The Autonomic Nervous System: Structure, Function, and Neurotransmission

Autonomic Nervous System (ANS)

Overview and General Organization

The Autonomic Nervous System (ANS) is a division of the peripheral nervous system responsible for involuntary regulation of internal organs and homeostatic processes. It controls visceral effectors such as smooth muscle, cardiac muscle, glands, and adipose tissue. The ANS operates through a two-neuron pathway: a preganglionic neuron originating in the CNS and a postganglionic neuron in the peripheral ganglia, which innervates the target organ.

• Visceral motor nuclei in the hypothalamus initiate autonomic responses.

• Preganglionic neurons originate in the brainstem or spinal cord.

• Autonomic ganglia house the cell bodies of postganglionic neurons.

• Postganglionic axons extend to visceral effectors.

Divisions of the ANS

The ANS is divided into two main branches with generally opposing effects:

• Sympathetic Division – Prepares the body for emergency situations ("fight or flight").

• Parasympathetic Division – Promotes maintenance activities during restful states ("rest and digest").

Sympathetic Division

Structure and Pathways

The sympathetic division is organized to mobilize the body's resources during stress or emergencies. Preganglionic fibers originate in the thoracic and lumbar segments (T1–L2) of the spinal cord. There are three main types of sympathetic ganglia:

• Chain ganglia – Located alongside the spinal cord; control effectors in the body wall, thoracic cavity, head, neck, and limbs.

• Collateral ganglia – Anterior to the spinal cord; control effectors in abdominopelvic tissues and organs.

• Adrenal medullae – Modified ganglia within the adrenal glands; release hormones directly into the bloodstream.

Key Characteristics

• Short preganglionic axons (originate from T1–L2).

• Ganglia are close to the spinal cord.

• Long postganglionic axons.

• One preganglionic neuron may innervate many targets.

Functions of the Sympathetic Division

Sympathetic activation produces a coordinated set of physiological changes:

• Heightened mental alertness

• Pupil dilation

• Increased metabolic rate and ATP production

• Reduced digestive and urinary functions

• Activation of energy reserves (glycogenolysis, lipolysis)

• Increased respiratory rate and dilation of airways

• Increased heart rate and blood pressure

• Activation of sweat glands

• Orgasm (sexual function)

Activation and Systemic Effects

• Sympathetic responses often involve the entire division activating simultaneously (mass activation).

• Results in increased alertness, energy, euphoria, and temporary insensitivity to pain.

• Mobilization of energy reserves (glycogen breakdown, lipid release).

Parasympathetic Division

Structure and Pathways

The parasympathetic division is responsible for conserving energy and promoting maintenance activities. Preganglionic fibers originate in the brainstem (cranial nerves III, VII, IX, X) and sacral spinal cord segments (S2–S4). Ganglia are located near or within target organs:

• Terminal ganglia – Near target organ

• Intramural ganglia – Embedded within the target organ's tissue

Key Characteristics

• Long preganglionic axons (from brainstem and sacral region)

• Ganglia within or adjacent to target organs

• Short postganglionic axons

• One neuron innervates specific targets (more localized effects)

Functions of the Parasympathetic Division

Parasympathetic activation supports rest, digestion, and energy storage:

• Decreased metabolic rate and ATP production

• Decreased heart rate and blood pressure

• Constriction of respiratory passages

• Increased secretion by salivary and digestive glands

• Increased blood flow to digestive tract

• Increased absorption of nutrients

• Stimulation of urination and defecation

• Constriction of pupils

• Sexual arousal and erection

Activation and Systemic Effects

• Parasympathetic functions are the default state and are not activated as a single unit.

• Focuses on relaxation, food processing, and nutrient absorption.

Neurotransmitters and Receptors in the ANS

Acetylcholine (ACh)

• Released by cholinergic neurons (all preganglionic neurons and parasympathetic postganglionic neurons).

• Effects are brief due to rapid breakdown by acetylcholinesterase (AChE).

• Nicotinic receptors – Found in autonomic ganglia and neuromuscular junctions; always excitatory (ionotropic).

• Muscarinic receptors – Found on parasympathetic target organs; can be excitatory or inhibitory (metabotropic, G-protein coupled).

Norepinephrine (NE) and Epinephrine

• Released by adrenergic neurons (most sympathetic postganglionic neurons).

• Effects last longer than ACh; NE is reabsorbed and degraded by monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT).

• Alpha (\( \alpha \)) receptors – \( \alpha_1 \) (excitatory), \( \alpha_2 \) (inhibitory, often inhibits parasympathetic activity).

• Beta (\( \beta \)) receptors – \( \beta_1 \) (increases heart rate/metabolism), \( \beta_2 \) (relaxes smooth muscle in airways), \( \beta_3 \) (stimulates lipolysis in adipose tissue).

Summary Table: Neurotransmitters and Receptors

Division	Preganglionic Neurotransmitter (Receptor)	Postganglionic Neurotransmitter (Receptor)
Sympathetic	Acetylcholine (Nicotinic)	Mostly Norepinephrine (Alpha and Beta)
Parasympathetic	Acetylcholine (Nicotinic)	Acetylcholine (Muscarinic)

Comparison of Sympathetic and Parasympathetic Divisions

Characteristic	Sympathetic	Parasympathetic
Origin of Preganglionic Neuron	Spinal Segments T1–L2	Cranial Nerves III, VII, IX, X; Sacral Segments S2–S4
Location of Peripheral Ganglia	Near spinal cord	Near or within target organ/tissue
Preganglionic Fiber Length	Short	Long
Postganglionic Fiber Length	Long	Short
Preganglionic Neurotransmitter (receptor)	ACh (Nicotinic)	ACh (Nicotinic)
Postganglionic Neurotransmitter (receptor)	Mostly Norepinephrine (Alpha and Beta)	Acetylcholine (Muscarinic)
Target Area	Broad	Specific
General Function	Stimulates metabolism, increases alertness, prepares for emergency	Promotes relaxation, nutrient uptake, energy storage