ANS

Chapter 14: The Autonomic Nervous System

Lecture Introduction

  • Course: BIOL 2401
  • Lecture Number: #20
  • Program: CTC A&P Program
  • Pages: 527-543

Place of the ANS in Nervous System Organization

  • Components of the Nervous System:
      - Central Nervous System (CNS)
      - Peripheral Nervous System (PNS)
        - Sensory (afferent) division
        - Motor (efferent) division
          - Somatic nervous system
          - Autonomic nervous system (ANS)
           - Sympathetic division
           - Parasympathetic division

Overview of the Autonomic Nervous System (ANS)

  • The ANS consists of motor neurons that:
      - Innervate smooth muscle, cardiac muscle, and glands
      - Make adjustments to ensure optimal support for body activities
      - Operate via subconscious control (involuntary nervous system or general visceral motor system)

Differences Between Somatic and Autonomic Nervous Systems

  • Both systems have motor fibers but differ in several key features:
      - Effectors:
        - Somatic nervous system: Skeletal muscles
        - Autonomic nervous system (ANS): Cardiac muscle, smooth muscle, glands
      - Efferent pathways and ganglia:
        - Somatic: Cell body in CNS; thick, myelinated, group A fiber extends directly to skeletal muscle
        - ANS: Two-neuron motor chain
          1. Preganglionic neuron (in CNS) with thin, lightly myelinated axon
          2. Postganglionic neuron in autonomic ganglion outside CNS with nonmyelinated axon extending to effector organ
      - Target organ responses to neurotransmitters:
        - Somatic: All motor neurons release acetylcholine (ACh) with an always stimulatory effect (contraction)
        - ANS:
          - Preganglionic fibers also release ACh
          - Postganglionic fibers release either norepinephrine (NE) or ACh; effects can be stimulatory or inhibitory depending on receptor type

Comparison of Motor Neurons in Somatic and Autonomic Systems

  • Somatic Nervous System vs. Autonomic Nervous System
      - Neurons:
        - Somatic: Single neuron from CNS to effectors, heavily myelinated axon
        - ANS: Two-neuron chain from CNS to effectors, lightly myelinated preganglionic axon, unmyelinated postganglionic axon
      - Neurotransmitter at Effector Organs:
        - Somatic: ACh
        - ANS: ACh, NE (with effects based on receptor)
      - Effect:
        - Somatic: Always stimulatory
        - ANS: Stimulatory or inhibitory depending on neurotransmitter and receptor type

Divisions of the Autonomic Nervous System

  • Two major divisions:
      - Parasympathetic Division:
        - Promotes maintenance activities (conserves energy)
        - Known as the "rest-and-digest system" ('D' division)
        - Effects: Directs digestion, diuresis, and defecation
          - Example: Following a meal, blood pressure, heart rate, and respiratory rates are low; GI tract activity is high; pupils are constricted; lens is accommodated for close vision

      - Sympathetic Division:
        - Mobilizes body during activity ("fight-or-flight" system) ('E' division)
        - Activated during exercise, excitement, emergencies, and embarrassment
        - Effects include increased heart rate, dry mouth, cold sweaty skin, dilated pupils, and deviation in blood flow towards skeletal muscles and heart, dilation of bronchioles, and mobilization of glucose from the liver

Anatomical Differences Between Sympathetic and Parasympathetic Divisions

  • Sites of Origin:
      - Parasympathetic fibers: Craniosacral (originate in brainstem and sacral spinal cord)
      - Sympathetic fibers: Thoracolumbar (originate in thoracic and lumbar regions of spinal cord)

  • Location of Ganglia:
      - Parasympathetic ganglia: Located in or near their effector organs
      - Sympathetic ganglia: Lie close to the spinal cord

  • Relative Lengths of Fibers:
      - Parasympathetic: Long preganglionic fibers and short postganglionic fibers
      - Sympathetic: Short preganglionic fibers and long postganglionic fibers

Cranial Part of the Parasympathetic Division

  • Cell Bodies: Located in the brain stem
  • Preganglionic fibers: Travel in cranial nerves:
      - Oculomotor (III): Controls intrinsic smooth muscle of the eye, constricts pupils, bulges lenses for focusing on close vision
      - Facial (VII): Stimulates large glands in head (nasal, lacrimal, submandibular, and sublingual salivary glands)
      - Glossopharyngeal (IX): Stimulates parotid salivary glands
      - Vagus (X): Accounts for ~90% of all preganglionic parasympathetic fibers; serves all thoracic and abdominal viscera
        - Preganglionic fibers arise from the medulla and synapse in terminal ganglia in walls of target organs
          - Cardiac plexus: Slows heart rate
          - Pulmonary plexus: Serves lungs and bronchi
          - Esophageal plexus: Sends branches to stomach, liver, gallbladder, pancreas, small intestine, and proximal large intestine

Sacral Part of the Parasympathetic Division

  • Serves pelvic organs and distal half of large intestine
  • Cell bodies: Originate in lateral gray horn of T12-L1
  • Axons travel in ventral roots of spinal nerves S2-S4
  • Synapse in ganglia in the pelvic floor (inferior hypogastric or pelvic plexus) and in intramural ganglia in the walls of distal half of the large intestine, urinary bladder, ureters, and reproductive organs

Sympathetic (Thoracolumbar) Division

  • More complex than the parasympathetic division and innervates more organs
  • Some organs, such as eccrine sweat glands and arrector pili muscles, are innervated only by the sympathetic division
  • Origin of Preganglionic Neurons: From spinal cord segments T1–L2, forming lateral gray horns of spinal cord
  • Preganglionic fibers pass through white rami communicantes to enter sympathetic trunk ganglia

Sympathetic Pathways and Ganglia

  1. Upon entering sympathetic trunk ganglia, preganglionic fibers may:
       - Synapse in sympathetic trunk ganglia (same trunk ganglion)
       - Ascend or descend in the sympathetic trunk to synapse in different trunk ganglia
       - Pass through trunk to synapse in collateral ganglia (abdominal/pelvic area)

  2. Pathways to the Head:
       - Fibers from T1 to T4 synapse in superior cervical ganglion
       - Innervate skin and blood vessels of the head, stimulate dilator muscles of the iris, inhibit nasal and salivary glands

  3. Pathways to the Thorax:
       - Preganglionic fibers from T1 to T6 synapse in middle and inferior cervical trunk ganglia
       - Innervate heart, thyroid gland, skin, lungs via the pulmonary plexus and esophagus

  4. Pathways with Synapses in Collateral Ganglia:
       - Fibers from T5 to L2 synapse in collateral ganglia (e.g., celiac, superior, and inferior mesenteric ganglia)
       - Postganglionic fibers travel to abdomen and pelvis
       - Serve stomach, intestines, liver, spleen, kidneys, urinary bladder, reproductive organs

  5. Pathways with No Synapse in the Trunk:
       - Some preganglionic fibers pass directly to the adrenal medulla without synapsing and cause the release of norepinephrine and epinephrine into the bloodstream ("adrenaline rush")

Visceral Reflexes

  • Similar components to somatic reflex arcs: receptor, sensory neuron, integration center, motor neuron, and effector
  • Differences:
      - Visceral reflex arc has two neurons (preganglionic and postganglionic)
      - Afferent fibers are visceral sensory neurons (info about chemical changes, stretch, irritation)
      - Cell bodies lie in cranial nerve sensory ganglia or dorsal root ganglia of spinal cord
      - Examples: Emptying of the rectum and urinary bladder

Neurotransmitters

  • Major neurotransmitters in ANS:
      - Acetylcholine (ACh) and Norepinephrine (NE)
      - ACh is released by cholinergic fibers found in:
        - All ANS preganglionic axons
        - All postganglionic parasympathetic axons
      - NE is released by adrenergic fibers in almost all sympathetic postganglionic axons (except those at sweat glands, which release ACh)
      - Effects depend on receptor type:
        - Cholinergic receptors (respond to ACh)
          - Nicotinic (always stimulatory)
          - Muscarinic (can be excitatory or inhibitory)
        - Adrenergic receptors (respond to NE)
          - Alpha (α) receptors (subclasses: α1, α2)
          - Beta (β) receptors (subclasses: β1, β2, β3)

Effects of Parasympathetic and Sympathetic Divisions

  • Dual Innervation: Most visceral organs receive signals from both divisions, allowing for dynamic antagonism and maintaining homeostasis
  • Sympathetic Effects: Increase heart and respiratory rates, inhibit digestion
  • Parasympathetic Effects: Decrease heart and respiratory rates, promote digestion and elimination waste

Sympathetic and Parasympathetic Tone

  • Sympathetic Tone:
      - Controls blood pressure even at rest
      - Vasomotor tone: partial constriction of blood vessels maintained by sympathetic fibers
  • Parasympathetic Tone:
      - Dominates heart and smooth muscle of digestive and urinary tracts, maintaining normal activity levels
      - These organs exhibit a state of parasympathetic tone (always slightly active)

Unique Roles of the Sympathetic Division

  1. Thermoregulatory responses:
       - Increases blood flow to the skin and activates sweat glands when temperatures rise
       - Constricts blood vessels when temperatures drop

  2. Renin release from kidneys:
       - Sympathetic stimulation activates renin release, increasing blood pressure

  3. Metabolic effects:
       - Increases metabolic rates of cells, raises blood glucose levels, mobilizes fats for fuel

Localized vs. Diffuse Effects

  • Parasympathetic Division: Short-lived, localized control; ACh quickly destroyed by acetylcholinesterase
  • Sympathetic Division: Longer-lasting, widespread effects; NE inactivated slowly, with prolonged effects from hormones released by the adrenal medulla

Control of ANS Function

  • ANS is under the control of CNS centers located in:
      - Brain stem
      - Spinal cord
      - Hypothalamus
      - Cerebral cortex
      - Hypothalamus: Main integrative center of ANS activity

Summary of Control Functions

  1. Brain stem centers control heart rate, blood vessel diameter, and GI activities
       - Medullary centers regulate these functions
  2. Midbrain controls pupil and lens muscles
  3. Spinal cord regulates defecation and urination, but functions can be consciously overridden
  4. Hypothalamic Controls direct parasympathetic and sympathetic functions
       - Controls body temperature, heart activity, blood pressure, water balance, and endocrine activity
       - Emotional responses stimulate fight-or-flight via limbic system activation