Human Anatomy 5th ed. 2005 Benjamin Cummings General Anatomy of the Autonomic Nervous System.

Human Anatomy 5th ed. 2005Benjamin Cummings

General Anatomy of the Autonomic Nervous System


Introduction to ANS

• Regulates activity of smooth muscle, cardiac muscle and glands

• Operates without conscious control

• Named autonomic because was thought to be AUTONOMUS (working without CNS)


Autonomic Nervous System

• But to operate it depends on continuous flow of sensory input from:

– Visceral organs and – Blood vessels(not consciously perceived) into

– INTEGRATING CENTERS IN THE CNS.



• Structurally then ANS includes:– Autonomic sensory neurons– Integrating centers in the CNS– Autonomic motor neurons to smooth

muscle, cardiac muscle and glands

Where in the CNS? Hypothalamus and brain stem


Comparing the ANS and Somatic NS

• Structurally then SNS includes:

– Somatic sensory neurons

– Integrating centers in the CNS (Cortex)

– Somatic motor neuron to skeletal muscles only


Comparing the ANS and Somatic NS

• Somatic- the effect of a motor neuron is always excitation

• Autonomic- the effect of a motor neuron is either excitatory or inhibitory



• Makes all routine adjustments in physiological systems.

• The ANS pathway from the CNS to the effector always involves 2 neurons synapsing in an autonomic ganglion


ANS

– Preganglionic (neuron #1) – cell body is in the CNS, axon extends to the ganglion outside the CNS

– Postganglionic (neuron #2) – cell body is in the ganglion, axon extends to the visceral effector


Nerve Fibers of the ANS

• Preganglionic (neuron #1)

– Always myelinated

– Neurotransmitter is always ACh

• Postganglionic (neuron #2)

– Always nonmyelinated

– Neurotransmitter is Ach or norepinephrine



Subdivisions of the ANS

• Sympathetic Division

– Fight-or-flight

• Parasympathetic Division

– Rest-and-digest

• These divisions are anatomically distinct


Sympathetic

• Sympathetic division (thoracolumbar) – Cell bodies for all the neurons #1 reside

in the thoracic and lumbar portions of the spinal cord.• T1 – L2


Sympathetic– Stimulates

• heart beat

• tissue metabolism,

• increases alertness,

• prepares the body to deal with emergencies

• (“fight or flight” division)


T1-L2


Parasympathetic

•Parasympathetic division (craniosacral)

–Cell bodies reside in the brain stem (cranial nerves) or in the sacral portion of the spinal cord.


Cranial & Sacral


Parasympathetic

– Slows the heart rate,

– inhibits senses,

– prepares the body for rest and relaxation; (“rest and digest” division).


The Sympathetic Division


Sympathetic Chain Ganglia

– Synapses of neurons #1 and #2 are in a chain of ganglia that run alongside the spinal cord

– Extends on both sides of the vertebral column

– Carries preganglionic fibers and cell bodies of postganglionic neurons


Ganglia


Anatomy of the chain

• Rami communicantes from the spinal nerves connect to the chain



A closer look at spinal nerves


Routes of Preganglionic Axons

• Cell bodies of neurons #1 lie in the lateral gray horns of the spinal cord

• The axons of neurons #1 leave the spinal cord via the ventral root

• These axons pass to the spinal nerve• Axons leave the spinal nerve via the white

branches (rami communicantes)• Connect with the sympathetic chain ganglia



• There are 3 possible routes that sympathetic neurons may follow

• Possibility #1: synapses within the ganglion at that level and

– Second neuron leaves at that level via the gray ramus communicans, exits to the visceral effector




• Possibility #2: neuron #1 goes up or down the chain and synapses at some other level.

– Second neuron: leaves at that other level via the gray ramus communicantes, and exits to the visceral effector.




• Possibility #3: neuron #1 does not synapse in the chain (exception!!) but exits and synapses in a collateral ganglion near a major blood vessel.

– Neuron #2 travels from that ganglion to the visceral effector.



Where are the Collateral Ganglia ?

• Location –Near a major blood vessel– Celiac ganglion

• Innervates upper abdominal viscera – Superior mesenteric

• Innervates middle abdominal viscera– Inferior mesenteric

• Innervates lower abdominal & pelvic organs


The Adrenal Medulla

• Yet another type of innervation:

– Going to the adrenal medulla

– No synapse in ganglia

– No synapse in collateral ganglia

– YES synapse in the adrenal medulla



Adrenal Medulla

• Only preganglionic neurons are in this pathway

• Neuron #1 stimulates the medulla,

• The medulla releases norepinephrine and epinephrine (adrenaline) to blood


Adrenal Medulla


Effects of Sympathetic Stimulation

• Widespread

– The sympathetic chain allows one preganglionic fiber to synapse with many postganglionic neurons

• Enhanced & prolonged by the adrenal medulla


Convergence

• See heart


Neurotransmitters of Sympathetic Division

• Preganglionic fibers release acetylcholine (Ach) Therefore they are called:– Cholinergic

• Postganglionic fibers (most) release norepinephrine (NE) (=noradrenaline)– Adrenergic

• Adrenal medulla releases norepinephrine and epinephrine (adrenalin)



Functions of the Sympathetic Division

• Heart: increases rate

• Lung bronchioles: dilates bronchioles

• Salivary glands: produce viscous fluid

• Stomach: decreases motility

• Pupil: dilates

• Sweat glands: produce secretions


Summary of Sympathetic Division

• Cell bodies are found in the thoracic and lumbar portions of the spinal cord

• Preganglionic fibers are short, connect to the sympathetic chain, and synapse with long postganglionic fibers

• Preganglionic fibers produce ACh, postganglionic fibers produce NE or Ach

• “Fight or flight” division



The Parasympathetic Division


Parasympathetic division

• Cell bodies are in the brain or in the gray matter of the spinal cord (sacral region)

• Neurons #1 exit the cranial region through cranial nerves 3, 7, 9, & 10 or

• Neurons #1 exit the spinal cord through the sacral spinal nerves


Parasympathetic


Parasympathetic

• Neurons #1 are long and synapse with neurons #2 (short) in ganglia

• Ganglia are found on, or

– near the visceral effector


Parasympathetic


Parasympathetic ganglia


Neurotransmitter of Parasympathetic Division

• Preganglionic fibers: Acetylcholine

• Postganglionic fibers: Acetylcholine



General Functions of the Parasympathetic

• Prepares the individual for rest and repose

• “Rest & digest” division


Effects on various organs:

Heart: decreases rate• Lung bronchioles: constricts bronchioles• Salivary glands: produces watery fluid

fluid• Stomach: increases motility• Pupil: constricts• Sweat glands: reduces secretions


Summary of the Parasympathetic Division

• Cell bodies are found in the brain and in the sacral region of the spinal cord

• Preganglionic fibers are long and synapse with short postganglionic fibers on or near the target viscera

• Both preganglionic and postganglionic fibers produce Ach

• “Rest & digest” division


Relationship Between the Sympathetic and Parasympathetic Divisions

• Most organs receive dual innervation

• It is a tug of war between the two


ANS either increases excitation or inhibits the activity

– Ex. Sympathetic fibers increase heart rate, parasympathetic fibers decrease heart rate.

– Homeostasis comes from the balance of the two.


ANS either increases excitation or inhibits the activity

Ex.#2 Sympathetic fibers decreases stomach motility.

Parasympathetic fibers increase stomach motitlity


Parasympathetic innervation

• The cranial nerve fibers involved are motor - control smooth muscle & glands in the upper body– Cranial nerve #3 – lens & pupil – Cranial nerve #7 – lacrimal glands,

submandibular & submaxillary glands (salivary)– Cranial nerve #9 – parotid gland (salivary)– Cranial nerve #10 - viscera of thorax & abdomen

• Sacral nerves innervate the kidneys, colon, & sex organs

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Human Anatomy 5th ed. 2005 Benjamin Cummings General Anatomy of the Autonomic Nervous System.

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