Autonomic Nervous System: Parasympathetic and Sympathetic Divisions

Autonomic Nervous System (ANS)

Overview of the ANS

The Autonomic Nervous System (ANS) is a division of the peripheral nervous system responsible for involuntary regulation of internal organs, smooth muscle, cardiac muscle, and glands. It maintains homeostasis by integrating signals from the central nervous system (CNS) and coordinating automatic body functions.

• Involuntary control of visceral functions (e.g., heart rate, digestion, respiratory rate).

• Mediated by the hypothalamus, brain stem, limbic system, spinal cord, and cerebrum.

• Divided into sympathetic and parasympathetic divisions.

Functional Organization of the Nervous System

The nervous system is organized into the central nervous system (CNS) and peripheral nervous system (PNS). The PNS is further divided into sensory (afferent) and motor (efferent) divisions. The motor division includes the somatic nervous system (voluntary control of skeletal muscle) and the autonomic nervous system (involuntary control of smooth/cardiac muscle and glands).

Somatic vs. Autonomic Nervous Systems

Comparison of Motor Pathways

• Somatic Nervous System: Single, heavily myelinated motor neuron extends from CNS to skeletal muscle; neurotransmitter is acetylcholine (ACh); effect is always stimulatory.

• Autonomic Nervous System: Two-neuron chain (preganglionic and postganglionic neurons); neurotransmitters include ACh and norepinephrine (NE); effect can be stimulatory or inhibitory depending on the receptor and target tissue.

Feature	Somatic Nervous System	Autonomic Nervous System
Type of Control	Voluntary (skeletal muscle)	Involuntary (smooth/cardiac muscle, glands)
Number of Neurons	One	Two (preganglionic and postganglionic)
Neurotransmitter	Acetylcholine (ACh)	ACh or norepinephrine (NE)
Effect on Effector	Always excitatory	Excitatory or inhibitory
Axon Properties	Thick, myelinated, fast conduction	Preganglionic: thin, lightly myelinated; Postganglionic: unmyelinated, slower conduction

Divisions of the Autonomic Nervous System

Sympathetic Division

The sympathetic division is responsible for the "fight-or-flight" response, preparing the body for situations involving stress, exercise, or emergencies (the 3 E's: exercise, excitement, emergency).

• Increases heart rate, blood pressure, and metabolic rate.

• Dilates pupils and bronchioles.

• Diverts blood flow to skeletal muscles.

Parasympathetic Division

The parasympathetic division is responsible for "resting and digesting" activities, conserving energy, and replenishing nutrient stores.

• Decreases heart rate and promotes digestion and glandular activity.

• Stimulates activities that occur when the body is at rest.

Dual Innervation and Control

• Dual innervation: Most organs receive input from both sympathetic and parasympathetic divisions, often with opposing effects (antagonistic control).

• Antagonistic control: Example—sympathetic increases heart rate, parasympathetic decreases heart rate.

• Tonic control: Some effectors (e.g., blood vessels) are regulated by varying levels of sympathetic activity alone.

Anatomical and Physiological Differences Between Divisions

Characteristic	Parasympathetic	Sympathetic
Origin	Craniosacral (brain stem nuclei of cranial nerves III, VII, IX, X; sacral spinal cord S2–S4)	Thoracolumbar (lateral horns of T1–L2 spinal cord)
Location of Ganglia	Near or within effector organs	Close to spinal cord (paravertebral or prevertebral ganglia)
Preganglionic Fiber Length	Long	Short
Postganglionic Fiber Length	Short	Long
Degree of Branching	Minimal	Extensive
Neurotransmitters	Both pre- and postganglionic release ACh	Preganglionic: ACh; Postganglionic: NE (most), ACh (sweat glands)
Functional Role	Maintenance, energy conservation	Mobilizes body for activity

Autonomic Efferent Pathway

General Structure

• Consists of two motor (efferent) neurons in series: preganglionic neuron (originates in CNS) and postganglionic neuron (synapses with effector).

• Synapse between the two neurons occurs at an autonomic ganglion.

Parasympathetic Pathway

• Preganglionic neuron: Long, myelinated, cholinergic (releases ACh); originates in brain stem (cranial nerves III, VII, IX, X) or sacral spinal cord (S2–S4).

• Ganglia: Terminal (at effectors) or intramural (within effectors); minimal divergence (1 preganglionic to 4–5 postganglionic neurons).

• Postganglionic neuron: Short, unmyelinated, cholinergic (releases ACh); synapses with single effector (smooth muscle, cardiac muscle, glands).

• Receptors: Nicotinic (on postganglionic neuron; ligand-gated ion channel) and muscarinic (on effector; G-protein coupled).

Sympathetic Pathway (Overview)

• Preganglionic neuron: Short, myelinated, cholinergic (releases ACh); originates in thoracolumbar spinal cord (T1–L2).

• Ganglia: Paravertebral (sympathetic chain) or prevertebral (collateral) ganglia; extensive divergence.

• Postganglionic neuron: Long, unmyelinated; most are adrenergic (release NE), some cholinergic (sweat glands).

Levels of ANS Control

• Hypothalamus: Main integration center for ANS activity; regulates visceral functions.

• Brain stem: Controls basic life-support functions (e.g., heart rate, blood pressure, respiration).

• Spinal cord: Mediates reflexes (e.g., urination, defecation, erection, ejaculation).

• Limbic system and cerebral cortex: Influence ANS activity via emotional and conscious input.

Summary Table: Key Anatomical Differences Between ANS Divisions

Feature	Parasympathetic	Sympathetic
Origin	Craniosacral (brain stem, S2–S4)	Thoracolumbar (T1–L2)
Ganglia Location	Near/within effector	Near spinal cord
Preganglionic Fiber	Long, myelinated, ACh	Short, myelinated, ACh
Postganglionic Fiber	Short, unmyelinated, ACh	Long, unmyelinated, NE (most)
Receptors	Nicotinic (ganglia), Muscarinic (effectors)	Nicotinic (ganglia), Adrenergic (effectors)

Example: Parasympathetic Pathway

• Preganglionic axon releases ACh, which binds to nicotinic receptors on the postganglionic neuron.

• Postganglionic axon releases ACh, which binds to muscarinic receptors on the target cell (e.g., smooth muscle, gland).

Additional info:

• Cholinergic neurons release acetylcholine (ACh); adrenergic neurons release norepinephrine (NE).

• Muscarinic receptors are G-protein coupled and mediate slower, prolonged responses compared to nicotinic receptors, which are ion channels.

• Parasympathetic effects are generally more localized and discrete, while sympathetic effects are more widespread due to greater divergence and adrenal medulla involvement.