Autonomic Nervous System (ANS): Structure, Function, and Comparison

Ch. 14 – Autonomic Nervous System (ANS) Study Guide

Contrast of Somatic and Autonomic Nervous Systems

The somatic nervous system (SNS) and autonomic nervous system (ANS) are two major divisions of the peripheral nervous system, each with distinct structures and functions.

• Number of Neurons Between CNS and Effector Organ:

  • SNS: Single neuron from CNS to effector (skeletal muscle).

  • ANS: Two-neuron chain: preganglionic neuron (CNS to ganglion) and postganglionic neuron (ganglion to effector).

• Location of Neuron Cell Bodies:

  • SNS: Cell bodies in the CNS (anterior horn of spinal cord).

  • ANS: Preganglionic cell bodies in CNS; postganglionic cell bodies in autonomic ganglia.

• Structure Each Innervates:

  • SNS: Skeletal muscle.

  • ANS: Cardiac muscle, smooth muscle, glands.

• Inhibitory or Excitatory Effects:

  • SNS: Always excitatory (stimulates muscle contraction).

  • ANS: Can be excitatory or inhibitory, depending on neurotransmitter and receptor type.

• Conscious and Unconscious Control:

  • SNS: Under voluntary (conscious) control.

  • ANS: Involuntary (unconscious) control.

Key Terms: Neurons and Ganglia

• Preganglionic Neuron: The first neuron in the ANS pathway, with its cell body in the CNS and axon projecting to an autonomic ganglion.

• Postganglionic Neuron: The second neuron, with its cell body in the autonomic ganglion and axon projecting to the effector organ.

• Ganglion (plural: ganglia): A cluster of neuronal cell bodies outside the CNS.

Comparison of Sympathetic and Parasympathetic Divisions

The ANS is divided into sympathetic and parasympathetic divisions, each with unique anatomical and functional characteristics.

• Cell Bodies:

  • Sympathetic: Preganglionic cell bodies in thoracic and lumbar spinal cord (thoracolumbar division).

  • Parasympathetic: Preganglionic cell bodies in brainstem and sacral spinal cord (craniosacral division).

• Preganglionic Neurons: Longer in parasympathetic, shorter in sympathetic division.

• Cranial Parasympathetic Ganglia: Include ciliary, pterygopalatine, submandibular, and otic ganglia (associated with cranial nerves III, VII, IX, X).

• Nerve Fibers:

  • Sympathetic: Short preganglionic, long postganglionic fibers.

  • Parasympathetic: Long preganglionic, short postganglionic fibers.

• Neurotransmitters:

  • Sympathetic: Preganglionic release acetylcholine (ACh); postganglionic usually release norepinephrine (NE).

  • Parasympathetic: Both pre- and postganglionic neurons release acetylcholine (ACh).

• Inhibitory or Excitatory: Both divisions can have inhibitory or excitatory effects depending on the target organ and receptor type.

• Organs Receiving Dual Innervation: Most organs receive both sympathetic and parasympathetic input, allowing for precise regulation.

Functional Generalizations about the ANS

The ANS regulates involuntary physiological functions through complex interactions between its two divisions.

• Stimulatory vs. Inhibitory: The effect on target organs depends on the neurotransmitter and receptor; for example, sympathetic stimulation increases heart rate, while parasympathetic stimulation decreases it.

• Dual Innervation: Most organs receive input from both divisions, often with opposing effects (e.g., sympathetic dilates pupils, parasympathetic constricts pupils).

• Opposite Effect: The two divisions often produce opposite responses in the same organ system.

• Cooperative Effect: In some cases, both divisions work together for a common function (e.g., sexual function: erection by parasympathetic, ejaculation by sympathetic).

• General vs. Localized Effect:

  • Sympathetic: More generalized, widespread effects due to branching and adrenal medulla involvement.

  • Parasympathetic: More localized, discrete effects.

• Functions During Activity vs. Rest:

  • Sympathetic: "Fight or flight"—prepares body for activity, increases heart rate, dilates bronchioles, inhibits digestion.

  • Parasympathetic: "Rest and digest"—promotes maintenance activities, conserves energy, stimulates digestion.

Table: Comparison of Somatic and Autonomic Nervous Systems

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

Example: Heart Rate Regulation

• Sympathetic stimulation: Increases heart rate and force of contraction.

• Parasympathetic stimulation: Decreases heart rate.

Additional info: The ANS also includes the enteric nervous system, which independently regulates digestive tract activity but is modulated by sympathetic and parasympathetic input.