Ch. 14 Autonomic Nervous System: Structure, Function, and Divisions

Autonomic Nervous System (ANS) in the Structural Organization of the Nervous System

Overview of Nervous System Organization

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The PNS consists of sensory (afferent) and motor (efferent) divisions. The motor division is further subdivided into the somatic nervous system (SNS) and the autonomic nervous system (ANS), with the ANS comprising the sympathetic and parasympathetic divisions.

• CNS: Brain and spinal cord

• PNS: Nerves outside the CNS

• Sensory Division: Transmits sensory information to CNS

• Motor Division: Transmits motor commands from CNS to effectors

• SNS: Controls voluntary movements (skeletal muscle)

• ANS: Controls involuntary functions (cardiac muscle, smooth muscle, glands)

Comparison: Somatic vs. Autonomic Nervous System

Effectors

The SNS innervates skeletal muscles, enabling voluntary movement. The ANS innervates cardiac muscle, smooth muscle, and glands, regulating involuntary functions.

• SNS Effectors: Skeletal muscles

• ANS Effectors: Cardiac muscle, smooth muscle, glands

Efferent Pathways and Ganglia

The SNS uses a single, thick myelinated axon from the CNS to the effector. The ANS uses a two-neuron chain: a preganglionic neuron (lightly myelinated) and a postganglionic neuron (nonmyelinated).

• SNS: Single neuron, direct pathway

• ANS: Preganglionic neuron (CNS origin) synapses in ganglion; postganglionic neuron extends to effector

Neurotransmitter Effects

• SNS: Releases acetylcholine (ACh); always excitatory

• ANS: Preganglionic fibers release ACh; postganglionic fibers release norepinephrine (NE) or ACh, with effects that may be excitatory or inhibitory depending on receptor type

Overlap of Somatic and Autonomic Function

Higher brain centers regulate both systems. Many nerves contain both somatic and autonomic fibers, and adaptations often involve both skeletal muscles and visceral organs.

• Example: During exercise, ANS increases heart rate and opens airways to support active muscles

Divisions of the Autonomic Nervous System

Parasympathetic Division

The parasympathetic division promotes maintenance functions and conserves energy. It is often referred to as the "rest-and-digest" system.

• Functions: Directs digestion, diuresis, defecation

• Effects: Lowers blood pressure, heart rate, and respiratory rate; increases gastrointestinal activity; constricts pupils

• Example: Relaxing after a meal

Sympathetic Division

The sympathetic division mobilizes the body during activity and is known as the "fight-or-flight" system.

• Functions: Responds to exercise, excitement, emergency, embarrassment

• Effects: Increases heart rate, dilates pupils, shunts blood to muscles, dilates bronchioles, releases glucose from liver

• Example: Vigorous physical activity or stressful situations

Dual Innervation and Dynamic Antagonism

Most visceral organs are innervated by both divisions, which typically have opposing effects. This dynamic antagonism maintains homeostasis.

• Dual Innervation: Both divisions serve all visceral organs

• Homeostasis: Maintained by balancing opposing actions

Anatomical Differences Between Sympathetic and Parasympathetic Divisions

Key Anatomical Differences

There are three main anatomical differences between the divisions:

• Sites of Origin: Parasympathetic fibers are craniosacral (brain and sacral spinal cord); sympathetic fibers are thoracolumbar (thoracic and lumbar spinal cord)

• Relative Lengths of Fibers: Parasympathetic has long preganglionic and short postganglionic fibers; sympathetic has short preganglionic and long postganglionic fibers

• Location of Ganglia: Parasympathetic ganglia are in or near visceral effectors; sympathetic ganglia are close to the spinal cord

Summary Table: Anatomical and Physiological Differences

Feature	Parasympathetic Division	Sympathetic Division
Site of Origin	Craniosacral (brain & sacral spinal cord)	Thoracolumbar (thoracic & lumbar spinal cord)
Fiber Length	Long preganglionic, short postganglionic	Short preganglionic, long postganglionic
Ganglia Location	In/near visceral effectors	Close to spinal cord
Neurotransmitter (postganglionic)	Acetylcholine (ACh)	Norepinephrine (NE)
General Function	Rest-and-digest	Fight-or-flight

Key Terms and Concepts

• Acetylcholine (ACh): Neurotransmitter released by all somatic motor neurons and parasympathetic postganglionic fibers

• Norepinephrine (NE): Neurotransmitter released by most sympathetic postganglionic fibers

• Preganglionic Neuron: First neuron in ANS pathway, originating in CNS

• Postganglionic Neuron: Second neuron in ANS pathway, extending to effector

• Dual Innervation: Both ANS divisions innervate most organs, producing opposing effects

Equations and Additional Info

Neurotransmitter Release

• Somatic Pathway: $\text{CNS} \xrightarrow{\text{ACh}} \text{Skeletal Muscle}$

• Autonomic Pathway: $\text{CNS} \xrightarrow{\text{ACh}} \text{Ganglion} \xrightarrow{\text{ACh or NE}} \text{Effector}$

Summary

The autonomic nervous system is essential for regulating involuntary physiological functions and maintaining homeostasis through its sympathetic and parasympathetic divisions. Understanding the anatomical and physiological differences between these divisions is crucial for comprehending their roles in health and disease.