Chapter 14: The Autonomic Nervous System – Study Guide & Learning Objectives

Chapter 14: The Autonomic Nervous System (ANS)

Major Divisions of the Autonomic Nervous System

The Autonomic Nervous System (ANS) is a critical component of the peripheral nervous system that regulates involuntary physiological processes. It is divided into two main branches:

• Sympathetic Division: Often referred to as the "fight or flight" system, it prepares the body for stressful or emergency situations by increasing heart rate, dilating airways, and mobilizing energy stores.

• Parasympathetic Division: Known as the "rest and digest" system, it conserves energy by slowing the heart rate, increasing intestinal and gland activity, and relaxing sphincter muscles.

Example: When startled, the sympathetic division increases your heart rate, while the parasympathetic division slows it down once the threat passes.

Anatomical Differences Between Sympathetic and Parasympathetic Pathways

• Origin of Fibers: Sympathetic fibers originate from the thoracolumbar region (T1-L2) of the spinal cord, while parasympathetic fibers arise from the brainstem and sacral spinal cord (craniosacral outflow).

• Length of Fibers: Sympathetic pathways have short preganglionic and long postganglionic fibers; parasympathetic pathways have long preganglionic and short postganglionic fibers.

• Location of Ganglia: Sympathetic ganglia are close to the spinal cord (paravertebral or prevertebral), whereas parasympathetic ganglia are near or within the target organs.

Additional info: The neurotransmitter released by preganglionic neurons in both divisions is acetylcholine (ACh), but postganglionic neurons release norepinephrine (sympathetic) or acetylcholine (parasympathetic).

Neurotransmitters and Receptors in the ANS

• Sympathetic Division: Preganglionic neurons release acetylcholine (ACh); postganglionic neurons primarily release norepinephrine (NE), which acts on adrenergic receptors (α and β types).

• Parasympathetic Division: Both pre- and postganglionic neurons release acetylcholine (ACh), which acts on muscarinic receptors in target organs.

Table: Main Neurotransmitters and Receptors in the ANS

Division	Preganglionic Neurotransmitter	Postganglionic Neurotransmitter	Target Receptor
Sympathetic	Acetylcholine (ACh)	Norepinephrine (NE)	Adrenergic (α, β)
Parasympathetic	Acetylcholine (ACh)	Acetylcholine (ACh)	Muscarinic

Dual Innervation and Its Effects

Most organs receive input from both sympathetic and parasympathetic divisions, a phenomenon known as dual innervation. This allows for precise regulation of organ function.

• Example: The heart receives both sympathetic (increases rate and force) and parasympathetic (decreases rate) innervation.

• Antagonistic Effects: The two divisions often have opposite effects on the same organ, maintaining homeostasis.

Additional info: Some organs, such as most blood vessels, are innervated only by the sympathetic division.

Role of the Autonomic Reflexes in Homeostasis

Autonomic reflexes are involuntary responses that help maintain internal stability (homeostasis). These reflexes involve sensory input, integration in the CNS, and motor output via the ANS.

• Examples: Regulation of blood pressure (baroreceptor reflex), heart rate, respiratory rate, and digestive processes.

• Mechanism: Sensory receptors detect changes, signals are processed in the CNS, and autonomic motor neurons adjust organ function accordingly.

Additional info: Disruption of autonomic reflexes can lead to disorders such as orthostatic hypotension or digestive dysfunction.