Chapter 4:  Autonomic (ANS) Pharmacology: Introduction

ANS Neurotransmitters: Effector Organs

• *An increase in the slope of phase 4 depolarization results in ENHANCED AUTOMATICITY.

• As a result of the increase in phase 4 slope the cell reaches threshold more often, with a higher heart rate as a result.

• Factors that increase phase 4 depolarization include

  •  mechanical stretch

  •  beta-adrenergic stimulation

  •  hypokalemia

•  Ischemia can induce abnormal automaticity, i.e. automaticity that occurs in cells not typically exhibiting pacemaker activity.

  • Acetylcholine is an example of an agent that decreases the slope of phase 4 depolarization and as a result, slows the heart rate.

Arterioles

Systemic Veins

Lung

Skin

Adrenal Medulla

Skeletal Muscle

Posterior Pituitary

Based on Table 6-1: Lefkowitz, R.J, Hoffman, B.B and Taylor, P. Neurotransmission: The Autonomic and Somatic Motor Nervous Systems, In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics, (Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) TheMcGraw-Hill Companies, Inc.,1996, pp.110-111

• Characteristics of Autonomic Organ Innervation

  • Usually, parasympathetic and sympathetic systems are physiological antagonists; that is, if one system facilitates or augments a process the other system inhibits the process.

  • Since most visceral organs are innervated by both system, the activity of the organ is influenced by both, even though one system may be dominant.

  • The general pattern of antagonism between sympathetic and parasympathetic systems is not always applicable. The interaction between sympathetic and parasympathetic systems may be independent or interdependent.

  • Examples of Antagonistic Interactions between Sympathetic and Parasympathetic Systems

    • Actions of sympathetic and parasympathetic influences on the heart.

    • Actions of sympathetic and parasympathetic influences on the iris.

  • Interdependent or Complementary Sympathetic and Parasympathetic Effects

    • Actions of sympathetic and parasympathetic systems on male sexual organs are complementary.

  • Independent Effects

    • Vascular resistance is mainly controlled by sympathetic tone.

• Fight or Flight: General Functions of the Autonomic Nervous System

  • ANS regulates organs/processes not under conscious control including:

    1. circulation

    2. digestion

    3. respiration

    4. temperature

    5. sweating

    6. metabolism

    7. some endocrine gland secretions

  • Sympathetic system is most active when the body needs to react to changes in the internal or external environment: The requirement for sympathetic activity is most critical for:

    1. temperature regulation

    2. regulation of glucose levels

    3. rapid vascular response to hemorrhage

    4. reacting to oxygen deficiency

  • During rage or fright the sympathetic system can discharge as a unit--affecting multiorgan systems.

    • Sympathetic fibers show greater ramification.

    • Sympathetic preganglionic fibers may traverse through many ganglia before terminiating at its post-ganglionic cell. Synaptic terminal arborization results in a single preganglionic fiber terminating on many post-ganglionic cells.

    • This anatomical characteristic is the basis for the diffuse nature of sympathic response in the human and other species.

  •  Sympathetic Responses

    1. heart rate increases

    2. blood pressure increases

    3. blood is shunted to skeletal muscles

    4. blood glucose increase

    5. bronchioles dilate

    6. pupils dilate

  • Parasympathetic responses

    1. slows heart rate

    2. protects retina from excessive light

    3. lowers blood pressure

    4. empties the bowel and bladder

    5. increases gastrointestinal motility

    6. promotes absorption of nutrients

Lefkowitz, R.J, Hoffman, B.B and Taylor, P. Neurotrasmission: The Autonomic and Somatic Motor Nervous Systems, In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics, (Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) TheMcGraw-Hill Companies, Inc.,1996, pp.108.