Autonomic Nervous System Overview

Overview

• The autonomic nervous system (ANS) is a component of the peripheral nervous system that regulates involuntary bodily functions.
• It operates without conscious control, managing processes like heart rate, digestion, and respiration.
• The system is divided into two branches: the sympathetic and parasympathetic nervous systems. Each branch has distinct roles and responses.

Sympathetic Nervous System

• The sympathetic nervous system is activated during stressful or emergency situations (the "fight-or-flight" response).
• It prepares the body for immediate action by increasing heart rate, blood pressure, and respiration.
• Key neurotransmitters in the sympathetic system include norepinephrine (noradrenaline).
• It stimulates the release of glucose from the liver.
• It dilates pupils and bronchi, increasing airflow to the lungs.
• Sympathetic neurons typically utilize nicotinic acetylcholine receptors at the synapse with the postganglionic neuron and norepinephrine at the target organ.
• Physiological responses triggered by this system are generally rapid, short-lived, and widespread throughout the body.

Parasympathetic Nervous System

• The parasympathetic nervous system is responsible for "rest-and-digest" functions.
• It promotes relaxation and conservation of energy by slowing heart rate, decreasing blood pressure, and stimulating digestion.
• It is often active during periods of rest or relaxation.
• Key neurotransmitters in the parasympathetic system include acetylcholine.
• It constricts pupils and bronchi, reduces heart rate, and stimulates digestion.
• Parasympathetic neurons use acetylcholine at both pre- and post-ganglionic synapses and at the target organ.
• Responses are more localized and longer-lasting compared to the sympathetic system.

Dual Innervation

• Many organs receive input from both the sympathetic and parasympathetic branches.
• This dual innervation allows for a fine-tuning of the organ's response, maintaining homeostasis.
• This means the body adjusts its activity of organ systems based on the current need and internal conditions.
• For example, heart rate can be sped up or slowed down to meet the body's demands.

Neurological Pathways

• Sympathetic pathways utilize ganglia (clusters of nerve cell bodies) located in the paravertebral chain.
• Parasympathetic pathways have ganglia closer to the target organ, which often leads to longer axons.
• This difference in ganglia location contributes to the responses' difference in speed and distribution.

Key Functions

• Heart: The sympathetic system increases heart rate and force of contraction, while the parasympathetic system slows them down.
• Lungs: The sympathetic system dilates the bronchioles (airways), while the parasympathetic system constricts them.
• Digestive System: The parasympathetic system stimulates digestive activity, while the sympathetic system inhibits it.
• Eyes: The sympathetic system dilates pupils, while the parasympathetic system constricts them.
• Urinary Bladder: The sympathetic system inhibits urination, while the parasympathetic system stimulates it.
• Blood Vessels: In most organs, sympathetic system generally has dominant control, leading to vasoconstriction. Parasympathetic influence on blood vessels is minimal.

Homeostasis

• The interplay between the sympathetic and parasympathetic systems plays a crucial role in maintaining homeostasis, a stable internal environment.
• The system constantly adjusts bodily functions to maintain optimal conditions for survival.

Clinical Significance

• Dysfunction in the ANS can lead to various health problems, including cardiovascular issues, digestive disorders, and neurological conditions.
• Understanding the ANS pathways and their components is vital for developing targeted treatments for these conditions.