The Autonomic Nervous System (ANS) | Part 1: Overview & Comparison

Autonomic Nervous System (ANS)

Part of the motor division of the PNS that controls smooth muscle, cardiac muscle, and glands automatically without conscious control.

Purpose of the ANS

Regulates slow, routine bodily functions and shifts body resources during emergencies or stress.

Conscious Control of ANS

Operates largely under subconscious control.

Effectors of the ANS

Smooth muscle, cardiac muscle, and glands.

Effectors of the Somatic Nervous System (SNS)

Skeletal muscles.

How the ANS Works

Uses motor neurons that innervate viscera and operate via subconscious reflexes.

Comparison: Somatic vs Autonomic

Somatic: voluntary skeletal control. Autonomic: involuntary control of viscera.

Somatic Motor Pathway

Single heavily myelinated neuron extends from CNS to effector.

Autonomic Motor Pathway

Two-neuron chain consisting of a preganglionic and a postganglionic neuron.

Preganglionic Neuron

First neuron in the ANS pathway; lightly myelinated axon that exits CNS to synapse in autonomic ganglion.

Postganglionic Neuron

Second neuron in ANS pathway; unmyelinated axon extends from ganglion to effector organ.

Synapse Location in ANS

In an autonomic ganglion between preganglionic and postganglionic neurons.

Neurotransmitter in SNS

All somatic motor neurons release acetylcholine (ACh), which always excites skeletal muscle.

Neurotransmitters in ANS

Preganglionic fibers release ACh; postganglionic fibers release either ACh or norepinephrine (NE).

Effect of Neurotransmitters in ANS

Either stimulatory or inhibitory depending on the receptor type and organ.

Summary: SNS vs ANS (Voluntary vs Involuntary)

SNS: voluntary skeletal muscle control. ANS: involuntary control of cardiac, smooth muscle, and glands.

Summary: SNS vs ANS (Neural Pathway)

SNS: one neuron from CNS to effector. ANS: two neurons with one synapse in ganglion.

Summary: SNS vs ANS (Myelination)

SNS: heavily myelinated axons. ANS: preganglionic axons lightly myelinated, postganglionic unmyelinated.

Summary: SNS vs ANS (Effect on Effector)

SNS responses always excitatory. ANS responses may be excitatory or inhibitory.

Main Divisions of ANS

Sympathetic division and parasympathetic division.

Sympathetic Division

Prepares the body for emergencies; "fight or flight" response.

Parasympathetic Division

Promotes rest, recovery, and energy conservation; "rest and digest" response.

Counterbalance Function of ANS

The two divisions (sympathetic and parasympathetic) generally oppose each other to maintain homeostasis.

Role of the ANS

Mobilizes energy in emergencies and conserves energy during relaxation.

“Fight or Flight” System

Term for the sympathetic division that activates body systems under stress.

“Rest and Digest” System

Term for the parasympathetic division that promotes calm and normal body maintenance.

Effectors in SNS vs ANS

SNS controls skeletal muscles; ANS controls smooth muscle, cardiac muscle, and glands.

Efferent Pathway Structure in SNS

Single, heavily myelinated axon from CNS to skeletal muscle.

Efferent Pathway Structure in ANS

Two-neuron chain: preganglionic neuron (CNS) → ganglion → postganglionic neuron → effector.

Preganglionic Fiber

Lightly myelinated axon extending from CNS to autonomic ganglion.

Postganglionic Fiber

Unmyelinated axon extending from ganglion to visceral effector organ.

Target Organ Response in SNS

Always excitatory, causing muscle contraction.

Target Organ Response in ANS

May be excitatory or inhibitory depending on neurotransmitter (ACh or NE) and receptor type.

Somatic vs Autonomic Review

Somatic: voluntary, skeletal muscle, one neuron. Autonomic: involuntary, visceral effectors, two-neuron chain.

Three Key Differences: SNS vs ANS

1. Effectors, 2. Efferent pathways, 3. Target organ responses.

SNS Neurotransmitter

ACh (acetylcholine) — always excitatory.

ANS Neurotransmitters

ACh (acetylcholine) and NE (norepinephrine) — can be excitatory or inhibitory.

Speed of Control

SNS provides faster control due to heavy myelination and direct connection to effectors.

Response Time of ANS

Slower than SNS due to use of two neurons and unmyelinated postganglionic fibers.

Summary of ANS Function

Coordinates involuntary processes like heart rate, blood pressure, respiration, digestion, and glandular activity.

Routine Autonomic Activity

Includes slow body functions such as digestion, urination, and gland secretion without conscious effort.

Emergency Autonomic Activity

Redirects energy and blood flow to muscles, heart, and lungs during stress or danger.