1. The Autonomic Nervous System
Chapter 15
The Autonomic Nervous System
Lecture slides prepared by Curtis DeFriez, Weber State University

2. Introduction to the ANS
In this chapter, we examine the structural and functional features of the autonomic nervous system (ANS) and compare the organization and actions of its two major parts, the sympathetic and parasympathetic divisions.
The autonomic nervous system contributes to homeostasis by responding to subconscious visceral sensations and exciting or inhibiting smooth muscle, cardiac muscle, and many glands.

3. Introduction to the ANS
Structurally, the ANS includes autonomic sensory neurons, integrating centers in the CNS, and autonomic motor neurons.
The enteric division is a specialized network of nerves and ganglia forming an independent nerve network within the wall of the gastrointestinal (GI) tract. The enteric division will not be further discussed in this chapter, but we will return to it in Chapter 24.

4. Introduction to the ANS
While both the ANS and the somatic nervous system (SNS) include sensory and motor neurons, the ANS has many distinctive features which set it apart.
Perhaps the biggest difference between these two systems is the involvement of conscious control.
In the SNS, feedback via tactile, thermal, pain, and proprioceptive sensations are consciously perceived, and skeletal muscle is the main tool used to provide reflexive and voluntary movement.

5. Introduction to the ANS
If a somatic motor neuron ceases to stimulate a muscle, the result is a paralyzed, limp muscle that has no tone.
Although we are generally not conscious of breathing, the muscles that generate respiratory movements are skeletal muscles controlled by somatic motor neurons.
If the respiratory motor neurons become inactive, breathing stops.

6. Introduction to the ANS
The ANS usually operates without conscious control, though centers in the hypothalamus and brain stem do provide regulation for ANS reflexes.
Sensory receptors called interoceptors located in blood vessels, visceral organs, muscles, and the nervous system monitor conditions in the internal environment.
Examples of interoceptors are chemoreceptors that monitor blood CO2 level and mechanoreceptors that detect the degree of stretch in the walls of organs or blood vessels.

7. ANS Motor Pathways
Autonomic motor neurons regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities in their effector tissues.
Because autonomic responses cannot be consciously altered to any great degree, some autonomic responses are the basis for polygraph (“lie detector”) tests.
However, practitioners of yoga and biofeedback techniques may learn how to regulate at least some of their autonomic activities through long practice.

8. Introduction to the ANS
The anatomy of all autonomic pathways can best be understood by picturing a double-barrelled neuronal construct consisting of
a preganglionic neuron
leading to an intermediate
ganglion that contains
the cell bodies of post-
ganglionic neurons
(that innervate an effector).

9. Introduction to the ANS

10. Introduction to the ANS Interactions Animation
The ANS: An Introduction Animation
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11. Divisions of the ANS
Most body organs have dual ANS innervation; that is, they receive impulses from both sympathetic and parasympathetic neurons.
Usually the nerve impulses from one division stimulate an organ, while impulses from the other division decrease activity.

12. Divisions of the ANS

13. Divisions of the ANS

14. Divisions of the ANS
Furthermore, the responses of the various organs to ANS stimulation neatly group into two functional categories :
Like children on a teeter-totter, the sympathetic divisions “fight or flight” response is balanced against the “rest and relax” (or rest and digest)
activities of the parasympathetic division.

15. The Sympathetic Division
The cell bodies of neurons which participate in motor responses of the sympathetic nervous system are located
in the lateral horns of the gray matter in the 12 thoracic segments and the first two lumbar segments of the cord.
Sympathetic preganglionic neurons exit the spinal cord only between levels T1-L2 (hence the name thoracolumbar division), though sympathetic ganglia extend in the vicinity of the cord from the cervical to the sacral region.

16. The Sympathetic Division
Some of the major groups of sympathetic ganglia include:
The sympathetic trunk
(vertebral chain) ganglia
Prevertebral ganglia
The celiac, superior
mesenteric, inferior
mesenteric, aorticorenal
and renal ganglia

17. The Sympathetic Division
Axons leave the sympathetic trunk in four possible ways:
They can enter and travel with spinal nerves.
They can form fine networks of periarterial preganglionic traveling cephalad to synapse in the cervical ganglia.
Postganglionic axons exiting the sympathetic trunk can form sympathetic nerves to the heart and lungs.
Preganglionic axons can leave the sympathetic trunk without synapsing and form splanchnic nerves.
Gray ramus: Axons of some postganglionic neurons leave the sympathetic trunk by entering a short pathway called a gray ramus and merge with the anterior ramus of a spinal nerve.
Gray rami communicantes: structures containing sympathetic postganglionic axons that connect the ganglia of the sympathetic trunk to spinal nerves.

18. The Sympathetic Division
Major groups of sympathetic ganglia.

19. The Sympathetic Division
A truism of the sympathetic division is that a single sympathetic preganglionic fiber synapses with many postganglionic branches (with 20 or more) to create a diverging circuit.
The postganglionic axons typically terminate in several different visceral effectors, making the effects of sympathetic stimulation a widespread massive response.
This is why anger can be hard to control – it is such a diffuse response.

20. The Sympathetic Division
This schematic
illustrates the outflow of the sympathetic division of the ANS via thoracolumbar pathways to the
many organs of the
body.

21. The Parasympathetic Division
The cell bodies of preganglionic neurons which participate in motor responses of the parasympathetic nervous system are located in nuclei of 4 cranial nerves in the brainstem (III, VII, IX and X) and in the lateral gray matter of sacral areas of the spinal cord (S2-S4).
The vagus nerve (CN X) carries nearly 80% of the total parasympathetic flow to the organs of the thorax and upper abdomen. Lower abdominal and pelvic organs are innervated by the sacral output.

22. The Parasympathetic Division
Parasympathetic ganglia are called terminal ganglia because they are located far from their origin at the “terminal” ends of the pathways (near the target organs).
Four pairs of cranial parasympathetic ganglia innervate structures in the head: The ciliary, pterygopalatine, submandibular, and otic ganglia.
The cranial-sacral division also has the ganglia associated with the vagus (X) nerve and the sacral nerves.

23. Most of the parasympathetic ganglia are located very close to the organs or intended action.

24. The Parasympathetic Division
The sacral preganglionic axons branch off of sacral spinal nerves to form pelvic splanchnic nerves which synapse with parasympathetic postganglionic neurons located in terminal ganglia in the walls
of the innervated viscera.
From the terminal ganglia,
postganglionic axons innervate
smooth muscle and glands in
the walls of the colon, ureters,
urinary bladder, and
reproductive organs

25. The Parasympathetic Division
In contrast to the sympathetic system, the parasympathetic response is more controlled.
Presynaptic parasympathetic neurons usually synapse with only 4–5 postsynaptic neurons, all of which supply a single visceral effector. Parasympathetic stimulation leads to a narrow, focused action on specific organs.
This is why it is possible to walk and chew gum at the same time (not really!)

26. The division of the sympathetic and parasympathetic divisions of the ANS are compared in Table 15.3

27. ANS Neurotransmitters
The total number of neurotransmitters used in the entire nervous system is not known, but is well over 100.
Despite the variety of possible chemicals that could be used to transmit chemical messages in the ANS, only 2, acetylcholine and norepinephrine, are used to any great degree.
Synapses at which ACh is used are termed cholinergic.
Synapses at which norepinephrine or epinephrine are used are termed adrenergic.

28. ANS Neurotransmitters
The neurotransmitter used in all of the synapses of sympathetic and parasympathetic ganglia (between the synapses of the preganglionic and postganglionic fibers) is acetylcholine.
Receptors that respond to Ach released by these cholinergic neurons are called cholinergic receptors and there are 2 subtypes: nicotinic receptors (found in the ganglia) and muscarinic receptors (found in the synapses with the effector organs).
There are also two subtypes of receptors that respond to norepinephrine: alpha and beta receptors that are widely scattered throughout the body. Even these subtypes of adrenergic receptors can be further subtyped.

29. ANS Neurotransmitters
Acetylcholine acts on a sub-type of cholinergic receptor (called nicotinic receptors) at ganglia of the ANS.

30. ANS Neurotransmitters
The neurotransmitter used at most sympathetic postganglionic synapses is norepinephrine.
The exception to this rule is that ACh is used at sympathetic postganglionic synapses for sweat glands.

31. ANS Neurotransmitters
The neurotransmitter used at all parasympathetic postganglionic synapses
is Ach.
These are all a variety
of cholinergic receptors
called muscarinic.

32. ANS Neurotransmitters
Neurons and Neurotransmitters of the Parasympathetic Nervous System
Preganglionic
Postganglionic
Cell body in brain or spinal cord
Cell body in intramural ganglion
Acetylcholine (ACh)

33. ANS Neurotransmitters
Neurons and Neurotransmitters of
the Sympathetic Nervous System
Preganglionic
Postganglionic
oCell body in lateral horn of ospinal cord
Cell body in sympathetico chain gangliono
oAcetylcholine (ACh)
(norepinephrine, NE) ol
except sweat glands (Ach) o

34. ANS Neurotransmitters Interactions Animation
ANS Neurotransmitters and Neurons Animation
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35. Physiology of the ANS
Sympathetic stimulation leads to secretion of norepinephrine by the adrenal glands, an increase in the rate and strength of the heartbeat, constriction of blood vessels of non-essential organs, dilation of vessels of essential organs (skeletal muscle and the cerebral cortex), an increase in the rate and depth of breathing, hepatic conversion of glycogen to glucose, and decrease in GI activity.

36. Physiology of the ANS

37. Physiology of the ANS

38. Physiology of the ANS

39. Physiology of the ANS
SLUDD is as an acronym used to describe the responses of the parasympathetic nervous system:
Salivation (increased)
Lacrimation (increased)
Urination (increased)
Digestion (increased)
Defecation (increased)
… and 3 decreases (in the rate and force of the heart beat, airway size and rate of breathing, and pupil size)

40. Physiology of the ANS Interactions Animation
The balance of autonomic sympathetic-parasympathetic tone is regulated by feedback loops between the spinal cord and brainstem, with input from the limbic system and oversight by the hypothalamus.
Physiological Effects of the ANS Animation
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41. End of Chapter 15
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