Study Notes: The Autonomic Nervous System (Chapter 15, BSC 2085: Anatomy & Physiology I)

The Autonomic Nervous System (ANS)

General Properties of the Autonomic Nervous System

The autonomic nervous system (ANS), also known as the visceral motor system, is a division of the peripheral nervous system that regulates involuntary physiological processes. It controls glands, cardiac muscle, and smooth muscle, and is essential for maintaining homeostasis.

• Primary target organs: Viscera of thoracic and abdominal cavities, cutaneous blood vessels, sweat glands, and arrector muscles.

• Involuntary control: Actions are carried out without conscious intent or awareness.

• Visceral effectors: Do not depend on the ANS for basic function, but the ANS adjusts their activity to meet the body's needs.

• Denervation hypersensitivity: Severing ANS nerves to smooth/cardiac muscles results in exaggerated responses.

Comparison: Somatic vs. Autonomic Nervous Systems

The efferent division of the nervous system is divided into somatic and autonomic branches, each with distinct roles and mechanisms.

Feature	Somatic Nervous System	Autonomic Nervous System (ANS)
Control	Conscious (voluntary)	Unconscious (involuntary)
Effectors	Skeletal muscles	Cardiac muscle, smooth muscle, glands, adipocytes
Survival Impact	Seldom affects long-term survival	Coordinates organ system functions vital for survival

Visceral Reflexes

Visceral reflexes are unconscious, automatic, and stereotyped responses to stimulation involving visceral receptors and effectors, controlled by the ANS.

• Reflex arc components:

  1. Receptors: Detect stretch, tissue damage, blood chemicals, temperature, and other internal stimuli.

  2. Afferent neurons: Carry signals to the CNS.

  3. Integrating center: Interneurons in the CNS process the information.

  4. Efferent neurons: Carry motor signals away from the CNS.

  5. Effectors: Carry out the end response.

Divisions of the Autonomic Nervous System

The ANS is subdivided into three main divisions: sympathetic, parasympathetic, and enteric.

Sympathetic Division

• Function: Prepares the body for physical activity (fight-or-flight response).

• Effects: Increases heart rate, blood pressure, airflow, blood glucose levels; reduces blood flow to skin and digestive tract.

Parasympathetic Division

• Function: Calms body functions, reduces energy expenditure, assists in bodily maintenance (rest-and-digest state).

• Effects: Promotes digestion and waste elimination.

Autonomic Tone

• Definition: The normal background rate of activity representing the balance of sympathetic and parasympathetic systems.

• Examples:

  • Parasympathetic tone maintains smooth muscle tone in intestines and holds resting heart rate down to about 70–80 beats per minute.

  • Sympathetic tone keeps most blood vessels partially constricted and maintains blood pressure.

• Opposing effects: Neither division is universally excitatory or inhibitory; effects depend on the organ and context.

Autonomic Output Pathways

The ANS has components in both the central and peripheral nervous systems, including control nuclei in the hypothalamus and brainstem, motor neurons in the spinal cord and peripheral ganglia, and nerve fibers traveling through cranial and spinal nerves.

Pathway	Somatic Motor	Autonomic Motor
Number of Neurons	One (from CNS to effector)	Two (preganglionic and postganglionic)
Synapse Location	No ganglion; direct to muscle	Autonomic ganglion between neurons
Neurotransmitters	ACh	ACh (preganglionic), ACh or NE (postganglionic)

The Sympathetic Division

Anatomy and Pathways

The sympathetic division is also called the thoracolumbar division because its pathways arise from the thoracic and lumbar regions of the spinal cord.

• Preganglionic neurons: Located in lateral horns of spinal cord segments T1–L2.

• Sympathetic chain ganglia (paravertebral ganglia): Series of ganglia adjacent to both sides of the vertebral column from cervical to coccygeal levels.

• Fiber lengths: Short preganglionic, long postganglionic (except at adrenal medulla).

• Distribution: Sympathetic nerve fibers reach every level of the body.

Responses to Increased Sympathetic Activity

1. Heightened mental alertness

2. Increased metabolic rate

3. Reduced digestive and urinary functions

4. Activation of energy reserves

5. Increased respiratory rate and dilation of respiratory passageways

6. Increased heart rate and blood pressure

7. Activation of sweat glands

Locations of Sympathetic Ganglionic Neurons

1. Sympathetic chain ganglia: On both sides of vertebral column; control effectors in body wall, thoracic cavity, head, and limbs.

2. Collateral ganglia: Anterior to vertebral bodies; innervate tissues and organs in abdominopelvic cavity.

3. Adrenal medullae: Endocrine cells innervated by preganglionic axons; release hormones into bloodstream.

Adrenal (Suprarenal) Glands

• Adrenal cortex: Outer layer; secretes steroid hormones.

• Adrenal medulla: Inner core; modified postganglionic neurons; secretes catecholamines (85% epinephrine, 15% norepinephrine) into the bloodstream.

Sympathetic Division Neurotransmitters

• Preganglionic neurons: Release acetylcholine (ACh) at synapses with ganglionic neurons (always excitatory).

• Postganglionic neurons: Mostly release norepinephrine (NE); some release ACh (cholinergic neurons).

• Varicosities: Axon terminals form branching networks (telodendria) with swollen segments packed with neurotransmitter vesicles, passing along or near effector cells.

Sympathetic Activation

• Global response: Activates entire division during crisis.

• Mechanisms: Releases NE at peripheral synapses, distributes E and NE via bloodstream, controlled by hypothalamic centers.

• Effects: Increased alertness, energy, changes in breathing, elevation in muscle tone, mobilization of energy reserves.

The Parasympathetic Division

Anatomy and Pathways

The parasympathetic division is also called the craniosacral division, with pathways arising from the brain and sacral regions of the spinal cord.

• Preganglionic neurons: Located in midbrain, pons, medulla (cranial nerves III, VII, IX, X) and sacral spinal cord segments S2–S4.

• Fiber lengths: Long preganglionic, short postganglionic.

• Terminal ganglia: Preganglionic fibers end in ganglia near or within target organs (intramural ganglia).

• Vagus nerve: Provides 75% of all parasympathetic outflow, innervating neck, thoracic, and abdominopelvic structures.

Neuromuscular and Neuroglandular Junctions

• Synaptic clefts: Small and narrow.

• Neurotransmitter: All preganglionic and postganglionic neurons release ACh.

• Duration: Effects are short-lived due to rapid inactivation by acetylcholinesterase (AChE) and tissue cholinesterase.

Responses to Increased Parasympathetic Activity

1. Decreased metabolic rate

2. Decreased heart rate and blood pressure

3. Increased secretion by salivary and digestive glands

4. Increased motility and blood flow in digestive tract

5. Stimulation of urination and defecation

Summary Table: Sympathetic vs. Parasympathetic Divisions

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (T1–L2)	Craniosacral (brainstem, S2–S4)
Preganglionic Fiber Length	Short	Long
Postganglionic Fiber Length	Long	Short
Main Neurotransmitter	NE (postganglionic)	ACh (postganglionic)
Main Effects	Fight-or-flight	Rest-and-digest

Key Terms and Definitions

• Autonomic ganglion: A cluster of neuronal cell bodies in the peripheral nervous system where preganglionic and postganglionic neurons synapse.

• Preganglionic fiber: The axon of a neuron that originates in the CNS and synapses in an autonomic ganglion.

• Postganglionic fiber: The axon of a neuron that originates in an autonomic ganglion and synapses on the target organ.

• Varicosity: Swollen segment of an axon terminal containing neurotransmitter vesicles, characteristic of autonomic neurons.

• Acetylcholine (ACh): A neurotransmitter released by all preganglionic neurons and parasympathetic postganglionic neurons.

• Norepinephrine (NE): A neurotransmitter released by most sympathetic postganglionic neurons.

Example: Autonomic Reflex Arc

When blood pressure rises, stretch receptors in the aorta send signals via afferent neurons to the CNS. The integrating center processes the information and sends efferent signals via autonomic neurons to the heart, causing it to slow down and lower blood pressure.

Equations and Mechanisms

• Neurotransmitter release: $\text{Preganglionic neuron} \xrightarrow{ACh} \text{Autonomic ganglion} \xrightarrow{ACh \text{ or } NE} \text{Target organ}$

Additional info:

• The enteric division, though not covered in detail in these slides, is a third division of the ANS and is responsible for the regulation of digestive tract motility and secretion.