Nurs 1002-Obj #14

Chapter 14: The Autonomic Nervous System

Overview of the Autonomic Nervous System (ANS)

The autonomic nervous system (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes, including those of smooth muscle, cardiac muscle, and glands. It operates largely below the level of consciousness to maintain homeostasis and respond to stress.

• Motor neurons innervate smooth muscle, cardiac muscle, and glands.

• Adjusts body functions such as heart rate, blood pressure, and digestion to support activity and rest.

• Functions via subconscious control.

• Also known as the involuntary nervous system or visceral (autonomic) motor system.

Sympathetic and Parasympathetic Nerve Pathways

The ANS is divided into two main branches: the sympathetic and parasympathetic divisions, which often have opposing effects on target organs.

• Parasympathetic division: Promotes maintenance functions and conserves energy ("rest and digest").

• Sympathetic division: Mobilizes the body during activity ("fight or flight").

• Dual innervation: Most visceral organs receive input from both divisions, allowing dynamic antagonism to maintain homeostasis.

• Dynamic antagonism:

  • Sympathetic division increases heart and respiratory rates, inhibits digestion and elimination.

  • Parasympathetic division decreases heart and respiratory rates, allows for digestion and discarding of wastes.

Visceral Effects of Sympathetic and Parasympathetic Innervation

Each division of the ANS has distinct effects on the body’s organs and tissues, depending on the situation and physiological needs.

Parasympathetic Division

• Keeps body energy use as low as possible during maintenance activities.

• Directs digestion, diuresis, and defecation.

• Referred to as the "rest-and-digest" system.

• Example: After a meal, blood pressure, heart rate, and respiratory rates are low; gastrointestinal activity is high; pupils are constricted.

Sympathetic Division

• Mobilizes body during activity, especially in stressful situations.

• Referred to as the "fight-or-flight" system.

• Exercise, excitement, emergency, and embarrassment activate the sympathetic system.

• Example: During vigorous activity, heart rate increases, skin becomes cold and sweaty, pupils dilate, blood is shunted to skeletal muscles and heart, bronchioles dilate, and the liver releases glucose.

Key Anatomical Differences Between Divisions

• Sites of origin:

  • Parasympathetic fibers are craniosacral (originate in brain and sacral spinal cord).

  • Sympathetic fibers are thoracolumbar (originate in thoracic and lumbar regions of spinal cord).

• Relative lengths of fibers:

  • Parasympathetic: long preganglionic, short postganglionic fibers.

  • Sympathetic: short preganglionic, long postganglionic fibers.

• Location of ganglia:

  • Parasympathetic ganglia are located in or near the visceral effectors.

  • Sympathetic ganglia lie close to the spinal cord.

Parasympathetic Pathways

• Cell bodies are located in the brain stem.

• Preganglionic fibers run in cranial nerves:

  • Oculomotor (III)

  • Facial (VII)

  • Glossopharyngeal (IX)

  • Vagus (X)

• Sacral nerves innervate pelvic organs (intestine, urinary bladder, ureters, reproductive organs).

Sympathetic Pathways

• More complex, innervates more organs than parasympathetic.

• Some structures (e.g., sweat glands, arrector pili muscles, blood vessel smooth muscle) are innervated only by sympathetic fibers.

• Sympathetic division is also called the thoracolumbar division.

• Typically, there are 23 sympathetic trunk ganglia:

  • 3 cervical

  • 11 thoracic

  • 4 lumbar

  • 4 sacral

  • 1 coccygeal

• Preganglionic axons may synapse:

  • At the same level in the trunk ganglion

  • At a higher or lower level in another trunk ganglion

  • In a collateral ganglion (abdomen and pelvis, forming splanchnic nerves)

Receptors and Neurotransmitters of the Two Divisions

The ANS and somatic nervous system differ in their effectors, number of neurons in the motor pathway, and neurotransmitter actions.

• Effectors:

  • Somatic: innervates skeletal muscles

  • ANS: innervates cardiac muscle, smooth muscle, and glands

• Motor pathway:

  • Somatic: single neuron from CNS to effector

  • ANS: two-neuron chain (preganglionic and postganglionic neurons)

Neurotransmitters

• Somatic nervous system: All motor neurons release acetylcholine (ACh) (always excitatory).

• ANS:

  • Preganglionic fibers release ACh

  • Postganglionic fibers release:

    • Norepinephrine (NE) at most sympathetic fibers

    • ACh at most parasympathetic fibers

  • Effect can be excitatory or inhibitory, depending on receptor type.

Visceral Reflex Arcs

• All reflex arcs begin with sensory input.

• Visceral reflex arcs receive information from visceral sensory neurons.

• Components: receptor, sensory neuron, integration center, motor neuron, effector.

• Differences from somatic reflex arcs:

  • Visceral reflex arcs have two consecutive neurons in the motor pathway.

  • Effectors are smooth muscle, cardiac muscle, and glands.

Examples of Visceral Reflexes

• Reflexes that empty the rectum and bladder.

• Three-neuron reflex arcs exist in the walls of the gastrointestinal tract.

• Involve the enteric nervous system (sensory neurons, interneurons, motor neurons).

Sympathetic Tone (Vasomotor Tone)

• Most blood vessel smooth muscle is innervated only by sympathetic fibers.

• Sympathetic tone: Continual state of partial constriction of blood vessels.

• If blood pressure drops, sympathetic fibers (vasomotor fibers) fire faster to increase constriction and raise blood pressure.

• If blood pressure rises, vasomotor fibers fire less, causing dilation and lowering blood pressure.

• Allows the sympathetic system to shunt blood where needed.

Table: Comparison of Sympathetic and Parasympathetic Divisions

Feature	Parasympathetic Division	Sympathetic Division
Site of Origin	Craniosacral (brain and sacral spinal cord)	Thoracolumbar (thoracic and lumbar spinal cord)
Length of Fibers	Long preganglionic, short postganglionic	Short preganglionic, long postganglionic
Location of Ganglia	In or near visceral effectors	Close to spinal cord
Main Neurotransmitter (Postganglionic)	Acetylcholine (ACh)	Norepinephrine (NE)
Main Function	Rest and digest	Fight or flight

Key Equations

• Blood Pressure Regulation (Vasomotor Tone):

$\text{Blood Pressure} \propto \text{Vasomotor Fiber Firing Rate}$

Additional info: The ANS is essential for maintaining homeostasis and responding to internal and external stimuli. Understanding its divisions, pathways, neurotransmitters, and effects is crucial for comprehending how the body regulates involuntary functions.