Autonomic Nervous System

Autonomic Nervous System Overview

  • The Autonomic Nervous System (ANS) regulates involuntary bodily functions, including heart rate, digestion, and respiratory rate.

  • It is divided into two main parts:

    • Parasympathetic Nervous System (PNS)

    • Sympathetic Nervous System (SNS)

Components of ANS

  • The ANS is part of the peripheral nervous system and is primarily composed of two main divisions:

    • Central Nervous System (CNS): Comprises the brain and spinal cord.

    • Peripheral Nervous System (PNS): Comprises all nerves that branch out from the CNS.

Hierarchical Structure

  • CNS:

    • Contains cell bodies responsible for processing input and generating output.

  • PNS:

    • Encompasses the autonomic and somatic divisions.

    • Sensory (afferent) Division: Sends signals from the body to the CNS.

    • Motor (efferent) Division: Sends signals from the CNS to muscles and glands.

    • Somatic Nervous System: Controls voluntary movements.

    • Autonomic Nervous System (ANS): Controls involuntary actions.

Differences Between Parasympathetic and Sympathetic

Anatomical Differences

  • Parasympathetic Division:

    • Originates from the brainstem and sacral spinal cord (S2-S4).

    • Ganglia: Located near or within target organs (intramural ganglia).

    • Preganglionic Fibers: Long, Postganglionic Fibers: Short.

  • Sympathetic Division:

    • Originates from thoracolumbar regions (T1-L2).

    • Ganglia: Close to spinal cord (sympathetic trunk ganglia and collateral ganglia).

    • Preganglionic Fibers: Short, Postganglionic Fibers: Long.

Functional Differences

  • Parasympathetic Division:

    • Known for "rest and digest" functions.

    • Maintains homeostasis, conserves energy.

    • Releases Acetylcholine (ACh) at both preganglionic and postganglionic synapses.

  • Sympathetic Division:

    • Known for "fight or flight" responses.

    • Prepares body for stressful situations, increases heart rate, and blood flow to muscles.

    • Preganglionic neurons release ACh; postganglionic neurons primarily release norepinephrine (NE).

Neurotransmitters

  • Parasympathetic:

    • ACh is the primary neurotransmitter. It can be stimulatory or inhibitory depending on the effector organ and the type of receptor.

  • Sympathetic:

    • Uses both ACh and NE; often stimulates and prepares body for rapid response.

    • ACh acts mostly on sweat glands, while NE is involved in blood vessel constriction.

Key Functions of the Divisions

  • Parasympathetic Effects on Organs:

    • Eye: Constricts pupils.

    • Heart: Decreases heart rate.

    • Digestion: Increases activity in gastrointestinal tract.

  • Sympathetic Effects on Organs:

    • Eye: Dilates pupils.

    • Heart: Increases heart rate and force.

    • Blood Vessels: Constricts most vessels, increases blood pressure.

Reflex Arc in ANS

  1. Stimulus: A change in internal or external environment.

  2. Receptors: Located in viscera, detect changes.

  3. Integration Center: CNS (brain or spinal cord) processes information.

  4. Motor Neurons: Preganglionic and postganglionic neurons transmit signals to effectors.

  5. Effectors: Target organs (muscles or glands) respond to signals.

Autonomic Ganglia Summary

  • Parasympathetic Ganglia: Intramural or near visceral organs; short postganglionic fibers.

  • Sympathetic Ganglia: Close to the spinal cords; long postganglionic fibers.

Clinical Implications

  • Understanding how the ANS functions can lead to better treatments for conditions like hypertension (high blood pressure) or digestive disorders.

  • Medications can either enhance or inhibit ANS function, which is crucial in managing various physiological conditions.