Autonomic Nervous System - Anatomy - Lecture Slides, Slides for Dental Anatomy. All India Institute of Medical Sciences
shailaja_987c15 October 2012

Autonomic Nervous System - Anatomy - Lecture Slides, Slides for Dental Anatomy. All India Institute of Medical Sciences

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Main topics of Human Anatomy are adult spinal cord, articulations, autonomic nervous system, blood, circulation, classification of joints, functions of heart, glands, gall bladder, general osteology, head and neck, human...
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Human Anatomy, First Edition McKinley&O'Loughlin


Human Anatomy

Chapter 18 :

Autonomic Nervous System


Autonomic Nervous System


 complex system of nerves

 controls involuntary actions.

 Works with the somatic nervous system (SNS)

 regulates body organs

 maintains normal internal functions.



 SNS and ANS are both part of the peripheral nervous system (PNS).

 SNS operates under our conscious control.

 ANS functions are involuntary.


Comparison of SNS and ANS

 SNS uses both somatic sensory and somatic motor neurons  Somatic sensory neurons conduct stimulus

information from a sensory receptor

 Somatic motor neurons innervate skeletal muscle fibers.

 ANS also utilizes sensory and motor neurons.  Visceral sensory neurons provide input to activate

the ANS

 Visceral motor neurons innervate smooth muscle, cardiac muscle, and glands



Neuron Chains in ANS

 Preganglionic neurons

 Before the ganglion

 Ganglion

 Synapse

 Grey matter

 Postganlionic neurons

 After the ganglion


Neuron Chains

 Neuronal convergence

 occurs when axons from numerous preganglionic cells synapse (converge) on a single postganglionic cell.

 Neuronal divergence

 occurs when axons from one preganglionic cell synapse on numerous postganglionic cells



Divisions of the ANS

 Two divisions  Parasympathetic division  Sympathetic division

 Divisions are similar:  both use a preganglionic neuron (cell body in the CNS)  Both use a postganglionic neuron (cell body in the

ganglion)  innervates muscles or glands.

 Both contain autonomic ganglia  house the cell body of the preganglionic neurons.

 Both are involuntary  Both are concerned with the body’s internal

environment. (homeostasis)

 Divisions perform dramatically different functions.


The Parasympathetic Division

 Also termed the craniosacral division.  Primarily concerned with:

 conserving energy  replenishing nutrient stores.

 Is most active when the body is at rest or digesting a meal.  nicknamed the “rest-and-digest” division

 Works with the sympathetic division in maintaining homeostasis (a constant internal environment).


The Sympathetic Division

 Also termed the thoracolumbar division.

 Primarily concerned with preparing the body for emergencies.

 referred to as the “fight-or-flight” division

 Increased sympathetic activity results in:  increased alertness

 Increased metabolic activity



Anatomic Differences

 Divisions are distinguished by several anatomic differences.

 Preganglionic neuron cell bodies are housed in different regions of the CNS.

 Parasympathetic preganglionic neurons originate in either:

 Brainstem

 lateral gray matter of the S2–S4 spinal cord regions.

 Sympathetic preganglionic neurons originate in:

 lateral horns of the T1–L2 spinal cord regions



Anatomic Differences

 Parasympathetic division is structurally simple.  Parasympathetic division is also termed the

craniosacral division because its preganglionic neurons are:  housed within nuclei in the brainstem  within the lateral gray regions of the S2–S4 spinal cord


 Postganglionic neurons in the parasympathetic division are found in  terminal ganglia: are located close to the

target organ  intramural ganglia: located within the wall of

the target organ


Cranial Nerves: parasympathetic division

 Associated with the parasympathetic division:

 the oculomotor (CN III)

 facial (CN VII)

 glossopharyngeal (CN IX)

 vagus (CN X)

 First three of these nerves convey parasympathetic innervation to the head.

 Vagus nerve is the source of parasympathetic stimulation for:

 thoracic organs

 most abdominal organs.


Spinal Nerves: parasympathetic division

 Target organs innervated include:  the distal portion of the large intestine  the rectum  most of the reproductive organs  the urinary bladder  the distal part of the ureter.

 Parasympathetic innervation causes  increased smooth muscle motility (muscle contraction) and

secretory activity in digestive tract organs  contraction of smooth muscle in the bladder wall  erection of the female clitoris and the male penis



Effects and General Functions

of the Parasympathetic Division

 Parasympathetic division is most active during times when the body must process nutrients and conserve energy.

 Lack of extensive divergence in preganglionic axons  prevents the mass activation seen in the sympathetic


 Effects of the parasympathetic nervous system tend to be discrete and localized.

 Parasympathetic activity can affect one group of organs without necessarily having to “turn on” all other organs


Organization and Anatomy of the Sympathetic Division

 Much more complex than the parasympathetic division.

 Sympathetic preganglionic neuron cell bodies  housed in the lateral horn of the T1–L2

 Preganglionic sympathetic axons:  travel with somatic motor neuron axons  exit the spinal cord

 enter first the anterior roots  then the T1–L2 spinal nerves.

 Preganglionic sympathetic axons remain with the spinal nerve for a short distance  they branch off and leave the spinal nerve


Left and Right Sympathetic Trunks

 Immediately anterior to the paired spinal nerves are the left and right sympathetic trunks.

 Each is located immediately lateral to the vertebral column.

 A sympathetic trunk is like a pearl necklace:

 the “string” of the “necklace” is composed of bundles of axons

 the “pearls” are the sympathetic trunk (or paravertebral) ganglia

 house sympathetic ganglionic neuron cell bodies


Left and Right Sympathetic Trunks

 One sympathetic trunk ganglion is approximately associated with each spinal nerve.

 Cervical portions

 three sympathetic trunk ganglia

 superior, middle, and inferior cervical ganglia

 opposed to the eight cervical spinal nerves.




White Rami

 Connecting the spinal nerves to each sympathetic trunk are rami communicantes.

 Carry preganglionic sympathetic axons from the T1–L2 spinal nerves to the sympathetic trunk.

 Associated only with the T1–L2 spinal nerves.

 Preganglionic axons are myelinated.

 the white ramus has a whitish appearance

 Similar to “entrance ramps” on a highway.


Gray Rami

 Carry postganglionic sympathetic axons

 from the sympathetic trunk to the spinal nerve.

 Axons are unmyelinated.

 gray rami have a grayish appearance

 Similar to “exit ramps” on a highway.

 Connect to all spinal nerves.

 Sympathetic information that starts in the thoracolumbar region can be dispersed to all parts of the body.


Splanchnic Nerves

 Composed of preganglionic sympathetic axons.

 Run anteriorly from the sympathetic trunk to most of the viscera.

 Should not be confused with the pelvic splanchnic nerves associated with the parasympathetic division.

 Larger splanchnic nerves have specific names:

 greater thoracic splanchnic nerves

 lesser thoracic splanchnic nerves

 least thoracic splanchnic nerves

 lumbar splanchnic nerves

 sacral splanchnic nerves


Splanchnic Nerves

 Terminate in prevertebral (or collateral) ganglia.

 Called “prevertebral” because they are immediately anterior to the vertebral column.

 Prevertebral ganglia typically cluster around the major abdominal arteries and are named for these arteries.

 Example: celiac ganglia cluster around the celiac trunk

 Sympathetic postganglionic axons extend away from the ganglia and innervate many of the abdominal organs.


Types of Prevertebral Ganglia

 Differ from the sympathetic trunk ganglia.

 Are single structures, rather than paired.

 Are anterior to the vertebral column on the anterior surface of the aorta.

 Located only in the abdominopelvic cavity.

 Prevertebral ganglia include:

 the celiac ganglion

 superior mesenteric ganglion

 interior mesenteric ganglion.




Sympathetic Pathways

 Spinal nerve pathway

 Postganglionic sympathetic nerve pathway

 The Splanchnic Nerve Pathway

 The Adrenal Medulla Pathway


Fight-or-Flight Function of the ANS

 May involve a single effector or many effectors.

 In mass activation, a large number of ganglionic neurons activate many effector organs.

 causes a heightened sense of alertness due to stimulation of the reticular activation system


Dual Innervation by the Parasympathetic and Sympathetic Divisions of the ANS

 Innervate organs through specific axon bundles called autonomic plexuses.

 Communication by chemical messengers, called neurotransmitters.

 specific in each division of the autonomic nervous system

 Usually all organs are innervated by both divisions of the autonomic nervous system.

 Maintains homeostasis through autonomic reflexes that occur in the innervated organs.


Autonomic Plexuses

 Collections of sympathetic postganglionic axons and parasympathetic preganglionic axons, as well as some visceral sensory axons.

 Close to one another, but they do not interact or synapse with one another.

 Provide a complex innervation pattern to their target organs.



Plexuses  Cardiac plexus

 increased sympathetic activity increases heart rate and blood pressure, while

 increased parasympathetic activity decreases heart rate  Pulmonary Plexus

 parasympathetic pathway causes bronchoconstriction and increased secretion from mucous glands of the bronchial tree

 sympathetic innervation causes bronchodilation  Esophageal Plexus

 parasympathetic axons control the swallowing reflex  Abdominal aortic plexus

 consists of the celiac plexus, superior mesenteric plexus, and inferior mesenteric plexus

 Hypogastric plexus


Neurotransmitters and Receptors

 Two neurotransmitters are used in the ANS.

 acetylcholine (ACh)

 norepinephrine (NE)

 Neurotransmitters are released by the presynaptic cell.

 Bind to specific receptors in the postsynaptic cell membrane.

 Binding has either an excitatory or an inhibitory effect on the effector, depending on the specific receptor.



 Both the preganglionic and postganglionic axons in the parasympathetic division release acetylcholine and thus are called cholinergic.

 The preganglionic axon and a few postganglionic axons in the sympathetic division are also cholinergic.

 Most of the postganglionic axons of the sympathetic division release norepinephrine and are called





Dual Innervation

 Many visceral effectors are innervated by postganglionic axons from both ANS divisions.

 Actions of the divisions usually oppose each other.

 exert antagonistic effects on the same organ

 Opposing effects are also achieved by increasing or decreasing activity in one division.


Autonomic Reflexes

 ANS helps maintain homeostasis through the involuntary activity of autonomic reflexes or visceral reflexes.

 Consist of smooth muscle contractions, cardiac muscle contractions, or secretion by glands that are mediated by autonomic reflex arcs in response to a specific stimulus.

 Example: micturition reflex, which partly controls the release of urine

 Other reflexes include alteration of heart rate, changes in respiratory rate and depth, regulation of digestive system activities, and alteration of pupil diameter.

 Comparable to spinal reflexes.

 Classic autonomic reflex involves the reduction of blood pressure.



CNS Control of Autonomic Function

 Autonomic function is influenced by the cerebrum, hypothalamus, brainstem, and spinal cord.

 Sensory processing in the thalamus and emotional states controlled in the limbic system directly affect the hypothalamus.  the integration and command center for autonomic functions

 contains nuclei that control visceral functions in both divisions of the ANS

 communicates with other CNS regions, including the cerebral cortex, thalamus, brainstem, cerebellum, and spinal cord


CNS Control of Autonomic Function  The hypothalamus is the central brain structure involved in emotions

and drives that act through the ANS.

 The brainstem nuclei in the mesencephalon, pons, and medulla oblongata mediate visceral reflexes.

 Reflex centers control accommodation of the lens, blood pressure changes, blood vessel diameter changes, digestive activities, heart rate changes, and pupil size.

 The centers for cardiac, digestive, and vasomotor functions are housed within the brainstem.

 Some responses (defecation and urination), are processed and controlled at the level of the spinal cord without the involvement of the brain.

 Higher centers in the brain may consciously inhibit these reflex



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