The Autonomic Nervous System and Homeostasis: Study Notes

Chapter 14: The Autonomic Nervous System and Homeostasis

Overview of the Autonomic Nervous System (ANS)

The Autonomic Nervous System (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes, including heart rate, blood pressure, digestion, and urinary functions. It operates largely without conscious control and is essential for maintaining homeostasis.

• Key Functions: Oversees vital functions such as heart rate, blood pressure, digestion, and urinary activities.

• Control Mechanism: Utilizes visceral reflex arcs to regulate target organs automatically.

Visceral Reflex Arcs

Visceral reflex arcs are the fundamental circuits through which the ANS exerts its effects on the body.

• Sensory Input: Sensory signals from the viscera and skin are transmitted by afferent sensory neurons to the central nervous system (CNS).

• Integration: The CNS integrates these stimuli.

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

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

Comparison of Somatic and Autonomic Nervous Systems

The nervous system's motor division is divided into the somatic and autonomic (visceral) branches, each with distinct structures and functions.

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

• Visceral Motor Division (ANS): Controls involuntary functions by innervating smooth and cardiac muscle cells, and glands. Utilizes a two-neuron chain:

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

  • Postganglionic Neuron: Cell body in the PNS autonomic ganglion; axon synapses on the target cell. Neurotransmitters: ACh or norepinephrine; 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	Always excitatory	Excitatory or inhibitory

Key Terms

• Autonomic Nervous System (ANS): The part of the nervous system responsible for control of the bodily functions not consciously directed.

• Visceral Reflex Arc: A neural pathway that controls an action reflex in internal organs.

• Preganglionic Neuron: The first neuron in the two-neuron chain of the ANS, with its cell body in the CNS.

• Postganglionic Neuron: The second neuron in the chain, with its cell body in an autonomic ganglion in the PNS.

• Afferent Neuron: A sensory neuron that carries nerve impulses from receptors or sense organs toward the CNS.

• Efferent Neuron: A motor neuron that carries neural impulses away from the CNS to effectors such as muscles or glands.

Example: Visceral Reflex Arc in Action

When blood pressure rises, baroreceptors in the blood vessels send sensory information to the CNS. The CNS processes this information and sends motor commands via the ANS to the heart and blood vessels, causing a decrease in heart rate and vasodilation to lower blood pressure.

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