Autonomic Nervous System (ANS) Overview

Introduction to the ANS

The autonomic nervous system (ANS) is a division of the peripheral nervous system responsible for regulating involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and glandular activity. It operates largely below the level of consciousness and is essential for maintaining homeostasis.

• Effectors: Smooth muscle, cardiac muscle, and glands

• Functions: Adjusts blood flow, heart rate, blood pressure, and digestive processes

• Control: Subconscious (involuntary)

• Alternate names: Involuntary nervous system, visceral motor system

Organization of the Nervous System

Central and Peripheral Divisions

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 (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of visceral effectors).

ANS vs. Somatic Nervous System

Key Differences

Although both systems have motor fibers, they differ in their effectors, neurotransmitters, and neural pathways.

• Effectors: Somatic system targets skeletal muscle; ANS targets cardiac muscle, smooth muscle, and glands.

• Neurotransmitters: Somatic neurons release acetylcholine (ACh), always excitatory. ANS preganglionic fibers release ACh; postganglionic fibers release ACh or norepinephrine (NE), which can be excitatory or inhibitory.

• Neural Pathways: Somatic system uses a single neuron from CNS to effector; ANS uses a two-neuron chain (preganglionic and postganglionic neurons).

Divisions of the Autonomic Nervous System

Parasympathetic and Sympathetic Divisions

The ANS is divided into two main branches with generally opposing effects:

• Parasympathetic Division: Promotes maintenance functions and conserves energy ("rest and digest").

• Sympathetic Division: Mobilizes the body during activity ("fight or flight").

Parasympathetic Division

The parasympathetic division maintains body functions during restful situations.

• Blood pressure, heart rate, and respiratory rate are low

• Gastrointestinal tract activity is high

• Pupils constricted, lenses accommodated for close vision

• Example: Relaxing and reading after a meal

Sympathetic Division

The sympathetic division prepares the body for stressful or energetic activity.

• Increases heart rate, dilates pupils, causes cold, sweaty skin

• Shunts blood to skeletal muscles and heart

• Dilates bronchioles, causes liver to release glucose

• Activated by exercise, excitement, emergency, embarrassment

Dual Innervation and Homeostasis

Dynamic Antagonism

Most visceral organs receive input from both sympathetic and parasympathetic divisions, which usually have opposite effects. This dynamic antagonism maintains homeostasis.

• Example: Sympathetic division increases heart rate; parasympathetic division decreases it.

• Iris of the eye: Sympathetic dilates pupil; parasympathetic constricts pupil (antagonistic effect).

• Salivary glands: Sympathetic stimulates mucus secretion; parasympathetic stimulates enzyme secretion (cooperative effect).

Neurotransmitters and Receptors of the ANS

Key Neurotransmitters

The effects of the ANS are determined by the neurotransmitters released and the receptors present on target organs.

• Acetylcholine (ACh): Released by all preganglionic neurons and parasympathetic postganglionic neurons (cholinergic).

• Norepinephrine (NE): Released by most sympathetic postganglionic neurons (adrenergic).

Anatomical Differences Between Sympathetic and Parasympathetic Divisions

Key Anatomical Features

• Sites of Origin: Parasympathetic fibers are craniosacral (brain and sacral spinal cord); sympathetic fibers are thoracolumbar (thoracic and lumbar spinal cord).

• Relative Lengths of Fibers: Parasympathetic has long preganglionic and short postganglionic fibers; sympathetic has short preganglionic and long postganglionic fibers.

• Location of Ganglia: Parasympathetic ganglia are in or near visceral effectors; sympathetic ganglia are close to the spinal cord.

Visceral Reflexes

Structure and Function

Visceral reflexes control involuntary functions such as digestion and blood pressure. They share basic components with somatic reflex arcs but have key differences.

• All reflex arcs begin with sensory input

• Components: receptor, sensory neuron, integration center, motor neuron, effector

• Differences from somatic reflexes:

  • Two consecutive neurons in the motor pathway

  • Afferent fibers are visceral sensory neurons

  • Effectors are smooth muscle, cardiac muscle, and glands

• Examples: Reflexes that empty the rectum and bladder; baroreflex for blood pressure regulation

Neural Pathways: Somatic vs. Autonomic

Pathway Structure

• Somatic Pathway: Single motor neuron from CNS to skeletal muscle

• Autonomic Pathway: Two-neuron chain (preganglionic and postganglionic) from CNS to effector

Clinical Considerations and Disorders of the ANS

Common Disorders

• Autonomic neuropathy: Damage to autonomic nerves, often due to diabetes mellitus. Symptoms include sexual dysfunction, dizziness, urinary incontinence, sluggish pupil reactions, and impaired sweating. Prevention focuses on good blood glucose control.

• Hypertension: Overactive sympathetic vasoconstrictor response to stress, leading to high blood pressure. Treated with adrenergic receptor-blocking drugs (beta blockers).

• Raynaud’s disease: Exaggerated vasoconstriction in fingers and toes, causing them to turn pale or cyanotic. Treated with vasodilators.

• Autonomic dysreflexia: Uncontrolled activation of autonomic neurons in individuals with spinal cord injuries, causing dangerous increases in blood pressure and risk of stroke.

Control of ANS Activity

• ANS is regulated by CNS centers: brainstem, spinal cord, hypothalamus, and cerebral cortex

• Hypothalamus is the main integrative center

• Cerebral input can modify ANS activity subconsciously via the limbic system

Summary Table: Comparison of Somatic and Autonomic Motor Neurons

Additional info: The enteric nervous system, a subdivision of the ANS, is involved in gastrointestinal reflexes and is composed of sensory neurons, interneurons, and motor neurons located in the walls of the GI tract.