The Autonomic Nervous System

The Autonomic Nervous System

1. Overview of the Autonomic Nervous System (ANS)

  • Definition: The autonomic nervous system is often referred to as the involuntary motor or visceral nervous system.

  • Components: It comprises two main divisions: the sympathetic and parasympathetic divisions.

  • Functions: The ANS innervates smooth muscle, cardiac muscle, and glands in the body.

2. Comparison between Somatic and Autonomic Nervous Systems

A. Effectors

  • Somatic Nervous System: The primary effectors are skeletal muscles.

  • Autonomic Nervous System: It innervates cardiac and smooth muscles as well as glands.

B. Efferent Pathways and Ganglia

  1. Somatic Nervous System:

    • Neurons have their cell bodies located in the spinal cord.

    • Axons extend directly to skeletal muscles they affect.

  2. Autonomic Nervous System:

    • Composed of a two-neuron chain:

      • The first neuron, known as the preganglionic neuron, has its cell body in the spinal cord and synapses with a second neuron, the postganglionic neuron, located in an autonomic ganglion outside the central nervous system (CNS).

C. Neurotransmitter Effects

  1. Somatic Motor Neurons:

    • Release acetylcholine (ACh), which always has an excitatory effect.

  2. ANS:

    • Can release epinephrine and acetylcholine, which may have either excitatory or inhibitory effects depending on the receptor and target.

D. Regulation and Coordination

  • Higher brain centers regulate and coordinate both the somatic and autonomic nervous systems, ensuring cooperation between skeletal muscle functions and visceral organ activities.

3. Divisions of the ANS

A. Parasympathetic Division

  • Known as the "rest-and-digest" system.

    • Functions:

    • Minimizes energy consumption.

    • Encourages digestion and waste elimination.

B. Sympathetic Division

  • Known as the "fight-or-flight" system.

    • Functions:

    • Prepares the body for stress and potential threats to homeostasis.

    • Promotes:

      • Adjustments in the cardiovascular and respiratory systems.

      • Sweat production, pupil dilation, and glucose release from the liver.

    • Inhibits nonessential activities such as gastrointestinal motility.

C. Differences in Sympathetic and Parasympathetic Divisions

  • Variations in sites of origin, relative lengths of fibers, and locations of ganglia.

4. Anatomy of the Parasympathetic Division

A. CNS Origin and Fiber Pathways

  • Also known as the craniosacral division.

    • Origin: Preganglionic fibers arise from the spinal cord segments in the brainstem and sacral regions (S2–S4).

    • Preganglionic axons extend toward the innervated structures, synapsing with postganglionic neurons in terminal ganglia.

B. Cranial Part of the Parasympathetic Division

  1. Oculomotor Nerve: Controls pupil constriction and lens shape.

  2. Facial Nerve: Activates nasal glands, lacrimal glands, submandibular, and sublingual salivary glands.

  3. Glossopharyngeal Nerve: Activates parotid salivary glands.

  4. Vagus Nerve: Supplies the heart (cardiac plexus), lungs (pulmonary plexus), and esophagus/stomach (esophageal plexus).

C. Sacral Part of the Parasympathetic Division

  • Innervates the distal half of the large intestine and pelvic organs, arising from the lateral gray matter of spinal cord segments S2–S4.

5. Anatomy of the Sympathetic Division

A. CNS Origin and Fiber Pathways

  • Known as the thoracolumbar division.

    • Origin: Preganglionic fibers originate from T1–L2 segments of the spinal cord.

    • Preganglionic body cells found in the lateral horns of the spinal cord, absent in parasympathetic areas.

B. Sympathetic Pathways

  • Exit Pathways:

    • Preganglionic fibers exit via the ventral root to join the sympathetic trunk ganglion along the vertebral column through white rami communicantes.

    • Ganglia Composition:

    • 23 ganglia in each sympathetic trunk:

      • 3 cervical

      • 11 thoracic

      • 4 lumbar

      • 4 sacral

      • 1 coccygeal

C. Pathway Characteristics

  1. Preganglionic axons can:

    • Synapse with postganglionic neurons in the same ganglion.

    • Travel to another ganglion to synapse.

    • Leave the trunk to form splanchnic nerves which synapse in collateral ganglia.

  2. Sympathetic ganglia are located close to the spinal cord, characterized by long postganglionic neurons.

  3. Gray Rami Communicates: Conduct postganglionic axons into nearby spinal nerves for directing neurons to their effector cells.

D. Pathways Serving Various Regions

  1. Head: Sympathetic pathways arise from T1–T4 and serve skin/blood vessels, iris, nasal/salivary glands, upper eyelid, heart.

  2. Thorax: Arise from T1–T6, interlinking pulmonary, esophageal, and cardiac plexuses.

  3. Abdomen: Pathways exit spinal segments T5–L2 and serve organs such as stomach, intestines, liver, spleen, kidneys.

  4. Pelvic Organs: Innervated by fibers originating from T10–L2, targeting large intestine, urinary bladder, reproductive organs.

  5. Adrenal Medulla: Thoracic splanchnic nerves travel to the adrenal medulla promoting release of epinephrine and norepinephrine.

6. Visceral Reflex Arcs

A. Components Comparison

  • Visceral reflex arcs share similar components with somatic reflex arcs but feature:

    • Two consecutive neurons in the motor components.

    • Visceral sensory neurons as part of afferent fibers, providing information on chemical changes, stretch, and irritation in the viscera.

7. Major Neurotransmitters of the ANS

A. Cholinergic Receptors

  1. Nicotinic Receptors:

    • Location: Found on all postganglionic neurons, hormone-producing cells of adrenal medulla, skeletal muscle cells at neuromuscular junction.

    • Functionality: Bind acetylcholine, always producing an excitatory effect.

  2. Muscarinic Receptors:

    • Location: Found on all parasympathetic target organs and some sympathetic targets (e.g., eccrine sweat glands).

    • Functionality: Bind acetylcholine, can yield excitatory or inhibitory responses.

B. Adrenergic Receptors

  1. Classes: Divided into alpha (α) and beta (β) receptors.

  2. Functionality: Bind norepinephrine to produce either excitatory or inhibitory responses.

8. Opposing Effects of the ANS Divisions

A. Dual Innervation

  • Most visceral organs are governed by both ANS divisions, creating a system of dynamic antagonism for precise control of visceral activities.

B. Sympathetic and Parasympathetic Tone

  1. Sympathetic Tone:

    • Maintains blood vessel diameter and regulates systemic blood pressure through firing rates of sympathetic neurons.

  2. Parasympathetic Tone:

    • Dominant in organs like the heart, digestive system, and urinary tract; maintains homeostatic levels unless overridden by sympathetic responses during stress.

C. Cooperative Effects

  • Both divisions cooperate in the genitalia during sexual excitement and orgasm.

D. Unique Roles of the Sympathetic Division

  1. Controls thermoregulation, renin release from kidneys, and influences metabolic rate.

9. Regulatory Centers for the ANS

A. Brain Stem and Hypothalamus

  • The brain stem directly influences many autonomic functions.

  • The hypothalamus serves as the main integration center for the ANS.

B. Cortical Control

  • Potential for voluntary control over some autonomic functions is possible.

C. Biofeedback Training

  • May enable individuals to modify certain involuntary physiological functions.