Autonomics: The Autonomic Nervous System (ANS) in Anatomy & Physiology

PNS II: Autonomics

Introduction

The autonomic nervous system (ANS) is a critical component of the peripheral nervous system (PNS) responsible for regulating involuntary physiological functions. This study guide covers the structure, function, and divisions of the ANS, focusing on its role in controlling visceral motor tissues and maintaining homeostasis.

Somatic vs. Visceral Tissue

Overview

The nervous system is divided into somatic and visceral components, each serving distinct functions and target tissues.

• Somatic Tissue: Refers to structures under voluntary control, primarily skeletal muscle.

• Visceral Tissue: Refers to structures under involuntary control, including cardiac muscle, smooth muscle, and glands.

Key Differences:

• Somatic Motor Pathways: Single neuron from CNS to skeletal muscle.

• Visceral Motor Pathways: Two-neuron chain (preganglionic and postganglionic) from CNS to target organ.

Example: Somatic motor neurons control voluntary movement (e.g., moving an arm), while visceral motor neurons regulate heart rate and digestion.

Visceral Motor Tissues

Types and Locations

Visceral motor tissues are found in various organs and structures throughout the body, primarily within body cavities.

• Cardiac Muscle: Found in the heart; responsible for pumping blood.

• Smooth Muscle: Found in the walls of blood vessels, digestive tract, respiratory tract, and other organs.

• Glands: Includes sweat glands, salivary glands, and others involved in secretion.

Note: Some smooth muscle and glands are also found in the body wall (e.g., arrector pili muscles, sweat glands).

Autonomic Chains

Structure of the Autonomic Pathway

The autonomic motor pathway consists of a two-neuron chain:

• Preganglionic Neuron: Cell body located in the CNS; axon projects to an autonomic ganglion.

• Autonomic Ganglion: Site of synapse between preganglionic and postganglionic neurons.

• Postganglionic Neuron: Cell body in the ganglion; axon projects to the target tissue (e.g., heart, smooth muscle, gland).

Example: In the sympathetic division, preganglionic neurons originate in the spinal cord and synapse in paravertebral ganglia.

Divisions of the Autonomic Nervous System (ANS)

Sympathetic vs. Parasympathetic

The ANS is divided into two main branches, each with distinct anatomical origins and functions.

• Sympathetic Division: Originates from the thoracolumbar region (T1-L2) of the spinal cord. Prepares the body for 'fight or flight' responses.

• Parasympathetic Division: Originates from the craniosacral regions (brainstem and S2-S4 spinal segments). Promotes 'rest and digest' activities.

Key Anatomical Differences:

Feature	Sympathetic	Parasympathetic
Origin	Lateral horn of spinal cord (T1-L2)	Brainstem (cranial nerves III, VII, IX, X) & sacral spinal cord (S2-S4)
Preganglionic Fiber Length	Short	Long
Postganglionic Fiber Length	Long	Short
Ganglia Location	Near spinal cord (paravertebral, prevertebral)	Near or within target organ

Functional Differences:

• Sympathetic: Increases heart rate, dilates pupils, inhibits digestion, mobilizes energy stores.

• Parasympathetic: Decreases heart rate, constricts pupils, stimulates digestion, conserves energy.

Example: During exercise, sympathetic activity increases heart rate and redirects blood flow to muscles.

Sympathetic Pathways

Tracing Sympathetic Neurons

Sympathetic neurons follow specific pathways from the spinal cord to their target tissues.

• Preganglionic Neurons: Exit the spinal cord via the ventral root, enter the spinal nerve, and pass through white rami communicantes to reach paravertebral ganglia.

• Postganglionic Neurons: Exit ganglia via gray rami communicantes to innervate target tissues.

Pathway Variations:

• Some preganglionic fibers synapse in paravertebral ganglia at the same level.

• Others ascend or descend to synapse at different levels.

• Some pass through paravertebral ganglia to synapse in prevertebral ganglia or the adrenal medulla.

Example: Sympathetic innervation to the heart involves preganglionic fibers synapsing in cervical ganglia, with postganglionic fibers traveling to cardiac tissue.

Parasympathetic Pathways

Origins and Distribution

Parasympathetic preganglionic neurons originate in the brainstem and sacral spinal cord.

• Cranial Outflow: Via cranial nerves III, VII, IX, and X to head, thoracic, and abdominal organs.

• Sacral Outflow: Via pelvic splanchnic nerves to lower abdominal and pelvic organs.

Example: The vagus nerve (cranial nerve X) provides extensive parasympathetic innervation to thoracic and abdominal organs.

Antagonism Between Sympathetic and Parasympathetic Systems

Functional Balance

Most organs receive dual innervation from both sympathetic and parasympathetic fibers, which often have opposing effects.

• Heart Rate: Sympathetic stimulation increases heart rate; parasympathetic stimulation decreases it.

• Digestive Activity: Sympathetic inhibits; parasympathetic stimulates.

Example: During stress, sympathetic activity predominates; during relaxation, parasympathetic activity restores baseline function.

Autonomic Reflexes

General Structure

Autonomic reflexes are involuntary responses that regulate internal organ function.

• Components: Sensory receptor, afferent neuron, integration center (CNS), efferent (autonomic) neuron, effector organ.

• Example: Baroreceptor reflex regulates blood pressure by adjusting heart rate and vessel diameter.

Clinical Application: Medications Affecting the ANS

Sympathetic Blockers

Medications for high blood pressure often block sympathetic effects, reducing heart rate and vascular resistance.

• Emergency Situations: Reduced ability to mount a 'fight or flight' response (e.g., less increase in heart rate or blood pressure).

• Stage Fright: Useful for reducing symptoms like rapid heartbeat and sweating.

• Adverse Effects: May cause fatigue, dizziness, or inability to respond to stress appropriately.

Additional info: Beta-blockers are a common class of medications that inhibit sympathetic stimulation of the heart.