CH 14

Chapter 14: The Autonomic Nervous System and Visceral Reflexes

I. General Properties of the Autonomic Nervous System

The autonomic nervous system (ANS) is a motor nervous system that controls glands, cardiac muscle, and smooth muscle. It operates largely without conscious awareness and is essential for maintaining homeostasis.

• Primary Target Organs: Viscera of the thoracic and abdominopelvic cavities, as well as some structures of the body wall (e.g., blood vessels, sweat glands, piloerector muscles).

• Functions: Regulates involuntary effectors such as heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal.

• Visceral Reflexes: Unconscious, automatic, stereotyped responses to stimulation involving receptors, afferent neurons, interneurons, efferent neurons, and effectors.

Example: Baroreceptor reflex helps regulate blood pressure by adjusting heart rate and vessel diameter in response to changes in blood pressure.

II. Anatomy of the Autonomic Nervous System

The ANS consists of two main divisions: the sympathetic and parasympathetic divisions, each with distinct anatomical and functional characteristics.

A. Sympathetic Division (Thoracolumbar Division)

• Originates from the thoracic and lumbar regions (T1–L2) of the spinal cord.

• Preganglionic fibers are short; postganglionic fibers are long.

• Sympathetic ganglia are located near the spinal cord in a chain (sympathetic trunk) or in collateral ganglia.

• Functions: Prepares the body for 'fight or flight' responses (e.g., increases heart rate, dilates bronchioles, inhibits digestion).

B. Parasympathetic Division (Craniosacral Division)

• Originates from the brainstem (cranial nerves III, VII, IX, X) and sacral spinal cord (S2–S4).

• Preganglionic fibers are long; postganglionic fibers are short.

• Parasympathetic ganglia are located near or within target organs.

• Functions: Promotes 'rest and digest' activities (e.g., decreases heart rate, stimulates digestion and glandular activity).

C. Comparison of Sympathetic and Parasympathetic Divisions

Feature	Sympathetic Division	Parasympathetic Division
Origin	Thoracolumbar (T1–L2)	Craniosacral (Brainstem, S2–S4)
Ganglia Location	Near spinal cord (sympathetic trunk/collateral)	Near or within target organs
Preganglionic Fiber Length	Short	Long
Postganglionic Fiber Length	Long	Short
General Function	Fight or flight	Rest and digest

III. Autonomic Effects on Target Organs

The ANS exerts its effects through neurotransmitters and receptors, primarily acetylcholine (ACh) and norepinephrine (NE).

A. Neurotransmitters and Receptors

• Cholinergic Fibers: Release acetylcholine (ACh). Found in all preganglionic neurons and parasympathetic postganglionic neurons.

• Adrenergic Fibers: Release norepinephrine (NE). Found in most sympathetic postganglionic neurons.

• Receptors:

  • Nicotinic Receptors: Found on all postganglionic neurons; always excitatory.

  • Muscarinic Receptors: Found on target cells of parasympathetic fibers; can be excitatory or inhibitory.

  • Adrenergic Receptors: Alpha (α) and beta (β) subtypes; effects depend on receptor type and location.

Example: Sympathetic stimulation of the heart increases heart rate via β1-adrenergic receptors; parasympathetic stimulation decreases heart rate via muscarinic receptors.

B. Dual Innervation and Autonomic Tone

• Most organs receive input from both sympathetic and parasympathetic fibers (dual innervation), which may have antagonistic or cooperative effects.

• Some organs (e.g., blood vessels) receive only sympathetic innervation and are regulated by sympathetic tone.

IV. Central Control of Autonomic Function

Although the ANS functions largely independently, it is influenced by higher brain centers, especially the hypothalamus, brainstem, and spinal cord.

• Hypothalamus: Major control center for visceral motor system; integrates input from the limbic system and other regions.

• Brainstem: Contains centers for cardiac, vasomotor, and digestive functions.

• Spinal Cord: Mediates autonomic reflexes such as urination, defecation, erection, and ejaculation.

Example: Emotional responses (e.g., fear, anxiety) can influence autonomic output via the limbic system and hypothalamus.

Key Terms and Concepts

• Visceral Reflex: An automatic response to a stimulus involving visceral effectors.

• Preganglionic Neuron: The first neuron in the autonomic pathway; cell body in CNS.

• Postganglionic Neuron: The second neuron; cell body in autonomic ganglion, axon extends to target organ.

• Autonomic Tone: The background rate of activity in the sympathetic and parasympathetic divisions.

Summary Table: Neurotransmitters and Receptors in the ANS

Division	Preganglionic Neurotransmitter	Postganglionic Neurotransmitter	Receptors on Target Organs
Sympathetic	Acetylcholine (ACh)	Norepinephrine (NE) or ACh (sweat glands)	Adrenergic (α, β), Muscarinic (sweat glands)
Parasympathetic	Acetylcholine (ACh)	Acetylcholine (ACh)	Muscarinic

Key Equations

• Blood Pressure Regulation (Baroreceptor Reflex):

$\text{Blood Pressure} = \text{Cardiac Output} \times \text{Peripheral Resistance}$

Additional info: The notes have been expanded with definitions, examples, and summary tables for clarity and completeness.