Peripheral Nervous System PNS | Parasympathetic and Sympathetic System
Peripheral Nervous System PNS | Parasympathetic and Sympathetic System I. Introduction to the Peripheral Nervous System (PNS) The Peripheral Nervous System (PNS) consists of all neural structures outside the brain and spinal cord (which constitute the Central Nervous System, CNS ). The PNS serves as a communication network, linking the CNS to the rest of the body.
1. Major Divisions of the PNS The PNS is broadly classified into:
Somatic Nervous System (SNS) – Controls voluntary skeletal muscle movements.Autonomic Nervous System (ANS) – Regulates involuntary functions (e.g., heart rate, digestion, glandular secretion).
Sympathetic Nervous System (SNS) – “Fight or Flight” response.Parasympathetic Nervous System (PSNS) – “Rest and Digest” response.
Enteric Nervous System (ENS) – The “second brain” governing gastrointestinal function.
2. Functional Overview of the ANS
The ANS maintains homeostasis by unconsciously adjusting bodily functions.
It operates via reflex arcs (sensory input → CNS → motor output).
Dual innervation : Most organs receive input from both sympathetic and parasympathetic systems, allowing fine-tuned regulation.
II. Sympathetic Nervous System (SNS) – “Fight or Flight” 1. Anatomical Organization
Origin : Preganglionic neurons arise from the thoracolumbar region (T1–L2) of the spinal cord.Ganglia :
Sympathetic chain ganglia (paravertebral ganglia) – Located along the spinal column.Prevertebral ganglia (e.g., celiac, superior mesenteric ganglia) – Near major abdominal arteries.
Adrenal Medulla : Modified sympathetic ganglion that secretes epinephrine (80%) and norepinephrine (20%) into the bloodstream.
2. Neurotransmitters & Receptors Neuron Type Neurotransmitter Receptor Preganglionic Acetylcholine (ACh) Nicotinic (Nn) Postganglionic Norepinephrine (NE) Adrenergic (α & β) Exceptions :
Sweat glands (postganglionic releases ACh, acting on muscarinic receptors).Adrenal medulla (no postganglionic neuron; secretes epinephrine directly).
3. Physiological Effects The SNS prepares the body for stress, danger, or intense activity :
Target Organ Effect Mechanism Heart ↑ Heart rate (tachycardia) & contractility β₁-adrenergic stimulation Blood Vessels Vasoconstriction (↑ BP) α₁-adrenergic (skin & gut) Vasodilation (skeletal muscles) β₂-adrenergic Lungs Bronchodilation (↑ airflow) β₂-adrenergic Liver Glycogenolysis (↑ blood glucose) β₂-adrenergic Pupils Dilation (mydriasis) α₁-adrenergic (radial muscle contraction) Digestive System ↓ Peristalsis, ↓ secretions α & β inhibition Adrenal Medulla Releases epinephrine (systemic effects) –
4. Clinical Relevance
Hypertension : Excessive sympathetic tone → chronic vasoconstriction.Asthma : Sympathetic drugs (β₂-agonists like albuterol) relieve bronchoconstriction.Shock : Sympathetic activation maintains blood pressure via vasoconstriction.
III. Parasympathetic Nervous System (PSNS) – “Rest & Digest” 1. Anatomical Organization
Origin : Preganglionic neurons arise from:
Cranial nerves (CN III, VII, IX, X) – Vagus nerve (CN X) is the major parasympathetic nerve (~75% of PSNS fibers).Sacral spinal cord (S2–S4) – Pelvic splanchnic nerves.
Ganglia : Located near or within target organs (terminal ganglia ).
2. Neurotransmitters & Receptors Neuron Type Neurotransmitter Receptor Preganglionic Acetylcholine (ACh) Nicotinic (Nn) Postganglionic Acetylcholine (ACh) Muscarinic (M1–M5)
3. Physiological Effects The PSNS conserves energy and promotes digestion, recovery, and relaxation :
Target Organ Effect Mechanism Heart ↓ Heart rate (bradycardia) M₂ muscarinic (vagal tone) Lungs Bronchoconstriction, ↑ mucus secretion M₃ muscarinic Digestive System ↑ Salivation, ↑ peristalsis, ↑ enzyme secretion M₃ muscarinic Pupils Constriction (miosis) M₃ (sphincter pupillae contraction) Bladder Contraction (urination) M₃ muscarinic Reproductive System Erection (via vasodilation) Nitric oxide (NO) release
4. Clinical Relevance
Bradycardia : Excessive vagal stimulation slows heart rate.Cholinergic Toxicity (e.g., organophosphate poisoning): Overstimulation of muscarinic receptors → diarrhea, salivation, bronchospasm.Dry Mouth (Xerostomia) : Anticholinergic drugs (e.g., atropine) block PSNS effects.
IV. Comparative Summary: Sympathetic vs. Parasympathetic Feature Sympathetic (SNS) Parasympathetic (PSNS) Origin Thoracolumbar (T1–L2) Craniosacral (CN III, VII, IX, X; S2–S4) Ganglia Location Near spinal cord Near or within target organs Neurotransmitter (Postganglionic) Norepinephrine (mostly) Acetylcholine Response Time Fast, systemic Slower, localized General Role Energy expenditure (stress response) Energy conservation (homeostasis)
V. Integration & Balance of ANS Activity
Cooperative Effects : Some organs require both systems (e.g., male sexual function: PSNS → erection, SNS → ejaculation).Tonic Activity : Both systems are always active but dominate under different conditions.Autonomic Dysreflexia : A dangerous condition (e.g., in spinal cord injury) where SNS overreacts below the injury level, causing severe hypertension.
VI. Conclusion & Key Takeaways
The PNS connects the CNS to the body, with the ANS regulating involuntary functions.
The Sympathetic system prepares for emergency responses (↑ HR, bronchodilation).
The Parasympathetic system promotes digestion, recovery, and relaxation (↓ HR, ↑ gut motility).
Dysregulation of the ANS leads to disorders like hypertension, asthma, and gastrointestinal dysfunction.