Lecture 19 - Autonomic Nervous System

Chapter 16 - The Autonomic Nervous System

  • Sympathetic Nervous system + Parasympathetic Nervous System

    • Dual Innervation: divisions of both reach the same organs - tight reguation of tissues

      • Includes smooth muscle of specific tissues, cardiac muscle & glands

    • Autonomic Nervous System - reflex loops, sense organ function & coordinate neural response

      • Maintains homeostasis

Organization

  • Peripheral Nervous System →

    1. Sensory/Afferent Division - transmits action potentials to CNS from Periphery

      1. Somatic Division - consciously perceived sensation (skin/special senses)

      2. Motor Divison - Unconsciously percieved visceral sensations w/in body

    2. Motor/Efferent Division - transmits action potentials from CNS to Effector organs (target tissues)

      1. Somatic Nervous System (Skeletal Muscle) - 1 neuron system

        • Cell body located in CNS, in motor nuclei of brain (cranial nerves), or anterior horn of spinal cord (spinal nerves)

        • Axon travels directly to target tissue

        • Always myelinated, only causes excitatory responses

      2. Autonomic Nervous System (Cardiac, smooth, glands) - 2 Neuron system

        • 1st neuron - preganglionic neuron (closest to CNS) - cell bodies w/in CNS located in autonomic nuclei of cranial nerves (brain stem), or lateral horn of spinal cord

          • Always myelinated

          • Synapses w/ postganglionic neuron

        • 2nd neuron - extends from synapse (ganglion outside of CNS) to effector/target tissue

          • Always unmyelinated

          • Can cause Excitatory response or Inhibitory response

Summary - Somatic/Autonomic Organization of Motor Division

  • Somatic:

    1. Cell bodies in anterior horn of spinal cord

    2. Exit through ventral root

    3. Join spinal nerve

    4. Travels to effector/target tissue (skeletal muscle)

    5. Neurotransmitter - acetylcholine → muscle contraction

  • Autonomic:

    1. Preganglionic neuron cell body in lateral gray horn (or in brainstem)

    2. leaves through ventral root → spinal nerve

    3. Synapse w/ postganglionic neuron in Autonomic Ganglion

      • Presynaptic neuron always myelinated, postganglionic neuron always unmyelinated

    4. Preganglionic neruon releases acetylcholine (neurotransmitter) → synapse w/ postganglionic neuron → synapse w/ effector organs (smooth muscle, cardiac muscle, glands)

      • Postganglionic neuron can release acetylcholine or norepinephrine

Divisions of Autonomic Nervous System

  • 2 Main Divisions:

    1. Sympathetic - Increases heart rate

    2. Parasympathetic - Decreases heart rate

  • Most organs have dual innervation from both divisions

  • Enertic Division (isnt really part of autonomic system, closely associated)

    • provides innervation to digestive tract

Sympathetic Nervous System - Thoracolumbar Division

  • Location of cell bodies of neurons w/in CNS

    • Originate in lateral horns of spinal cord at T1-L2

    • Neurons leave spinal cord through ventral route → join spinal nerve → 2 types of sympathetic ganglia:

  • Sympathetic Trunk Ganglia - Chains on either side of vertebral column

    • Sympathetic chain ganglion

    • Collection of postganglionic cell bodies closer to spinal cord

  • Prevertebral Ganglia/Collateral Ganglia - Collection of postganglionic cell bodies - closer to effectors/target tissues


  • 4 Main Routes for Sympathetic Axons - After leaving chain ganglion

    • Axons of preganglionic neuron can exit ventral root, form spinal nerve → exit spinal nerve & enter sympathetic chain ganglion

      • synapse at same level or travel up/down chain ganglion, synapse w/ postganlionic axon on different level

      • Postganglionic axon leaves sympathetic chain → effector

    • Move through chain w/o synapsing → collateral ganglion → synapse there w/ postganglionic neuron → effector


Routes of Sympathetic Axons

  1. Axons of preganglionic neuron → ventral root → spinal nerve

    • Spinal nerve → sympathetic chain ganglion

    • Synapse @ same level or different level (travel) → postganglionic neruon → leave sympathetic chain → re-enter spinal nerve in anterior ramus

      • In spinal nerve, present at each spinal level → effector

        • skin from the neck down, sweat glands, smooth muscle for blood vessels, smooth muscles for arrector pili muscles of skin (goosebumbs)

  2. Presynaptic ganglionic neuron → ventral root → spinal nerve → sympathetic chain ganglion

    • Synapse at same level or travel up/down

    • Postganglionic neuron does NOT re-enter spinal nerve

    • Leaves chain ganglion → directly to target tissue

    • Sympathetic Nerve - Postganglionic axon w/ this configuration

      • Target tissues: organs of thoracic cavity (heart, lungs)

  3. Preganglionic axon → sympathetic chain ganglion → through chain ganglion w/o synapsing → collateral ganglion

    • Splanchnic Nerve - nerve, after it leaves sympathetic chain ganglion

    • Synapses w/ postganglionic neuron in collateral ganglion

    • Effectors: abdominopelvic cavity

      • Eg. GI tract

  4. (Travels Directly to Adrenal Gland) Preganglionic axon bypasses sympathetic chain ganglion & collateral ganglion → center of adrenal gland (Adrenal medulla)

    • Synapses w/ specialized cells (modified cluster of postganglionic cell bodies) - considered second neuron

    • Releases epinephrine & norepinephrine into blood

Routes of Sympathetic Axons - Summary

  1. Synapse in chain ganglion - exit via Spinal Nerve

    • Targets skin of neck, trunk, limbs

  2. Synapse in chain ganglion - exit via sympathetic Nerve

    • Targets heart & lungs

  3. Pass through chain ganglion, synapse in collateral ganglion - exit via Splanchnic Nerve

    • Targets abdominopelvic organs

  4. Pass through chain & collateral ganglia - synapse w/ cells in Adrenal Medulla → cells = “postsynaptic neurons” - releases Epinephrine & Norepinephrine

Parasympathetic Nervous System - Craniosacral Divison

  • Cell bodies of parasympathetic neurons associated w/ nuclei of 4 different cranial nerves

    1. Cranial Nerve 3 - Oculomotor Nerve

    2. Cranial Nerve 7 - Facial Nerve

    3. Cranial Nerve 9 - Glossopharyngeal Nerve

    4. Cranial Nerve 10 - Vagus Nerve

  • Contains cell bodies in lateral gray horn of sacral region of spinal cord (S2-S4)

    • Pelvic Splanchnic Nerves (S2-S4) - Smooth muscle, glands of colon, bladder & reproductive organs

  • Terminal Ganglia: Where preganglionic axons synapse with postganglionic axons

    • usually closer to effector tissues

  • Cranial Nerve 3 - Oculomotor Nerve

    • stimulates ciliary muscles of eye & sphincter pupilae

  • Cranial nerve 7 - Facial Nerve

    • Glands for tears, salivary glands, & glands for nasal secretions

  • Cranial Nerve 9 - Gloosopharyngeal Nerve

    • Parotid salivary gland

  • Cranial Nerve 10 - Vagus Nerve

    • (Most important, many branches)

    • Heart, pulmonary system, GI tract (upto midpoint for colon)

  • Pelvic Splanchnic Nerves:

    • Supply smooth muscle glands of colon from midpoint on, uterus, bladder, reproductive organs

Neurotransmitters

  • Sympathetic & parasympathetic neurons secret either:

    • Acetylcholine (Cholinergic)

    • Norepinephrine (Adrenergic)


  • Sympathetic:

    • Preganglionic neuron → cholinergic (Acetylcholine) → Postganglionic Neuron → Adreneric (Norepinephrine)

      • Exception: Some sweat glands of sympathetic nervous system release acetylcholine

  • Parasympathetic:

    • Preganglionic neuron → Cholinergic (Acetylcholine) → Postganglionic Neuron → Cholinergic (Acetylcholine)

Receptors - Neurotransmitters

  • Cholinergic Receptors:

    • 2 Classes:

      • Nicotinic Receptors - excitatory - bind Na+ channels

      • Muscarinic Receptors - excitatory or inhibitory - G-protein

  • Nicotinic Receptors:

    • Postganglionic Neurons in autonomic ganglia

    • Plasma membrane of skeletal muscle (neuromuscular junction)

      • If action potential reaches autonomic ganglion, only option is to excite postganglionic neuron/skeletal muscle

  • Muscarinic Receptors:

    • Effector cells that respond to acetylchoine from postganglionic neurons

    • Plasmamembrane of ALL parasympathetic nervous system effectors/target tissues

      • Smooth muscle, cardiac muscle, glands

    • Binding to receptor → G Protein signalling pathway begins → excitatory/inhibitory response


  • Adrenergic Receptors:

    • 2 classes:

      • Alpha Receptors (A1, A2) -

        • A1 - excitatory

        • A2 - inhibitory

      • Beta Receptors (B1, B2) -

        • B1 - excitatiory

        • B2 - inhibitory

    • Binds to norepinephrine or epinephrine

    • Most of our sympathetic nervous system effectors (except sweat glands w/ exception - acetylcholine)

      • Sympathetic nervous system postganglionic neurons → release norepinephrine


Receptor Location

  • Preganglionic neuron → acetylcholine

    • (All sympathetic or parasympathetic preganglionic neurons)

  • Nicotinic Receptors → postganglionic neurons cell body & dendrites

    • Excitatory response (sympathetic & parasympathetic)


  • Muscarinic receptors → target tissue (Postganglionic neuron of parasympathetic nervous system - released acetylcholine)

    • excitatory/inhibitory response

    • Some exceptions in sympathetic -sweat glands


  • Adrenergic receptors → target tissues of sympathetic nervous system

    • Alpha/Beta receptors

    • Excitatory/inhibitory response


  • All preganglionic neurons are myelinated, all postganglionic neurons are unmyelinated


ANS Regulation

  • ANS - Homeostasis

    • Ability to maintain homeostaiss dependent on ANS & autonomic reflexes

    • Loops from direct link between visceral sensory input & autonomic nervous system motor responses

      • Eg. Baroreceptors - receptors in walls of larger arteries, detect stretch & changes in BP

  • Regulating Blood Pressure:

    • Baroreceptors in walls of carotid artery

      • Arteries that lead to brain, maintain blood flow

    • Increase in BP → detected as “stretch” on baroreceptors

      • Sensory signal sent to medulla oblongata through sensory fiber of glossopharyngeal nerve

    • @ medulla oblongata → integration of action potentials → reflex action & action potential → sent through vagus nerve

    • Vagus nerve acts as parasympathetic preganglionic neuron → wall of heart → synapse w/ short postganglionic neuron (also within wall of heart) → releases acetylcholine

      • Decreases heart rate → decreases blood pressure


    • Blood pressure decreased → less stretch on baroreceptors → decreased firing rate on sensory portion of glassopharyngeal nerve

      • Reaches medulla oblongata → interpreted differently

    • decreased rate of action potentials → different response

      • Signal moves down spinal cord → thoracolumbar region/sympathetic nervous system division

    • Preganglionic neuron leaves spinal cord → synapse in sympathetic chain ganglion → postganglionic neuron extends as sympathetic nerve to walls of heart → releases norepinephrine → increase heart rate

    • increases force of contraction of heart → increases BP

Generalization of ANS

  1. Both divisions produce stimulatory/inhibitory effects

  2. Most organs have dual innervation (sympathetic & parasympathetic)

  3. Opposing effects (parasympathetic vs sympathetic)

  4. Produce cooperative effects

  5. General (sympathetic) versus localized (parasympathetic) effects

  6. Rest (parasympathetic - rest/digestion) vs Activity (sympathetic)

    Parasympathetic

    • SLUDD

      • Salivation

      • Lacrimation

      • urination

      • digestion

      • defecation

      3 Ds

      • decreased heart rate

      • Decreased diameter of airway

      • decreased diameter of pupil

    Sympathetic - stress

    • E situations

      • Emergency

      • embarassment

      • excitement

      • exercise

    • Flight/fight response

      • Increased heart rate

      • increased blood pressure

      • increased force of heart contraction

      • pupil dilation

      • decreased blood flow to nonessential organs (gut)

      • increased blood flow to skeletal muscle & heart

      • Airways will dilate → increase in respiratory rate

      • increased blood glucose concentration

Exception: Paradoxical Fear - Fear there’s no escape route/no way to in

  • Mass action effect of parasympahtetic nervous system

  • Lose control of things like urination/defecation when you’d expect sympathetic nervous system to dominate


Enertic Nervous System

  • Nerve plexuses in walls of digestive tract

  • These plexuses contain nerve cell bodies, not just axons (compared to spinal nerves)

  • 3 Points of Nervous Input - contribute to plexuses

    1. Sensory neurons → connect digestive system to CNS

      • Sends info about homeostasis of digestive system to CNS

    2. ANS motor neurons connect CNS → digestive tract

      • control smooth muscles & gland secretions

    3. Enteric Neurons - control through autonomic reflexes

      • Found only within the plexus

      • capable of controling digestive homeostasis without input from CNS

      • Works through local reflex loops

  • Major Types of Enteric Neurons

    1. Enteric Sensory Neurons - stretch, chemical composition

      • Maintaining homeostasis by detecting stretch on walls of digestive tract

      • Detects chemical composition of contents in digestive tract

    2. Enteric Motor Neurons - Stimulate smooth muscle, glands

      • controls secretions from glands

      • stimulates/inhibits smooth muscle of digestive tract

      • → helps breakdown & move food through digestive tract

    3. Enteric Interneurons - Connects sensory & motor enteric neurons

      • Creates reflex loops → rapid responses to sensory information