autonomic nervous system

• Autonomic Nervous System (ANS)


  Regulates involuntary physiologic processes, including heart rate, blood pressure, respiration, digestion, and sexual arousal
• ANS


  • Consists of motor neurons that innervate smooth and cardiac muscle and glands
  • Makes adjustments to ensure optimal support for body activities
  • Operates via subconscious control
• Autonomic Nervous System


  Also called the involuntary nervous system or general visceral motor system
• Somatic Nervous System


  Has motor fibers that innervate skeletal muscles
• Autonomic Nervous System


  Has motor fibers that innervate cardiac muscle, smooth muscle, and glands
• Somatic Nervous System vs Autonomic Nervous System

  • Both have motor fibers
  • Differ in effectors, efferent pathways and ganglia, and target organ response to neurotransmitters
• Effectors of Somatic Nervous System

  Skeletal muscles
• Effectors of Autonomic Nervous System

  Cardiac muscle, smooth muscle, glands
• Somatic Nervous System Pathway

  1. Cell body in CNS

  2. Thick, myelinated, group A fibers extend in spinal or cranial nerve to skeletal muscle
• Autonomic Nervous System Pathway

  1. Preganglionic neuron (in CNS, thin, lightly myelinated axon)

  2. Postganglionic neuron (in autonomic ganglion outside CNS, nonmyelinated axon extending to effector organ)
• Somatic Nervous System


  All somatic motor neurons release acetylcholine (ACh), effects are always stimulatory (contraction)
• Autonomic Nervous System


  Preganglionic fibers release ACh, postganglionic fibers release norepinephrine (NE) or ACh at effectors, effects are either stimulatory or inhibitory depending on receptors
• Major Divisions of Autonomic Nervous System

  • Parasympathetic Division
  • Sympathetic Division
• Parasympathetic Division


  Promotes maintenance functions, conserves energy
• Sympathetic Division


  Mobilizes body during activity
• Dynamic Antagonism


  Between parasympathetic and sympathetic divisions maintains homeostasis
• Dual Innervation


  Both divisions serve all visceral organs but cause opposite effects
• Parasympathetic Division


  Referred to as the "rest-and-digest" system, promotes maintenance activities and conserves body energy
• Sympathetic Division


  Referred to as the "fight-or-flight" system, mobilizes body during activity
• Parasympathetic Effects


  • Increased digestion, diuresis, defecation
  • Decreased heart rate, blood pressure, respiratory rate
  • Constricted pupils, accommodated lenses for close vision
• Sympathetic Effects


  • Increased heart rate, dry mouth
  • Cold, sweaty skin
  • Dilated pupils, flattened lenses for distant vision
  • Increased blood flow to skeletal muscle and heart
  • Dilated bronchioles
  • Increased liver glucose release
• Parasympathetic Fibers


  Originate in brain stem and sacral spinal cord (craniosacral division), have long preganglionic and short postganglionic fibers
• Sympathetic Fibers


  Originate in thoracic and lumbar spinal cord (thoracolumbar division), have short preganglionic and long postganglionic fibers
• Major Cranial Nerves Serving Parasympathetic Division

  • Oculomotor (III)
  • Facial (VII)
  • Glossopharyngeal (IX)
  • Vagus (X)
• Oculomotor Nerve (III)


  Controls smooth muscles of the eye (iris and ciliary muscle) for pupil constriction and lens accommodation
• Facial Nerve (VII)


  Stimulates large glands in the head (nasal, lacrimal, submandibular, sublingual)
• Glossopharyngeal Nerve (IX)


  Stimulates parotid salivary glands
• Vagus Nerve (X)


  Accounts for 90% of all preganglionic parasympathetic fibers, serves thoracic and abdominal viscera
• Effectors Served by Sacral Parasympathetic Division

  • Pelvic organs
  • Distal half of large intestine
• Sympathetic Preganglionic Fibers


  Originate in spinal cord segments T1-L2, pass through white rami communicantes to enter sympathetic trunk ganglia
• Pathways of Sympathetic Preganglionic Fibers


  1. Synapse in sympathetic trunk ganglia

  2. Ascend/descend trunk to synapse in another trunk ganglion

  3. Pass through trunk without synapsing, synapse in collateral ganglia (splanchnic nerves)
• Sympathetic Postganglionic Fibers


  Enter ventral rami via gray rami communicantes, innervate sweat glands, arrector pili, and vascular smooth muscle
• Pathways of Sympathetic Fibers

  • Pathways to the head (T1-T4 to superior cervical ganglion)
  • Pathways to the thorax (T1-T6 to middle/inferior cervical trunk ganglia)
  • Pathways to the abdomen and pelvis (T5-L2 to collateral ganglia)
• Adrenal Medulla


  Some sympathetic preganglionic fibers synapse directly, causing release of norepinephrine and epinephrine ("adrenaline rush")
• The visceral reflex arc has the same components as other reflex arcs
• Thyroid gland and the skin


  • Innervated by
• Lungs


  • Innervated via the pulmonary plexus and esophagus
• Collateral ganglia


  Ganglia outside of the trunk where most fibers from T5 to L2 synapse, forming splanchnic nerves
• Splanchnic nerves


  1. Interweave and contain several important ganglia

  2. Celiac and superior and inferior mesenteric ganglia

  3. Postganglionic fibers from these ganglia then travel pathways to the abdomen and pelvis
• Preganglionic fibers from T5 to L2


  Serve the stomach, intestines, liver, spleen, and kidneys