Autonomic Nervous System

Autonomic Nervous System

Introduction to the Autonomic Nervous System (ANS)

• The Autonomic Nervous System (ANS) is a critical component of the nervous system.

  • Regulates involuntary bodily functions (e.g., heart rate, digestion).

• The ANS works alongside the endocrine system to maintain homeostasis.

Main Components of the ANS

• The ANS is divided into three major categories:

  • Enteric Nervous System (ENS): Controls the gastrointestinal system.

  • Parasympathetic Nervous System: Often referred to as the "rest and digest" system, conserves energy and promotes maintenance activities.

  • Sympathetic Nervous System: Often referred to as the "fight or flight" system, prepares the body for action during stressful situations.

Mechanism of Action of Autonomic Drugs

• Autonomic drugs can act in two main ways:

  • Agonists: Stimulate receptors on autonomic nerves, enhancing the effects of neurotransmitters.

  • Antagonists: Block receptors, reducing the actions of neurotransmitters.

Comparative Functions of the Parasympathetic and Sympathetic Systems

Parasympathetic Nervous System (PSNS)

• Functions:

  • Constricts pupils: Reduces the amount of light entering the eye.

  • Stimulates saliva flow: Aids in digestion.

  • Constricts bronchi: Decreases air passage to the lungs.

  • Slows heartbeat: Helps to conserve energy.

  • Stimulates peristalsis and secretion: Aids in the movement of food through the digestive tract.

  • Stimulates bile release: Important for fat digestion.

  • Contracts bladder: Facilitates urination.

  • Stimulates orgasm: Related to sexual arousal and response.

Sympathetic Nervous System (SNS)

• Functions:

  • Dilates pupils: Increases the amount of light for better vision in low-light conditions.

  • Inhibits salivation: Redirects energy away from digestion during stress.

  • Accelerates heartbeat: Prepares the body for quick response to threats.

  • Relaxes bronchi: Increases airflow to the lungs.

  • Inhibits peristalsis and secretion: Reduces digestive activity.

  • Stimulates glucose production and release: Increases energy availability during stress.

  • Secretion of adrenaline and noradrenaline: Intensifies the body’s response to stress.

  • Inhibits bladder contraction: Prevents urination during emergencies.

Organs Affected by the ANS

• The ANS impacts various organs through specific pathways:

  • Trachea: Airway management influenced by both PSNS (constriction) and SNS (dilation).

  • Gallbladder: Bile storage and release affected by autonomic responses.

  • Pancreas: Influences insulin and glucagon release.

  • Sphincters: Controls passage through digestive and excretory systems.

Neurotransmission in the Autonomic Nervous System

• Neurotransmission within the ANS involves chemical signaling between cells:

  • Hormones: Regulatory substances such as insulin and glucagon.

  • Local mediators: Examples include histamine and prostaglandins which influence local responses.

  • Neurotransmitters: Key neurotransmitters include:

  • Acetylcholine: Used by PSNS to communicate.

  • Norepinephrine: Common in SNS activity.

  • Epinephrine: Hormone and neurotransmitter, involved in the fight or flight response.

Drugs Affecting the Autonomic Nervous System

• Adrenergic Agonists:

  • Promote the release of norepinephrine.

  • Directly bind to adrenergic receptors and mimic effects of norepinephrine or epinephrine.

  • Types of Adrenergic Receptors:

  • Alpha ₁ (α₁): Primarily involved in vasoconstriction and increased peripheral resistance.

  • Alpha ₂ (α₂): Regulates neurotransmitter release and can act as an inhibitory mediator.

  • Beta ₁ (β₁): Primarily affects the heart, increasing heart rate and contraction force.

  • Beta ₂ (β₂): Influences smooth muscle relaxation (e.g., bronchodilation).