Chapter 2: Autonomic Nervous System

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1

What are the two components of the motor (efferent) nervous system?

  1. somatic
  2. autonomic
2

What is the somatic nervous system?

It is a voluntary motor system under conscious control that innervates skeletal muscle.

3

What does a pathway in the somatic nervous system consist of?

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A single motoneuron and the skeletal muscle fibers it innervates.

4

What neurotransmitter is released from presynaptic terminals of motoneurons?

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acetylcholine

5

What is the autonomic nervous system?

An involuntary motor system that controls and modulates the functions primarily of visceral organs.

6

What does a pathway in the autonomic nervous system consist of?

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A preganglionic neuron and a postganglionic neuron, which synapse on the effector organ.

7

What neurotransmitter is released from presynaptic terminals of preganglionic neurons?

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acetylcholine

8

What neurotransmitter is released from presynaptic terminals of postganglionic neurons?

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Either acetylcholine, norepinephrine or neuropeptides.

9

What are the two major divisions of the autonomic nervous system?

  1. sympathetic
  2. parasympathetic
10

What is the enteric nervous system?

A third, minor division of the autonomic nervous system located in plexuses of the gastrointestinal tract.

11

Where do preganglionic neurons in the sympathetic division originate?

In the thoracolumbar spinal cord.

12

Where do preganglionic neurons in the parasympathetic division originate?

In the brain stem and sacral spinal cord.

13

What are adrenergic neurons and what type of receptor do they act on?

They are neurons that release norepinephrine, which acts on adrenoreceptors.

14

What are cholinergic neurons and what type of receptor do they act on?

They are neurons that release acetylcholine (ACh), which acts on cholinoreceptors.

15

What are nonadrenergic, noncholinergic neurons?

Some postganglionic parasympathetic neurons of the gastrointestinal tract that release peptides or other substances besides ACh (e.g. substance P, nitric oxide)

16

What are neuro­effector junctions?

The junctions between postganglionic autonomic neurons and their effectors (target tissues).

17

Describe three ways in which neuro­effector junctions differ from neuromuscular junctions.

  1. postganglionic neurons form diffuse, branching networks with varicosities
  2. branching networks from different postganglionic neurons overlap
  3. there is no region like the motor end plate; receptors are widely distributed
18

What is the overall function of the sympathetic nervous system?

To mobilize the body for activity.

19

What is the “fight or flight” response?

A response mediated by the sympathetic nervous system that includes increased arterial pressure, blood flow to active muscles, metabolic rate, blood glucose concentration, and mental activity and alertness.

20

What are paravertebral ganglia?

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Ganglia near the spinal cord that form the sympathetic chain (e.g. superior cervical ganglion).

21

What are prevertebral ganglia?

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Ganglia that supply visceral organs, glands, and the enteric nervous system of the gastrointestinal tract (e.g. celiac, superior mesenteric, and inferior mesenteric).

22

Why is the sympathetic nervous system referred to as "thoracolumbar"?

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Because preganglionic neurons of the sympathetic division arise from nuclei in the thoracic and lumbar spinal cord segments (T1–L3).

23

How do preganglionic and postganglionic nerve axons compare in the sympathetic nervous system?

Because the sympathetic ganglia are located near the spinal cord, the preganglionic nerve axons are short and the postganglionic nerve axons are long.

24

What type of receptors do preganglionic neurons in the sympathetic nervous system an act on?

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They are cholinergic, releasing ACh, which interacts with nicotinic (N2) receptors.

25

What type of receptors do postganglionic neurons in the sympathetic nervous system an act on?

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They are adrenergic, releasing norepinephrine, which interacts with adrenoreceptors (except in the sweat glands where they are cholinergic, acting on muscarinic cholinoreceptors).

26

What is contained in the small dense-core vesicles of sympathetic adrenergic varicosities?

They contain norepinephrine and ATP (said to be “colocalized” with norepinephrine).

27

What is contained in the large dense-core vesicles of sympathetic adrenergic varicosities?

They contain neuropeptide Y, which augments the actions of norepinephrine.

28

What is the adrenal medulla?

A specialized ganglion in the sympathetic division of the autonomic nervous system.

29

What are chromaffin cells?

Cells of the the adrenal medulla that secrete catecholamines (epinephrine and norepinephrine).

30

Why does the adrenal medulla secrete mainly epinephrine, whereas sympathetic postganglionic neurons release only norepinephrine?

Because of phenylethanolamine-N-methyltransferase (PNMT) in the adrenal medulla, which catalyzes the conversion of norepinephrine to epinephrine.

31

What is a pheochromocytoma?

A tumor of the adrenal medulla.

32

Why does a pheochromocytoma secrete maimly norepinephrine, whereas the adrenal medulla secretes mainly epinephrine?

Because they are located too far from the adrenal cortex to receive the cortisol that is required for the function of PNMT.

33

What is the overal function of the parasympathetic nervous system?

To restore and conserve energy.

34

Why is the parasynpathetic nervous sytem reffered to as "craniosacral"?

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Because preganglionic neurons of the parasympathetic division arise from nuclei of cranial nerves (CNs) III, VII, IX, and X or from sacral spinal cord segments S2–S4.

35

How do preganglionic and postganglionic nerve axons compare in the parasympathetic nervous system?

Because parasympathetic ganglia are located near or in the effector organs, preganglionic neurons have long axons and postganglionic neurons have short axons.

36

What type of receptors do preganglionic neurons in the sympathetic nervous system an act on?

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They are cholinergic, releasing ACh, which interacts at nicotinic (N2) receptors.

37

What type of receptors do postganglionic neurons in the sympathetic nervous system an act on?

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Most are cholinergic, releasing ACh, which interacts at muscarinic (M) receptors.

38

What is contained in the small, clear vesicles of parasympathetic adrenergic varicosities?

They contain ACh.

39

What is contained in the large dense-core vesicles of parasympathetic adrenergic varicosities?

They contain peptides (e.g. VIP), which augment the actions of ACh.

40

How does the autonomic nervous system act on the sinoatrial node of the heart?

An increase in sympathetic activity of the SA node increases heart rate; increase in parasympathetic activity of the SA node decreases heart rate.

41

How does the autonomic nervous system act on the urinary bladder when it is filling?

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Increased sympathetic activity produces relaxation of the detrusor muscle, via β2 receptors, and contraction of the internal sphincter muscle, via α1 receptors.

42

How does the autonomic nervous system act on the urinary bladder when it is full?

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Increased parasympathetic activity produces contraction of the detrusor muscle and relaxation of the internal sphincters (micturition reflex).

43

How does the autonomic nervous system act on the pupil during dilation?

The pupillary dilator muscle, controlled by sympathetic innervation through α1 receptors, contracts to produce dilation of the pupil, or mydriasis.

44

How does the autonomic nervous system act on the pupil during constriction?

The pupillary constrictor muscle, controlled by parasympathetic innervation through M receptors, contracts to produce con­striction of the pupil, or miosis.

45

What is the pupillary light reflex?

Light on the retina activates activates parasympathetic preganglionic nerves in the Edinger-Westphal nucleus, which cause pupillary constriction.

46

What is the accommodation response?

A blurred retinal image activates parasympathetic neurons in the Edinger-Westphal nuclei and leads to pupillary constriction.

47

What is Horner's syndrome?

A classic triad of a drooping eyelid (ptosis), constriction of the pupil (miosis), and lack of sweating (anhidrosis).

48

What is Shy-Drager syndrome?

A rare, progressive disease of the central autonomic nervous system associated with degeneration of preganglionic neurons resulting in impotence, difficulty urinating, and heat intolerance are all explained by sympathetic and parasympathetic failures.

49

What is the role of the hypothalamus and brain stem in autonomic function?

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They coordinate the autonomic regulation of organ system functions.

50

How does the β1 receptor demonstrate the tissue-specificity of autonomic receptors?

The β1 receptor in the SA node is coupled to mechanisms that increase heart rate, while the same receptor in ventricular muscle is coupled to mechanisms that increase contractility.

51

Why are autonomic receptors called G protein–linked receptors?

Because they are coupled to GTP-binding proteins (G proteins).

52

What are G proteins?

Heterotrimeric proteins composed of α, β, and γ subunits, that couple G protein–linked receptors to enzymes that execute physiologic actions.

53

How are G proteins activated?

The α subunit binds either GDP or GTP; when GDP is bound, the α subunit is inactive; when GTP is bound, the α subunit is active.

54

What are the four types of adrenoreceptors?

  1. α1
  2. α2
  3. β1
  4. β2
55

Where are α1 receptors found?

In vascular smooth muscle of the skin, skeletal muscle, in sphincters of the gastrointestinal tract and bladder, and in the radial muscle of the iris.

56

What does activation of α1 receptors cause?

Smooth muscle contraction.

57

Decribe the mechanism of α1 receptors in seven steps.

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  1. α1 receptor is activated by norepinephrine
  2. αq subunit of Gq protein binds GTP
  3. αq-GTP complex activates phospholipase C
  4. phospholipase C liberates IP3 and DAG
  5. IP3 causes the release of Ca2+ from ER or SR
  6. Ca2+ and DAG activate protein kinase C
  7. protein kinase C phosphorylates proteins
58

Where are α2 receptors located?

On presynaptic and postsynaptic adrenergic and cholinergic nerve terminals and in the gastrointestinal tract.

59

What does activation of α2 receptors cause?

Inhibition of target tissues.

60

What are the two forms of α2 receptors?

  1. autoreceptors
  2. heteroreceptors
61

What are autoreceptors?

α2 receptors present on sympathetic postganglionic nerve terminals that inhibit release of norepinephrine from presynaptic nerve terminals.

62

What are heteroreceptors?

α2 receptors present on parasynpathetic postganglionic nerve terminals that inhibit release of ACh from presynaptic nerve terminals.

63

Describe the mechanism of α2 receptors in four steps.

  1. α2 receptor is activated by norepinephrine
  2. αi subunit of Gi protein binds GTP
  3. αi-GTP complex inhibits adenylyl cyclase
  4. cAMP levels decrease
64

Where are β1 receptors found?

They are prominent in the heart, in the SA node, atrioventricular (AV) node, and ventricular muscle.

65

What does activation of β1 receptors cause?

Increase in heart rate in the SA node, increase in conduction velocity in the AV node, and increase in contractility in ventricular muscle.

66

Describe the mechanism of β1 and β2 receptors in five steps.

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  1. β1 receptor is activated by norepinephrine
  2. αs subunit of Gs protein binds GTP
  3. αs-GTP complex activates adenylyl cyclase
  4. adenylyl cyclase converts of ATP to cAMP
  5. cAMP activates protein kinases
67

Where are β2 receptors found?

In the vascular smooth muscle of skeletal muscle, in the walls of the gastrointestinal tract and bladder, and in the bronchioles.

68

What does activation of β1 receptors cause?

Smooth muscle relaxation or dilation.

69

What are the two types of cholinoreceptors?

  1. nicotinic
  2. muscarinic
70

Where are nicotinic receptors found?

On the motor end plate of skeletal muscle (N1), on all postganglionic neurons of the SNS and PNS (N2), and on the chromaffin cells of the adrenal medulla (N2).

71

What is nicotine?

A nicotinic receptor agonist.

72

What is curare?

A nicotinic receptor antagonist.

73

What is hexamethonium?

A ganglionic-blocking agent, which produces different responses in different organs depending on whether sympathetic or parasympathetic control is dominant.

74

What is the the mechanism of nicotinic receptors?

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Binding of ACh to the nicotinic receptor induces a conformational change opens its central core, allowing Na+ and K+ flow down their electrochemical gradients, causing depolarization.

75

Where are muscarinic receptors found?

In all of the effector organs of the parasympathetic nervous system (M1, M3, M5) and specific sympathetic effector organs, namely the sweat glands (M2, M4).

76

What is the mechanism of action of M1, M3, and M5 receptors?

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They follow the same pathway as α1 adrenoreceptors, acting through G-protein activation of phospholipase C, which generates IP3 and DAG, which increase Ca2+.

77

What is the mechanism of action of M4 receptors?

They inhibit adenylyl cyclase to decrease cAMP levels.

78

What is the mechanism of action of M2 receptors?

They alter physiologic processes via a direct action of the G protein.

79

How does scopolamine work?

It blocks cholinergic muscarinic receptors in target tissues.