1. Chapter 15

2. Autonomic Nervous System and Visceral Reflexes
Autonomic nervous system (ANS)
general properties
anatomy
Autonomic effects on target organs
Central control of autonomic function

3. ANS - General Properties
Motor nervous system controls glands, cardiac and smooth muscle
also called visceral motor system
Regulates unconscious processes that maintain homeostasis
BP, body temperature, respiratory airflow
ANS actions are automatic
biofeedback techniques
train people to control hypertension, stress and migraine headaches

4. Visceral Reflexes
Unconscious, automatic responses to stimulation of glands, cardiac or smooth muscle
Receptors
detect internal stimuli -- stretch, blood chemicals, etc.
Afferent neurons
connect to interneurons in the CNS
Efferent neurons
carry motor signals to effectors
ANS is the efferent neurons of these reflex arcs
Effectors
glands, smooth or cardiac muscle
ANS modifies effector activity

5. Visceral Reflex to High BP
High blood pressure detected by arterial stretch receptors (1), afferent neuron (2) carries signal to CNS, efferent (3) signals travel to the heart (4), heart slows reducing BP

6. Divisions of ANS Two divisions innervate same target organs
may have cooperative or contrasting effects
Sympathetic division
prepares body for physical activity
increases heart rate, BP, airflow, blood glucose levels, etc
Parasympathetic division
calms many body functions and assists in bodily maintenance
digestion and waste elimination
Autonomic tone is the normal rate of activity that represents the balance of the two systems
Effects of each depend upon neurotransmitters released

7. Somatic versus Autonomic Pathways
ANS = 2 neurons from CNS to effectors
presynaptic neuron cell body in CNS
postsynaptic neuron cell body in peripheral ganglion

8. Sympathetic Nervous System
Origin of presynaptic neurons
lateral horns of spinal cord (T1-L2)
Sympathetic chain ganglia (paravertebral)
3 cervical, 11 thoracic, 4 lumbar, 4 sacral and 1 coccygeal ganglia
white and gray communicating rami suspend ganglia from spinal nerve
pathways of preganglionic fibers
enter ganglia and synapse on postganglionic cell
travel to higher or lower ganglia and synapse
pass through chain without synapsing to reach collateral ganglia via splanchnic nerves

9. Sympathetic Nervous System
Neuronal divergence predominates
each preganglionic cell branches and synapses on multiple postganglionic cells
produces widespread effects on multiple organs

10. Efferent Pathways

11. Preganglionic Pathways

12. Ganglia and Abdominal Aortic Plexus

13. Sympathetic Innervation
Effectors in body wall are innervated by sympathetic fibers in spinal nerves
Effectors in head and thoracic cavity are innervated by fibers in sympathetic nerves
Effectors in abdominal cavity are innervated by sympathetic fibers in splanchnic nerves
celiac, superior and inferior mesenteric ganglion

14. Adrenal Glands Paired glands sit on superior pole of each kidney
Cortex (outer layer)
secretes steroid hormones
Medulla (inner core)
a modified sympathetic ganglion
stimulated by preganglionic sympathetic neurons
secretes neurotransmitters (hormones) into blood
catecholamines (85% epinephrine and 15% norepinephrine)
Sympathoadrenal system is the closely related functioning adrenal medulla and symphathetic nervous system

15. Parasympathetic Nervous System
Origin of preganglionic fibers
pons and medulla (for cranial nerve nuclei)
sacral spinal cord segments S2-S4
Pathways of preganglionic fibers
cranial nerves III, VII, IX and X
arising from sacral spinal cord
pelvic splanchnic nerves and inferior hypogastric plexus
Terminal ganglia in/near target organs
long preganglionic, short postganglionic fibers

16. Efferent Pathways

17. Parasympathetic Cranial Nerves
Oculomotor nerve (III)
narrows pupil and focuses lens
Facial nerve (VII)
tear, nasal and salivary glands
Glossopharyngeal (IX)
parotid salivary gland
Vagus nerve (X)
viscera as far as proximal half of colon
Cardiac, pulmonary, and esophageal plexus

18. Enteric Nervous System
Nervous system of the digestive tract
Composed of 100 million neurons found in the walls of the digestive tract (no components in CNS)
Has its own reflex arcs
Regulates motility of viscera and secretion of digestive enzymes and acid in concert with the ANS

19. Neurotransmitters and Receptors
Effects of ANS
determined by types of neurotransmitters released and types of receptors on target cells
Sympathetic has longer lasting effects
neurotransmitters persist in synapse and some reach the bloodstream
Many substances released as neurotransmitters
enkephalin, substance P, neuropeptide Y, neurotensin, nitric oxide (NO)
NO inhibits muscle tone in BV walls (vasodilation)

20. Neurotransmitters and Receptors

21. Cholinergic Receptors for ACh
Acetylcholine (Ach) binds to 2 classes of receptors
nicotinic receptors
on all ANS postganglionic neurons, in the adrenal medulla, and at neuromuscular junctions (skeletal muscle)
excitatory when ACh binding occurs
muscarinic receptors
on all gland, smooth muscle and cardiac muscle cells that receives cholinergic innervation
excitatory or inhibitory due to subclasses of muscarinic receptors

22. Adrenergic Receptors for NE
Norepinephrine binds to 2 classes of receptors
alpha adrenergic receptors (often excitatory)
beta adrenergic receptors (often inhibitory)
Exceptions
existence of subclasses of each receptor type
alpha 1 and 2; beta 1 and 2
Function by means of 2nd messengers
cyclic AMP and alpha 1 receptors

23. Dual Innervation
Most of viscera receive nerve fibers from both parasympathetic and sympathetic divisions
Both divisions do not normally innervate an organ equally

24. Dual Innervation Antagonistic effects oppose each other
exerted through dual innervation of same effector
heart rate decreases (parasympathetic)
heart rate increases (sympathetic)
exerted because each division innervates different cells
pupillary dilator muscle (sympathetic) dilates pupil
constrictor pupillae (parasympathetic) constricts pupil

25. Dual Innervation
Cooperative effects seen when 2 divisions act on different effectors to produce a unified effect
parasympathetics increase salivary serous cell secretion
sympathetics increase salivary mucous cell secretion

26. Dual Innervation of the Iris

27. Without Dual Innervation
Some effectors receive only sympathetic
adrenal medulla, arrector pili muscles, sweat glands and many blood vessels
Sympathetic tone
a baseline firing frequency
vasomotor tone provides partial constriction
increase in firing frequency = vasoconstriction
decrease in firing frequency = vasodilation
can shift blood flow from one organ to another as needed
sympathetic stimulation increases blood to skeletal and cardiac muscles -- reduced blood to skin

28. Sympathetic and Vasomotor Tone
Sympathetic division prioritizes blood vessels to skeletal muscles and heart in times of emergency.
Blood vessels to skin vasoconstrict to minimize bleeding if injury occurs during stress or exercise.

29. Control of Autonomic Function
ANS regulated by several levels of CNS
cerebral cortex has an influence
hypothalamus (major visceral motor control center)
nuclei for primitive functions – hunger, thirst
midbrain, pons, and medulla oblongata
nuclei for cardiac and vasomotor control, salivation, swallowing, sweating, bladder control, and pupillary changes
spinal cord reflexes
defecation and micturition reflexes integrated in cord
brain can inhibit these responses consciously

30. Drugs Sympathomimetics enhance sympathetic activity
stimulate receptors or norepinephrine release
Sympatholytics suppress sympathetic activity
block receptors or inhibit norepinephrine release
Parasympathomimetics enhance activity while parasympatholytics suppress activity
Management of clinical depression
Prozac blocks reuptake of serotonin to prolong its mood-elevating effect
MAO inhibitors interfere with breakdown of monoamine neurotransmitters
Caffeine competes with adenosine (inhibitory; causes sleepiness) by binding to its receptors