Understanding the Autonomic Nervous System

1. 14 The Autonomic Nervous System

2. Autonomic Nervous System (ANS) • The ANS consists of motor neurons that: • Innervate smooth and cardiac muscle and glands • Make adjustments to ensure optimal support for body activities • Operate via subconscious control

3. Autonomic Nervous System (ANS) • Other names • Involuntary nervous system • General visceral motor system

4. Central nervous system (CNS) Peripheral nervous system (PNS) Sensory (afferent) division Motor (efferent) division Somatic nervoussystem Autonomic nervous system (ANS) Sympathetic division Parasympathetic division Figure 14.1

5. Somatic and Autonomic Nervous Systems • The two systems differ in • Effectors • Efferent pathways (and their neurotransmitters) • Target organ responses to neurotransmitters

6. Effectors • Somatic nervous system • Skeletal muscles • ANS • Cardiac muscle • Smooth muscle • Glands

7. Neuro- transmitter at effector Cell bodies in central nervous system Effector organs Peripheral nervous system Effect Single neuron from CNS to effector organs ACh + SOMATIC NERVOUS SYSTEM Stimulatory Heavily myelinated axon Skeletal muscle Two-neuron chain from CNS to effector organs NE ACh Unmyelinated postganglionic axon Ganglion SYMPATHETIC Lightly myelinated preganglionic axons + Epinephrine and norepinephrine ACh Stimulatory or inhibitory, depending on neuro- transmitter and receptors on effector organs AUTONOMIC NERVOUS SYSTEM Adrenal medulla Blood vessel ACh ACh Smooth muscle (e.g., in gut), glands, cardiac muscle PARASYMPATHETIC Lightly myelinated preganglionic axon Unmyelinated postganglionic axon Ganglion Acetylcholine (ACh) Norepinephrine (NE) Figure 14.2

8. Divisions of the ANS • Sympathetic division • Parasympathetic division • Dual innervation • Almost all visceral organs are served by both divisions, but they cause opposite effects

9. Role of the Parasympathetic Division • Promotes maintenance activities and conserves body energy • Its activity is illustrated in a person who relaxes, reading, after a meal • Blood pressure, heart rate, and respiratory rates are low • Gastrointestinal tract activity is high • Pupils are constricted and lenses are accommodated for close vision

10. Role of the Sympathetic Division • Mobilizes the body during activity; is the “fight-or-flight” system • Promotes adjustments during exercise, or when threatened • Blood flow is shunted to skeletal muscles and heart • Bronchioles dilate • Liver releases glucose

11. Parasympathetic Sympathetic Eye Eye Brain stem Salivary glands Skin* Cranial Salivary glands Sympathetic ganglia Heart Cervical Lungs Lungs T1 Heart Stomach Thoracic Stomach Pancreas Liver and gall- bladder Pancreas L1 Adrenal gland Liver and gall- bladder Lumbar Bladder Bladder Genitals Genitals Sacral Figure 14.3

12. Spinal cord Dorsal root Ventral root Rib Sympathetic trunk ganglion Sympathetic trunk Ventral ramus of spinal nerve Gray ramus communicans White ramus communicans Thoracic splanchnic nerves (a) Location of the sympathetic trunk Figure 14.5a

13. Visceral Reflexes • Visceral reflex arcs have the same components as somatic reflexes • Main difference: visceral reflex arc has two neurons in the motor pathway • Visceral pain afferents travel along the same pathways as somatic pain fibers, contributing to the phenomenon of referred pain

14. Stimulus Dorsal root ganglion Sensory receptor in viscera 1 Spinal cord Visceral sensory neuron 2 Integration center • May be preganglionic neuron (as shown) • May be a dorsal horn interneuron • May be within walls of gastrointestinal tract 3 Autonomic ganglion Efferent pathway (two-neuron chain) • Preganglionic neuron • Ganglionic neuron 4 Visceral effector 5 Response Figure 14.7

15. Referred Pain • Visceral pain afferents travel along the same pathway as somatic pain fibers • Pain stimuli arising in the viscera are perceived as somatic in origin

16. Heart Lungs and diaphragm Liver Heart Gallbladder Liver Appendix Stomach Pancreas Small intestine Ovaries Colon Kidneys Urinary bladder Ureters Figure 14.8

17. Two-neuron chain from CNS to effector organs NE ACh Unmyelinated postganglionic axon Ganglion SYMPATHETIC Lightly myelinated preganglionic axons + Epinephrine and norepinephrine ACh Stimulatory or inhibitory, depending on neuro- transmitter and receptors on effector organs AUTONOMIC NERVOUS SYSTEM Adrenal medulla Blood vessel ACh ACh Smooth muscle (e.g., in gut), glands, cardiac muscle PARASYMPATHETIC Lightly myelinated preganglionic axon Unmyelinated postganglionic axon Ganglion Acetylcholine (ACh) Norepinephrine (NE) Figure 14.2

18. Sympathetic Tone • Sympathetic division controls blood pressure, even at rest • Sympathetic tone (vasomotor tone) • Keeps the blood vessels in a continual state of partial constriction

19. Parasympathetic Tone • Parasympathetic division normally dominates the heart and smooth muscle of digestive and urinary tract organs • Slows the heart • Dictates normal activity levels of the digestive and urinary tracts • The sympathetic division can override these effects during times of stress • Drugs that block parasympathetic responses increase heart rate and block fecal and urinary retention

20. Cooperative Effects • Best seen in control of the external genitalia • Parasympathetic fibers cause vasodilation; are responsible for erection of the penis or clitoris • Sympathetic fibers cause ejaculation of semen in males and reflex contraction of a female’s vagina

21. Unique Roles of the Sympathetic Division • The adrenal medulla, sweat glands, arrector pili muscles, kidneys, and most blood vessels receive only sympathetic fibers • The sympathetic division controls • Thermoregulatory responses to heat • Release of renin from the kidneys • Metabolic effects • Increases metabolic rates of cells • Raises blood glucose levels • Mobilizes fats for use as fuels

22. Localized Versus Diffuse Effects • Parasympathetic division: short-lived, highly localized control over effectors • Sympathetic division: long-lasting, bodywide effects

23. Control of ANS Functioning • Hypothalamus—main integrative center of ANS activity • Subconscious cerebral input via limbic lobe connections influences hypothalamic function

24. Communication at subconscious level Cerebral cortex (frontal lobe) Limbic system (emotional input) Hypothalamus Overall integration of ANS, the boss Brain stem (reticular formation, etc.) Regulation of pupil size, respiration, heart, blood pressure, swallowing, etc. Spinal cord Urination, defecation, erection, and ejaculation reflexes Figure 14.9

25. Hypothalamic Control • Control may be direct or indirect (through the reticular system) • Centers of the hypothalamus control • Heart activity and blood pressure • Body temperature, water balance, and endocrine activity • Emotional stages (rage, pleasure) and biological drives (hunger, thirst, sex) • Reactions to fear and the “fight-or-flight” system

26. Developmental Aspects of the ANS • Effects of age on ANS • Constipation • Dry eyes • Frequent eye infections • Orthostatic hypotension • Low blood pressure occurs because aging pressure receptors respond less to changes in blood pressure with changes in body position and because of slowed responses by sympathetic vasoconstrictor centers