Unit 5.4

Unit 5: Lesson 4 - Autonomic Nervous System (NURS 106)

Overview of the Nervous System

  • Central Nervous System (CNS): Comprises the brain and spinal cord.
  • Peripheral Nervous System (PNS): Includes sensory information from outside the CNS and the enteric nervous system (ENS).
  • Information Processing:
    • Integrates, processes, and coordinates sensory input and motor commands.
  • Motor Commands:
    • Efferent division can be divided into:
    • Somatic Nervous System (SNS): Controls voluntary skeletal muscles.
    • Autonomic Nervous System (ANS): Controls involuntary actions.

Introduction to the Autonomic Nervous System (ANS)

  • Functions outside of conscious awareness.
  • Involuntary Control: Manages major body functions relevant for homeostasis, including:
    • Cardiovascular system
    • Respiratory system
    • Digestive system (involving smooth muscle)
    • Endocrine function (glands)
    • Urinary function
    • Reproductive system
    • Electrolyte, water, and gas concentrations
    • Adipose tissue concerning energy storage.
  • Directed by the Hypothalamus:
    • Regulates autonomic tone continuously.
    • Directs visceral reflex responses.
  • Divided into:
    • Sympathetic Nervous System (SNS)
    • Parasympathetic Nervous System (PNS)
  • Resting State: Parasympathetic NS predominates.
  • Crisis Mode: Sympathetic NS takes over.

Comparison of Somatic and Autonomic Nervous Systems

Somatic Nervous System
  • Consciously Perceived Sensations: Sensory input is consciously processed.
  • Motor Neurons: Carry information from CNS to skeletal muscles.
  • Neural Structure: Involves one motor neuron connecting the CNS to the effector.
Autonomic Nervous System
  • Involuntary Response: Controls excitation or inhibition of smooth muscle, cardiac muscle, or glandular secretion.
  • Motor Neurons: Transmit information through:
    • Preganglionic neurons (from CNS to autonomic ganglia).
    • Postganglionic neurons (from ganglia to effector organs).
  • Neural Structure: Requires two motor neurons to connect the CNS to effector or organ (preganglionic and postganglionic).
Major Differences Between Autonomic and Somatic Systems
  • Effectors:
    • SNS: Skeletal muscle.
    • ANS: Smooth muscle, cardiac muscle, and glands.
  • Neurotransmitters:
    • SNS: Primarily Acetylcholine (ACh) at neuromuscular junctions.
    • ANS: Various neurotransmitters including Acetylcholine (ACh) and Norepinephrine (NE).

Anatomy of the ANS

  • Preganglionic Neuron:
    • Cell body located in the brain or spinal cord.
    • Axon is myelinated (Type B Fiber) terminating at the autonomic ganglion.
  • Postganglionic Neuron:
    • Cell body located outside the CNS in an autonomic ganglion.
    • Axon is unmyelinated (Type C Fiber) terminating at a visceral effector.

Divisions of the ANS

Sympathetic Nervous System (SNS)
  • Also known as Thoracolumbar Division.
  • Preganglionic cell bodies located in T1-L2 segments of spinal cord.
  • Characterized by short preganglionic fibers.
Parasympathetic Nervous System (PNS)
  • Also known as Craniosacral Division.
  • Preganglionic cell bodies found in four cranial nerves and S2-S4 segments of spinal cord.
  • Characterized by long preganglionic fibers.
  • Most organs receive dual innervation from both branches to maintain homeostasis, with few exceptions like arteries and sweat glands.

Functional Regulation of Organs

Parasympathetic Functions (Rest & Digest)
  • Eyes: Constrict pupils.
  • Salivary Glands: Stimulates salivation.
  • Heart: Slows heartbeat.
  • Lungs: Constrict bronchi.
  • Stomach: Stimulates digestion.
  • Liver: Stimulates bile release.
  • Intestines: Stimulates peristalsis and secretion.
  • Bladder: Contracts bladder.
Sympathetic Functions (Fight-or-Flight)
  • Eyes: Dilate pupils.
  • Salivary Glands: Inhibit salivation.
  • Heart: Accelerates heartbeat.
  • Lungs: Dilate bronchi.
  • Stomach: Inhibits digestion.
  • Liver: Stimulates glucose release.
  • Kidneys: Stimulate epinephrine and norepinephrine release.
  • Intestines: Inhibit peristalsis and secretion.
  • Bladder: Relaxes bladder.

Preganglionic and Postganglionic Neurons

  • Sympathetic NS:
    • Preganglionic neurons synapse at:
    • Sympathetic Chain Ganglia: Affect body wall, thoracic cavity, limbs.
    • Collateral Ganglia: Innervation of abdominopelvic tissues.
    • Adrenal Medulla: Secretes neurotransmitters into blood (epinephrine & norepinephrine).
  • Parasympathetic NS:
    • Preganglionic neurons release ACh which binds to nicotinic receptors on postganglionic neurons. Postganglionic fibers release ACh at the neuroeffector junctions.

Neurotransmitters in ANS

  • Acetylcholine (ACh):
    • Released by sympathetic and parasympathetic preganglionic neurons and parasympathetic postganglionic neurons.
    • Considered cholinergic neurotransmitters; have an excitatory effect.
  • Norepinephrine (NE):
    • Released from most ganglionic neurons and classified as adrenergic neurotransmitters.
    • Binds to adrenergic receptors (alpha and beta) and has a longer-lasting effect than ACh.

Blood Pressure Regulation in Sympathetic NS

  • Epinephrine: Non-selective action, increases blood pressure.
  • Norepinephrine: Has a higher affinity for alpha receptors, selectively increases blood pressure.

Higher Order Functions and Mind Functioning

  • Characteristics:
    • Require the cerebral cortex.
    • Involve conscious and unconscious information processing.
    • Subject to adjustment over time.
  • Examples: Memory, consciousness.

Aging and the Nervous System

  • Anatomical Changes:
    • Memory consolidation becomes more difficult.
    • Diminished acuity in hearing, balance, vision, smell, taste.
    • Slowed reaction times and weakened reflexes.
    • Decrease in brain size, neuron quantity, blood flow, and changes in synaptic organization.
    • Accumulation of tangles and plaques, altering normal brain function.

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