Chapter 17: The Autonomic Nervous System



Chapter 15: The Autonomic Nervous System

Chapter Objectives

COMPARISON OF SOMATIC AND AUTONOMIC NERVOUS SYSTEMS

1. List the structural and functional characteristics of the autonomic nervous system.

2. Distinguish between the pre- and postganglionic neurons, in terms of location.

3. Specify the different origins and destination ganglia for the sympathetic (thoracolumbar) and parasympathetic (craniosacral) preganglionic neurons.

4. Describe the differing locations of the ganglia of the sympathetic and parasympathetic systems.

PHYSIOLOGICAL EFFECTS OF THE ANS

5. Discuss the primary purpose of the sympathetic division and the general body functions it directs.

6. Describe specific responses of effectors due to increased sympathetic stimulation.

7. Discuss the primary purpose of the parasympathetic division and the general body functions it directs.

8. Describe specific responses of effectors due to increased parasympathetic stimulation.

ANS NEUROTRANSMITTERS AND RECEPTORS

9. Identify the cholinergic neurons, receptors, and neurotransmitters.

10. Identify the adrenergic neurons, receptors, and neurotransmitters.

Chapter Lecture Notes

Characteristics of the autonomic nervous system (Table 15.1)

Involuntary control

Sensory input mostly from interoceptors

Motor pathways divided into sympathetic and parasympathetic divisions

Two-neuron pathway (Fig 15.1)

Preganglionic

Postganglionic

Neurotransmitters

Preganglionic – acetylcholine

Postganglionic – acetylcholine (parasympathetic and sympathetic to sweat glands (except on palms and soles), blood vessels in skeletal muscle and arrector pili muscles) or norepinephrine (remainder of sympathetic)

Effects smooth muscle, cardiac muscle and glands

Physiological effects of the autonomic nervous system (Table 15.4)

Sympathetic – “E” situations (exercise, emergency, excitement and embarrassment) - fight or flight response

Pupils dilate

Heart rate, force of contraction and blood pressure increase

Airways dilate

Blood vessels to kidneys and gastrointestinal tract constrict

Blood vessels to skeletal muscles, cardiac muscle, liver and adipose tissue dilate

Liver cells perform glycogenolysis and lipid cell perform lipolysis

Release of glucose by the liver

Parasympathetic – rest and digest response

Increased salivation, lacrimation, urination, digestion and defecation

Decreased heart rate, diameter of airways and diameter of pupils (constriction)

Anatomical Differences between the Parasympathetic and Sympathetic Divisions (Table 15.3)

|Characteristics |Parasympathetic |Sympathetic |

|Origin (Fig 15.2 & 15.3) |Craniosacral outflow: brainstem nuclei of |Thoracolumnar outflow: lateral horn of gray |

| |cranial nerves III, VII, IX and X; spinal cord |matter of spinal cord segments T1-L2 |

| |segments S2-S4 | |

|Location of ganglia |Ganglia in (intramural= terminal) or close to |Ganglia within a few cm of CNS: alongside |

| |visceral organ served |vertebral column (paravertebral ganglia=chain) |

| | |and anterior to vertebral column (prevertebral |

| | |ganglia) |

|Relative length of pre- and postganglionic |Long preganglionic; short postganglionic |Short preganglionic; long postganglionic |

|fibers | | |

Sympathetic ganglia (Fig 15.5)

Paravertebral ganglia = chain ganglia

Innervate organs above the diaphragm like the heart and lungs

Prevertebral ganglia = collateral ganglia

Prevertebral ganglia surround the following arteries that branch from the aorta

Celiac artery (celiac ganglia forms the solar plexus)

Superior mesenteric artery

Inferior mesenteric artery

Neurotransmitters and receptors (Table 15.2 & Fig 15.7)

Cholinergic neurons – release acetylcholine (all preganglionic neurons, select sympathetic postganglionic neurons and all parasympathetic postganglionic neurons)

Cholinergic receptors – receptors on the postsynaptic membrane that bind acetylcholine

Nicotinic receptor

Found in the dendrites and cell bodies of sympathetic and parasympathetic postganglionic neurons, the motor end plate in the neuromuscular junction and adrenal medullary cells (excitatory)

Is mimicked by nicotine

Muscarinic receptor

Found in all parasympathetic target organs (excitatory in all but cardiac muscle where it is inhibitory), sweat glands (activation) and some blood vessels in skeletal muscle (inhibition; vasodilation) and arrector pili muscle (excitatory)

Is mimicked by muscarine (a mushroom poison)

Adrenergic neurons – release norepinephrine (most sympathetic postganglionic neurons)

Adrenergic receptors – bind norepinephrine and epinephrine

α1

Smooth muscle of blood vessels (excitation; vasoconstriction)

radial muscles of eye (dilation of pupil)

sphincter muscles of stomach and urinary bladder (closing)

salivary gland cells (decreased salivation)

sweat glands on palms and soles (increased sweating)

α2

Smooth muscle in some blood vessels (inhibition; vasodilation)

beta cells in pancreatic islets (decrease insulin secretion)

pancreatic acinar cells (inhibition of digestive enzyme secretion)

blood platelets (aggregation)

β1

Cardiac muscle (excitation; increased force and rate of contraction)

posterior pituitary (secretion of antidiuretic hormones)

adipose cells (breakdown of triglycerides)

β2

Smooth muscle in airways (relaxation; bronchodilation)

blood vessels (relaxation; vasodilation)

walls of internal organs (relaxation)

cillary muscles (inhibition; relaxation)

hepatocytes (glycogenolysis)

β3 – Brown adipose tissue (thermogenesis)

Sympathetic Responses

|Target Organ |Ganglia Location |Effect |

|Blood vessels of skeletal and cardiac |Paravertebral |Dilation |

|muscle | | |

|Blood vessels of skin, gut and kidneys |Paravertebral |Constriction |

|Arrector pili muscle |Paravertebral |Contraction |

|Sweat Glands |Paravertebral |Sweat production |

|Eyes |Paravertebral |Dilates pupils and relaxes ciliary muscles for far vision |

|Lungs |Paravertebral |Dilates bronchi |

|Heart |Paravertebral |Increased rate and force of contraction |

|Liver |Prevertebral |Breakdown of glycogen to glucose and triglycerides to fatty acids |

|Small and Large Intestines |Paravertebral |Slows digestion, stops secretions and contracts sphincters |

|Urinary Bladder |Paravertebral |Contracts sphincter |

|Adrenal Medulla | |Stimulates release of norepinephrine and epinephrine |

Parasympathetic Responses

|Target Organ |Origin |Effect |

|Eyes |Cranial Nerve III |Constricts pupils and ciliary muscles for near vision and |

| | |stimulates tears |

|Salivary Glands |Cranial Nerve VII&IX |Stimulate salivation |

|Lungs |Cranial Nerve X |Constricts bronchi and stimulates secretion |

|Heart |Cranial Nerve X |Decreased rate and force of contraction |

|Gall Bladder |Cranial Nerve X |Constriction |

|Pancreas |Cranial Nerve X |Stimulation of exocrine secretions and release of insulin |

|Small and Large Intestines |Cranial Nerve X and Sacral Nerves |Increases motility and secretions and relaxes sphincters for |

| | |increase defecation |

|Urinary Bladder |Sacral Nerves |Contracts bladder (micturition) and relaxes sphincter |

|Genitals |Sacral Nerves |Erection |

Drugs that Effect the Autonomic Nervous System

|Drug |Mechanism |Action |Uses |

|Drugs that Stimulate the Sympathetic Nervous System (Sympathomimetic) |

|Phenylephrine |Stimulates α1 receptors |Elevates blood pressure; stimulates|Nasal decongestant and to elevate |

| | |smooth muscle |blood pressure |

|Clonidine |Stimulates α2 receptors |Lowers blood pressure |Treatment of high blood pressure |

|Isoproterenol |Stimulates β receptors |Stimulates heart rate; dilates |Treatment of respiratory disorders |

| | |respiratory passages |and as a cardiac stimulant during |

| | | |cardiac resuscitation |

|Albuteral, terbutaline |Stimulates β2 receptors |Dilates respiratory passages |Treatment of asthma, severe |

| | | |allergies and other respiratory |

| | | |disorders |

|Ephedrine, psuedoephedrine |Stimulates NE release at synapses |Similar to epinephrine |Nasal decongestant and to elevate |

| | | |blood pressure or dilate |

| | | |respiratory passages |

|MAO Inhibitors |Increases NE levels in the synapse |Boosts mood in the brain |Antidepressant |

| |by inhibiting the enzyme that | | |

| |breaks it down | | |

|Tricyclic antidepressants (Moxadil,|Increases NE levels in the synapse |Boosts mood in the brain |Antidepressant |

|Endep) |by blocking reuptake | | |

|Drugs that Stimulate the Parasympathetic Nervous System (Parasympathomimetic) |

|Muscarine, pilocarpine, |Stimulates muscarinic receptors |Similar to effects of Ach |Applied topically to cornea of eye |

|methacholine | | |to cause pupil contraction |

|Physostigmine, neostigmine, |Block action of |Increase Ach concentrations at |Stimulate digestive tract and |

|malathion, nerve gases |acetylcholinesterase |synapses |smooth muscles of urinary bladder |

|Drugs that Block the Sympathetic Nervous System |

|Prazosin (Minipress) |Blocks (1 receptors |Lowers blood pressure |Treatment of high blood pressure |

|Tamsulosin (Flomax) |Blocks (1a receptors |Relaxes smooth muscle surrounding |Treatment of benign prostrate |

| | |the urethra |hyperplasia |

|Yohimbine (herbal supplement) |Blocks (2 receptors |Dilates blood vessels and relaxes |Aphrodisiac |

| | |smooth muscles | |

|Propranolol (Inderal) |Blocks (1 and (2 receptors |Reduces metabolic activity in |Treatment of high blood pressure: |

| | |cardiac muscle but may constrict |used to reduce heart rate and force|

| | |respiratory passageways: slows |of contraction in heart disease |

| | |heart rate | |

|Metoprolol (Lopressor), atenolol |Blocks (1 receptors |Reduces metabolic activity in |Similar to those of Inderal but has|

| | |cardiac muscle |less of an effect on respiratory |

| | | |muscles |

|Drugs that Block the Parasympathetic Nervous System |

|Atropine, related drugs |Block muscarinic receptors |Inhibit parasympathetic activity |Treating diarrhea; dilating pupils;|

| | | |raising heart rate; blocking |

| | | |secretions of digestive and |

| | | |respiratory tracts prior to |

| | | |surgery; used to treat accidental |

| | | |exposure to anticholinesterase |

| | | |drugs, such as pesticides or |

| | | |military nerve gases |

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