Autonomic Nervous System Overview

Autonomic Nervous System (ANS)

Regulates involuntary physiological functions through 2 divisions:

• Sympathetic Nervous System
• Fight-or-flight response
• Increases heart rate, dilates bronchi, etc.
• Parasympathetic Nervous System
• Rest-and-digest response
• Decreases heart rate, stimulates digestion, etc.

Objectives:

Key Terms:

Autonomic Reflexes: Control organ functions without conscious input (e.g., heart rate regulation)

Dual Innervation: Most organs receive input from both sympathetic and parasympathetic systems

Functions of the ANS:

Controls involuntary activities such as heart rate, digestion, and respiration.

Visceral Reflex Pathway:

Sympathetic vs Parasympathetic:

Sympathetic: Thoracolumbar outflow (T1-L2)

• Preganglionic fibers synapse near the spinal cord
• Widespread, generalized responses (e.g., blood vessel constriction)

Parasympathetic: Craniosacral outflow (brain stem, S2-S4)

• Preganglionic fibers synapse near or within target organs
• Localized responses (e.g., bladder contraction)

Preganglionic and Postganglionic Fibers:

Preganglionic: From CNS to ganglion (cholinergic fibers, acetylcholine)

Postganglionic: From ganglion to target organ

• Sympathetic: Adrenergic fibers (norepinephrine)
• Parasympathetic: Cholinergic fibers (acetylcholine)

Neurotransmitters:

Acetylcholine (ACh): Released by cholinergic fibers, binds to cholinergic receptors (muscarinic and nicotinic)

Norepinephrine: Released by adrenergic fibers, binds to adrenergic receptors (alpha and beta types)

Cholinergic Receptors:

Muscarinic: Located on target organs (e.g., decrease heart rate)

Nicotinic: Found in ganglia and on skeletal muscles, leading to contraction

Adrenergic Receptors:

Alpha-adrenergic: Generally cause vasoconstriction

Beta-adrenergic:

• Beta 1: Increase heart rate and contraction strength
• Beta 2: Dilate bronchioles, increase airflow

Autonomic Tone and Vasomotor Tone:

Autonomic Tone: Background activity of the ANS

Vasomotor Tone: Continuous sympathetic activity keeps blood vessels constricted or dilated as needed

Pharmacological Terms:

Sympathomimetic: Mimics sympathetic activity (e.g., increasing heart rate)

Sympatholytic: Inhibits sympathetic activity

Parasympathomimetic: Mimics parasympathetic activity (e.g., increases digestion)

Parasympatholytic: Inhibits parasympathetic activity (e.g., decreases salivation)

Clinical Relevance:

Conditions such as autonomic dysreflexia highlight the importance of the ANS in maintaining normal physiological function.

Aging Effects on ANS:

Decreased nerve conduction speed

Increased synaptic delay

Increased risk of fainting due to autonomic dysfunction

Knowledge Check:

Be prepared to distinguish between sympathetic and parasympathetic roles, differentiate between receptors, and apply pharmacological terms in clinical scenarios.