Unit #11 Chapter 14 - Autonomic Nervous System

Autonomic Nervous System

Learning Outcome

  • Define autonomic nervous system and explain its relationship to the peripheral nervous system.

    • Definition: The autonomic nervous system (ANS) is also known as the involuntary motor or visceral nervous system. It comprises two main divisions: the sympathetic and parasympathetic divisions.

    • Function: The ANS innervates the smooth muscle, cardiac muscle, and glands within the body.

    • Difference from Somatic Nervous System: The ANS can either stimulate or inhibit its effectors.

Comparison: Somatic and Autonomic Nervous Systems

Learning Outcome

  • Compare the somatic and autonomic nervous systems relative to effectors, efferent pathways, and neurotransmitters released. A. Effectors

    • Somatic Nervous System:

    • Effectors include skeletal muscles.

    • Autonomic Nervous System:

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

B. Efferent Pathways and Ganglia

  1. Somatic Nervous System:

    • Neuron cell bodies reside in the spinal cord.

    • Axons extend directly to the skeletal muscles they innervate.

  2. Autonomic Nervous System:

    • The structure consists of a two-neuron chain:

      • Preganglionic Neuron: Cell body in spinal cord.

      • Postganglionic Neuron: Cell body in autonomic ganglion outside the CNS.

C. Neurotransmitter Effects

  1. Somatic Motor Neurons release the neurotransmitter acetylcholine; has an excitatory effect.

  2. Autonomic Nervous System can release:

    • Epinephrine

    • Acetylcholine

    • Effects may be either excitatory or inhibitory.

D. Regulation

  • Higher brain centers manage and coordinate both somatic and autonomic systems, facilitating cooperation between skeletal muscle and visceral organs.

Divisions of the Autonomic Nervous System

Learning Outcome

  • Compare and contrast the functions of the parasympathetic and sympathetic divisions. A. Parasympathetic Division

    • Known as the “rest and digest” system.

    • Functions to maintain low energy usage while promoting digestion and the elimination of feces and urine.

B. Sympathetic Division

  • Known as the “fight or flight” system.

  • Prepares the body for potential threats to homeostasis by:

    • Promoting adjustments in cardiovascular and respiratory systems.

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

    • Inhibiting non-essential functions like gastrointestinal motility.

C. Differences Between Divisions

  • Variations in site of origin, fiber lengths, and ganglia location.

Parasympathetic Division Structure

Learning Outcome

  • For the parasympathetic division, describe the site of CNS origin, locations of ganglia, and general fiber pathways. A. Craniosacral Division

    • Preganglionic fibers arise from spinal cord segments in the brain stem and sacral regions.

B. Preganglionic Pathways

  • Axons extend almost entirely to the target structures before synapsing with postganglionic neurons located in terminal ganglia.

Cranial Part of Parasympathetic Division

  1. Oculomotor nerve fibers control pupil constriction and lens shape.

  2. Facial nerve fibers target:

    • Nasal glands

    • Lacrimal glands

    • Submandibular and sublingual salivary glands.

  3. Glossopharyngeal fibers stimulate parotid salivary glands.

  4. Vagus nerve fibers innervate:

    • Cardiac plexus (heart)

    • Pulmonary plexus (lungs)

    • Esophageal plexus (esophagus and stomach).

Sacral Part of Parasympathetic Division

  • Serves the distal half of the large intestine and pelvic organs originating from neurons in lateral gray matter of spinal cord segments S2–S4.

Sympathetic Division Structure

Learning Outcome

  • For the sympathetic division, describe the site of CNS origin, locations of ganglia, and general fiber pathways. A. Thoracolumbar Division

    • Preganglionic fibers originate in spinal segments T1–L2.

    • Lateral horns contain cell bodies of sympathetic preganglionic neurons.

    • No lateral horns exist for parasympathetic neurons due to their lower quantity.

B. Pathway Characteristics

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

    1. Sympathetic trunk consists of 23 ganglia (3 cervical, 11 thoracic, 4 lumbar, 4 sacral, 1 coccygeal).

    2. Preganglionic fibers arise solely from segments T1 to L2, despite sympathetic trunks spanning neck to pelvis.

    3. Preganglionic axons can:

      • Synapse in the same trunk ganglion.

      • Travel to another trunk ganglion to synapse.

      • Leave the trunk to form splanchnic nerves for synapsing in collateral ganglia.

C. Sympathetic Ganglia Location

  • Sympathetic ganglia are positioned near the spinal cord with long postganglionic neurons.

D. Gray Rami Communicantes

  • Conduct postganglionic axons into adjacent spinal nerves, directing postganglionic neurons to effector cells.

E. Pathways to Different Regions

  1. Head: Sympathetic neurons from T1–T4 serve skin, blood vessels, iris of the eye, nasal and salivary glands, upper eyelids, and the heart.

  2. Thorax: Sympathetic neurons from T1–T6 navigate through pulmonary, esophageal, and cardiac plexuses.

  3. Abdomen: Neurons from T5–L2 anticipate serving the stomach, most intestines, liver, spleen, and kidneys.

  4. Pelvic Organs: Neurons originate from T10–L2, innervating the latter half of the large intestine, urinary bladder, and reproductive organs.

  5. Thoracic Splanchnic Nerves: Travel to the adrenal medulla after departing ganglia without synapsing, inducing secretion of epinephrine and norepinephrine.

Visceral Reflex Arcs

Learning Outcome

  • Compare visceral reflexes to somatic reflexes. A. Differences in Components

    • Visceral reflex arcs consist of two consecutive neurons in their motor components, contrasting with somatic motor reflex arcs.

    • Afferent fibers in visceral reflexes are visceral sensory neurons, which relay information regarding chemical changes, stretch, and irritation within the viscera.

Major Neurotransmitters of ANS

Learning Outcomes

  • Define cholinergic and adrenergic fibers, and list receptor types

  • Describe clinical implications of adrenergic and cholinergic drugs.
    A. Cholinergic Receptors

    1. Nicotinic Receptors:

    • Found on all postganglionic neurons, adrenal medulla hormone-producing cells, and skeletal muscle cells at neuromuscular junctions.

    • Bind acetylcholine.

    • Always exert excitatory effects.

    1. Muscarinic Receptors:

    • Located on all parasympathetic target organs and some sympathetic targets like eccrine sweat glands.

    • Bind acetylcholine with varying effects of either excitatory or inhibitory nature.

B. Adrenergic Receptors

  1. Characterized into two classes: alpha (α) and beta (β).

  2. Bind norepinephrine with responses that may be either excitatory or inhibitory.

Sympathetic and Parasympathetic Effects on Organs

Learning Outcome

  • State effects on heart, blood vessels, gastrointestinal tract, lungs, adrenal medulla, and external genitalia. A. Dual Innervation

    • Most visceral organs receive input from both divisions of the ANS, creating dynamic antagonism for precise control.

B. Sympathetic and Parasympathetic Tone

  1. Sympathetic tone regulates systemic blood pressure through vessel diameter control.

  2. Parasympathetic tone is dominant in heart and digestive systems, stabilizing normal homeostatic functions unless stressors necessitate sympathetic override.

C. Cooperative Effects

  • Sympathetic and parasympathetic systems collaborate during sexual excitement and release.

D. Unique Sympathetic Roles

  • Contributes to thermoregulation, controls renin release from kidneys, and modulates metabolic rate.

Autonomic Nervous System Control

Learning Outcome

  • Describe how ANS functions are controlled. A. Brain Stem Influence

    • Directly influences autonomic functions.

B. Hypothalamus Control

  • Main integration center for coordinating ANS activity.

C. Cortical Control

  • Possible voluntary control over autonomic processes.

D. Biofeedback Potential

  • Biofeedback training may allow individuals to modify certain involuntary functions.

ANS Disorders

Learning Outcome

  • Explain specific disorders' relationships to autonomic function abnormalities. A. Hypertension

    • Can stem from excessive sympathetic vasoconstrictor response, often linked to chronic stress.

B. Raynaud’s Disease

  • Characterized by episodes where fingers and toes become pale, cyanotic, and painfully symptomatic.

C. Autonomic Dysreflexia

  • Life-threatening condition with uncontrolled activation of both somatic and autonomic motor neurons.

Developmental Aspects of the ANS

A. Embryonic Development

  1. ANS preganglionic and somatic motor neurons originate from the embryonic neural tube.

  2. ANS structures in PNS (postganglionic neurons, adrenal medulla, autonomic ganglia) come from the neural crest.

  3. Nerve Growth Factor (NGF): Protein that directs axon growth toward target cells.

B. Youth Dysfunction

  • Often results from spinal cord injuries or damage to autonomic nerves.

C. Aging Effects

  • ANS efficiency declines with age due to structural changes in preganglionic axon terminals, leading to issues such as:

    • Constipation

    • Dry eyes and frequent eye infections

    • Orthostatic hypotension.

Cross References

  • Additional information can be found in the following chapters:

    1. Chapter 3: Membrane functions; membrane receptors.

    2. Chapter 4: Nervous tissue.

    3. Chapter 5: Sympathetic control of sweat glands.

    4. Chapter 11: Membrane potentials; neuronal integration; serial and parallel processing; synapses; neurotransmitters.

    5. Chapter 12: Ascending and descending tracts of spinal cord; spinal roots; gray and white matter.

    6. Chapter 16: Sympathetic control of adrenal medulla.

    7. Chapter 18: Role of sympathetic and parasympathetic pathways in cardiac rate control.

    8. Chapter 19: Sympathetic control of blood vessel diameter.

    9. Chapter 20: Nervous control of lymphoid organs.

    10. Chapter 22: Neural control of bronchoconstriction.

    11. Chapter 23: Sympathetic and parasympathetic control of digestive processes.

    12. Chapter 25: Sympathetic kidney blood vessel control; parasympathetic pelvic splanchnic nerves in urinary functions.

    13. Chapter 27: Human sexual response controlled by sympathetic and parasympathetic pathways.