Chapter 9: The Autonomic Nervous System (March 18, 2025)

  • Overview nervous system (ANS) manages our physiology.

    • By regulating organs and organ systems, and their smooth muscles and glands.

  • ANS Control of Smooth Muscle

    • Smooth muscle maintains resting tone in absence of nerve stimulation.

      • Smooth becomes more sensitive when ANS input is cut (=denervation hypersensitivity)

    • Many types of smooth are spontaneously active and contract rhythmically without ANS input.

      • ANS input simply increases or decreases intrinsic activity.

  • Autonomic Neurons

    • ANS has 2 neurons in its efferent pathway.

    • 1st neuron (=preganglionic neuron) has cell body in brain or spinal cord.

      • Synapses with 2nd neuron (=postganglionic neuron) in an autonomic ganglion.

    • Postganglionic axon extends from autonomic ganglion to target tissue.

  • Divisions of the ANS

    • ANS has sympathetic and parasympathetic divisions.

    • Usually have antagonistic effects.

      • These coordinate physiology with what’s going on in person’s life.

      • Sympathetic mediates “fight, flight, and stress” reactions.

        • Parasympathetic mediates “rest and digest” reactions.

  • Sympathetic Dvisions

    • Is also called thoracolumbar division because its pregpreganglionics anglionics exit spinal cord from T1 to L2.

      • Most then synapse on postganglionic in the paravertebral ganglia.

        • Which form chain of interconnected ganglia paralleling spinal cord.

    • Divergence: preganglionic branch to synapse with number of postganglionic neurons.

    • Convergence: Postganglionic receive synaptic input from large number of preganglionic.

    • Some postganglionics do not synapse in paravertebral ganglion but go to outlying collateral ganglion.

  • Sympathoadrenal System

    • The adrenal medulla, located in adrenal gland on top of kidney, appears to be a modified collateral ganglion.

      • Its secretory cells appear to be modified postganglionic.

      • That release 85% epinephrine (Epi) and 15% norepinephrine (Norepi) into blood in response to preganglionic stimulation.

        • Stimulated during mass activation.

    • Epi is made by methylating Norepi.

  • Parasympathetic Division

    • Is also called craniosacral division because long preganglionic originate in midbrain, medulla, pons, and S2-S4.

      • Synapse on postganglionic in terminal ganglia located next to or within target organ.

        • Postganglionic has short axon that innervates target.

    • The long vagus nerve carries most Parasymp fibers.

      • Innervates heart, lungs, esophagus, stomach, pancreas, liver, small intestine, and upper half of the large intestine.

    • Preganglionic fibers from S2-4 innervate lower half of large intestine, rectum, urinary and reproductive systems.

  • ANS Overview

  • ANS Neurotransmitters

    • Both Symp and Parasymp preganglionics release ACh.

    • Parasymp postganglionics also release ACh.

      • Called cholinergic synapses.

    • Most Symp postganglionic release Norepi (Noradrenaline).

      • Called adrenergic synapses.

      • A small number release ACh

    • Postganglionic have unusual synapses called varicosities.

      • Which release NTs along a length of axon.

      • = synapses en passant

  • Adrenergic Stimulation

    • Many useful drugs have been developed to affect ANS receptors.

      • Drugs that promote actions of a NT are agonists

      • Drugs that inhibit actions of a NT are antagonists

  • Cholinergic Stimulation

    • ACh is used at all motor neuron synapses on skeletal muscle, all preganglionic, and Parasymp postganglionics.

    • Cholinergic receptors have 2 subtypes:

      • Nicotinic which is stimulated by nicotine; blocked by curare.

      • and muscarinic which is stimulated by muscarine (from poisonous mushrooms); blocked by atropine.

  • Other ANS NTs

    • Some postganglionic do not use Norepi or ACh.

    • Called nonadrenergic, noncholinergic fibers.

    • Appear to use ATP, VIP, or NO as NTs.

      • NO produces smooth muscle relaxation in many tissues.

  • Organs With Dual Innervation

    • Most visceral organs receive dual innervation (supplied by both Symp and Parasymp)

    • While 2 branches are usually antagonistic, such as their effects on heart rate.

      • Can be complementary (causes similar effects) such as with salivation.

      • Or copperative (produce different effects that work together to cause desired effect) such as with micturition, and sexual function.