Autonomic Nervous System Overview

Autonomic Nervous System

• The autonomic nervous system (ANS) is part of the peripheral nervous system
• It is divided into two branches: sympathetic and parasympathetic
• Sympathetic: "fight or flight" response
• Parasympathetic: "rest and digest" response

Sympathetic Nervous System

• Physiological Effects:
  • Increased heart rate
  • Increased blood pressure
  • Dilation of pupils
  • Bronchodilation
  • Increased blood sugar
  • Reduced digestion
  • Increased sweating
  • Stress response
• Origin: T1-L2 of the spinal cord
• Ganglia: Close to spinal cord; short preganglionic fibers
• Neurotransmitter: Acetylcholine (ACh) at ganglion, Norepinephrine (NE) at target organ
• Receptor types: Primarily adrenergic (α and β) receptors
• Anatomy:
  • Most organs receive dual nerve supply, except some.
  • Includes dilator pupillae M, erector pili M, adrenal medulla, sweat glands, ventricles, blood vessels
• Function:
  • Activated in response to threatening stressors
  • Mobilizes body resources for action

Parasympathetic Nervous System

• Physiological Effects:
  • Decreased heart rate
  • Decreased blood pressure
  • Constriction of pupils
  • Bronchoconstriction
  • Increased digestion
  • Increased urination
• Origin: Cranial nerves (III, VII, IX, X) and sacral spinal cord segments (S2-S4)
• Ganglia: Near or embedded in organs; long preganglionic fibers
• Neurotransmitter: Acetylcholine (ACh) at ganglion and target organ
• Receptor types: Primarily cholinergic (muscarinic and nicotinic) receptors.
• Anatomy:
  • Near or embedded in organs
• Function:
  • Activates relaxation response

Neurotransmitters

• Sympathetic:
  • Ganglia: Acetylcholine (ACh)
  • Postganglionic: Norepinephrine (NE)
• Parasympathetic:
  • Ganglia: Acetylcholine (ACh)
  • Postganglionic: Acetylcholine (ACh)

Receptors

• Autonomic receptors - cholinergic and adrenergic

• Cholinergic receptors - respond to acetylcholine

• Adrenergic receptors - respond to norepinephrine and epinephrine

• α1 and α2: primarily vasoconstriction

• β1: Primarily cardiac stimulation

• β2: Primarily bronchodilation, relaxation of smooth muscles, uterus

Physiology (Sympathetic and Parasympathetic)

• Tone: The basal level of activity in the system
• Blood vessels: Sympathetic: vasoconstriction; Parasympathetic: no direct innervation
• Heart: Sympathetic: increased rate and contractility; Parasympathetic: decreased rate
• Blood pressure: Sympathetic: increased; Parasympathetic: decreased
• Eye: Sympathetic: mydriasis; Parasympathetic: miosis
• Bronchi: Sympathetic: bronchodilation; Parasympathetic: bronchoconstriction
• GIT, urinary tract: Sympathetic: inhibits tone and motility, contracts sphincter; Parasympathetic: increases tone and motility, relaxes sphincters
• Urinary bladder: Sympathetic: contracts sphincter; Parasympathetic: relaxes wall
• Uterus: Sympathetic: relaxation; Parasympathetic: no direct innervation

Autonomic Drugs

• Drugs that mimic or block the actions of the autonomic nervous system.
• Sympathomimetics: Mimic effects of the sympathetic nervous system (e.g., epinephrine, norepinephrine).
• Sympatholytics: Block effects of the sympathetic nervous system (e.g., beta-blockers).
• Parasympathomimetics: Mimic effects of the parasympathetic nervous system (e.g., pilocarpine).
• Parasympatholytics: Block effects of the parasympathetic nervous system (e.g., atropine).

Classification of Autonomic Drugs

• Mechanism of action: Direct-acting agonists (e.g., epinephrine), indirect-acting agonists (e.g., cocaine), mixed-action agonists (ephedrine).
• Chemical Structure: Catecholamines (e.g., epinephrine, norepinephrine) and non-catecholamines (e.g., ephedrine).

Endogenous Catecholamines (Epinephrine, Norepinephrine)

• Epinephrine: Direct effect on α1, α2, β1, β2, β3 adrenoceptors.
• Norepinephrine: Direct effect on α1, α2, β1 adrenoceptors (no β2)

Actions and Uses

• Local Actions: Decongestants, hemostasis, delayed absorption of local anesthetics, prolonged duration.

• Systemic Actions (CVS, Resp.):

  • Cardiovascular system: Increased heart rate, contractility, conduction velocity, bronchodilation, blood pressure, vasodilation/constriction.
  • Respiratory system: bronchodilation.
  • Other targets: smooth muscle relaxation/contraction in several organs etc., uterine relaxation, allergic reactions

Adverse Effects, Contraindications, and Preparations

• Adverse Effects:
  • Tachycardia, arrhythmias, hypertension, angina, cerebral hemorrhage.
• Contraindications:
  • Coronary heart disease, hypertension, arrhythmias, peripheral vascular disease.
• Preparations & dosage - varies by drug

Specific Drugs

• Epinephrine (adrenaline): Uses in anaphylactic shock, local vasoconstriction, cardiac arrest.
• Norepinephrine (noradrenaline): Uses in hypotensive states after sympathectomy or spinal anesthesia.
• Midodrine: Treatment of hypotension
• Xylometazoline and oxymetazoline: Topical decongestants
• Isoprenaline: Use in heart block, bronchial asthma
• Dobutamine: Use in acute heart failure
• Salbutamol, terbutaline: Short-acting bronchodilators
• Formoterol and salmeterol: Long-acting bronchodilators
• Ritodrine: Uterine relaxant
• Amphetamine: CNS stimulant, increase wakefulness

Note: Further details on specific drugs' actions, uses, dosages, etc., need to be studied separately within each topic for thorough understanding.