Chapter 14: The Autonomic Nervous System – Structure, Function, and Regulation

Autonomic Nervous System (ANS)

Overview of the ANS

The Autonomic Nervous System (ANS) is a division of the peripheral nervous system responsible for regulating involuntary physiological functions. It primarily controls smooth muscle, cardiac muscle, and glands, ensuring optimal support for body activities through subconscious control.

• Effectors: Smooth muscle, cardiac muscle, glands

• Control: Operates via subconscious mechanisms

• Effectors Location: Most effectors are viscera (internal organs)

Organization of the Nervous System

ANS in the Nervous System

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The PNS is further divided into sensory and motor divisions. The motor division includes the somatic nervous system (SNS) and the autonomic nervous system (ANS), which is subdivided into sympathetic and parasympathetic divisions.

• Somatic Nervous System (SNS): Controls voluntary movements (skeletal muscle)

• Autonomic Nervous System (ANS): Controls involuntary functions (smooth/cardiac muscle, glands)

Sympathetic vs Parasympathetic Divisions

Functional Differences

• Sympathetic Division: "Fight or flight" – prepares the body for emergency situations

• Parasympathetic Division: "Rest and digest" – conserves energy and maintains routine functions

ANS Versus Somatic Nervous System (SNS)

Key Differences

• Effectors: SNS targets skeletal muscle; ANS targets smooth muscle, cardiac muscle, and glands

• Efferent Pathways: SNS uses a single, heavily myelinated neuron; ANS uses a two-neuron chain (preganglionic and ganglionic neurons)

• Target Organ Responses: SNS always excitatory; ANS can be excitatory or inhibitory depending on neurotransmitter and receptor

Effectors

• SNS Effectors: Skeletal muscles

• ANS Effectors: Cardiac muscle, smooth muscle, glands

• Functional Role: Sympathetic division activates the organism; parasympathetic division restores normalcy

Efferent Pathways

• SNS: Single, heavily myelinated axon from CNS to effector

• ANS: Two-neuron chain:

  • Preganglionic neuron: Lightly myelinated

  • Ganglionic neuron: Unmyelinated, extends to effector organ

Comparison of Somatic and Autonomic Systems

System	Neurotransmitter	Effector Organs	Pathway
Somatic	Acetylcholine (ACh)	Skeletal muscle	Single neuron, heavily myelinated
Autonomic – Sympathetic	ACh (preganglionic), NE/EPI (postganglionic)	Smooth muscle, glands, cardiac muscle	Two-neuron chain
Autonomic – Parasympathetic	ACh (both pre- and postganglionic)	Smooth muscle, glands, cardiac muscle	Two-neuron chain

Neurotransmitter Effects

• SNS: All motor neurons release ACh (always excitatory)

• ANS:

  • Preganglionic fibers: Release ACh

  • Postganglionic fibers: Release NE or ACh (effect can be stimulatory or inhibitory)

  • Effect depends on neurotransmitter and receptor type

Divisions of the ANS: Functions

• Sympathetic: Mobilizes body during extreme situations (fight, flight, fright, sex)

• Parasympathetic: Maintains body functions and conserves energy (rest and digest)

• The two divisions counterbalance each other

Role of the Parasympathetic Division

• Maintains low energy use

• Involves "D" activities: digestion, defecation, diuresis

• Illustrated by relaxation after a meal: low blood pressure, heart rate, and respiratory rate; high GI activity; warm skin; constricted pupils

Role of the Sympathetic Division

• "Fight-or-flight" system

• Involves "E" activities: exercise, excitement, emergency, embarrassment

• Promotes increased blood flow to muscles, increased heart rate, rapid breathing, cold/sweaty skin, dilated pupils

Anatomy of the ANS

Division	Origin of Fibers	Length of Fibers	Location of Ganglia
Sympathetic	Thoracolumbar region of spinal cord	Short preganglionic, long postganglionic	Close to spinal cord
Parasympathetic	Brain and sacral spinal cord (craniosacral)	Long preganglionic, short postganglionic	In visceral effector organs

Parasympathetic Division Outflow

Cranial Nerve	Ganglion	Effector Organ(s)
Oculomotor (III)	Ciliary	Eye
Facial (VII)	Pterygopalatine, Submandibular	Salivary, nasal, lacrimal glands
Glossopharyngeal (IX)	Otic	Parotid salivary glands
Vagus (X)	Located within walls of target organs	Heart, lungs, most visceral organs
S2–S4 (sacral)	Located within walls of target organs	Large intestine, urinary bladder, ureters, reproductive organs

Sympathetic Trunks and Pathways

• Preganglionic fibers may:

  • Synapse with ganglionic neuron in the same ganglion

  • Ascend/descend the sympathetic chain to another ganglion

  • Pass through the chain ganglion and emerge without synapsing

Adrenal Medulla Pathway

• Thoracic splanchnic nerve fibers pass directly to the adrenal medulla

• Stimulation causes secretion of norepinephrine (NE) and epinephrine (EPI) into the blood

Referred Pain

• Pain from viscera perceived as somatic in origin

• Occurs because visceral pain afferents travel along the same pathways as somatic pain fibers

Neurotransmitters and Receptors

• Acetylcholine (ACh) and norepinephrine (NE) are the main ANS neurotransmitters

• Cholinergic fibers: Release ACh

• Adrenergic fibers: Sympathetic postganglionic axons that release NE

• Effects depend on receptor type (can be excitatory or inhibitory)

Cholinergic Receptors

• Two types: nicotinic and muscarinic

• Named after drugs that mimic ACh (nicotine, muscarine)

Nicotinic Receptors

• Found on motor end plates, all ganglionic neurons, adrenal medulla cells

• ACh binding is always stimulatory

Muscarinic Receptors

• Found on all effector cells stimulated by postganglionic cholinergic fibers

• ACh binding can be inhibitory or excitatory, depending on target organ receptor type

Adrenergic Receptors

• Two main types: alpha (α) and beta (β), each with subclasses (α1, α2, β1, β2, β3)

• NE binding to α receptors is generally stimulatory; to β receptors is generally inhibitory (except β1 in the heart, which is stimulatory)

Effects of Drugs on the ANS

• Atropine: Blocks parasympathetic effects (opposes ACh)

• Neostigmine: Inhibits acetylcholinesterase, increasing ACh effects (used for myasthenia gravis)

• Tricyclic antidepressants: Prolong NE activity

• OTC cold/allergy drugs: Stimulate α-adrenergic receptors (vasoconstriction, less mucus)

• Beta-blockers: Attach to β1 receptors, reduce heart rate, prevent arrhythmias

Interactions of the Autonomic Divisions

• Most visceral organs receive dual innervation (sympathetic and parasympathetic)

• Dynamic antagonism allows precise control of visceral activity

• Sympathetic: Increases heart/respiratory rates, inhibits digestion/elimination

• Parasympathetic: Decreases heart/respiratory rates, allows digestion/waste elimination

Sympathetic Tone

• Maintains blood vessel constriction (vasomotor tone)

• Increases blood pressure as needed; prompts dilation if pressure must decrease

• Alpha-blockers treat hypertension by interfering with vasomotor fibers

Parasympathetic Tone

• Slows the heart

• Controls normal digestive and urinary system activity

• Sympathetic division can override during stress

• Drugs blocking parasympathetic responses increase heart rate, block fecal/urinary retention

Cooperative Effects

• Seen in control of external genitalia

• Parasympathetic: Vasodilation, erection of penis/clitoris

• Sympathetic: Ejaculation in males, reflex peristalsis in females

Unique Roles of the Sympathetic Division

• Regulates functions not influenced by parasympathetic division (adrenal medulla, sweat glands, arrector pili, kidneys, most blood vessels)

• Controls thermoregulation, renin release, metabolic effects

Thermoregulatory Responses to Heat

• Heat causes reflex dilation of blood vessels

• Elevated body temperature leads to widespread vasodilation and sweating

• Cold causes vasoconstriction, retaining blood in vital organs

Release of Renin from the Kidneys

• Sympathetic impulses trigger renin release

• Renin increases blood pressure

Metabolic Effects

• Increases metabolic rate

• Raises blood glucose

• Mobilizes fat as energy

• Stimulates reticular activating system (RAS) for alertness

Localized vs Diffuse Effects

• Parasympathetic: Short-lived, localized control

• Sympathetic: Long-lasting, diffuse effects (due to slower NE inactivation, second-messenger systems, and adrenal medulla hormone release)

Levels of ANS Control

• Hypothalamus: Main integration center for ANS activity

• Receives input from limbic system, cerebral cortex, reticular formation, and spinal cord

• Controls heart activity, blood pressure, body temperature, water balance, endocrine activity, emotional states, biological drives, and fear responses