Chapter 15

The Autonomic Nervous System

 

Lecture Outline

INTRODUCTION

•      The autonomic nervous system (ANS) operates via reflex arcs.

•      Operation of the ANS to maintain homeostasis, however, depends on a continual flow of sensory afferent input, from receptors in organs, and efferent motor output to the same effector organs.

•      Structurally, the ANS includes autonomic sensory neurons, integrating centers in the CNS, and autonomic motor neurons.

•      Functionally, the ANS usually operates without conscious control.    

•      The ANS is regulated by the hypothalamus and brain stem.

Chapter 15
The Autonomic Nervous System

•      Regulate activity of smooth muscle, cardiac muscle & certain glands

•      Structures involved

–   general visceral afferent neurons

–   general visceral efferent neurons

–   integration center within the brain

•      Receives input from limbic system and other regions of the cerebrum

SOMATIC AND AUTONOMIC NERVOUS SYSTEMS

•      The somatic nervous system contains both sensory and motor neurons.

•      The somatic sensory neurons receive input from receptors of the special and somatic senses.

•      These sensations are consciously perceived.

•      Somatic motor neurons innervate skeletal muscle to produce conscious, voluntary movements.

•      The effect of a motor neuron is always excitation.

SOMATIC AND AUTONOMIC NERVOUS SYSTEMS

•      The autonomic nervous system contains both autonomic sensory and motor neurons.

–   Autonomic sensory neurons are associated with interoceptors.

–   Autonomic sensory input is not consciously perceived.

•      The ANS also receives sensory input from somatic senses and special sensory neurons.

•      The autonomic motor neurons regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities of cardiac muscle, smooth muscle, and glands.

–   Most autonomic responses can not be consciously altered or suppressed.

SOMATIC vs AUTONOMIC NERVOUS SYSTEMS

•      All somatic motor pathways consist of a single motor neuron

•      Autonomic motor pathways consists of two motor neurons in series

–   The first autonomic neuron motor has its cell body in the CNS and its myelinated axon extends to an autonomic ganglion.

•   It may extend to the adrenal medullae rather than an autonomic ganglion

–   The second autonomic motor neuron has its cell body in an autonomic ganglion; its nonmyelinated axon extends to an effector.

Somatic versus Autonomic NS

Basic Anatomy of ANS

•      Preganglionic neuron

–   cell body in brain or spinal cord

–   axon is myelinated type B fiber that extends to autonomic ganglion

•      Postganglionic neuron

–   cell body lies outside the CNS in an autonomic ganglion

–   axon is unmyelinated type C fiber that terminates in a visceral effector

Sympathetic vs. Parasympathetic NS

AUTONOMIC NERVOUS SYSTEM

•      The output (efferent) part of the ANS is divided into two principal parts:

–   the sympathetic division

–   the parasympathetic division

–   Organs that receive impulses from both sympathetic and parasympathetic fibers are said to have dual innervation.

•      Table 15.1 summarizes the similarities and differences between the somatic and autonomic nervous systems.

Sympathetic ANS vs. Parasympathetic ANS

Divisions of the ANS

•       2 major divisions

–    parasympathetic

–    sympathetic

•       Dual innervation

–    one speeds up organ

–    one slows down organ

–    Sympathetic NS increases heart rate

–    Parasympathetic NS decreases heart rate

Divisions of the ANS

•       2 major divisions

–    parasympathetic

–    sympathetic

•       Dual innervation

–    one speeds up organ

–    one slows down organ

–    Sympathetic NS increases heart rate

–    Parasympathetic NS decreases heart rate

Autonomic Ganglia

Sympathetic Ganglia

•      These ganglia include the sympathetic trunk or vertebral chain or paravertebral ganglia that lie in a vertical row on either side of the vertebral column (Figures 15.2).

•      Other sympathetic ganglia are the prevertebral or collateral ganglia that lie anterior to the spinal column and close to large abdominal arteries.

–   celiac

–   superior mesenteric

–   inferior mesenteric ganglia

–   (Figures 15.2 and 15.4).

Parasympathetic Ganglia

•      Parasympathetic ganglia are the terminal or intramural ganglia that are located very close to or actually within the wall of a visceral organ.

•      Examples of terminal ganglia include (Figure 15.3)

–   ciliary,

–   pterygopalatine,

–   submandibular,

–   otic ganglia

Sympathetic ANS vs. Parasympathetic ANS

Dual Innervation, Autonomic Ganglia

•       Sympathetic (thoracolumbar) division

–    preganglionic  cell bodies in thoracic and first 2 lumbar segments of spinal cord

•       Ganglia

–    trunk (chain) ganglia near vertebral bodies

–    prevertebral ganglia near large blood vessel in gut (celiac, superior mesenteric, inferior mesenteric)

•       Parasympathetic (craniosacral) division

–    preganglionic cell bodies in nuclei of 4 cranial nerves and the sacral spinal cord

•       Ganglia

–    terminal ganglia in wall of organ

 

Autonomic Plexuses

•      These are tangled networks of sympathetic and parasympathetic neurons (Figure 15.4) which lie along major arteries.

•      Major autonomic plexuses include

–   cardiac,

–   pulmonary,

–   celiac,

–   superior mesenteric,

–   inferior mesenteric,

–   renal and

–   hypogastric

Autonomic Plexuses

•       Cardiac plexus

•       Pulmonary plexus

•       Celiac (solar) plexus

•       Superior mesenteric

•       Inferior mesenteric

•       Hypogastric

Autonomic Plexuses

•       Cardiac plexus

•       Pulmonary plexus

•       Celiac (solar) plexus

•       Superior mesenteric

•       Inferior mesenteric

•       Hypogastric

Structures of Sympathetic NS

•      Preganglionic cell bodies at  T1 to L2

•      Rami communicantes

–   white ramus = myelinated = preganglionic fibers

–   gray ramus = unmyelinated = postganglionic fibers

•      Postganglionic cell bodies

–   sympathetic chain ganglia along the spinal column

–   prevertebral ganglia at a distance from spinal cord

•   celiac ganglion

•   superior mesenteric ganglion

•   inferior mesenteric ganglion

Postganglionic Neurons: 
Sympathetic vs. Parasympathetic

•      Sympathetic preganglionic neurons pass to the sympathetic trunk. They may connect to postganglionic neurons in the following ways. (Figure 17.5).

–   May synapse with postganglionic neurons in the ganglion it first reaches.

–   May ascend or descend to a higher of lower ganglion before synapsing with postganglionic neurons.

–   May continue, without synapsing, through the sympathetic trunk ganglion to a prevertebral ganglion where it synapses with the postganglionic neuron.

•      Parasympathetic preganglionic neurons synapse with postganglionic neurons in terminal ganglia (Figure 17.3).

Pathways of Sympathetic Fibers

•       Spinal nerve route

–    out same level

•       Sympathetic chain route

–    up chain & out spinal nerve

•       Collateral ganglion route

–    out splanchnic nerve to collateral ganglion

 

 

Organs Innervated by Sympathetic NS

•      Structures innervated by each spinal nerve

–   sweat glands, arrector pili mm., blood vessels to skin & skeletal mm.

•      Thoracic & cranial plexuses supply:

–   heart, lungs, esophagus & thoracic blood vessels

–   plexus around carotid artery to head structures

•      Splanchnic nerves to prevertebral ganglia supply:

–   GI tract from stomach to rectum, urinary & reproductive organs

Ganglia & Plexuses of Sympathetic NS

Circuitry of Sympathetic NS

•      Divergence = each preganglionic cell synapses on many postganglionic cells

•      Mass activation due to divergence

–   multiple target organs

–   fight or flight response explained

•      Adrenal gland

–   modified cluster of postganglionic cell bodies that release epinephrine & norepinephrine into blood

Application

•      In Horner’s syndrome, the sympathetic innervation to one side of the face is lost.

Structure of the Parasympathetic Division

•      The cranial outflow consists of preganglionic axons that extend from the brain stem in four cranial nerves. (Figure 15.3). 

–   The cranial outflow consists of four pairs of ganglia and the plexuses associated with the vagus (X) nerve.

•      The sacral parasympathetic outflow consists of preganglionic axons in the anterior roots of the second through fourth sacral nerves and they form the pelvic splanchnic nerve. (Figure15.3)

Anatomy of Parasympathetic NS

•      Preganglionic cell bodies found in

–    4 cranial nerve nuclei in brainstem

–    S2 to S4 spinal cord

•      Postganglionic cell bodies very near or in the wall of the target organ in a terminal ganglia

Parasympathetic Cranial Nerves

•       Oculomotor nerve

–    ciliary ganglion in orbit

–    ciliary muscle & pupillary constrictor muscle inside eyeball

•       Facial nerve

–    pterygopalatine and submandibular ganglions

–    supply tears, salivary & nasal secretions

•       Glossopharyngeal

–    otic ganglion supplies parotid salivary gland

•       Vagus nerve

–    many brs supply heart, pulmonary and GI tract as far as the midpoint of the colon

Parasympathetic Sacral Nerve Fibers

•       Form pelvic splanchnic nerves

•       Preganglionic fibers end on terminal ganglia in walls of target organs

•       Innervate smooth muscle and glands in colon, ureters, bladder & reproductive organs

ANS NEUROTRANSMITTERS AND RECEPTORS

ANS Neurotransmitters

•       Classified as either cholinergic or adrenergic neurons based upon the neurotransmitter released

 

•       Adrenergic

 

 

 

•       Cholinergic

 

Cholinergic Neurons and Receptors

•      Cholinergic neurons release acetylcholine

–   all  preganglionic neurons

–   all parasympathetic postganglionic neurons

–   few sympathetic postganglionic neurons (to most sweat glands)

 

•      Excitation or inhibition depending upon receptor subtype and organ involved.

Cholinergic Neurons and Receptors

•      Cholinergic receptors are integral membrane proteins in the postsynaptic plasma membrane.

•      The two types of cholinergic receptors are nicotinic and muscarinic receptors (Figure 15.6 a , b).

–   Activation of nicotinic receptors causes excitation of the postsynaptic cell.

•   Nicotinic receptors are found on dendrites & cell bodies of autonomic NS cells (and at NMJ.)

–   Activation of muscarinic receptors can cause either excitation or inhibition depending on the cell that bears the receptors.

•   Muscarinic receptors are found on plasma membranes of all parasympathetic effectors

 

Adrenergic Neurons and Receptors

•      Adrenergic neurons release norepinephrine (NE) )

–   from postganglionic
sympathetic neurons only

 

•      Excites or inhibits organs depending on receptors

 

•      NE lingers at the synapse until enzymatically inactivated by monoamine oxidase (MAO) or catechol-O-methyltransferase (COMT)

Adrenergic Neurons and Receptors

•      The main types of adrenergic receptors are alpha and beta receptors.  These receptors are further classified into subtypes.

–   Alpha1 and Beta1 receptors produce excitation

–   Alpha2 and Beta2 receptors cause inhibition

–   Beta3 receptors (brown fat) increase thermogenesis

•      Effects triggered by adrenergic neurons typically are longer lasting than those triggered by cholinergic neurons.

•      Table 15.2 describes the location of the subtypes of cholinergic and adrenergic receptors and summarizes the responses that occur when each type of receptor is activated.

Receptor Agonists and Antagonists

•      An agonist is a substance that binds to and activates a receptor, mimicking the effect of a natural neurotransmitter or hormone.

•      An antagonist is a substance that binds to and blocks a receptor, preventing a natural neurotransmitter or hormone from exerting its effect.

•      Drugs can serve as agonists or antagonists to selectively activate or block ANS receptors.

Physiological Effects of the ANS

•      Most body organs receive dual innervation

–   innervation by both sympathetic & parasympathetic

•      Hypothalamus regulates balance (tone) between  sympathetic and parasympathetic activity levels

•      Some organs have only sympathetic innervation

–   sweat glands, adrenal medulla, arrector pili mm & many blood vessels

–   controlled by regulation of the “tone” of the sympathetic system

Sympathetic Responses

•      Dominance by the sympathetic system is caused by physical or emotional stress -- “E situations”

–    emergency, embarrassment, excitement, exercise

•      Alarm reaction = flight or fight response

–   dilation of pupils

–   increase of heart rate, force of contraction & BP

–   decrease in blood flow to nonessential organs

–   increase in blood flow to skeletal & cardiac muscle

–   airways dilate & respiratory rate increases

–   blood glucose level increase

•      Long lasting due to lingering of NE in synaptic gap and release of norepinephrine by the adrenal gland

 

Parasympathetic Responses

•      Enhance “rest-and-digest” activities

•      Mechanisms that help conserve and restore body energy during times of rest

•      Normally dominate over sympathetic impulses

•      SLUDD type responses = salivation, lacrimation, urination, digestion & defecation and 3 “decreases”--- decreased HR, diameter of airways and diameter of pupil

•       Paradoxical fear when there is no escape route or no way to win

–    causes massive activation of parasympathetic division

–    loss of control over urination and defecation

PHYSIOLOGICAL EFFECTS OF THE ANS - Summary

•      The sympathetic responses prepare the body for emergency situations (the fight-or-flight responses).

•      The parasympathetic division regulates activities that conserve and restore body energy (energy conservation-restorative system).

 

•      Table 15.4 summarizes the responses of glands, cardiac muscle, and smooth muscle to stimulation by the ANS.

INTEGRATION AND CONTROL OF AUTONOMIC FUNCTIONS

Autonomic or Visceral Reflexes

•      A visceral autonomic reflex adjusts the activity of a visceral effector, often unconsciously.

–   changes in blood pressure, digestive functions etc

–   filling & emptying of bladder or defecation

•      Autonomic reflexes occur over autonomic reflex arcs. Components of that reflex arc:

–   sensory receptor

–   sensory neuron

–   integrating center

–   pre & postganglionic motor neurons

–   visceral effectors

          Control of Autonomic NS

•      Not aware of autonomic responses because control center is in lower regions of the brain

•      Hypothalamus is major control center

–   input: emotions and visceral sensory information

•   smell, taste, temperature, osmolarity of blood, etc

–   output: to nuclei in brainstem and spinal cord

–   posterior & lateral portions control sympathetic NS

•   increase heart rate, inhibition GI tract, increase temperature

–   anterior & medial portions control parasympathetic NS

•   decrease in heart rate, lower blood pressure, increased GI tract secretion and mobility

 

Autonomic versus Somatic NS - Review

•      Somatic nervous system

–   consciously perceived sensations

–   excitation of skeletal muscle

–   one neuron connects CNS to organ

•      Autonomic nervous system

–   unconsciously perceived visceral sensations

–   involuntary inhibition or excitation of smooth muscle, cardiac muscle or glandular secretion

–   two neurons needed to connect CNS to organ

•   preganglionic and postganglionic neurons

DISORDERS

•      Raynaud’s phenomenon is due to excessive sympathetic stimulation of smooth muscle in the arterioles of the digits as a result the digits become ischemic after exposure to cold or with emotional stress.

 

Autonomic Dysreflexia

•      Exaggerated response of sympathetic NS in cases of spinal cord injury above T6

•      Certain sensory impulses trigger mass stimulation of sympathetic nerves below the injury

•      Result

–   vasoconstriction which elevates blood pressure

–   parasympathetic NS tries to compensate by slowing heart rate & dilating blood vessels above the injury

–   pounding headaches, sweating warm skin above the injury and cool dry skin below

–   can cause seizures, strokes & heart attacks

 

 

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