Chapter 17- The Autonomic Nervous System: Structure, Function, and Divisions

Autonomic Nervous System (ANS)

Overview of the ANS

The Autonomic Nervous System (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and glandular activity. It innervates adipose tissue, smooth muscle, cardiac muscle, and glands. The ANS operates via a two-neuron efferent pathway: a preganglionic fiber and a postganglionic fiber.

• Preganglionic fiber: Originates in the CNS and synapses in a ganglion.

• Postganglionic fiber: Extends from the ganglion to the effector organ.

In contrast, the Somatic Nervous System controls voluntary movements of skeletal muscle and uses a single motor neuron in its efferent pathway.

Divisions of the ANS

Sympathetic and Parasympathetic Divisions

The ANS is divided into two main subdivisions, each with distinct anatomical and functional characteristics:

• Sympathetic Division: Prepares the body for 'fight-or-flight' responses during stress or activity.

• Parasympathetic Division: Promotes 'rest-and-digest' activities during periods of rest and recovery.

Enteric Nervous System

The Enteric Nervous System is a complex network of neurons within the walls of the digestive tract. It can function independently of the CNS to coordinate local reflexes related to digestion.

Sympathetic Division

Anatomy and Pathways

The sympathetic division has three main locations for its ganglia:

• Sympathetic chain ganglia: Located on both sides of the vertebral column.

• Collateral ganglia: Positioned anterior to the vertebral column; postganglionic fibers from these ganglia innervate abdominopelvic organs.

• Adrenal medulla: The inner region of the adrenal gland, which releases hormones into the bloodstream.

Preganglionic neurons originate in the thoracic and lumbar regions of the spinal cord, a pattern known as thoracolumbar outflow.

Neurotransmitters and Receptors

• Preganglionic fibers: Release acetylcholine (ACh), a cholinergic neurotransmitter, which excites ganglionic neurons.

• Postganglionic fibers: Release norepinephrine (NE), an adrenergic neurotransmitter, which can have excitatory or inhibitory effects depending on the receptor type (alpha or beta) on the effector cell.

Anatomical Distribution

Sympathetic postganglionic fibers are distributed to various organs, including the heart, lungs, digestive organs, and blood vessels. Splanchnic nerves serve abdominopelvic organs.

Adrenal Medulla

The adrenal medulla is a modified sympathetic ganglion. When stimulated by sympathetic preganglionic fibers, it secretes epinephrine and norepinephrine into the bloodstream, acting as hormones to produce widespread effects.

Effects of Sympathetic Stimulation

• Increased alertness and energy

• Elevated heart rate, blood pressure, and respiratory rate

• Increased muscle tone

• Mobilization of energy reserves (glucose and lipids)

Sympathetic Motor Pathways

Sympathetic motor pathways involve the release of ACh at the ganglion and NE at the effector. The effect on the target cell depends on the receptor type (alpha or beta).

Parasympathetic Division

Anatomy and Pathways

Parasympathetic ganglia are located within or near the target organs, resulting in more specific and localized effects. Preganglionic neurons originate in the brainstem and sacral spinal cord, known as craniosacral outflow.

Neurotransmitters and Receptors

• Preganglionic fibers: Release ACh (cholinergic), exciting ganglionic neurons.

• Postganglionic fibers: Also release ACh, which can excite or inhibit the effector depending on the receptor type:

  • Nicotinic receptors: Always excitatory; found on ganglionic neurons and skeletal muscle.

  • Muscarinic receptors: Can be excitatory or inhibitory; found on parasympathetic target organs.

Effects of Parasympathetic Stimulation

• Pupil constriction

• Stimulation of digestive gland secretion

• Promotion of nutrient absorption

• Increased smooth muscle activity in the digestive tract

• Defecation and urination

• Constriction of airways

• Reduced heart rate and contraction force

• Sexual arousal and stimulation

Anatomical Distribution

Parasympathetic output is distributed to organs such as the eye, salivary glands, heart, lungs, digestive organs, and reproductive organs.

Dual Innervation

Antagonistic Effects

Most vital organs receive input from both sympathetic and parasympathetic divisions, often producing opposite effects. For example, the sympathetic division increases heart rate, while the parasympathetic division decreases it.

Comparison of Sympathetic and Parasympathetic Divisions

Key Differences

The following table summarizes the main differences between the sympathetic and parasympathetic divisions of the ANS:

Characteristic	Sympathetic Division	Parasympathetic Division
Location of CNS Visceral Motor Neurons	Lateral horns of spinal segments T1–L2	Brainstem and spinal segments S2–S4
Location of PNS Ganglia	Sympathetic chain, collateral ganglia, adrenal medulla	Near or within target organs
Preganglionic Fiber Length	Relatively short, myelinated	Relatively long, myelinated
Postganglionic Fiber Length	Relatively long, unmyelinated	Relatively short, unmyelinated
Neurotransmitter Released (Preganglionic)	Acetylcholine (ACh)	Acetylcholine (ACh)
Neurotransmitter Released (Postganglionic)	Norepinephrine (NE), sometimes ACh or NO	Acetylcholine (ACh)
General Functions	Stimulates metabolism, increases alertness, prepares for emergency ('fight or flight')	Promotes relaxation, nutrient uptake, energy storage ('rest and digest')

Visceral Reflexes

Autonomic Reflexes

Visceral reflexes are automatic, unconscious responses to stimulation of visceral organs. Many are coordinated locally within the organ system, sometimes bypassing the central nervous system entirely.