Autonomic Nervous System: Structure, Function, and Integration

Autonomic Nervous System (ANS)

Overview of the Nervous System

The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The PNS is further subdivided into afferent (sensory) and efferent (motor) divisions. The efferent division includes the somatic nervous system (controls skeletal muscle) and the autonomic nervous system (regulates smooth muscle, cardiac muscle, and glands).

• Central Nervous System (CNS): Brain and spinal cord.

• Peripheral Nervous System (PNS): All neural tissue outside the CNS.

• Afferent Division: Brings sensory information to the CNS.

• Efferent Division: Carries motor commands from the CNS to effectors.

• Somatic Nervous System: Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System (ANS): Controls involuntary functions by regulating smooth muscle, cardiac muscle, and glands.

Divisions of the Autonomic Nervous System

The ANS is divided into the sympathetic and parasympathetic divisions, which generally have opposing effects on target organs.

• Sympathetic Division (Thoracolumbar): Prepares the body for 'fight or flight' responses.

• Parasympathetic Division (Craniosacral): Promotes 'rest and digest' activities.

Anatomy of the Autonomic Pathways

Sympathetic Pathway

• Preganglionic fibers: Short, release acetylcholine (ACh).

• Postganglionic fibers: Long, release norepinephrine (NE) at most effectors.

• Adrenal Medulla: Preganglionic fibers synapse directly with chromaffin cells in the adrenal medulla (no ganglia involved), causing release of epinephrine (E) and norepinephrine (NE) into the bloodstream.

Parasympathetic Pathway

• Preganglionic fibers: Long, release acetylcholine (ACh).

• Postganglionic fibers: Short, release acetylcholine (ACh) at effectors.

Neurotransmitters of the Autonomic Nervous System

• PREGANGLIONIC axons (both SNS and PSNS): Release acetylcholine (ACh) – cholinergic.

• POSTGANGLIONIC axons:

  • Sympathetic: Most release norepinephrine (NE) – adrenergic.

  • Parasympathetic: Release acetylcholine (ACh).

Adrenal Medulla and Catecholamines

Structure and Function

• Location: Sits atop each kidney.

• Function: Major organ of the sympathetic nervous system; secretes large amounts of epinephrine and some norepinephrine (collectively called catecholamines).

• Stimulation: Chromaffin cells are stimulated by preganglionic sympathetic fibers.

Receptors in the Autonomic Nervous System

Cholinergic Receptors

• Nicotinic Receptors:

  • Found at neuromuscular junctions of skeletal muscle, all postganglionic neurons (both sympathetic and parasympathetic), and chromaffin cells of the adrenal medulla.

  • Binding of ACh is always stimulatory.

• Muscarinic Receptors:

  • Found on all effector cells stimulated by postganglionic cholinergic fibers (parasympathetic target organs, some sympathetic targets like sweat glands).

  • Binding of ACh can be stimulatory or inhibitory depending on the target organ.

Adrenergic Receptors

• Alpha (α) Receptors:

  • α1: Usually excitatory (e.g., constricts blood vessels, contracts sphincters).

  • α2: Usually inhibitory (e.g., inhibits NE release, promotes blood clotting).

• Beta (β) Receptors:

  • β1: Excitatory (increases heart rate, stimulates renin release, lipolysis).

  • β2: Inhibitory (relaxes/dilates bronchioles, relaxes smooth muscle in digestive and urinary tracts).

  • β3: Involved in relaxation of urinary bladder, lipolysis, and thermogenesis.

Note: The effect of a neurotransmitter depends on the type of receptor present on the target organ, not just the neurotransmitter itself.

Types of Actions at Receptor Sites

• Agonist: Binds to a receptor and activates it, mimicking the effect of the endogenous neurotransmitter.

• Antagonist: Binds to a receptor but does not activate it, blocking the action of the neurotransmitter or agonist.

Examples of Pharmacological Agents

Type	Example
Sympathomimetic	Beta-adrenergic agonist, Alpha-adrenergic agonist
Sympatholytic	Beta-blocker, Alpha-blocker
Parasympathomimetic	Cholinergic agonist (muscarinic or nicotinic), Anticholinesterases
Parasympatholytic	Antimuscarinic, Antinicotinic

Localized vs. Diffuse Effects

• Parasympathetic: Effects are localized and short-lived due to rapid breakdown of ACh by acetylcholinesterase.

• Sympathetic: Effects are diffuse and long-lasting, especially when the adrenal medulla releases catecholamines into the bloodstream.

Functional Effects of the ANS

Sympathetic Nervous System (SNS)

• Dilates pupils

• Increases heart rate and force of contraction

• Increases blood pressure

• Relaxes airways (bronchodilation)

• Constriction of blood vessels to non-essential organs (kidneys, GI tract)

• Stimulates glycogenolysis and lipolysis (increases blood glucose and fatty acids)

• Inhibits digestive and urinary functions

Parasympathetic Nervous System (PSNS)

• SLUDD: Salivation, Lacrimation, Urination, Digestion, Defecation

• Decreases heart rate

• Constricts airways (bronchoconstriction)

• Constricts pupils

SLUDGE (side effects of cholinergics): Salivation, Lacrimation, Urinary incontinence, Diarrhea, Gastrointestinal cramps, Emesis

Central Control of the ANS

The Hypothalamus

• Main integration center for autonomic functions.

• Coordinates heart activity, blood pressure, body temperature, water balance, and endocrine activity.

• Receives input from the limbic system (emotional/visceral brain), cerebral cortex, and other brain regions.

The Enteric Nervous System (ENS)

Structure and Function

The ENS is a third component of the ANS, sometimes called the "intrinsic nervous system" of the gut. It is responsible for regulating digestive processes independently of the CNS, though it can be influenced by both sympathetic and parasympathetic divisions.

• Contains as many neurons as the spinal cord.

• Controls motility, secretion, absorption, and blood flow in the GI tract.

Plexuses of the ENS

• Myenteric (Auerbach's) Plexus: Located between longitudinal and circular muscle layers; controls GI tract motility.

• Submucosal (Meissner's) Plexus: Located in the submucosa; regulates gastrointestinal blood flow and epithelial cell function.

Types of Neurons in the ENS

• Sensory Neurons: Detect mechanical, thermal, osmotic, and chemical stimuli; monitor gut contents and wall state.

• Motor Neurons: Control GI motility and secretion by acting on smooth muscle and secretory cells.

• Interneurons: Integrate sensory input and coordinate motor output.

Visceral (Autonomic) Reflexes

Visceral reflex arcs are similar to somatic reflex arcs but involve a two-neuron motor pathway (preganglionic and postganglionic fibers). They regulate functions such as defecation, urination, blood pressure, and digestion.

• Components: Receptor, sensory neuron, integration center, two-motor neuron chain, effector.

• Examples: Defecation reflex, micturition reflex, blood pressure regulation.

Summary Table: Comparison of Sympathetic and Parasympathetic Divisions

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (T1-L2)	Craniosacral (brainstem, S2-S4)
Preganglionic Fiber	Short	Long
Postganglionic Fiber	Long	Short
Neurotransmitter (Preganglionic)	Acetylcholine	Acetylcholine
Neurotransmitter (Postganglionic)	Norepinephrine (most)	Acetylcholine
General Function	"Fight or flight"	"Rest and digest"
Effect Duration	Diffuse, long-lasting	Localized, short-lived

Key Terms

• Cholinergic: Refers to neurons or receptors that use acetylcholine.

• Adrenergic: Refers to neurons or receptors that use norepinephrine or epinephrine.

• Agonist: A substance that activates a receptor.

• Antagonist: A substance that blocks a receptor.

• Catecholamines: Hormones produced by the adrenal medulla (epinephrine and norepinephrine).

Additional info: The notes also reference pharmacological agents and clinical acronyms (SLUDGE) relevant for understanding drug effects on the ANS, which is important for applied physiology and pharmacology.