Chapter 14: The Autonomic Nervous System – Structure, Function, and Neurotransmission

Chapter 14: The Autonomic Nervous System

Overview of the Autonomic Nervous System (ANS)

The autonomic nervous system (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes, including those of smooth muscle, cardiac muscle, and glands. It operates largely below the level of consciousness to maintain homeostasis and respond to stress.

• Motor neurons innervate smooth muscle, cardiac muscle, and glands.

• Adjusts body functions to support activity (e.g., shunts blood, regulates heart rate, blood pressure, digestion).

• Operates via subconscious control.

• Also called the involuntary nervous system or visceral (autonomic) motor system.

Sympathetic and Parasympathetic Nerve Pathways

The ANS is divided into two main branches: the sympathetic and parasympathetic divisions. These divisions often have opposing effects on target organs, allowing precise control of visceral functions.

• Parasympathetic division: Promotes maintenance functions and conserves energy ("rest-and-digest").

• Sympathetic division: Mobilizes the body during activity ("fight-or-flight").

• Dual innervation: Most visceral organs are served by both divisions, which maintain homeostasis through dynamic antagonism.

Example: During exercise, sympathetic nerves increase heart rate and open airways to supply active muscles with more oxygen and glucose.

Visceral Effects of Sympathetic and Parasympathetic Innervation

Each division of the ANS has distinct effects on body organs and systems, depending on the situation and physiological needs.

• Parasympathetic Division:

  • Keeps body energy use low during maintenance activities.

  • Directs digestion, diuresis, and defecation.

  • Blood pressure, heart rate, and respiratory rates are low; gastrointestinal activity is high; pupils constricted.

• Sympathetic Division:

  • Mobilizes body during activity, especially in stressful situations.

  • Increases heart rate, dilates pupils, shunts blood to skeletal muscles and heart, dilates bronchioles, and causes liver to release glucose.

  • Inhibits digestion and elimination.

Structural Differences Between Sympathetic and Parasympathetic Divisions

There are three main anatomical differences between the two divisions:

• Sites of origin:

  • Parasympathetic fibers are craniosacral (originate in brain and sacral spinal cord).

  • Sympathetic fibers are thoracolumbar (originate in thoracic and lumbar regions of spinal cord).

• Relative lengths of fibers:

  • Parasympathetic: long preganglionic, short postganglionic fibers.

  • Sympathetic: short preganglionic, long postganglionic fibers.

• Location of ganglia:

  • Parasympathetic ganglia are located in or near the visceral effectors.

  • Sympathetic ganglia lie close to the spinal cord.

Parasympathetic Pathways and Nerves

Parasympathetic cell bodies are located in the brain stem and sacral spinal cord. Preganglionic fibers run in:

• Oculomotor Nerves (III)

• Facial Nerves (VII)

• Glossopharyngeal Nerves (IX)

• Vagus Nerves (X)

• Sacral nerves (to pelvic floor, intestine, urinary bladder, ureters, reproductive organs)

Sympathetic Pathways and Trunk Ganglia

The sympathetic division is more complex and innervates more organs than the parasympathetic division. It is also called the thoracolumbar division.

• Some structures (e.g., sweat glands, arrector pili muscles, blood vessel smooth muscle) are innervated only by sympathetic fibers.

• Typically, there are 23 sympathetic trunk ganglia in each trunk:

  • 3 cervical

  • 11 thoracic

  • 4 lumbar

  • 4 sacral

  • 1 coccygeal

Once a preganglionic axon reaches a sympathetic trunk ganglion, it can:

• Synapse at the same level

• Synapse at a higher or lower level

• Synapse in a collateral ganglion (in abdomen and pelvis, forming splanchnic nerves)

Receptors and Neurotransmitters of the Two Divisions

The ANS and somatic nervous system both have motor fibers but differ in their effectors, number of neurons, and neurotransmitter actions.

• Somatic nervous system:

  • Innervates skeletal muscles

  • Uses a single neuron from CNS to effector

  • All motor neurons release acetylcholine (ACh) (always excitatory)

• Autonomic nervous system:

  • Innervates cardiac muscle, smooth muscle, and glands

  • Uses a two-neuron chain:

    • Preganglionic neuron: cell body in CNS, lightly myelinated axon to ganglion

    • Postganglionic neuron: cell body in autonomic ganglion, nonmyelinated axon to effector

  • Preganglionic fibers release ACh

  • Postganglionic fibers release norepinephrine (NE) (sympathetic) or ACh (parasympathetic)

  • Effect can be excitatory or inhibitory, depending on receptor type

Visceral Reflex Arcs

Visceral reflex arcs control many involuntary functions and differ from somatic reflex arcs in having two consecutive motor neurons.

• Components: receptor, sensory neuron, integration center, motor neuron, effector

• Effectors are smooth muscle, cardiac muscle, and glands

• Examples: reflexes that empty rectum and bladder; three-neuron reflex arcs in gastrointestinal tract walls

• Involve the enteric nervous system (sensory neurons, interneurons, motor neurons)

Sympathetic Tone and Blood Pressure Regulation

Most blood vessel smooth muscle is innervated only by sympathetic fibers, which maintain vasomotor tone (partial constriction of blood vessels).

• If blood pressure rises, vasomotor fibers fire less, causing dilation and lowering blood pressure.

• If blood pressure falls, vasomotor fibers fire more, causing constriction and raising blood pressure.

• Allows the sympathetic system to shunt blood where needed.

Summary Table: Comparison of Sympathetic and Parasympathetic Divisions

Feature	Sympathetic Division	Parasympathetic Division
Origin	Thoracolumbar (T1-L2)	Craniosacral (brainstem & S2-S4)
Fiber Lengths	Short preganglionic, long postganglionic	Long preganglionic, short postganglionic
Ganglia Location	Close to spinal cord	In/near effector organs
Main Neurotransmitters	ACh (preganglionic), NE (postganglionic)	ACh (preganglionic & postganglionic)
General Function	"Fight-or-flight"; mobilizes body	"Rest-and-digest"; conserves energy

Additional info: The enteric nervous system is sometimes considered a third division of the ANS, especially in the context of gastrointestinal reflexes.