome (CRPS): A chronic pain condition that can involve sympathetic nerve dysfunction, leading to pain, swelling, and changes in skin temperature and color.
• Parasympathetic Dysfunction:
  • Gastroparesis: Damage to the vagus nerve can impair stomach emptying, leading to nausea, vomiting, and abdominal pain.
  • Neurogenic Bladder: Damage to the parasympathetic nerves that control bladder function can lead to urinary retention or incontinence.

Conclusion

The sympathetic and parasympathetic divisions of the autonomic nervous system play crucial roles in maintaining homeostasis and regulating involuntary bodily functions. The sympathetic division prepares the body for action during stress, while the parasympathetic division conserves energy and promotes relaxation. Understanding the specific regions and functions of these divisions is essential for comprehending how the body responds to various stimuli and maintains its internal balance. Proper functioning of both divisions is vital for overall health and well-being.