Ch 15 - ANS

• Components of the reflex arc

  1. Sensory receptor (mostly interoceptors)

  2. Sensory neurons (from receptor to CNS)

  3. Integrating centers in the CNS (brain stem and hypothalamus)

  4. Motor neurons (2 in series; pre and postganglionic motor neurons)

  5. Visceral effectors (smooth and cardiac muscles, and glands)
• CNS to Effector Pathways

  Somatic motor pathways = single motor neuron. Autonomic motor pathways consist of two motor neurons in series: Pre-ganglionic neuron has its cell body in the CNS and its myelinated axon extends to an autonomic ganglion. Post-ganglionic neuron has its cell body in the autonomic ganglion and its nonmyelinated axon extends to an effector
• The input (afferent) information of the ANS is mostly unconsciously perceived visceral sensations
• The Autonomic Nervous System operates via reflex arcs to regulate activity of smooth muscle, cardiac muscle & certain glands, often unconsciously
• ANS: Dual Innervation

  The sympathetic division and the parasympathetic division generally send opposite signals. Sympathetic input increases heart rate; parasympathetic input decreases heart rate. Sympathetic input decreases urine production; parasympathetic input increases urine production. Most organs receive impulses from both sympathetic and parasympathetic fibers; this is called dual innervation
• Different from the somatic nervous system, where sensory input is consciously perceived
• Sympathetic responses are long-lasting and widespread due to divergence of postganglionic axons, lingering of norepinephrine in synaptic cleft, and release of norepinephrine by the adrenal gland
• Parasympathetic input

  Increases urine production
• Parasympathetic input

  Decreases heart rate
• Hypothalamus is a major control center
• Sympathetic input

  Decreases urine production
• Not aware of autonomic responses because control center is in lower regions of the brain (not in cerebral cortex)
• Most organs receive impulses from both sympathetic and parasympathetic fibers; this is called dual innervation
• Sympathetic Nervous Responses

  1. Emergency, embarrassment, excitement, exercise lead to "E situations"

  2. Dilation of pupils

  3. Increase of heart rate, force of contraction, and blood pressure

  4. Decrease in blood flow to non-essential organs (kidney, GI)

  5. Increase in blood flow to skeletal and cardiac muscle

  6. Airways dilate and respiratory rate increases

  7. Blood glucose level increases
• Parasympathetic Responses

  1. Dominance by the parasympathetic system occurs during times of rest (status quo)

  2. Enhance "rest-and-digest" activities

  3. Mechanisms that help conserve and restore body energy

  4. Enhance "SLUDD" type responses (Salivation, Lacrimation, Urination, Digestion & Defecation)

  5. 3 "decreases" lead to decreased heart rate, diameter of airways, and diameter of pupil
• Sympathetic Preganglionic Neurons

  • May synapse with postganglionic neurons in the chain ganglia at the same level
  • May synapse with postganglionic neurons in a ganglion higher up or lower down the chain
  • May pass through the chain and synapse with a postganglionic neuron in a pre-vertebral ganglion
  • May go to adrenal medulla
• Postganglionic neurons from the renal ganglion

  Innervate the kidney and ureters
• Postganglionic neurons of the thoracic ganglia

  Innervate the heart, lungs, bronchi, skin (glands, arrector pili)
• Postganglionic neurons innervating organs below the diaphragm


  1. Postganglionic neurons have many axon collaterals (20+!)

  2. One axon affects many organs
• Postganglionic neurons from the cervical ganglia

  Innervate the head (sweat, lacrimal, nasal, salivary glands, pupil and lens, face) and the heart
• Postganglionic neurons from the inferior mesenteric ganglion

  Innervate the colon, rectum, bladder, and genital organs
• Postganglionic neurons from the celiac ganglion

  Innervate the stomach, spleen, liver, kidney, and small intestine
• Pre-ganglionic Neurons

  • Ciliary muscle (lens) and pupillary constrictor muscle of iris
  • Tears, salivary (submandibular and sublingual glands), nasal secretions
  • Parotid salivary glands
  • Heart, airways, liver, gallbladder, stomach, pancreas, small intestine, colon
  • Colon, ureters, bladder & reproductive organs
• The parasympathetic ganglia are located very near or in the wall of the target organs. They are called terminal ganglia
• Visceral effectors

  • sweat glands
  • blood vessels
  • arrector pili muscles of skin of neck, trunk, and limbs
• Pre-ganglionic Neurons

  • Oculomotor nerve (CN III)
  • Facial nerve (CN VII)
  • Glossopharyngeal (CN IX)
  • Vagus nerve (CN X)
  • S2 to S4 spinal cord
• Visceral effectors

  • eyes
  • lacrimal glands
  • salivary glands
  • pineal gland
  • nasal mucosa
  • sweat glands
  • blood vessels
  • arrector pili muscles of skin of face
• All (cranial and sacral) preganglionic neurons end in terminal ganglia located in walls of target organs
• Parasympathetic Sacral Nerves


  Preganglionic neurons in S2-S4 spinal nerves branch off to form the pelvic splanchnic nerves. They synapse with postganglionic neurons in terminal ganglia located in walls of target organs (colon, ureters, bladder & reproductive organs)
• All preganglionic neurons are cholinergic neurons
• Parasympathetic preganglionic neurons

  End in terminal ganglia located in walls of target organs such as colon, ureters, bladder & reproductive organs
• A few sympathetic postganglionic neurons (to most sweat glands) are cholinergic neurons
• Norepinephrine (NE) can cause excitation or inhibition of the postsynaptic cell, depending upon the receptor subtype
• Adrenergic Neurons and Receptors

  • Norepinephrine is released as a neurotransmitter and as a hormone, while epinephrine is released as a hormone. The main types of adrenergic receptors are alpha and beta receptors, which are further classified into subtypes. Effects triggered by adrenergic neurons are typically longer lasting than those triggered by cholinergic neurons
• Neurotransmitters

  • Cholinergic neurons release acetylcholine (ACh), while adrenergic neurons release norepinephrine (NE)
• Acetylcholine (ACh) can cause excitation or inhibition of the postsynaptic cell, depending upon the cell type and receptor subtype
• All parasympathetic postganglionic neurons are cholinergic neurons
• Parasympathetic ANS


  • The parasympathetic ganglia are located very near or in the wall of the target organs, called terminal ganglia. They contain the cell bodies of parasympathetic postganglionic neurons with short, unmyelinated axons
• Cholinergic Neurons and Receptors

  • Cholinergic receptors are integral membrane proteins in the postsynaptic plasma membrane. There are two types: nicotinic and muscarinic. Activation of nicotinic receptors causes excitation, while activation of muscarinic receptors can cause either excitation or inhibition depending on the cell that bears the receptors
• Only sympathetic postganglionic neurons are adrenergic neurons