8.2 Autonomic Nervous System: Sympathetic vs. Parasympathetic

8.2 Autonomic Nervous System: Sympathetic vs. Parasympathetic

The Autonomic Nervous System (ANS) is the involuntary visceral motor division of the peripheral nervous system. It operates continuously below conscious awareness to regulate the internal environment, maintaining physiological homeostasis across fluctuating external conditions. The ANS innervates three broad categories of visceral effectors: cardiac muscle in the heart wall, smooth muscle within hollow organs and blood vessels, and glandular epithelium (including salivary, gastric, sweat, and endocrine glands).

---

Somatic Motor System vs. Autonomic Nervous System

To master autonomic physiology, one must clearly contrast the structural and chemical differences separating the somatic motor system from the autonomic nervous system:

+-----------------------------------------------------------------------------+
|                 SOMATIC MOTOR VS. AUTONOMIC EFFERENT CHAINS                 |
|                                                                             |
|  SOMATIC MOTOR SYSTEM:                                                      |
|  [ CNS ] ===== Heavily Myelinated Axon =====> (ACh) -> [ Skeletal Muscle ]  |
|          (Single Lower Motor Neuron)                   (Nicotinic Receptor) |
|                                                        *Always Excitatory*  |
|                                                                             |
|  AUTONOMIC: SYMPATHETIC DIVISION:                                           |
|  [ CNS ] == Lightly Myelinated ==> [ Ganglion ] -------- Unmyelinated ----> |
|          (Short Preganglionic)      (ACh / Nic)  (Long Postganglionic)      |
|                                                  Effector: NE -> [Adrenergic]|
|                                                  *Excitatory or Inhibitory* |
|                                                                             |
|  AUTONOMIC: PARASYMPATHETIC DIVISION:                                       |
|  [ CNS ] ========= Lightly Myelinated =========> [ Ganglion ] -- Unmyel. -> |
|          (Long Preganglionic)                     (ACh / Nic)   (Short Post)|
|                                                   Effector: ACh -> [Muscar.]|
|                                                   *Excitatory or Inhibitory*|
+-----------------------------------------------------------------------------+

1. Number of Neurons in the Efferent Pathway

• Somatic Motor System: Consists of a single lower motor neuron. Its cell body resides within the anterior gray horn of the spinal cord or a brainstem motor nucleus. A single, uninterrupted, heavily myelinated axon extends all the way from the CNS to the target skeletal muscle fibers.
• Autonomic Nervous System: Utilizes an obligate two-neuron efferent chain to link the CNS to visceral target tissues:
  1. Preganglionic Neuron: The first neuron in the chain. Its cell body resides within the gray matter of the central nervous system (brainstem motor nuclei or spinal cord lateral gray horns). Its axon—the preganglionic fiber—is a thin, lightly myelinated Type B fiber that exits the CNS via cranial nerves or ventral spinal roots to synapse within an autonomic ganglion outside the CNS.
  2. Postganglionic (Ganglionic) Neuron: The second neuron in the chain. Its cell body is located entirely within an autonomic ganglion in the peripheral nervous system. Its axon—the postganglionic fiber—is a very thin, unmyelinated Type C fiber that extends from the ganglion to terminate directly upon visceral effector cells.

2. Neurotransmitters & Effector Receptors

• Somatic Motor System: Somatic motor axon terminals release exclusively Acetylcholine (ACh) at the neuromuscular junction. ACh binds to nicotinic cholinergic receptors ($N_1/N_m$) on the skeletal muscle motor endplate, triggering rapid $Na^+$ influx, motor endplate depolarization, and muscle contraction. The somatic response is always excitatory; skeletal muscle relaxation occurs solely through the cessation of motor neuron firing.
• Autonomic Nervous System: Autonomic neuroeffector junctions do not form discrete endplates. Instead, postganglionic axons branch into widespread terminal networks studded with bulbous swellings called varicosities, which release neurotransmitters broadly across visceral tissue beds ("synapses en passant"):
  • All autonomic preganglionic neurons (both sympathetic and parasympathetic) release Acetylcholine (ACh), which binds to nicotinic cholinergic receptors ($N_2/N_n$) on postganglionic neuronal cell bodies, always exciting the postganglionic neuron.
  • Parasympathetic postganglionic neurons release Acetylcholine (ACh), which binds to muscarinic cholinergic receptors ($M_1, M_2, M_3$) on target cells.
  • Most sympathetic postganglionic neurons release Norepinephrine (NE), which binds to alpha ($\alpha_1, \alpha_2$) or beta ($\beta_1, \beta_2, \beta_3$) adrenergic receptors on target cells. (A critical exception is sympathetic innervation of eccrine sweat glands, which uses ACh onto muscarinic receptors).
  • Visceral effector responses can be excitatory or inhibitory, depending entirely on the specific receptor subtype expressed on the target cell membrane.

---

The Sympathetic Division (Thoracolumbar Division)

The sympathetic division mobilizes bodily resources during physical activity, psychological stress, and life-threatening crises. It is classically termed the "fight-or-flight" system, or the "E division" (governing Exercise, Excitement, Emergency, and Embarrassment).

Anatomical Outflow & Ganglionic Architecture

The sympathetic division is termed the thoracolumbar division because all its preganglionic cell bodies reside within the lateral gray horns of the 12 thoracic and upper two lumbar spinal cord segments (T1 through L2).

Preganglionic sympathetic axons exit the spinal cord through ventral roots, enter mixed spinal nerves, and rapidly diverge through small connecting pathways called white rami communicantes (myelinated) to enter the sympathetic ganglia. Sympathetic ganglia are organized into two major categories:

1. Sympathetic Trunk (Chain) Ganglia (Paravertebral Ganglia): A paired, longitudinal string of 22 to 24 interconnected ganglia running vertically along both lateral flanks of the vertebral column, extending from the base of the skull down to the coccyx (where they fuse at the ganglion impar). Upon entering a sympathetic trunk ganglion, a preganglionic axon follows one of three distinct pathways:

   • Synapse at the same level: Synapses immediately with a postganglionic neuron in that specific trunk ganglion.
   • Ascend or descend the chain: Travels superiorly or inferiorly within the sympathetic trunk to synapse in a chain ganglion at a different vertebral level (e.g., fibers originating in upper thoracic segments ascend into cervical trunk ganglia—superior, middle, and inferior cervical ganglia—to supply the head, neck, and dilated pupils).
   • Pass through without synapsing: Traverses the chain ganglion without synapsing and leaves as part of a splanchnic nerve (still a preganglionic fiber) that terminates in prevertebral ganglia located anterior to the abdominal aorta.
   • Postganglionic axons emerging from trunk ganglia re-enter adjacent spinal nerves via gray rami communicantes (unmyelinated) to distribute to vascular smooth muscle, sweat glands, and arrector pili muscles across all body dermatomes.

2. Prevertebral (Collateral) Ganglia: Unpaired ganglia situated anterior to the vertebral column and abdominal aorta, clustered around the origins of major arterial branches supplying the digestive organs. They include:

   • Celiac ganglion: Postganglionic fibers supply the stomach, liver, gallbladder, spleen, and pancreas.
   • Superior mesenteric ganglion: Postganglionic fibers supply the small intestine and proximal large intestine.
   • Inferior mesenteric ganglion: Postganglionic fibers supply the distal large intestine, rectum, urinary bladder, and reproductive organs.

Because sympathetic chain ganglia lie immediately adjacent to the spinal cord, preganglionic sympathetic fibers are characteristically short, whereas postganglionic sympathetic fibers are characteristically long, traveling extensive distances to reach peripheral effectors.

The Adrenal Medulla: A Modified Sympathetic Ganglion

A unique and vital component of the sympathetic division is the adrenal medulla, the inner neuroendocrine core of the adrenal gland situated atop each kidney. Embryologically and anatomically, the adrenal medulla is a specialized, modified sympathetic ganglion derived from neural crest tissue.

Preganglionic sympathetic fibers from the lower thoracic spinal cord travel through the thoracic splanchnic nerves and the celiac ganglion without synapsing, terminating directly upon the hormone-secreting cells of the adrenal medulla, termed chromaffin cells. Chromaffin cells are essentially modified postganglionic sympathetic neurons that lack axons. Upon stimulation by preganglionic acetylcholine, chromaffin cells release catecholamine hormones directly into the bloodstream:

• Approximately 80% Epinephrine (Adrenaline)
• Approximately 20% Norepinephrine (Noradrenaline)

Because these catecholamines are dumped directly into systemic circulation rather than across a microscopic synaptic cleft, they circulate throughout the entire body, binding to adrenergic receptors on tissues lacking direct sympathetic innervation. This systemic endocrine surge produces a widespread, coordinated, and prolonged sympathetic response that persists for several minutes after neural stimulation ceases, until circulating catecholamines are degraded (largely by the liver).

Physiological Actions: Fight-or-Flight Responses

Activation of the sympathetic nervous system triggers physiological adaptations designed to maximize physical performance and survival:

• Ocular System: Contraction of the radial dilator pupillae muscle produces pupillary dilation (mydriasis), maximizing ambient light entry to broaden the visual field; relaxation of the ciliary muscle flattens the lens for distance vision.
• Cardiovascular System: Stimulation of myocardial $\beta_1$ adrenergic receptors increases heart rate (positive chronotropy), accelerates AV nodal conduction velocity (positive dromotropy), and augments myocardial contractile force (positive inotropy), dramatically increasing cardiac output. Stimulation of vascular $\alpha_1$ adrenergic receptors produces profound vasoconstriction in cutaneous, renal, and splanchnic vascular beds. Simultaneously, local metabolic factors and $\beta_2$ receptors mediate vasodilation in coronary arteries and skeletal muscle capillary beds, effectively shunting blood flow away from non-essential viscera toward exercising skeletal muscles, the heart, and the brain, while driving up mean arterial blood pressure.
• Respiratory System: Stimulation of bronchial smooth muscle $\beta_2$ adrenergic receptors triggers profound bronchodilation, expanding airway caliber, decreasing airway resistance, and maximizing alveolar gas exchange; respiratory rate and depth increase.
• Metabolic & Hepatic System: Stimulates hepatic glycogenolysis (breakdown of glycogen into glucose) and gluconeogenesis (synthesis of new glucose), surging circulating blood glucose levels to fuel cerebral and muscular metabolism. Stimulates lipolysis in adipose tissue via $\beta_3$ receptors, releasing free fatty acids for cellular energy production.
• Gastrointestinal & Urinary Systems: Inhibits gastrointestinal peristalsis and smooth muscle motility; constricts gastrointestinal sphincters (ileocecal, pyloric, anal sphincters); diminishes digestive enzyme and acid secretions. In the urinary system, relaxes the detrusor muscle of the bladder wall (via $\beta_2/\beta_3$) and constricts the smooth muscle internal urethral sphincter (via $\alpha_1$), preventing micturition (urination) during acute fight-or-flight emergencies.

---

The Parasympathetic Division (Craniosacral Division)

The parasympathetic division acts as the physiological counterbalance to the sympathetic division. It is classically termed the "rest-and-digest" system, responsible for conserving and restoring bodily energy reserves during non-stressful periods. It maintains baseline organ function and orchestrates the "SLUDD" responses: Salivation, Lacrimation, Urination, Digestion, and Defecation.

Anatomical Outflow & Ganglionic Architecture

The parasympathetic division is designated the craniosacral division because its preganglionic neuronal cell bodies reside within two widely separated CNS regions:

1. Cranial Outflow: Brainstem motor nuclei of Cranial Nerves III, VII, IX, and X.
2. Sacral Outflow: Lateral gray matter of sacral spinal cord segments S2, S3, and S4.

Unlike sympathetic ganglia, parasympathetic ganglia lie far away from the central nervous system, situated either immediately adjacent to their target organs (terminal ganglia) or embedded directly within the connective tissue walls of the organs themselves (intramural ganglia). Consequently, preganglionic parasympathetic fibers are characteristically long, extending uninterrupted from the CNS nearly to the effector, whereas postganglionic parasympathetic fibers are characteristically short, spanning only millimeters to reach target cells.

Cranial Outflow Pathways

Four cranial nerves distribute parasympathetic preganglionic fibers:

• Oculomotor Nerve (CN III): Preganglionic fibers originate in the Edinger-Westphal nucleus of the midbrain and synapse in the ciliary ganglion within the orbit. Short postganglionic fibers innervate the sphincter pupillae muscle (triggering pupillary constriction [miosis]) and the ciliary muscle (causing the lens to bulge/round up for near-vision accommodation).
• Facial Nerve (CN VII): Preganglionic fibers originate in the superior salivatory nucleus of the pons and split into two pathways:
  • Fibers traveling via the greater petrosal nerve synapse in the pterygopalatine ganglion; postganglionic fibers stimulate the lacrimal (tear) glands and nasal mucosal glands.
  • Fibers traveling via the chorda tympani synapse in the submandibular ganglion; postganglionic fibers stimulate secretion from the submandibular and sublingual salivary glands.
• Glossopharyngeal Nerve (CN IX): Preganglionic fibers originate in the inferior salivatory nucleus of the medulla and synapse in the otic ganglion; postganglionic fibers innervate and stimulate secretion from the massive parotid salivary gland.
• Vagus Nerve (CN X): The dominant parasympathetic conduit of the human body, conveying approximately 75% to 90% of all parasympathetic preganglionic fibers. Arising from the dorsal motor nucleus of the vagus and nucleus ambiguus in the medulla, vagal preganglionic fibers descend through the neck to form cardiac, pulmonary, esophageal, and abdominal autonomic plexuses. They synapse within microscopic intramural ganglia embedded directly within target viscera, providing extensive parasympathetic regulation to:
  • The heart (slows heart rate and decreases AV nodal conduction via $M_2$ receptors).
  • The lungs (triggers bronchoconstriction and stimulates airway mucus secretion).
  • The digestive tract (esophagus, stomach, liver, gallbladder, pancreas, small intestine, and proximal two-thirds of the large intestine, ending at the splenic flexure).

Sacral Outflow Pathways

Preganglionic cell bodies in the lateral gray matter of sacral spinal cord segments S2, S3, and S4 exit via ventral roots and branch into pelvic splanchnic nerves. These nerves enter the inferior hypogastric plexus to synapse within intramural ganglia located within the distal pelvic viscera. Postganglionic parasympathetic fibers supply:

• The distal one-third of the large intestine (splenic flexure, descending colon, sigmoid colon, and rectum).
• The urinary bladder: Stimulates contraction of the detrusor muscle and coordinates relaxation of the involuntary internal urethral sphincter, driving bladder emptying (micturition).
• The reproductive organs and external genitalia: Mediates vasodilation of penile and clitoral erectile cavernous tissues, stimulating erection (recalled by the clinical aphorism: "Point and Shoot"—Parasympathetic governs erection [Point], Sympathetic governs ejaculation [Shoot]).

Physiological Actions: Rest-and-Digest Responses

Parasympathetic stimulation promotes vegetative functions, nutrient assimilation, and energy storage, characterized by the SLUDD responses and the "Three Decreases":

• The "Three Decreases":
  1. Decreased Heart Rate: Vagal postganglionic fibers release ACh onto muscarinic $M_2$ receptors on the sinoatrial (SA) node and atrioventricular (AV) node, increasing $K^+$ permeability to hyperpolarize pacemaker cells, slowing heart rate (negative chronotropy) and resting cardiac workload.
  2. Decreased Airway Diameter (Bronchoconstriction): Constricts smooth muscle rings of bronchioles via muscarinic $M_3$ receptors, reducing dead space ventilation during resting states.
  3. Decreased Pupil Diameter (Miosis): Constricts the circular sphincter pupillae muscle of the iris via $M_3$ receptors, protecting the retina from excessive illumination.
• Gastrointestinal Activation: Stimulates peristaltic contractions of the stomach and intestinal muscularis, accelerates gastric emptying, relaxes gastrointestinal sphincters, and enhances copious secretion of saliva, gastric hydrochloric acid, pancreatic digestive enzymes, and bile.
• Bladder Emptying & Defecation: Coordinates rhythmic detrusor contraction with internal sphincter relaxation to execute micturition; stimulates rectal smooth muscle contraction and internal anal sphincter relaxation to facilitate defecation.

---

Dual Innervation & Autonomic Tone

Most visceral organs in the human body receive dual innervation—meaning they are innervated by postganglionic fibers from both the sympathetic and parasympathetic divisions. Typically, the two divisions produce opposing (antagonistic) physiological effects on a given organ (e.g., sympathetic accelerates heart rate, while parasympathetic decelerates it).

Autonomic Tone: Basal Homeostatic Activity

Even in resting, unstressed states, neither autonomic division is ever completely quiescent. Instead, both divisions exhibit continuous, background basal firing known as autonomic tone:

• Vagal (Parasympathetic) Tone on the Heart: The intrinsic electrical pacing rate of the cardiac sinoatrial (SA) node in the complete absence of neural input is approximately 100 action potentials per minute. Under normal resting conditions, continuous parasympathetic firing delivered via the vagus nerve exerts a dominant inhibitory brake (vagal tone), depressing resting heart rate to 70-75 beats per minute. If pharmacological antagonism (e.g., atropine) blocks muscarinic receptors, or if the vagus nerves are surgically severed, vagal tone is abolished, and resting heart rate immediately jumps to approximately 100 bpm.
• Sympathetic (Vasomotor) Tone on Blood Vessels: Vascular smooth muscle encircling systemic arterioles is maintained in a state of continuous partial contraction by constant low-frequency firing of sympathetic fibers, known as sympathetic vasomotor tone. Because blood vessels lack parasympathetic innervation, arterial blood pressure is regulated simply by modulating this sympathetic firing frequency: increasing sympathetic firing produces vasoconstriction and elevates blood pressure, whereas decreasing sympathetic firing permits smooth muscle relaxation, resulting in vasodilation and reduced blood pressure.

Structures Receiving Exclusive (Single) Sympathetic Innervation

While dual innervation is the general anatomical rule, several clinically crucial effector organs receive exclusive innervation from the sympathetic division, with no opposing parasympathetic fibers whatsoever:

1. Adrenal Medulla: Innervated solely by preganglionic sympathetic fibers; releases circulating epinephrine and norepinephrine.
2. Eccrine Sweat Glands: Widely distributed throughout the skin; innervated exclusively by sympathetic postganglionic fibers that uniquely release Acetylcholine (ACh) onto muscarinic receptors to stimulate thermoregulatory sweating.
3. Arrector Pili Muscles: Tiny smooth muscles attached to hair follicles in the dermis; innervated exclusively by sympathetic fibers; contraction pulls hair erect ("goosebumps") in response to cold or fright.
4. Most Systemic Blood Vessels: Arteriolar and venous smooth muscle throughout the cutaneous, renal, and splanchnic circulations receives exclusively sympathetic vasomotor fibers governing systemic vascular resistance and blood pressure.
5. Kidneys: Sympathetic fibers stimulate the juxtaglomerular apparatus to release renin, initiating the renin-angiotensin-aldosterone system (RAAS) to elevate blood pressure.

---

Comprehensive Sympathetic vs. Parasympathetic Comparison Table