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Autonomic Nervous System (ANS)

Overview

  • The Autonomic Nervous System (ANS) consists of motor neurons that innervate:
      - Smooth muscle
      - Cardiac muscle
      - Glands
  • Functions in a subconscious manner to make automatic adjustments that ensure optimal body activity.
  • Also referred to as:
      - Involuntary nervous system
      - General visceral motor system

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Differences Between Somatic and Autonomic Nervous Systems

Motor Fibers

  • Both Somatic and Autonomic systems have motor fibers.

Key Differences

  1. Effectors:
       - Somatic: Skeletal muscle
       - Autonomic: Cardiac muscle, smooth muscle, glands
  2. Efferent Pathways and Ganglia:
       - Somatic: One neuron pathway from CNS to effectors.
       - Autonomic: Two neurons (preganglionic and postganglionic) pathway.
  3. Target Organ Responses to Neurotransmitters:
       - Somatic: Always acetylcholine (ACh), effects are always stimulatory.
       - Autonomic: ACh or norepinephrine (NE), effects can be stimulatory or inhibitory depending on receptor types.

Pathway Structures

Somatic System
  • Origin: Anterior horn of spinal cord
  • Number of Neurons: One
  • Ganglia: No motor ganglia
  • Termination:
      - At neuromuscular junction (NMJ) for skeletal muscle.
Autonomic Pathway
  • Origin:
      - Lateral horn (sympathetic)
      - Craniosacral (parasympathetic)
  • Number of Neurons: Two (preganglionic and postganglionic)
  • Ganglia: One (autonomic motor ganglia)
  • Termination:
      - Involuntary targets (not NMJs) using varicosities.

Efferent Pathways and Ganglia

Somatic Nervous System

  • Originates in the CNS with:
      - A thick, myelinated fiber.
      - Projecting through spinal nerve to skeletal muscle.
      - Also cranial nerves to skeletal muscle.

Autonomic Nervous System

  • Pathway comprises a two-neuron chain:
  1. Preganglionic Neuron:
       - Located in CNS
       - Thin, lightly myelinated axon.
  2. Postganglionic (Ganglionic) Neuron:
       - Located in autonomic ganglia outside CNS.
       - Nonmyelinated axon extends to effector organ.

Autonomic Ganglia

  • Contains cell bodies.
  • Post-ganglionic Fiber:
       - Non-myelinated, appears gray.
  • Pre-ganglionic Fiber:
       - Myelinated, appears white.

Neurotransmitter Effects

Somatic Nervous System

  • All somatic motor neurons release acetylcholine (ACh).
  • Effects are consistently stimulatory.

Autonomic Nervous System (ANS)

  • Preganglionic fibers release ACh.
  • Postganglionic fibers release either norepinephrine or ACh at effectors.
  • The effect can be either stimulatory or inhibitory, based on the type of receptors.
Comparison of Division Responses

  • Parasympathetic:
      - “Rest and Digest”
      - Long preganglionic fiber terminal synapsing and releasing ACh.
      - Short postganglionic fiber also releases ACh.

  • Sympathetic:
      - “Fight or Flight”
      - Short preganglionic fiber synapsing and releasing ACh.
      - Long postganglionic fiber releases NE.

Divisions of the ANS

Major Divisions

  • Sympathetic Division:
       - Mobilizes body in emergencies and physical activity.
  • Parasympathetic Division:
       - Promotes maintenance activities, conserves body energy.

Dual Innervation

  • Most visceral organs are served by both divisions causing opposite effects.
  • Homeostasis: Achieved through dynamic antagonism between two divisions.

Physiological States

Role of Parasympathetic Division
  • Directs:
       - Digestion
       - Diuresis
       - Defecation
  • Physiological State:
      - Low blood pressure, heart and respiratory rates high, pupils constricted, lenses accommodated for close vision.
Role of Sympathetic Division
  • Activates systems during strenuous activities (exercise, emergency); symptoms include:
      - Increased heart rate
      - Dry mouth
      - Cold, sweaty skin
      - Dilated pupils.

Regional Anatomy

Parasympathetic (Craniosacral) Division
  • Long preganglionic fibers from:
      - Brain stem (nerves III, VII, IX, X).
      - Sacrum (S2-S4)
  • Postganglionic Fibers:
      - Short, synapsing right at or within target organs.
Sympathetic (Thoracolumbar) Division
  • Preganglionic neurons in spinal cord segments T1-L2.
  • Form lateral horns of spinal cord.
  • Pass through white rami communicantes to enter sympathetic trunk ganglia.

Sympathetic Pathway Details

Pathways and Ganglia

  • Paravertebral ganglia have varying sizes, positions, and numbers (23 in the sympathetic trunk).
Types of Synapsing in Sympathetic Pathways
  1. Same Level Synapse:
       - Preganglionic fiber synapses with ganglionic neuron.
  2. Ascending or Descending:
       - Preganglionic fiber synapses in another ganglion.
  3. No Synapse:
       - Preganglionic fibers may pass without synapsing, commonly in the abdomen.

Adrenal Gland Interaction

  • The adrenal medulla acts as a ganglion and releases norepinephrine and epinephrine.

Visceral Reflexes

Components

  • Visceral reflex arcs have similar components to somatic reflex arcs but include two neurons.
  • Visceral pain afferents: Travel alongside somatic pain fibers, contributing to referred pain.

Reflex Arc Structure

  • Stimulus triggers receptor in viscera -> Impulse travels through sensory neuron -> Visceral afferent -> Integration center (First synapse) -> Motor neuron cell body in lateral horn (Preganglionic fiber) -> Visceral efferent (Second synapse) -> Post-ganglionic fiber runs to involuntary effector.
  • All are polysynaptic.

Neurotransmitter Types

Cholinergic Fibers

  • Release acetylcholine (ACh) and include:
      - All ANS preganglionic axons
      - All parasympathetic postganglionic axons at the effector.

Adrenergic Fibers

  • Release norepinephrine (NE) and include:
      - Most sympathetic postganglionic axons.
      - Exception: sympathetic postganglionic fibers at sweat glands release ACh.

Cholinergic and Adrenergic Receptors

Cholinergic Receptors

Types
  1. Nicotinic Receptors:
       - Found on:
         - Skeletal muscle cells at NMJ
         - All postganglionic neurons
         - Hormone-producing cells of adrenal medulla
       - Effect: Always stimulatory.
  2. Muscarinic Receptors:
       - Found on all effector cells stimulated by postganglionic cholinergic fibers.
       - Effect: Can be either inhibitory or excitatory dependent on target organ receptor type.

Adrenergic Receptors

Types
  • Alpha Receptors
  • Beta Receptors
  • Effect of NE varies based on which subclass predominates on target organs.

Interactions of Autonomic Divisions

Dual Innervation

  • Most organs have both sympathetic and parasympathetic innervation allowing:
       - Sympathetic: Increases heart rate, respiratory rates; inhibits digestion and elimination.
       - Parasympathetic: Decreases heart rate and respiratory rates; promotes digestion and elimination.

Sympathetic Tone

  • Controls blood pressure even at rest.
  • Vascular innervation is primarily sympathetic.
  • Vasomotor Tone:
       - Maintains vessels in a partial constricted state.
  • Blood Pressure Regulation:
       - Elevated: sympathetic fibers fire rapidly.
       - Decreased: sympathetic fibers fire less rapidly for dilation.

Parasympathetic Tone

  • Usually dominates:
      - Slows the heart
      - Promotes functions in digestive and urinary organs
      - Exceptions found in glands (adrenal and sweat).
  • Drugs blocking parasympathetic control may increase heart rate and cause urinary retention.
Cooperative Effects
  • Notably observed in external genitalia control:
       - Vasodilation by parasympathetic fibers responsible for erections.
       - Sympathetic fibers prompt ejaculation.

Unique Sympathetic Division Roles

Functions

  • Controls the following:
      - Adrenal medulla (releases hormones)
      - Sweat glands
      - Arrector pili muscles
      - Kidneys
      - Most blood vessels
Additional Effects
  • Thermoregulation: Controls heat responses.
  • Renin Release: From kidneys to help regulate blood pressure.
  • Metabolic: Increases metabolic rates of cells, raises blood glucose levels, mobilizes fats for energy.

Response Duration and Localization

Parasympathetic Division

  • Causes short-lived responses; highly localized effect control.
  • ACh is rapidly inactivated by acetylcholinesterase.

Sympathetic Division

  • Longer-lasting responses; effects are systemic.
  • NE inactivated slower than ACh.
  • Hormones from adrenal medulla (NE and epinephrine) prolong effects.

Control of ANS Function

Integration

  • Hypothalamus: Main integrative center of ANS activity.
  • Subconscious Input: From limbic structures (emotional input).
  • Other Controls: From cerebral cortex, reticular formation, and spinal cord.

Homeostatic Imbalances of the ANS

  • Hypertension: Overactivity in sympathetic responses to stress.
  • Raynaud's Disease: Exaggerated vasoconstriction causing pale, cyanotic, and painful extremities.
  • Autonomic Dysreflexia: Uncontrolled activation in quadriplegics or those with spinal cord injuries above T6 can lead to life-threatening high blood pressure.

Additional Learning Resources

  • For further exploration, consider the following videos:
      - YouTube: Crash Course Anatomy: Autonomic Nervous System (about 8 minutes)
      - YouTube: Crash Course Anatomy: Sympathetic Nervous System (about 11 minutes)
      - YouTube: Crash Course Anatomy: Parasympathetic Nervous System (about 10 minutes)