Autonomic Nervous System: Structure, Function, and Regulation

Autonomic Nervous System

Overview

The autonomic nervous system (ANS) is a critical component of the peripheral nervous system (PNS) responsible for regulating involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and glandular activity. It operates largely outside of conscious control and maintains homeostasis by coordinating the function of internal organs.

Organization of the Nervous System

Main Divisions

• Central Nervous System (CNS): Composed of the brain and spinal cord; processes and integrates information.

• Peripheral Nervous System (PNS): Consists of cranial and spinal nerves; connects the CNS to limbs and organs.

Functional Divisions of the PNS

• Sensory (afferent) division: Nerve fibers that carry information to the CNS from sensory receptors (input from somatic and special senses).

• Motor (efferent) division: Nerve fibers that carry impulses away from the CNS to effector organs. Subdivided into:

  • Somatic division: Controls voluntary movements of skeletal muscles.

  • Autonomic division: Controls involuntary actions of smooth muscle, cardiac muscle, glands, and adipose tissue.

Somatic vs. Autonomic Nervous System

Comparison Table

Feature	Somatic	Autonomic
Control	Voluntary	Involuntary
Effector Organs	Skeletal muscle	Smooth muscle, cardiac muscle, glands, adipose tissue
Neural Pathway	Single motor neuron (monosynaptic)	Two motor neurons (disynaptic): preganglionic and postganglionic
Neurotransmitter	Acetylcholine	Acetylcholine or norepinephrine
Effect	Always excitatory	Excitatory or inhibitory
Control Centers	Cerebrum (conscious) or spinal cord (reflex)	Homeostatic centers in brain (pons, hypothalamus, medulla oblongata; unconscious)
Influence	Conscious thought	Can be influenced by thought and emotion

Autonomic Motor Neurons

Structure

• Somatic neuron: One neuron extends from the CNS to skeletal muscle.

• Autonomic neuron: Two-neuron chain:

  • Preganglionic neuron: From CNS to autonomic ganglion.

  • Postganglionic neuron: From ganglion to target tissue.

Divisions of the Autonomic Nervous System

Sympathetic vs. Parasympathetic

• Both divisions innervate most of the same organs but usually have antagonistic effects.

• Some structures are innervated by only one division.

• Both divisions are efferent (motor) systems, regulating the internal environment autonomously.

• Both use a two-neuron chain and innervate glands, smooth muscle, and cardiac muscle.

Location of Preganglionic Cell Bodies

Division	Location
Sympathetic	Thoracolumbar region (lateral horns of spinal cord segments T1–L2)
Parasympathetic	Craniosacral region (brainstem nuclei of cranial nerves III, VII, IX, X and sacral spinal cord S2–S4)

Site of Peripheral Ganglia

• Sympathetic: Ganglia are close to the spinal cord (sympathetic chain ganglia).

• Parasympathetic: Ganglia are near or within the target organs.

Relative Lengths of Neurons

• Sympathetic: Short preganglionic, long postganglionic neurons.

• Parasympathetic: Long preganglionic, short postganglionic neurons.

Enteric Division (ENS)

• Extensive network of neurons (about 100 million) in the walls of the gastrointestinal tract.

• Regulates and coordinates complex visceral activities, often independently of the CNS.

• Influenced by both sympathetic and parasympathetic divisions.

• Uses neurotransmitters similar to those in the brain.

Additional info: The ENS is sometimes called the "second brain" due to its autonomy and complexity.

Physiological Responses of the ANS

Sympathetic Division: "Fight or Flight"

• Heightened mental alertness

• Increased metabolic rate

• Decreased digestive and urinary functions

• Mobilization of energy reserves

• Dilation of respiratory passageways and increased respiratory rate

• Increased heart rate and blood pressure

• Activation of sweat glands

Parasympathetic Division: "Rest and Digest"

• Decreased metabolic rate

• Decreased heart rate and blood pressure

• Increased secretion by salivary and digestive glands

• Increased motility and blood flow to the digestive tract

• Stimulation of urination and defecation

Innervation Patterns

Sympathetic vs. Parasympathetic Innervation

Target Tissues	Sympathetic	Parasympathetic
Distribution	All tissues (due to vascular smooth muscle)	Organs of head, neck, trunk, external genitalia; never reaches limbs or body wall (except external genitalia)
Special Structures	Adrenal medulla, sweat glands, arrector pili muscles	Distinct pelvic nerves, vagus nerve (cranial nerve X)

Dual Innervation and Antagonistic Control

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

• Some structures receive input from only one division (e.g., sweat glands: sympathetic only).

• Dual innervation allows for precise regulation of organ function.

Exception: Adrenal Gland

• Adrenal medulla is innervated only by sympathetic preganglionic neurons.

• Releases epinephrine (80%) and norepinephrine (20%) directly into the bloodstream, causing widespread sympathetic effects ("adrenaline rush").

• This is an exception to the typical two-neuron chain; the preganglionic neuron synapses directly in the gland.

Neurotransmitters and Receptors

Sympathetic Division

• Neurotransmitters: Norepinephrine (NE) and epinephrine (E)

• Receptors: Adrenergic receptors (alpha and beta types)

• Adrenergic receptors are named for their response to drugs that mimic NE and E.

Parasympathetic Division

• Neurotransmitter: Acetylcholine (ACh)

• Receptors: Cholinergic receptors (nicotinic and muscarinic types)

• Nicotinic receptors are always excitatory; muscarinic receptors can be excitatory or inhibitory.

Functional Generalizations About the ANS

• Both divisions can produce stimulatory and inhibitory effects, depending on the target organ.

• Most organs are innervated by both divisions, usually with opposite effects.

• Either division alone or both together can coordinate organ activities.

• Sympathetic division produces more generalized, body-wide effects.

• Parasympathetic division is more important for resting and restorative functions.

Role of the ANS in Homeostasis

• Vital physiological processes continue without conscious awareness (e.g., in coma patients).

• The ANS adjusts activities of the digestive, cardiovascular, respiratory, and reproductive systems automatically.

• ANS output impacts virtually every body system, maintaining internal balance (homeostasis).