Download Autonomic Nervous System (Ch. 14)

Crafton Hills College
Human Anatomy & Physiology 150
Autonomic Nervous System (Ch. 14)
A. Introduction
1. General Characteristics
a. Consists of motor neurons that:
i. Innervate smooth & cardiac muscle & glands
ii. Optimize (adjustments) to body activities
iii. Subconscious control
iv. Area of Effect: Viscera (primarily)
b. Localized vs. Difuse Effects
i. Parasympathetic div: Short, very localized control
ii. Sympathetic div: NE - Long-lasting, diffuse effects
1) Effects inactivated more slowly than Ach
2) Indirect Neurotrans: Uses 2nd messenger sys
3) Epinephrine released into blood; remains until destroyed by liver
2. Levels of ANS Control
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Subconscious cerebral input via limbic lobe connections influences hypothalamic function
a. Hypothalamus
i. Main integration center of ANS activity
ii. Centers for control:
1) Heart activity & blood pressure
2) Body temp., water balance, & endocrine activity
3) Emotional stages (rage, pleasure) & Biological drives (hunger, thirst, sex)
4) Reactions to fear - "fight-or-flight" system
B. ANS Versus Somatic Nervous System (SNS)
1. Functional differences in the following three areas
a. Effectors
i. Effectors of the SNS are skeletal muscles
ii. The effectors of the ANS are cardiac muscle, smooth muscle, and glands
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b. Efferent Pathways
i. Heavily myelinated axons of the somatic motor neurons extend from CNS to the effector
ii. Axons of the ANS are a two-neuron chain
1) Preganglionic (first) neuron has a lightly myelinated axon
2) Ganglionic (second) neuron extends to an effector organ
2. Neurotransmitter Effects
a. Somatic motor neurons release Acetylcholine (ACh), (excitatory effect)
b. In the ANS:
i. Preganglionic fibers release Ach
ii. Postganglionic fibers release norepinephrine or Ach
iii. Effect: Stimulatory or Iinhibitory
C. Divisions of the ANS
1. Sympathetic and Parasympathetic
a. Sympathetic: Mobilizes body during extreme situations (fight or flight)
b. Parasympathetic: Performs maintenance activities & conserves body energy
c. These two divisions counterbalance each other
2. Parasympathetic Division
a. Keeps body energy use low
b. D activities: Digestion, Defecation, & Diuresis
c. Example: Physiology of person relaxing after meal
i. Low: Blood pressure, heart & respiratory rates
ii. High: Gastrointestinal tract activity
iii. Skin is warm & pupils constricted
3. Sympathetic division ("fight-or-flight" system)
a. Involves E activities - exercise, excitement, emergency, and embarrassment
b. Promotes changes during exercise: blood flow to organs reduced, flow to muscles increased
c. Its activity is illustrated by a person who is threatened
i. Heart rate increases, and breathing is rapid and deep
ii. The skin is cold and sweaty, and the pupils dilate
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D. Neurotransmitters and Receptors and Drugs
1. Neurotransmitters
a. Acetylcholine (ACh) & norepinephrine (NE)
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2 major neurotransmitters of the ANS
i. Acetylcholine (ACh):
1) Released by all preganglionic axons and all parasympathetic postganglionic axons
2) Cholinergic fibers - ACh-releasing fibers
ii. Norepinephrine (NE)
1) Adrenergic fibers: Sympathetic postganglionic axons that release NE
2) Neurotransmitter effects: Excitatory or Inhibitory depending upon receptor type
2. Receptors
a. Cholinergic
i. Nicotinic & Muscarinic: 2 receptor types that bind ACh
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Named after drugs that bind to them and mimic ACh effects
b. Nicotinic
i. Location:
1) Motor end plates (somatic targets)
2) Sympathetic & parasympathetic division neurons
3) Hormone-producing cells of the adrenal medulla
ii. ACh binding to Nicotinic receptors: Stimulatory
c. Muscarinic
i. All effector cells stimulated by postganglionic cholinergic fibers
ii. Effect of ACh binding:
1) Inhibitory or Excitatory: Depends on receptor type at the target organ
d. Adrenergic
i. 2 types: Alpha & Beta
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Each type has two or three subclasses
ii. Effects of NE binding to:
1) " receptors: Generally Stimulatory
2) $ receptors: Generally Inhibitory
3. Drug Effects
a. Atropine: Blocks parasympathetic effects
b. Over-the-counter drugs (colds, allergies, nasal congestion): Stimulate (-adrenergic receptors
c. Beta-blockers: Mainly to (1 receptors & reduce heart rate / prevent arrhythmias
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E. Interactions of Autonomic Divisions
1. General Characteristics
a. Most visceral organs innervated by both sympathetic and parasympathetic fibers
b. Result: Dynamic antagonisms that precisely control visceral activity
i. Sympathetic fibers: Increase heart & respiratory rates - Inhibit digestion & elimination
ii. Parasympathetic fibers: Decrease heart & respiratory rates, Allow for digestion &
discarding of wastes
2. Referred Pain
a. Pain stimuli from viscera perceived as somatic in origin
b. Possibly because visceral pain afferents travel along the same pathways as somatic pain fibers
3. Sympathetic Tone (vasomotor tone)
a. Sympathetic division controls blood pressure; keeps vessels partially constricted
i. Constricts vessels: Blood pressure rises
ii. Dilates vessels: Blood pressure drops
b. Alpha-blocker drugs: Interfere with vasomotor fibers - used to treat hypertension
4. Parasympathetic tone:
a. Slows the heart
b. Governs normal digestive and urinary processes
- Sympathetic division: Can override parasympathetic during times of stress
5. Cooperative Effects
a. Best Example: Control of external genitalia
i. Parasympathetic fibers: Cause vasodilation for erection of penis & clitoris
ii. Sympathetic fibers: Cause ejaculation of semen in males; reflex peristalsis in females
F. Unique Roles of Sympathetic Division
1. General
- Adrenal medulla, sweat glands, arrector pili muscles, kidneys, & most blood vessels
2. Thermoregulatory Responses to Heat
a. Heating causes reflex dilation of blood vessels
i. Systemic heat = systemic dilation , local heat = local dilation
ii. Warm blood to surface & activates sweat glands (evaporative cooling)
b. Cold has opposite effect: Blood is retained in deeper vital organs
3. Release of Renin from the Kidneys
a. Drop in blood pressure can cause kidneys to release renin
b. Renin: An enzyme that promotes increased blood pressure
4. Metabolic Effects
a. Promotes some metabolic effects that are not reversed by parasympathetic division
i. Increases metabolic rate of body cells
ii. Raises blood glucose levels
iii. Uses fat as a food source
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