Dr. Jacki Houghton, jhoughton@vcccd.edu ( 818)-397-2506 (TEXT ONLY)           Links:  HOME   UNIT 1   UNIT 2   UNIT 3   UNIT 4   UNIT 5   UNIT 6            Lessons: 1   2   3   4

The Autonomic Nervous System and Visceral Sensory Neurons

Autonomic Nervous System (ANS)


•      The motor portion of the ANS consists of motor neurons that:

•    Innervate smooth, cardiac muscle and glands

•    Make adjustments to ensure optimal support for body activities

•    Operate via subconscious control

•    Have viscera as most of their effectors


  • nMonitor temperature, pain, irritation, chemical changes and stretch in the visceral organs
  • qBrain interprets as hunger, fullness, pain, nausea, well-being
  • nReceptors widely scattered – localization poor (e.g. which part is giving you the gas pain?)
  • nVisceral sensory fibers run within autonomic nerves, especially vagus and sympathetic nerves
  • qSympathetic nerves carry most pain fibers from visceral organs of body trunk
  • nSimplified pathway: sensory neurons to spinothalamic tract to thalamus to cerebral cortex
  • nVisceral pain is induced by stretching, infection and cramping of internal organs but seldom by cutting (e.g. cutting off a colon polyp) or scraping them

ANS Versus Somatic Nervous System (SNS)

•      The ANS differs from the Somatic NS in the following three areas

•    Effectors

•    Efferent pathways

•    Target organ responses


•      The effectors of the Somatic NS are skeletal muscles

•      The effectors of the ANS are cardiac muscle, smooth muscle, and glands (visceral)

Efferent Pathways

•      Heavily myelinated axons of the somatic motor neurons extend from the CNS to the effector (skeletal Muscle)

•      Axons of the ANS are a two-neuron chain instead of one in the somatic neurons

•    The preganglionic (first) neuron with a lightly myelinated axon

•    The gangionic (second) neuron that extends to an effector organ

Neurotransmitter Effects

•      All somatic motor neurons release ACh, which has an excitatory effect

•      In the ANS:

•    Preganglionic fibers release ACh

•    Postganglionic fibers release norepinephrine or ACh and the effect is either stimulatory or inhibitory

•    ANS effect on the target organ is dependent upon the neurotransmitter released and the receptor type of the effector

Divisions of the ANS

•      The two divisions of the ANS are the sympathetic and parasympathetic

•      The sympathetic mobilizes the body during extreme situations and upregulates in an ongoing homeostatic function

•      The parasympathetic performs maintenance activities and conserves body energy

•      The two divisions counterbalance each other’s activity

Role of the Parasympathetic Division

•      Concerned with keeping body energy use low

•      Involves the D activities – digestion, defecation, and diuresis

•      Its activity is illustrated in a person who relaxes after a meal

•    Blood pressure, heart rate, and respiratory rates are low

•    Gastrointestinal tract activity is high

•    The skin is warm and the pupils are constricted

Role of the Sympathetic Division

•      The sympathetic division is the “fight-or-flight” system

•      Involves E activities – exercise, excitement, emergency, and embarrassment

•      Promotes adjustments during exercise – blood flow to organs is reduced, flow to muscles is increased

•      Its activity is illustrated by a person who is threatened

•    Heart rate increases, and breathing is rapid and deep

•    The skin is cold and sweaty, and the pupils dilate

Sympathetic Outflow

•      Is from nerves T1 through L2

•      Sympathetic neurons produce the lateral horns of the spinal cord

•      Preganglionic fibers pass through the white rami communicantes and synapse in the paravertebral ganglia

•      Fibers from T5-L2 form splanchnic nerves and synapse in collateral ganglia

•      Postganglionic fibers innervate the numerous organs of the body

Pathways with Synapses in a Chain Ganglion

•      Postganglionic axons enter the ventral rami via the gray rami communicantes

•      These fibers innervate sweat glands and arrector pili muscles

•      Rami communicantes are associated only with the sympathetic division

Pathways with Synapses in the Adrenal Medulla

•      Fibers of the thoracic splanchnic nerve pass directly to the adrenal medulla

•      Upon stimulation, medullary cells secrete norepinephrine and epinephrine into the blood

Visceral Reflexes

•      Visceral reflexes have the same elements as somatic reflexes

Referred Pain

•      Pain arising from the viscera but is perceived as somatic in origin

•      This may be due to the fact that visceral pain afferents travel along the same pathways as somatic pain fibers

Interactions of the Autonomic Divisions

•      Most visceral organs are innervated by both sympathetic and parasympathetic fibers

•      This results in dynamic antagonisms that precisely control visceral activity

•      Sympathetic fibers increase heart and respiratory rates, and inhibit digestion and elimination

•      Parasympathetic fibers decrease heart and respiratory rates, and allow for digestion and the discarding of wastes

Sympathetic Tone

•      The sympathetic division controls blood pressure and keeps the blood vessels in a continual state of partial constriction

•      This sympathetic tone (vasomotor tone):

•    Constricts blood vessels and causes blood pressure to rise as needed

•    Prompts vessels to dilate if blood pressure is to be decreased

•      Alpha-blocker drugs interfere with vasomotor fibers and are used to treat hypertension

Parasympathetic Tone

•      Parasympathetic tone:

•    Slows the heart

•    Dictates normal activity levels of the digestive and urinary systems

•      The sympathetic division can override these effects during times of stress

•      Drugs that block parasympathetic responses increase heart rate and block fecal and urinary retention

Thermoregulatory Responses to Heat

•      Applying heat to the skin causes reflex dilation of blood vessels

•      Systemic body temperature elevation results in widespread dilation of blood vessels

•      This dilation brings warm blood to the surface and activates sweat glands to cool the body

•      When temperature falls, blood vessels constrict and blood is retained in deeper vital organs

Localized Versus Diffuse Effects

•      The parasympathetic division exerts short-lived, highly localized control

•      The sympathetic division exerts long-lasting, diffuse effects

Effects of Sympathetic Activation

•      Sympathetic activation is long-lasting because NE:

•    Is inactivated more slowly than ACh

Levels of ANS Control

•      The hypothalamus is the main integration center of ANS activity

•      Subconscious cerebral input via limbic lobe connections influences hypothalamic function

•      Other controls come from the cerebral cortex, the reticular formation, and the spinal cord

Hypothalamic Control

•      Centers of the hypothalamus control:

•    Heart activity and blood pressure

•    Body temperature, water balance, and endocrine activity

•    Emotional stages (rage, pleasure) and biological drives (hunger, thirst, sex)

•    Reactions to fear and the “fight-or-flight” system



The ANS  approx 25 minutes

Powerpoint here