Sympathetic Nervous System: Structure, Function, and Pathways

Sympathetic Nervous System: Overview

Function of the Sympathetic Nervous System

The sympathetic nervous system is a division of the autonomic nervous system that prepares the body for physical activity or stress, often referred to as the "fight or flight" response. It regulates various organ systems to optimize performance during emergencies.

• Key Role: Mobilizes energy and resources for rapid action.

• Examples of Effects: Increases heart rate, dilates bronchioles, stimulates glucose release, contracts arrector pili muscles, decreases digestive activity, causes vasoconstriction, dilates pupils, and stimulates sweat glands.

Table: Effects of Sympathetic Nervous System on Body Structures

Structure	Effect of Sympathetic Nervous System
Cardiac Muscle	Increases heart rate
Bronchioles of the Lungs	Dilates bronchioles
Liver	Stimulates release of glucose into blood
Arrector Pili Muscles	Contracts arrector pili (causing goosebumps)
Gastrointestinal Tract	Decreases activity
Blood Vessels	Vasoconstriction in visceral vessels; vasodilation in skeletal muscles
Pupils	Dilates pupils
Sweet Glands	Stimulates sweat glands

Sympathetic Effects on Digestion and Blood Flow

During sympathetic activation, blood is shunted away from the digestive tract and toward skeletal muscles to prioritize physical activity. Digestive activity decreases as energy is redirected.

• Reason for Decreased Digestive Activity: Energy is required elsewhere in the body for essential functions during stress.

• Blood Flow: Vasodilation occurs in skeletal muscles, while vasoconstriction occurs in visceral organs.

Sympathetic Nervous System: Structure and Pathways

The "Sympathetic Subway" Analogy

The pathway of nerve impulses in the sympathetic division can be compared to a subway train system:

• Nerve Impulses: Trains traveling on tracks

• Spinal Cord: Central terminal where all trains originate

• Sympathetic Trunk: Parallel track running alongside the spinal cord

• Ganglia: Train stations where impulses change trains (synapse)

• Effectors: Destinations (organs/tissues)

Each impulse from the spinal cord must "change trains" (synapse) once, traveling through exactly one ganglion.

Structure of the Sympathetic Nervous System

Sympathetic fibers arise from thoracolumbar segments of the spinal cord. The sympathetic trunk is a bundle of nervous fibers flanking both sides of the vertebral column, resembling a chain of beads.

• Preganglionic Fibers: Short fibers that synapse in the sympathetic trunk ganglia.

• Postganglionic Fibers: Long fibers that extend from ganglia to effectors.

• White Ramus Communicans: Carries preganglionic fibers from spinal nerve to sympathetic ganglia.

• Gray Ramus Communicans: Carries postganglionic fibers from sympathetic ganglia to spinal nerves.

Pathway Table: Sympathetic Impulse Route

Step	Structure
1	Spinal cord
2	Preganglionic fiber
3	Ganglion (synapse)
4	Postganglionic fiber
5	Effector (target organ)

Pathways of Sympathetic Innervation

A preganglionic neuron can synapse with a postganglionic neuron in one of three ways:

1. Synapse in trunk ganglion at the same level

2. Synapse in trunk ganglion at a higher or lower level

3. Pass through trunk ganglion and synapse in a collateral ganglion

Splanchnic Nerves and the Adrenal Medulla

Splanchnic nerves are formed by preganglionic fibers that bypass the sympathetic trunk and synapse in collateral ganglia, innervating visceral organs in the abdomen. The adrenal medulla is a modified sympathetic ganglion that secretes epinephrine and norepinephrine when stimulated.

• Splanchnic Nerves: Innervate abdominal viscera

• Adrenal Medulla: Located in the adrenal gland; releases hormones into the bloodstream

Table: Splanchnic Nerves and Innervation

Structure	Innervated by Splanchnic Nerves?
Liver	Yes
Stomach	Yes
Spleen	Yes
Adrenal Medulla	Yes
Kidney	Yes
Large Intestine	Yes
Rectum	Yes
Genitals	No

Key Terms and Definitions

• Sympathetic Trunk: A chain of ganglia running parallel to the spinal cord.

• Preganglionic Fiber: The neuron that originates in the spinal cord and synapses in a ganglion.

• Postganglionic Fiber: The neuron that extends from the ganglion to the effector organ.

• Ganglion: A cluster of neuron cell bodies where synapses occur.

• Effector: The target organ or tissue that responds to neural stimulation.

• Splanchnic Nerves: Nerves that innervate abdominal viscera via collateral ganglia.

• Adrenal Medulla: Inner part of the adrenal gland, secreting epinephrine and norepinephrine.

Practice Questions and Answers

• Why does the sympathetic nervous system shunt blood to skeletal muscles?

  • To prepare the body for intense physical activity.

• Why are many preganglionic fibers in the sympathetic nervous system short?

  • Because they synapse at the sympathetic trunk, which is close to the spinal cord.

• Which of the following statements is true?

  • The parasympathetic system causes us to get goosebumps.

• Collateral ganglia contain which type of cell bodies?

  • Postganglionic sympathetic.

Summary Table: Sympathetic Pathways

Pathway	Description
Same Level Synapse	Preganglionic neuron synapses in trunk ganglion at same level
Higher/Lower Level Synapse	Preganglionic neuron synapses in trunk ganglion at different level
Collateral Ganglion Synapse	Preganglionic neuron passes through trunk ganglion and synapses in collateral ganglion

Equations and Scientific Notation

• General Pathway Equation: $\text{Spinal cord} \rightarrow \text{Preganglionic fiber} \rightarrow \text{Ganglion} \rightarrow \text{Postganglionic fiber} \rightarrow \text{Effector}$

Additional info:

• Some content and terminology have been expanded for clarity and completeness.

• Tables have been recreated and summarized for study purposes.