Somatic vs. Autonomic Nervous System

Overview of Peripheral Nervous System Subsystems

The peripheral nervous system (PNS) is divided into two major subsystems: the Somatic Nervous System (SNS) and the Autonomic Nervous System (ANS). Each system has distinct roles in controlling body functions.

• Somatic Nervous System:

  • Innervates skeletal muscle.

  • Controls voluntary movement.

  • Efferent pathway consists of one neuron.

• Autonomic Nervous System:

  • Innervates smooth muscle, cardiac muscle, and glands.

  • Controls involuntary functions (e.g., heart rate, digestion).

  • Efferent pathway consists of two neurons (preganglionic and postganglionic).

Example: Actions coordinated by the SNS include knee jerk reflex and pupil dilation; actions coordinated by the ANS include heart rate increase, secreting insulin, and sudden jump in response to a loud noise.

Comparison Table:

Practice: The primary functional difference is that the SNS controls voluntary muscle, while the ANS controls involuntary muscle.

Divisions of the Autonomic Nervous System (ANS)

Sympathetic and Parasympathetic Divisions

The ANS is organized into two divisions, each with opposing effects on target organs:

• Sympathetic Division:

  • Activated when we are threatened, excited, or stressed.

  • Known as the "Fight or Flight" system.

  • Helps maintain homeostasis during activity.

• Parasympathetic Division:

  • Activated when we are relaxed and at rest.

  • Known as the "Rest and Digest" system.

  • Maintains homeostasis during rest.

These divisions usually have antagonistic effects on the same organs. If one stimulates an effector, the other usually inhibits it.

Example: The sympathetic division increases heart rate, while the parasympathetic division decreases it.

Practice: After a large meal, the parasympathetic system becomes active to aid digestion, while the sympathetic system decreases heart rate and sweat gland activity.

Relevant Brain Anatomy in Autonomic Control

Key Brain Structures and Their Functions

Several brain structures are essential for autonomic regulation:

Example: A lesion in the anterior hypothalamus would most affect the parasympathetic division of the ANS.

Levels of Control in the ANS

Integration Centers and Pathways

The hypothalamus is the main integration center of the ANS. The ANS operates in two primary ways:

1. Maintaining Homeostasis

   • Pathway: Stimulus → Hypothalamus → Brain Stem (Reticular Formation) → Spinal Cord

2. Responding to Emotional/Stressful Events

   • Pathway: Stimulus → Cerebral Cortex → Limbic System → Hypothalamus → Brain Stem (Reticular Formation) → Spinal Cord

Example: The hypothalamus acts as an integration center between areas of the brain that govern emotion and regions that control visceral functions.

Function of the Sympathetic Nervous System

Effects on Target Organs

The sympathetic nervous system prepares the body for physical activity or stress. Its effects include:

Example: The sympathetic nervous system shunts blood to skeletal muscles to prepare the body for intense physical activity.

Practice Questions and Explanations

• Sympathetic activation decreases digestive tract activity because energy is required elsewhere for essential functions during stress.

• Parasympathetic system increases secretion from digestive glands; sympathetic system causes goosebumps via arrector pili muscles.

The Sympathetic Subway: Pathways and Synapses

Train System Analogy for Sympathetic Pathways

The sympathetic division can be visualized as a subway train system:

• Nerve Impulses: Trains traveling on tracks.

• Spinal Cord: Central terminal where all trains originate.

• Sympathetic Trunk: Track that all trains run along.

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

• Effectors: Destinations (organs, muscles, glands).

All impulses from the spinal cord must synapse once in a ganglion before reaching effectors. This ensures each impulse travels through exactly two fibers (preganglionic and postganglionic).

Example: In the sympathetic nervous system, a ganglion is a place where impulses synapse.

Summary Table: Somatic vs. Autonomic Nervous System

Key Terms and Definitions

• Somatic Nervous System (SNS): The part of the PNS that controls voluntary movements via skeletal muscles.

• Autonomic Nervous System (ANS): The part of the PNS that regulates involuntary functions such as heart rate, digestion, and glandular activity.

• Sympathetic Division: Prepares the body for "fight or flight" responses.

• Parasympathetic Division: Promotes "rest and digest" activities.

• Ganglion: A cluster of neuron cell bodies where synapses occur in the ANS pathway.

• Homeostasis: The maintenance of stable internal conditions.

Equations and Pathways

General ANS Pathway:

• Preganglionic neuron (CNS) → Ganglion (synapse) → Postganglionic neuron → Effector organ

Neural Pathway Equation:

Additional info: The notes have been expanded with academic context, definitions, and tables for clarity and completeness.