Chapter 14: The Autonomic Nervous System and Homeostasis – Study Notes

The Autonomic Nervous System and Homeostasis

Introduction

The Autonomic Nervous System (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and sexual arousal. The ANS operates largely below the level of consciousness to maintain homeostasis and respond to stress.

Functions of the Autonomic Nervous System and Reflex Arcs

Overview of the ANS

• Vital Functions: The ANS oversees essential bodily functions such as heart rate, blood pressure, digestion, and urinary processes without conscious control.

• Visceral Reflex Arcs: These are neural circuits that control involuntary functions through a series of steps:

  • 1. Sensory Input: Sensory signals from the viscera (internal organs) and skin are sent by afferent sensory neurons to the brain or spinal cord.

  • 2. Integration: The central nervous system (CNS) integrates these stimuli.

  • 3. Motor Output: Motor impulses from the CNS are sent via efferent motor neurons in cranial and spinal nerves to autonomic ganglia.

  • 4. Effector Response: Autonomic ganglia send impulses via other efferent neurons to target organs, triggering a motor response in the target cells.

Comparison of Somatic and Autonomic Nervous Systems

Somatic vs. Autonomic Motor Division

• Somatic Motor Division: Controls voluntary movements by directly innervating skeletal muscle fibers. Uses acetylcholine (ACh) as a neurotransmitter, which is always excitatory.

• Visceral Motor Division (ANS): Controls involuntary functions by innervating smooth muscle, cardiac muscle, and glands. Involves two neurons in series:

  • Preganglionic Neuron: Cell body located in the CNS; its axon synapses on the cell body of a postganglionic neuron.

  • Postganglionic Neuron: Cell body located in an autonomic ganglion in the PNS; its axon synapses on the target cell. Neurotransmitters can be ACh or norepinephrine, and effects can be excitatory or inhibitory.

Table: Comparison of Somatic and Autonomic Nervous Systems

Feature	Somatic Nervous System	Autonomic Nervous System
Control	Voluntary	Involuntary
Effector Organs	Skeletal muscle	Smooth muscle, cardiac muscle, glands
Number of Neurons	One (from CNS to effector)	Two (preganglionic and postganglionic)
Neurotransmitter	Acetylcholine (ACh)	ACh or norepinephrine
Effect on Target	Always excitatory	Excitatory or inhibitory

Example:

• Somatic: Deciding to move your arm to pick up a book.

• Autonomic: Your heart rate increases when you are startled, without conscious thought.

Additional info: The ANS is further divided into the sympathetic and parasympathetic divisions, which generally have opposing effects on target organs to maintain homeostasis.