Autonomic Division of the Nervous System (Chapter 14) – Study Notes

Autonomic Division of the Nervous System

I. Basic Overview of the ANS

The Autonomic Nervous System (ANS) is a major division of the peripheral nervous system responsible for regulating involuntary physiological functions. It operates largely without conscious control and is essential for maintaining homeostasis.

• Organization of the Nervous System: The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The ANS is part of the PNS and consists of afferent (sensory) and efferent (motor) pathways.

• ANS Location: Autonomic neural pathways connect regulatory centers in the CNS to effector organs in the periphery. These centers include the Vasomotor and Cardiac Centers (Medulla), and the Apneustic and Pneumotaxic Centers (Pons).

• Functions of the ANS:

  • Monitors organ system variables and body fluid composition.

  • Initiates rapid, involuntary reflexes for visceral functions: cardiovascular, respiratory, digestive, urinary, and reproductive systems.

  • Enables the body to adapt to changing environments, balancing between "Fight and flight" and "Resting and digesting" states.

• Anatomy of Autonomic Pathways:

  • Typically involves two motor neurons from CNS to effector organ: preganglionic and postganglionic neurons.

  • Synapse occurs in a ganglion (collection of cell bodies outside the CNS).

• Comparison: Somatic vs. Autonomic Nervous Systems

  Feature	Somatic Nervous System (SNS)	Autonomic Nervous System (ANS)
  Effector Organs	Skeletal muscle	Cardiac muscle, smooth muscle, glands
  Neurotransmitters	Acetylcholine (ACh)	Acetylcholine (ACh) & Norepinephrine (NE)
  Effects	Excitation only	Excitation & Inhibition

II. The Sympathetic Division

The Sympathetic Division of the ANS prepares the body for stressful or emergency situations, often referred to as the "fight or flight" response.

• Anatomy:

  • Preganglionic neurons exit the spinal cord between segments T1 and L2.

  • Ganglia are located close to the spinal cord:

    • Paravertebral ganglia (sympathetic chain ganglia) – either side of vertebral column.

    • Prevertebral (collateral) ganglia – in front of vertebral column.

    • Some preganglionic fibers reach target organs directly (e.g., adrenal medulla).

• Generalizations:

  • Preganglionic fibers are short; postganglionic fibers are long.

  • Preganglionic fibers release Acetylcholine (ACh).

  • Most postganglionic fibers release Norepinephrine (NE).

• Neurotransmitters:

  • Presynaptic neuron releases ACh at the ganglion.

  • Postsynaptic neuron releases NE or Epinephrine at the effector organ.

• Physiological Effects:

  • Increased alertness and mental activity

  • Increased heart rate (HR) and blood pressure (BP)

  • Enhanced respiration (increased rate and bronchiole dilation)

  • Elevation in muscle tone

  • Dilated pupils

  • Inhibition of digestive system

III. The Parasympathetic Division

The Parasympathetic Division of the ANS is responsible for conserving energy and promoting "rest and digest" activities during normal, non-stressful conditions.

• Anatomy:

  • Preganglionic neurons (long) travel via Cranial Nerves III, VII, IX, X and Sacral Spinal S2–S4 nerves.

  • Ganglia are located close to or within the wall of the target organ.

  • Postganglionic fibers (short) extend from ganglia to target tissue.

• Neurotransmitters:

  • Both preganglionic and postganglionic fibers release Acetylcholine (ACh).

• Physiological Effects:

  • Decreased heart rate and blood pressure

  • Constriction of bronchioles (airways)

  • Stimulation of digestion and absorption

  • Constriction of pupils

IV. Dual Innervation of Organ Systems

Most internal organs receive input from both sympathetic and parasympathetic divisions, allowing for precise regulation of physiological functions.

• Opposing Effects:

  • Heart: Stimulated by sympathetic, inhibited by parasympathetic (parasympathetic usually dominates at rest).

  • Blood vessels: Typically innervated only by the sympathetic division; dilation or constriction depends on the target tissue.

Autonomic Control Centers

Autonomic functions are regulated by specific centers in the brain, primarily the brainstem and hypothalamus.

• Brainstem: Origin of visceral reflexes controlling blood pressure, heart rate, and respiration.

• Hypothalamus:

  • Interacts with other brain regions.

  • Part of the limbic system, involved in processing emotions.

  • Tightly connected to the brainstem, where most autonomic control centers are located.

Key Terms and Definitions

• Autonomic Nervous System (ANS): The part of the nervous system that controls involuntary bodily functions.

• Preganglionic Neuron: The first neuron in the autonomic pathway, originating in the CNS.

• Postganglionic Neuron: The second neuron in the pathway, located in the PNS and innervating the target organ.

• Ganglion: A cluster of neuronal cell bodies outside the CNS.

• Acetylcholine (ACh): A neurotransmitter used by both divisions of the ANS.

• Norepinephrine (NE): A neurotransmitter primarily used by the sympathetic division.

Summary Table: Sympathetic vs. Parasympathetic Divisions

Feature	Sympathetic Division	Parasympathetic Division
Origin	Thoracolumbar (T1–L2)	Craniosacral (CN III, VII, IX, X; S2–S4)
Preganglionic Fiber Length	Short	Long
Postganglionic Fiber Length	Long	Short
Main Neurotransmitter (Postganglionic)	Norepinephrine (NE)	Acetylcholine (ACh)
Main Effects	"Fight or flight" (↑HR, ↑BP, dilated pupils)	"Rest and digest" (↓HR, ↑digestion, constricted pupils)

Example: Autonomic Regulation of the Heart

• Sympathetic stimulation: Increases heart rate and force of contraction.

• Parasympathetic stimulation: Decreases heart rate.

Additional info: The ANS also plays a role in thermoregulation, metabolic rate, and sexual function. Disorders of the ANS can lead to conditions such as orthostatic hypotension, autonomic dysreflexia, and various forms of dysautonomia.