The Autonomic Nervous System (ANS)

Overview of the ANS

The autonomic nervous system (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes. It consists of motor neurons that innervate smooth muscle, cardiac muscle, and glands, and operates largely below the level of conscious control.

• Effectors: The ANS controls smooth muscle, cardiac muscle, and glands, while the somatic nervous system (SNS) controls skeletal muscle.

• Functions: The ANS adjusts heart rate, blood pressure, digestive processes, and shunts blood to areas of need to maintain homeostasis.

• Control: Operates via subconscious (involuntary) control, also known as the involuntary nervous system or general visceral motor system.

Structural Organization of the Nervous System

Placement of the ANS

The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The PNS is further divided into sensory (afferent) and motor (efferent) divisions. The motor division includes the somatic nervous system and the autonomic nervous system, which itself has sympathetic and parasympathetic divisions.

• CNS: Brain and spinal cord

• PNS: Nerves outside the CNS

• Motor (efferent) division: Somatic (voluntary) and autonomic (involuntary)

• ANS divisions: Sympathetic and parasympathetic

ANS Versus Somatic Nervous System

Comparison of Effectors, Pathways, and Neurotransmitters

Although both the ANS and SNS have motor fibers, they differ in their effectors, efferent pathways, and neurotransmitter effects.

• Effectors:

  • SNS: Innervates skeletal muscles

  • ANS: Innervates cardiac muscle, smooth muscle, and glands

• Efferent Pathways and Ganglia:

  • SNS: Single, thick, myelinated group A axon from CNS to skeletal muscle

  • ANS: Two-neuron chain:

    1. Preganglionic neuron: Cell body in CNS, thin, lightly myelinated axon extends to ganglion

    2. Postganglionic (ganglionic) neuron: Cell body outside CNS in autonomic ganglion, nonmyelinated axon extends to effector organ

• Neurotransmitters and Target Organ Responses:

  • SNS: All somatic motor neurons release acetylcholine (ACh); effect is always stimulatory

  • ANS: Preganglionic fibers release ACh; postganglionic fibers release norepinephrine (NE) or ACh at effectors; effect can be stimulatory or inhibitory depending on receptor type

Overlap of Somatic and Autonomic Function

Integration and Coordination

Higher brain centers regulate and coordinate both the somatic and autonomic nervous systems. Many spinal and cranial nerves contain both somatic and autonomic fibers, and most body adaptations involve both skeletal muscles and visceral organs.

• Example: During exercise, active muscles require more oxygen and glucose, so ANS nerves increase heart rate and open airways to meet these demands.

Summary Table: Key Differences Between SNS and ANS