Cholinergic receptors

							Chapter 14              Autonomic Nervous System (ANS)

Objectives
 1. Define autonomic nervous system and explain its relationship to the peripheral nervous system.
 2. Compare the somatic and autonomic nervous systems relative to effectors, efferent pathways, and
    neurotransmitters released.
 3. Compare and contrast the functions of the parasympathetic and sympathetic divisions.
 4. For the parasympathetic and sympathetic divisions, describe the site of CNS origin, locations of ganglia, and
    general fiber pathways.
 5. Define cholinergic and adrenergic fibers, and list the different types of their receptors.
 6. Describe the clinical importance of drugs that mimic or inhibit adrenergic or cholinergic effects.
 7. State the effects of the parasympathetic and sympathetic divisions on the following organs: heart, blood vessels,
    gastrointestinal tract, lungs, adrenal medulla, and external genitalia.
 8. Describe autonomic nervous system controls.
 9. Define neurotransmitter and name several classes of neurotransmitters [Objective 16 from chapter 11].

I Introduction
[Note: Yes, life can be grand! Just think back to one of our sunny weekends when you had time (3.5 minutes) to linger
over a delicious brunch. Your ANS was taking care of digesting your food, keeping hour heart beating, etc., all without
any conscious effort on your part (autonomic means "self-governed"). Lie detectors tests are essentially checks on
your ANS, since it is very difficult to consciously alter or suppress autonomic responses. Remember that the ANS is
part of the PNS and it is often covered with the PNS and not as a separate chapter.

        A. ANS (Fig. 14.1)




        B. Comparison of the Somatic & ANS (Fig. 14.2)
              1. Effectors, 2. Efferent pathways & ganglia, & 3. Neurotransmitter effects
        C. ANS Divisions: General Functions
               1. Role of parasympathetic: "Rest & digest" ("D" division [digestion, defecation, & diuresis (urination)];
               2. Role of sympathetic: "Fight-or-fight" ("E" division; "EEEE")

II ANS Anatomy (Table 14.1 & Fig. 14.3)
        A. Intro
                 1. Origin sites
                          a. Parasympathetic: Craniosacral
                             [Parasympathetic fibers leave the brainstem by way of 4 cranial nerves (III, VII, IX, X (90%
                             of all parasympathetic preganglionic fibers travel in the vagus) and from sacral nerves 2-4
                             of the spinal cord.]
                          b. Sympathetic: Thoracolumbar
                 2. Length of fibers
                          a. Parasympathetic: long pre & short post-ganglionic fibers
                          b. Sympathetic: opposite of parasympathetic
                 3. Location of ganglia
                          a. Parasympathetic: effector organs
                          b. Sympathetic: near spinal cord
B. Parasympathetic division (Craniosacral) (Fig. 14.4)




C. Sympathetic division (Thoracolumbar) (Fig. 14.6)




        1. Ganglionic neurons (Fig. 14.5)
               a. Sympathetic trunk (also chain [paravertebral] ganglia)
                        * Targets: Thoracic cavity, head, body wall, limbs
               b. Collateral (prevertebral) ganglia
                        * Targets: Abdominopelvic cavity
               c. Adrenal medulla
                        * Targets: Organs & systems throughout the body
2. Ventral root (Fig. 14.5)
        a. Rami communicates
                  * White ramus (preganglionic fibers)
                  * Gray ramus (postganglionic fibers)

3. Pathways with synapses in trunk ganglia (head, thorax)

4. Pathways with synapses in collateral ganglia (Figs. 14.5 & 14.6)
        a. Splanchnic (splanchn += viscera) nerves
        b. Abdominal aortic plexus

                * Ganglia: celiac (SEE-lee-ac), superior & inferior mesenteric
                  [Note: the solar plexus is regarded as a collective designation for the celiac and
                  superior mesenteric ganglia by some authorities and as a synonym for only the
                  celiac by others]

5. Pathways with synapses in the adrenal medulla (Fig. 14.6)
        a. Norepinehrine (NE) & epinephrine (E) as hormones
       D. Visceral reflexes (Fig. 14.7)




III Neurotransmitters and Their Receptors [pp. 413-421, Chapter 11]

       A. Classification of Neurotransmitters by Chemical Structure
               1. Acetylcholine (ACh) (activity terminated by acetylcholinesterase)
               2. Biogenic amines (dopamine, norepinephrine (NE), epinephrine (E), serotonin, histamine)
               3. Amino acids (gamma []-aminobutyric acid or GABA, glycine, aspartate, glutamate)
               4. Peptides (substance P, endorphins, enkephalins)
               5. Purines (ATP, adenosine)
               6. Gases and lipids (nitric oxide or NO, carbon monoxide or CO)

       B. Classification of Neurotransmitters by Function
               1. Effects: Excitatory versus Inhibitory
               2. Actions: Direct (open ion channels) versus Indirect (alter metabolism of cell)
IV ANS Physiology

       A. Neurotransmitters and receptors

               1. Acetylcholine (Ach) & cholinergic fibers

                       a. All ANS preganglionic axons

                       b. All parasympathetic postganglionic axons

               2. Biogenic amines

                       a. Norepinephrine (NE) & adrenergic fibers

                               * Most sympathetic postganglionic axons

       B. Receptors (see pp. 420-421 and Table 14.2, p. 536)

               1. Channel-linked (ionotropic) receptors

                       a. Cholinergic (nicotinic) receptors

               2. G-protein-linked (metabotropic) receptors (Fig. 11.20, p. 420)

                       a. Second messengers (e.g., camp)

                       b. Cholinergic (muscarinic) receptors

                       c. Adrenergic receptors (Table 14.2)

                               * Alpha receptors

                               * Beta receptors




       C. Effects of drugs (Table 14.3)
               1. "Better living through chemistry…"
D. Interactions of the autonomic divisions (Table 14.4)

        1. Dual innervation (e.g., gastrointestinal tract, heart, urinary bladder, reproductive tract; note: dual
           innervation is not universal; sweat glands and blood vessels are innervated by sympathetic neurons
           almost exclusively. Also, one division may be more extensive and exhibit a greater influence than
           the other (e.g., parasympathetic's role on the digestive system)

        2. Antagonistic Interactions
           (In a few cases, the effects of the two divisions is not clearly opposite; e.g., salivary secretion:
           parasympathetic produces a thin, watery saliva and sympathetic = viscous saliva)

        3. Sympathetic tone (the vasomotor tone of blood vessels) & parasympathetic tone

        4. Cooperative effects

        5. Unique roles of the sympathetic division
                a. Thermoregulatory responses to heat
                b. Release of rennin from kidneys
                c. Metabolic effects

        6. Localized vs. diffuse effects
           [The sympathetic division has a more general effect because its activation often causes secretion of
           NE & E from the adrenal medulla. These circulating hormones also last longer, i.e., the need for a
           "coming down" period after a grueling A & P exam.)

E. Control of autonomic functioning (Fig. 14.9)

        1. Hypothalamus: "boss"

        2. Brain stem (reticular formation): most direct influence
I. Introduction (pp. 526–528, Figs. 14.1–14.2)
   A. Comparison of the Somatic and Autonomic Nervous Systems (pp. 526–527; Fig. 14.2)
       1.   The somatic nervous system stimulates skeletal muscles, while the ANS innervates cardiac and smooth
            muscle and glands.
       2.   In the somatic nervous system, the cell bodies of the neurons are in the spinal cord and their axons extend
            to the skeletal muscles they innervate. The ANS consists of a two-neuron chain.
       3.   The neurotransmitter released by the somatic motor neurons is acetylcholine, which always has an
            excitatory effect; the neurotransmitters released by the ANS are epinephrine and acetylcholine, and both
            may have either an excitatory or an inhibitory effect.
       4.   There is overlap between the somatic and autonomic nervous systems, and most body responses to
            changing internal and external stimuli involve both skeletal muscle activity and visceral organ responses.
   B. Divisions of the Autonomic Nervous System (pp. 527–528; Fig. 14.1)
       1.   The parasympathetic division keeps body energy use as low as possible while directing digestion and
            elimination activities.
       2.   The sympathetic division prepares the body to respond to an emergency or threatening situation (or
            vigorous exercise).

II. ANS Anatomy (pp. 528–535; Figs. 14.3–14.8; Table 14.1)
   A. Parasympathetic (Craniosacral) Division (pp. 529–530; Figs. 14.3–14.4; Table 14.1)
       1.   The preganglionic axons extend from the CNS nearly all the way to the structures to be innervated, where
            they synapse with ganglionic neurons in the terminal ganglia.
       2.   The cranial outflow consists of preganglionic fibers that run in the oculomotor, facial, glossopharyngeal,
            and vagus cranial nerves.
       3.   The rest of the large intestine and the pelvic organs are served by the sacral outflow, which arises from
            neurons located in the lateral gray matter of spinal cord segments S 2–S4.
   B. Sympathetic (Thoracolumbar) Division (pp. 530–534; Figs. 14.3, 14.5–14.6; Table 14.1)
       1.   The sympathetic division supplies the visceral organs in the internal body cavities but also all visceral
            structures in the somatic part of the body.
       2.   When synapses are made in chain ganglia, the postganglionic axons enter the ventral (or dorsal) ramus of
            the adjoining spinal nerves by way of communicating branches called gray rami communicantes.
       3.   The preganglionic fibers from T5 down synapse in collateral ganglia; thus these fibers enter and leave the
            sympathetic chains without synapsing.
       4.   Some fibers of the thoracic splanchnic nerves terminate by synapsing with the hormone-producing
            medullary cells of the adrenal cortex.
   C. The visceral sensory neurons are the first link in autonomic reflexes, sending information concerning chemical
      changes, stretch, and irritation of the viscera (pp. 534–535; Figs. 14.7–14.8).

III. ANS Physiology (pp. 535–540; Fig. 14-9; Tables 14.2–14.5)
   A. Neurotransmitters and Receptors (pp. 535–536; Table 14.2)
       1.   Cholinergic receptors, such as nicotinic and muscarinic receptors, bind acetylcholine.
       2.   Adrenergic receptors alpha and beta bind to epinephrine.
   B. Knowing the locations of the cholinergic and adrenergic receptor subtypes allows specific drugs to be
      prescribed to obtain desired inhibitory or stimulatory effects on target organs (pp. 535–536; Table 14.3).
   C. Interactions of the Autonomic Divisions (pp. 536–539; Table 14.4)
       1.   Most visceral organs receive dual innervation by both ANS divisions, allowing for a dynamic antagonism
            to exist between the divisions and precise control of visceral activity.
       2.   The sympathetic division increases heart rate, dilates airways, and inhibits digestion and elimination
            while the body is under stress.
       3.   After the stress has passed, the parasympathetic division returns heart rate and airway diameter to
            normal, and allows digestion and elimination to resume.
       4.   Sympathetic tone throughout the vascular system allows the firing rate of sympathetic neurons to control
            the diameter of blood vessels, regulating systemic blood pressure.
       5.   Parasympathetic tone is usually dominant in the heart, digestive system, and urinary tracts, maintaining
            normal homeostatic levels of function unless overridden by the sympathetic system during stress.
       6.   The divisions of the autonomic nervous system may work together, rather than antagonistically, as in
            sexual arousal.
       7.   The sympathetic system has a unique role in control of the adrenal medulla, sweat glands, arrector pili
            muscles of the skin, the kidneys, and most blood vessels.
       8.   The parasympathetic division exerts short-lived, localized control over its effectors, whereas effects of the
            sympathetic division are persistent and widespread.
   D. Control of Autonomic Functioning (pp. 539–540, Fig. 14.9)
       1.   The brain stem appears to exert the most direct influence over autonomic functions.
       2.   The hypothalamus is the main integration center for the autonomic nervous system.
       3.   Cortical or voluntary control of the autonomic nervous system does appear to be possible.