Study Notes: The Autonomic Nervous System (ANS)

The Autonomic Nervous System (ANS)

Divisions of the ANS

The Autonomic Nervous System is a component of the peripheral nervous system that regulates involuntary physiological processes. It is divided into two main branches:

• Sympathetic Nervous System (SNS): Prepares the body for 'fight or flight' responses during stressful situations.

• Parasympathetic Nervous System (PSNS): Promotes 'rest and digest' activities that conserve and restore energy.

Target Tissues of the ANS

• Cardiac muscle (heart)

• Smooth muscle (walls of blood vessels, digestive tract, respiratory tract, etc.)

• Glands (salivary, sweat, digestive glands, etc.)

Role of the ANS in Survival

The ANS is essential for survival because it regulates vital involuntary functions such as heart rate, blood pressure, digestion, and respiratory rate. Visceral effectors (organs) require the ANS to maintain homeostasis and respond to changes in the internal and external environment.

Roles of the SNS and PSNS

• SNS: Mobilizes body systems during activity (increases heart rate, dilates pupils, inhibits digestion).

• PSNS: Conserves energy and promotes housekeeping functions during rest (slows heart rate, stimulates digestion).

• Example: During exercise, the SNS increases heart rate and redirects blood flow to muscles, while the PSNS predominates during sleep to promote digestion and energy storage.

Excitatory vs. Inhibitory Effects

The statement "The SNS is always excitatory, while the PSNS is always inhibitory" is incorrect. Both systems can have excitatory or inhibitory effects depending on the target tissue and receptor type.

• Example: SNS excites cardiac muscle (increases heart rate) but inhibits digestive activity; PSNS excites digestive activity but inhibits heart rate.

Preganglionic and Postganglionic Neurons

• Preganglionic neuron: The first neuron in the ANS pathway; cell body is in the CNS.

• Postganglionic neuron: The second neuron; cell body is in an autonomic ganglion, axon extends to the effector organ.

• PSNS preganglionic cell bodies: Located in the brainstem and sacral spinal cord (craniosacral outflow).

ANS Fiber Pathways

• SNS fibers: Originate from thoracolumbar spinal cord (T1-L2); short preganglionic, long postganglionic fibers.

• PSNS fibers: Originate from craniosacral regions; long preganglionic, short postganglionic fibers.

Pathways and Neurotransmitters

• Typical SNS pathway: Preganglionic neuron (ACh) → autonomic ganglion → postganglionic neuron (NE) → effector organ.

• Typical PSNS pathway: Preganglionic neuron (ACh) → autonomic ganglion → postganglionic neuron (ACh) → effector organ.

Cholinergic and Adrenergic Receptors

• Cholinergic receptors: Bind acetylcholine (ACh). Types: nicotinic and muscarinic.

• Adrenergic receptors: Bind norepinephrine (NE) or epinephrine. Types: alpha (α) and beta (β).

Dual Innervation

Most organs receive input from both SNS and PSNS, allowing precise regulation. Some tissues (e.g., sweat glands, most blood vessels) receive only SNS innervation.

Definition: Innervation

Innervation refers to the supply of nerves to a specific body part, allowing it to receive signals from the nervous system.

Effects of SNS and PSNS Activation

Organ/System	SNS Effect	PSNS Effect
Heart	Increases rate and force	Decreases rate
Airways	Bronchodilation	Bronchoconstriction
Digestive tract	Decreases motility/secretion	Increases motility/secretion
Pupils	Dilation (mydriasis)	Constriction (miosis)
Bladder wall/Sphincter	Relaxes wall, contracts sphincter	Contracts wall, relaxes sphincter
Uterus	Varies (relaxation/contraction)	Minimal effect
Piloerector muscles	Contracts (goosebumps)	No effect
Blood vessels	Constriction (most)	Minimal effect
Sweat glands	Increases secretion	No effect
Other glands	Varies	Stimulates secretion

Removal of Neurotransmitters

• Acetylcholine (ACh): Removed by enzymatic breakdown (acetylcholinesterase).

• Norepinephrine (NE): Removed by reuptake into nerve terminals, diffusion, or enzymatic breakdown (monoamine oxidase, MAO).

Regions of the CNS Influencing the ANS

• Hypothalamus: Main integration center for ANS activity.

• Brainstem: Regulates heart rate, blood pressure, respiration.

• Spinal cord: Mediates reflexes.

Summary Table: Comparison of SNS and PSNS

Feature	Sympathetic (SNS)	Parasympathetic (PSNS)
Origin	Thoracolumbar (T1-L2)	Craniosacral (brainstem, S2-S4)
Preganglionic fiber length	Short	Long
Postganglionic fiber length	Long	Short
Main neurotransmitter (postganglionic)	Norepinephrine (NE)	Acetylcholine (ACh)
General function	"Fight or flight"	"Rest and digest"

Additional info: Some content was expanded for clarity and completeness, including definitions, examples, and summary tables.