Autonomic Nervous System

Autonomic Nervous System Overview

Introduction to the ANS

  • Definition: The Autonomic Nervous System (ANS) is a system of motor neurons that innervates smooth muscle, cardiac muscle, and glands to ensure optimal body function.
  • Functions:
    • Regulates heart rate, blood pressure, digestive processes, etc.
    • Operates largely subconsciously; referred to as the involuntary nervous system or general visceral motor system.

Differences Between ANS & SNS

Motor Fibers
  • Effectors:
    • SNS: Innervates skeletal muscles.
    • ANS: Innervates cardiac, smooth muscle, and glands.
Efferent Pathways and Ganglia
  • SNS:
    • Single, thick myelinated axon directly to muscle (from CNS).
  • ANS:
    • Two-neuron chain:
    • Preganglionic neuron: Cell body in CNS, thin lightly myelinated axon.
    • Postganglionic neuron: Nonmyelinated axon synapses at an autonomic ganglion.
Target Organ Responses
  • SNS: All motor neurons release ACh; effect is always stimulatory.
  • ANS:
    • Preganglionic fibers release ACh.
    • Postganglionic fibers release either norepinephrine or ACh; effects can be stimulatory or inhibitory depending on receptor type.

Overlapping of ANS & SNS

  • Regulate and coordinate between both systems.
  • Many spinal and cranial nerves have both somatic and autonomic fibers.
  • Adaptations involve both systems; e.g., increased heart rate and bronchodilation during muscle activity.

Divisions of the ANS

Two Major Divisions
  • Parasympathetic Division: Promotes maintenance and conserves energy.
  • Sympathetic Division: Mobilizes body during activity.
  • Both divisions innervate all visceral organs, causing opposing effects for homeostasis (dynamic antagonism).

Roles of the ANS Divisions

Parasympathetic Division
  • Function: Conserves energy, directs digestion, and maintains low body energy use.
  • Known as the rest-and-digest system; e.g., low heart rate and high gastrointestinal activity during relaxation.
Sympathetic Division
  • Function: Prepares the body for activity or emergencies (fight-or-flight).
  • Increases heart rate, dilates bronchioles, and mobilizes energy.

Anatomical Differences in Divisions

Sympathetic vs. Parasympathetic

  1. Sites of Origin:

    • Parasympathetic: Craniosacral (brain and sacral spinal cord).
    • Sympathetic: Thoracolumbar (thoracic and lumbar regions).

  2. Fiber Lengths:

    • Parasympathetic: Long preganglionic, short postganglionic fibers.
    • Sympathetic: Short preganglionic, long postganglionic fibers.

  3. Ganglia Location:

    • Parasympathetic: Located near or within the target organ.
    • Sympathetic: Close to the spinal cord (sympathetic trunk or collateral ganglia).
Comparison of Fiber Lengths and Ganglia Locations
CharacteristicParasympatheticSympathetic
OriginCraniosacralThoracolumbar
GangliaNear organsClose to CNS
Fiber LengthLong pre, short postShort pre, long post
Functional RoleRest and DigestFight or Flight

Neural Pathways in ANS

Parasympathetic Pathways
  • Cranial origin (Oculomotor, Facial, Glossopharyngeal, Vagus) provides a majority of fibers.
  • Sacral origin (S2-S4) targets pelvic organs.
Sympathetic Pathways
  • Preganglionic fibers may:
    • Synapse in the trunk ganglia.
    • Ascend/descend the sympathetic trunk.
    • Synapse in collateral ganglia.

Visceral Reflexes

  • Visceral reflex arcs consist of:
    1. Receptor: In viscera.
    2. Sensory neuron: Conveys information to CNS.
    3. Integration center: Consists of two neurons in the motor pathway.
    4. Motor neuron: Preganglionic and postganglionic nerves.
    5. Effector: Smooth/cardiac muscle or glands.

Neurotransmitters of the ANS

  • Major Neurotransmitters: ACh and NE.
    • ACh is released by cholinergic fibers (all preganglionic fibers).
    • NE released by adrenergic fibers (almost all sympathetic postganglionic fibers).
Receptor Types
Cholinergic Receptors
  • Nicotinic: Stimulatory, found on all postganglionic neurons.
  • Muscarinic: Could be excitatory or inhibitory depending on target organ (example: heart rate vs. intestinal motility).
Adrenergic Receptors
  • Alpha (α) and Beta (β) receptors with different effects based on subtype and target organ.

Control of ANS Function

  • CNS Centers:
    • Brain stem: Regulates autonomic functions such as heart rate and gastrointestinal activity.
    • Hypothalamus: Integrative center for autonomic activity.
    • Cerebral Cortex: Can exert voluntary control over some autonomic functions.

Disorders of ANS

  • Common Disorders:
    • Hypertension: Overactive sympathetic tone; manage with adrenergic blockers.
    • Raynaud’s Disease: Exaggerated vasoconstriction; treated with vasodilators.
    • Autonomic Dysreflexia: Dangerous autonomic response in spinal cord injury, requiring immediate treatment.

Developmental Aspects

  • ANS structures derive from the neural tube and neural crest during embryonic development.
  • Efficiency declines with age, affecting organ systems (e.g., decreased gastrointestinal motility, blood pressure regulation issues).