1. The Autonomic Nervous System
Chapter 15

2. Introduction
The Autonomic Nervous System (ANS) is the system of motor neurons that innervate the smooth muscle, cardiac muscle, and glands of the body
By controlling these effectors, the ANS regulates such visceral functions as …
Heart rate
Blood pressure
Digestion
Urination

3. Introduction
The ANS is the general visceral motor division of the peripheral nervous system and is distinct from the general somatic motor and brachial motor divisions which innervate skeletal muscles

4. Introduction
The general visceral sensory system continuously monitors the activities of the visceral organs so that the autonomic motor neurons can make adjustments as necessary to ensure optimal performance of the visceral organs

6. Introduction
The stability of our internal environment depends largely on the autonomic nervous system
Autonomic nervous system(ANS) receives signals from visceral organs
The ANS makes adjustments as necessary to ensure optical support for body systems

7. Comparison of ANS & PNS
Recall that the somatic motor system innervates skeletal muscles
Each somatic motor neuron runs from the central nervous system all the way to the muscle being innervated, and that each motor unit consists of a single neuron plus the skeletal muscle cells it innervates
Typical somatic motor axons are thick, heavily myelinated fibers that conduct nerve impulses rapidly

8. Comparison of ANS & PNS

9. Comparison of ANS & PNS In the somatic system
Cell bodies are within the central nervous system
Axons extend to the muscles they serve
Somatic motor fibers are thick, heavily myelinated Type A fibers that conduct impulses very rapidly

10. Comparison of ANS & PNS In the autonomic nervous system
The motor unit is a two neuron chain
The cell body of the first neuron, the preganglionic neuron, resides in the brain or spinal cord
Its axon, the preganglionic axon, synapses with the second motor neuron, the post- ganglionic neuron, in an autonomic ganglion outside the central nervous system
The postganglionic axon then extends to the effector organ

11. Comparison of ANS & PNS
Compare the one motor neuron of the somatic motor division with the two neuron chain of the autonomic nervous system

12. Efferent Pathways and Ganglia
Axons of most preganglionic neurons run from the CNS to synapse in a peripheral autonomic ganglion with a postganglionic neuron

13. Efferent Pathways and Ganglia
Axons of postganglionic neurons run from the ganglion to the effectors (cardiac and smooth muscle fibers and glands)

14. Efferent Pathways and Ganglia
Preganglionic axons are lightly myelinated thin fibers
Postganglionic axons are even thinner and are unmyelinated
Conduction though the autonomic chain is slower than through the somatic motor
Many pre and postganglionic fibers are incorporated into spinal or cranial nerves for most of their course

15. Efferent Pathways and Ganglia
Remember that autonomic ganglion are motor ganglia, containing the cell bodies of motor neurons
They are sites of synapse and information transmission from pre to postganglionic neurons
Also note that the somatic motor division lacks ganglia entirely

16. Neurotransmitter Effects
All somatic motor neurons release acetylcholine at their synapses with their effectors, skeletal muscle fibers
The effect is always excitatory, and if stimulation reaches threshold, the skeletal muscle fibers contacts

17. Neurotransmitter Effects
Neurotransmitters released onto visceral effector organs by postganglionic autonomic fibers include
Norepinephrine secreted by most sympathetic fibers
Acetylcholine released by parasympathetic fibers
Depending on the receptors present on the target organ, its response to these neuro- transmitters may be either excitation or inhibition

18. Overlap of Somatic & Autonomic
Higher brain centers regulate and coordinate both somatic and visceral motor activities
Nearby spinal nerves and many cranial nerves contain both somatic and autonomic fibers
Most of the body’s adaptations to changing internal and external conditions involve both skeletal activity and enhanced response of visceral organs

19. Divisions of ANS There are two division of the ANS
Parasympathetic
Sympathetic
Generally the two divisions have chains of two motor neurons that innervate same visceral organs but cause essentially opposite effects
If one division stimulates certain smooth muscle to contract or a gland to secrete, the other division inhibits that action
Through this process of duel innervation the two systems counterbalance each other

20. Divisions of ANS
The sympathetic part mobilizes the body during extreme situations (such as fear, exercise or rage)
The parasympathetic division allows us to unwind as it performs maintenance activities and conserves body energy

21. Divisions of ANS
Both the sympathetic and parasympathetic divisions issue from the brain and spinal cord
Two neuron pathways are shown for both divisions
Solid lines indicate pre-ganglionic axons while broken lines indicate post-ganglionic axons

22. Sympathetic Division
The sympathetic division is responsible for the “fight, flight, or fright” response
Its activity is evident during vigorous exercise, excitement, or emergencies
Physiological changes like a pounding heart, fast and deep breathing, dilated eye pupils, and cold, sweaty skin are signs of the mobilization of the sympathetic division, which help us survive danger

23. Sympathetic Division
Sympathetic responses prepare our bodies to cope with physiological stressors
While sympathetic response may increases the capacities of some systems they may in fact inhibit “non-essential” functions such as digestion and urinary tract motility

24. Sympathetic Division
The sympathetic system also innervates blood vessels, sending signals to the smooth muscles in their walls
Even though sympathetic input causes the smooth muscle in some vessels (in skeletal muscle) to relax so that the vessel dilates, the bulk of sympathetic input signals smooth muscle in blood vessels to contract, producing vasoconstriction

25. Sympathetic Division
Vasoconstriction results in the narrowing of vessel diameter which forces the heart to work harder to pump blood around the vascular circuit
As a result sympathetic activity results in blood pressure to rise during excitement and stress

26. Role of Sympathetic Division
During exercise the sympathetic division also promotes physiological adjustments
Visceral blood supply is diminished
Blood is shunted to working musculature
Bronchioles of the lungs dilate to increase ventilation
Liver releases more sugar into blood stream to support metabolism

27. Role of Sympathetic Division
Its activity is evident when we are excited or find ourselves in emergency or threatening situations (frightened)
Pounding heart; rapid, deep breathing; cold, sweaty skin; and dilated eyes are signs
Also changes in brain wave patterns
Its function is to provide the optimal conditions for an appropriate response to some threat (run / see / think)

28. Parasympathetic Division
The parasympathetic division is most effective in non-stressful situations
This division is chiefly concerned with keeping body energy use as low as possible, even as it directs body processes such as digestion and elimination
Resting and digesting division

29. Autonomic Homeostasis
Autonomic homeostasis is the dynamic counteraction between the two divisions such that they balance each other during times when we are neither highly excited nor completed at rest

30. Divisions of ANS
In addition to the functional differences between the parasympathetic and sympathetic divisions , there are also anatomical and biochemical differences

31. Divisions of ANS
The two divisions issue from different regions of the CNS
The sympathetic can also be called the thoracolumbar division because its fibers emerge from the thoracic and lumbar parts of the spinal cord

32. Divisions of ANS
The parasympathetic division can also be termed the craniospinal division because its fibers emerge from the brain and spinal cord (sacral)

33. Comparison of ANS & PNS
A second difference between the two divisions is that sympathetic pathways have short pre-ganglionic fibers and long post-ganglionic fibers

34. Comparison of ANS & PNS
Parasympathetic pathways in contrast have long pre-ganglionic fibers and short post-ganglionic fibers

35. Divisions of ANS
Therefore, all sympathetic ganglia lie near the spinal cord and vertebral column, and all parasympathetic ganglia lie far from the CNS, in or near the organs innervated

36. Divisions of ANS
The third anatomical difference between the two divisions is that sympathetic axons branch profusely, while parasympathetic fibers do not

37. Divisions of ANS
Extensive branching allows each sympathetic neuron to influence a number of different visceral organs, enabling many organs to mobilize simultaneously during the “fight, flight or fright” response
Parasympathetic effects, by contrast are more localized and discrete

38. Divisions of ANS
The main biochemical difference between the two divisions involves the neurotransmitter release by the postganglionic axons

39. Divisions of ANS
In the sympathetic division, most postganglionic axons release norepinephine (also called noradrenaline) these fibers are termed adrenergic
The postganglionic neurotransmitter in the parasympathetic division is acetycholine (Ach) these fibers are termed cholinergic
The preganglionic axon terminals of both divisions always release acetylcholine

40. Divisions of ANS
The main anatomical and physiological differences between the parasympathetic and sympathetic divisions are summarized in Table 15.1

41. Anatomy of ANS
The sympathetic and parasympathetic divisions are distinguished by
Unique sites of origin
Different lengths of their fibers
Location of their ganglia

42. Anatomy of ANS Unique origin sites
Parasympathetic fibers emerge from the brain and from the spinal cord at the sacral level
Sympathetic fibers originate from the thoracic and lumbar regions of the spinal cord

43. Anatomy of ANS Different Lengths of their Fibers
Parasympathetic division has long preganglionic and short postganglionic fibers
Sympathetic is the opposite with short preganglionic and long postganglionic fibers

44. Anatomy of ANS Length of their Ganglia
Most parasympathetic ganglia are located in the visceral effector organs
Sympathetic ganglia lie close to the spinal cord

45. Parasympathetic Division
The parasympathetic emerge from opposite ends of the central nervous system
The preganglionic axons extend from the CNS nearly all the way to the structures to be innervated

46. Parasympathetic Division
The preganglionic neurons synapse with the ganglionic neurons located in terminal ganglia
Very short post ganglionic axons issue from the terminal ganglia and synapse with effector cells in their immediate area

47. Parasympathetic Division
Several cranial nerves contain outflow of the parasympathetic
Preganglionic fibers run in the oculomotor, facial, glossopharyngeal, and vagus nerve

48. Cranial Outflow Oculomotor nerve III
The parasympathetic fibers of the oculomotor nerves innervate smooth muscles of the eye
Constrictor muscles of iris cause pupil to constrict
Ciliary muscle within the orbits of the eye controls lense shape for visual focusing
Allow the eye to focus on close objects in the visual field

50. Cranial Outflow Facial Nerves VII
The parasympathetic fibers of the facial nerves stimulate the secretory activity of many large glands of the head
The pathway activates the nasal glands and the lacrimal glands of the eyes
The preganglionic fibers then run to synapse with ganglionic neurons in the pterygopalatine ganglia stimulating the submandibular and sublingual salivary glands

52. Cranial Outflow Glossopharyngeal (IX)
The parasympathetic nerves originate in the medulla and activate the parotid salivary gland

53. Cranial Outflow Vagus nerves (X)
The major portion of the parasympathetic cranial outflow is via the vagus nerves
The two vagus nerves account for an estimated 90% of all preganglionic parasympathetic fibers in the body
They provide fibers to the neck and contribute to nerve plexuses that serve virtually every organ in the thoracic and abdominal cavity

54. Cranial Outflow
The vagus nerve fibers arise from the dorsal motor of the medulla and terminate by synapsing in terminal ganglia that are usually located in the walls of the target organ

55. Cranial Outflow
Most of the terminal ganglia are not individually named; instead they are collectively called intramural ganglia, literally ganglia “within the walls”
As the vagus nerves passes into the thorax, they send branches to autonomic plexuses
Cardiac plexuses
Pulmonary plexuses
Esophageal plexuses

56. Cranial Outflow
When the vagus nerves reach the esophagus, their fibers intermingle to form the anterior and posterior vagal trunks
Each trunk carries fibers from both vargus nerves

57. Cranial Outflow
The vagal trunks ride the esophagus down to enter the abdominal cavity
They send fibers to form the aortic plexuses (formed by the celiac, superior mesenteric and hypogastric)

58. Cranial Outflow
Abdominal organs which receive vagal innervation include the liver, gallbladder, stomach, small intestine, kidneys, pancreas, and the proximal half of the large intestine
The rest of the cavity are innervated by the sacral outflow

59. Sacral Outflow
The sacral outflow arises from neurons located in the lateral horn of the spinal cord at S2 - S4
The axons of these neurons run in the ventral roots of the spinal nerves to the ventral rami

60. Sacral Outflow
From the ventral rami the neurons branch to form the pelvic splanchnic nerves
Most neurons synapse in the intramural ganglia located in the walls of the distal large intestine, urinary bladder and reproductive organs

61. Sympathetic Division The sympathetic division innervates more organs
It supplies not only the visceral organs in the internal body cavities, but also the visceral structures in the superficial part of the body
Sweat glands
Arrector pili
Arteries and veins

62. Sympathetic Division
All preganglionic fibers in the sympathetic division arise from cell bodies of preganglionic neurons located in spinal cord segments from T1 through L2
It is also called the thoracolumbar

63. Sympathetic Division
After leaving the cord via the ventral root, the preganglionic sympathetic fibers pass through a white ramus communicans to enter the adjoining chain (paravertebral) ganglion forming part of the sympathetic trunk or chain

64. Sympathetic Division
The sympathetic trunks flank each side of the vertebral column and appear as strands of white beads

65. Sympathetic Division
Although the sympathetic trunks extend from the neck to the pelvis, sympathetic fibers arise only from the thoracic and lumbar spinal cord segments

66. Sympathetic Division
The ganglia vary in size, position, and number, but there are typically 23 ganglia in each sympathetic chain…
3 cervical
11 thoracic
4 lumbar
4 sacral
1 coccygeal

67. Sympathetic Division
Once a preganglionic axon reaches a paravertebral ganglion one of three things can happen to it

68. Sympathetic Division
Once a preganglionic axon reaches a paravertebral ganglion one of three things can happen to it…
1. It can synapse with a ganglionic neuron within the same chain ganglion

69. Sympathetic Division
Once a preganglionic axon reaches a paravertebral ganglion one of three things can happen to it…
2. It can ascend or descend the sympathetic chain to synapse in another chain ganglion

70. Sympathetic Division
Once a preganglionic axon reaches a paravertebral ganglion one of three things can happen to it…
3. It can pass through the chain ganglion and emerge from the sympathetic chain without synapsing

71. Sympathetic Division
Preganglionic fibers which emerge from the sympathetic chain without synapsing help to form the splanchnic nerves which synapse with prevertebral or collateral ganglia

72. Sympathetic Division
The prevertebral ganglia are located anterior to the vertebral column

73. Sympathetic Division
Unlike the paravertebral ganglia the prevertebral ganglia . . .
Are neither paired nor segmentally arranged
They occur only in the abdomen and pelvis

74. Sympathetic Division Note:
Regardless of where the synapse occurs, all sympathetic ganglia lie close to the spinal cord
The postganglionic fibers which run from the ganglion to the organs are typically much longer than the preganglionic fibers

75. Visceral Reflexes
The visceral sensory neurons are the first link in the autonomic reflexes
These neurons send information concerning chemical changes, stretch, and irritation of the viscera

76. Visceral Reflexes
Visceral reflex arcs have essentially the same components as somatic reflex arcs
Receptor
Sensory neuron
Integration center
Motor neuron
Effector

77. Visceral Reflexes
Visceral reflex arcs differ in that they have a two-neuron chain

78. Visceral Reflexes
Nearly all sympathetic and parasympathetic fibers are accompanied by afferent fibers conducting sensory impulses from glands or muscular structures
Thus, peripheral processes of visceral sensory neurons are found in cranial nerves, VII, IX, and X, the splanchnic nerves, and the sympathetic trunk, as well as the spinal nerves

79. Visceral Reflexes
Like sensory neurons serving somatic structures (skeletal muscles and skin)
The cell bodies of visceral sensory neurons are located in the sensory ganglia of associated cranial nerves or in the dorsal root ganglia of the spinal cord

80. Visceral Reflexes
Visceral sensory reflexes are also found within sympathetic ganglia where synapses with preganglionic neurons occur
Complete three-neuron reflex arcs (sensory, motor, and intrinsic neurons) exist within the walls of the gastro-intestinal tract
Enteric nervous system
Controls gastrointestinal activity

81. Visceral Reflexes
The fact that visceral pain travels along the same pathways as somatic pain fibers helps to explain the phenomenon of referred pain in which pain stimuli arising in the viscera is perceived as somatic in origin

82. Visceral Reflexes
A heart attach may produce a sensation of pain that radiates to the superior thoracic wall and along the medial aspect of the left arm

83. Visceral Reflexes
Since the same spinal segments (T1-T5) innervate both the heart and the regions to which pain signals from heart tissue are referred, the brain interprets most such inputs as coming from the somatic pathway

84. Visceral Reflexes
Additional cutaneous areas to which visceral pain is referred

85. Overview of the ANS The autonomic nervous system differs in…
Its effectors
Its efferent pathways
Its target organs

86. Effectors of ANS
The somatic nervous system stimulates skeletal muscles
The ANS innervates cardiac and smooth muscles and glands