Autonomic Nervous System Overview

Autonomic Nervous System Overview

• The autonomic nervous system (ANS) is part of the peripheral nervous system, controlling involuntary functions.
• It is divided into two branches: sympathetic and parasympathetic.

Sympathetic Nervous System

• Function: "Fight or flight" response
• Origin: Thoracic and lumbar spinal cord
• Ganglia: Close to spinal cord; short preganglionic fibers
• Neurotransmitter: Acetylcholine (ACh) preganglionic, norepinephrine (NE) postganglionic
• Innervation: Most organs receive dual nerve supply, except specific ones
• Effects: Increased heart rate, blood pressure, breathing rate, pupil dilation, etc.

Parasympathetic Nervous System

• Function: "Rest and digest" response
• Origin: Cranial nerves (III, VII, IX, X) and sacral spinal cord
• Ganglia: Near or embedded in organs; long preganglionic fibers
• Neurotransmitter: ACh both preganglionic and postganglionic
• Innervation: Most organs receive dual nerve supply, except specific ones
• Effects: Decreased heart rate, blood pressure, digestion, and pupil constriction,

Anatomy of Sympathetic and Parasympathetic Systems

Neurotransmitters

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

Autonomic Receptors

• Cholinergic
• Adrenergic (alpha and beta)

Physiology

• Tone:
  • Sympathetic: maintains blood vessel tone and sweat gland function
  • Parasympathetic: controls most organ function when not actively stimulated by stress.
• Heart:
  • Sympathetic: increases all cardiac properties
  • Parasympathetic: no direct innervation
• Blood vessels:
  • Sympathetic: constricts skin and mucous membrane vessels, dilates coronary and skeletal muscle vessels.
  • Parasympathetic: no direct innervation.
• Blood pressure:
  • Sympathetic: increases, mainly due to vasoconstriction (alpha receptors)
  • Parasympathetic: no direct effect but may indirectly affect through vagal reflex
• Eye:
  • Sympathetic: pupil dilation (mydriasis)
  • Parasympathetic: pupil constriction (miosis)
• Bronchi:
  • Sympathetic: dilation (beta2)
  • Parasympathetic: constriction
• GIT:
  • Sympathetic: inhibits tone and motility, constricts sphincters (alpha1)
  • Parasympathetic: stimulates motility and relaxes sphincters
• Urinary bladder:
  • Sympathetic: relaxes wall and constricts sphincters (alpha1)
  • Parasympathetic: relaxes sphincters and stimulates wall contraction
• Uterus:
  • Sympathetic: relaxes (beta2)
  • Parasympathetic: no direct effect

Adrenergic Receptors

• α1 (Gq): vasoconstriction, mydriasis, increased bladder tone
• α2 (Gi): inhibits NE release, reduced heart rate
• β1 (Gs): increases heart rate, contractility, and conduction velocity; increases renin release
• β2 (Gs): bronchodilation, relaxes smooth muscles
• β3 (Gs): lipolysis, increases renin release,

Autonomic System Drugs

• Sympathomimetics: drugs mimicking sympathetic effects, such as epinephrine and norepinephrine
• Sympatholytics: drugs blocking sympathetic effects
• Parasympathomimetics: drugs mimicking parasympathetic effects
• Parasympatholytics: drugs blocking parasympathetic effects

Categories of Autonomic Drugs

• Direct-acting agonists: mimic neurotransmitters (e.g., epinephrine)
• Indirect-acting agonists: enhance neurotransmitter release (e.g., amphetamines)
• Mixed-action agonists: both direct and indirect (e.g., ephedrine)

Epinephrine (Adrenaline)

• Mechanism: Direct acting on α1, α2, β1, β2, β3 adrenoceptors
• Pharmacological actions: Local (vasoconstriction, hemostasis, anesthetic delay), cardiovascular (increase in heart rate, contractility, conduction velocity), blood vessels (vasoconstriction/vasodilation depending on receptor), respiration (bronchodilation),
• Therapeutic uses: Anaphylaxis, local anesthetic, cardiac arrest

Norepinephrine (Noradrenaline)

• Mechanism: Direct acting on α1, α2, β1 adrenoceptors (not β2)
• Pharmacological actions: Cardiovascular (increase in blood pressure, peripheral resistance), heart (increases contractility)
• Therapeutic uses: Hypotensive states (after sympathectomy)

Synthetic Sympathomimetics

• α1-selective agonists: Midodrine (hypotension treatment), Xylometazoline/Oxymetazoline (nasal decongestants)

Beta-Agonists

• Non-selective: Isoprenaline (bronchodilation, heart stimulation)
• β1-Selective: Dobutamine (increases heart contractility)
• β2-Selective: Salbutamol, Terbutaline, Formoterol, Salmeterol (bronchodilation)

Pharmacokinetic Considerations

• Absorption (oral/parenteral)
• Distribution (throughout body, including BBB)
• Excretion (urine)
• Metabolism (COMT and MAO)

Adverse Effects & Contraindications

• Adverse Effects: Tachycardia, arrhythmias, hypertension, angina, tremors, anxiety, convulsions, insomnia and others
• Contraindications: Coronary heart disease, hypertension, arrhythmias, peripheral vascular disease.

Additional Information

• Catecholamines (e.g., epinephrine, norepinephrine): quickly metabolized, short-lived effects, not absorbed well orally
• Non-catecholamines (e.g., ephedrine): longer-lasting effects, absorbed orally.

Explore the key components of the autonomic nervous system, including its division into the sympathetic and parasympathetic systems. Understand their functions, origins, and effects on the body. This quiz will enhance your comprehension of involuntary physiological processes.