Chapter 17 The Autonomic Nervous System

1. Chapter 17 The Autonomic Nervous System • Regulate activity of smooth muscle, cardiac muscle & certain glands • Structures involved • general visceral afferent neurons • general visceral efferent neurons • integration center within the brain • Receives input from limbic system and other regions of the cerebrum

2. Autonomic versus Somatic NS • Somatic nervous system • consciously perceived sensations • excitation of skeletal muscle • one neuron connects CNS to organ • Autonomic nervous system • unconsciously perceived visceral sensations • involuntary inhibition or excitation of smooth muscle, cardiac muscle or glandular secretion • two neurons needed to connect CNS to organ • preganglionic and postganglionic neurons

3. Autonomic versus Somatic NS • Notice that the ANS pathway is a 2 neuron pathway while the Somatic NS only contains one neuron.

4. Basic Anatomy of ANS • Preganglionic neuron • cell body in brain or spinal cord • axon is myelinated type B fiber that extends to autonomic ganglion • Postganglionic neuron • cell body lies outside the CNS in an autonomic ganglion • axon is unmyelinated type C fiber that terminates in a visceral effector

5. Divisions of the ANS • 2 major divisions • parasympathetic • sympathetic • Dual innervation • one speeds up organ • one slows down organ • Sympathetic NS increases heart rate • Parasympathetic NS decreases heart rate

6. Sources of Dual Innervation • Sympathetic (thoracolumbar) division • preganglionic cell bodies in thoracic and first 2 lumbar segments of spinal cord • Parasympathetic (craniosacral) division • preganglionic cell bodies in nuclei of 4 cranial nerves and the sacral spinal cord

7. Locations of Autonomic Ganglia • Sympathetic Ganglia • trunk (chain) ganglia near vertebral bodies • prevertebral ganglia near large blood vessel in gut • celiac • superior mesenteric • inferior mesenteric • Parasympathetic Ganglia • terminal ganglia in wall of organ

8. Autonomic Plexuses • Cardiac plexus • Pulmonary plexus • Celiac (solar) plexus • Superior mesenteric • Inferior mesenteric • Hypogastric

9. Structures of Sympathetic NS • Preganglionic cell bodies at T1 to L2 • Rami communicantes • white ramus = myelinated = preganglionic fibers • gray ramus = unmyelinated = postganglionic fibers • Postganglionic cell bodies • sympathetic chain ganglia along the spinal column • prevertebral ganglia at a distance from spinal cord • celiac ganglion • superior mesenteric ganglion • inferior mesenteric ganglion

10. Ganglia & Plexuses of Sympathetic NS

11. Pathways of Sympathetic Fibers • Spinal nerve route • out same level • Sympathetic chain route • up chain & out spinal n • Collateral ganglion route • out splanchnic n to collateral ganglion

12. Organs Innervated by Sympathetic NS • Structures innervated by each spinal nerve • sweat glands, arrector pili mm., blood vessels to skin & skeletal mm. • Thoracic & cranial plexuses supply: • heart, lungs,esophagus & thoracic blood vessels • plexus around carotid artery to head structures • Splanchnic nerves to prevertebral ganglia supply: • GI tract from stomach to rectum, urinary & reproductive organs

13. Circuitry of Sympathetic NS • Divergence = each preganglionic cell synapses on many postganglionic cells • Mass activation due to divergence • multiple target organs • fight or flight response explained • Adrenal gland • modified cluster of postganglionic cell bodies that release epinephrine & norepinephrine into blood

14. Anatomy of Parasympathetic NS • Preganglionic cell bodies found in • 4 cranial nerve nuclei in brainstem • S2 to S4 spinal cord • Postganglionic cell bodies very near or in the wall of the target organ in a terminal ganglia

15. Parasympathetic Cranial Nerves • Oculomotor nerve • ciliary ganglion in orbit • ciliary muscle & pupillary constrictor muscle inside eyeball • Facial nerve • pterygopalatine and submandibular ganglions • supply tears, salivary & nasal secretions • Glossopharyngeal • otic ganglion supplies parotid salivary gland • Vagus nerve • many brs supply heart, pulmonary and GI tract as far as the midpoint of the colon

16. Parasympathetic Sacral Nerve Fibers • Form pelvic splanchnic nerves • Preganglionic fibers end on terminal ganglia in walls of target organs • Innervate smooth muscle and glands in colon, ureters, bladder & reproductive organs

17. ANS Neurotransmitters • Classified as either cholinergic or adrenergic neurons based upon the neurotransmitter released • Adrenergic • Cholinergic

18. Cholinergic Neurons and Receptors • Cholinergic neurons release acetylcholine from preganglionic neurons & from parasympathetic postganglionic neurons • Excites or inhibits depending upon receptor type and organ involved • Nicotinic receptors are found on dendrites & cell bodies of autonomic NS cells and at NMJ • Muscarinic receptors are found on plasma membranes of all parasympathetic effectors

19. Adrenergic Neurons and Receptors • Adrenergic neurons release norepinephrine (NE) ) • from postganglionicsympathetic neurons only • Excites or inhibits organs depending on receptors • Alpha1 and Beta1 receptors produce excitation • Alpha2 and Beta2 receptors cause inhibition • Beta3 receptors(brown fat) increase thermogenesis • NE lingers at the synapse until enzymatically inactivated by monoamine oxidase (MAO) or catechol-O-methyltransferase (COMT)

20. Physiological Effects of the ANS • Most body organs receive dual innervation • innervation by both sympathetic & parasympathetic • Hypothalamus regulates balance (tone) between sympathetic and parasympathetic activity levels • Some organs have only sympathetic innervation • sweat glands, adrenal medulla, arrector pili mm & many blood vessels • controlled by regulation of the “tone” of the sympathetic system

21. Sympathetic Responses • Dominance by the sympathetic system is caused by physical or emotional stress -- “E situations” • emergency, embarrassment, excitement, exercise • Alarm reaction = flight or fight response • dilation of pupils • increase of heart rate, force of contraction & BP • decrease in blood flow to nonessential organs • increase in blood flow to skeletal & cardiac muscle • airways dilate & respiratory rate increases • blood glucose level increase • Long lasting due to lingering of NE in synaptic gap and release of norepinephrine by the adrenal gland

22. Parasympathetic Responses • Enhance “rest-and-digest” activities • Mechanisms that help conserve and restore body energy during times of rest • Normally dominate over sympathetic impulses • SLUDD type responses = salivation, lacrimation, urination, digestion & defecation and 3 “decreases”--- decreased HR, diameter of airways and diameter of pupil • Paradoxical fear when there is no escape route or no way to win • causes massive activation of parasympathetic division • loss of control over urination and defecation

23. Autonomic or Visceral Reflexes • Autonomic reflexes occur over autonomic reflex arcs. Components of that reflex arc: • sensory receptor • sensory neuron • integrating center • pre & postganglionic motor neurons • visceral effectors • Unconscious sensations and responses • changes in blood pressure, digestive functions etc • filling & emptying of bladder or defecation

24. Control of Autonomic NS • Not aware of autonomic responses because control center is in lower regions of the brain • Hypothalamus is major control center • input: emotions and visceral sensory information • smell, taste, temperature, osmolarity of blood, etc • output: to nuclei in brainstem and spinal cord • posterior & lateral portions control sympathetic NS • increase heart rate, inhibition GI tract, increase temperature • anterior & medial portions control parasympathetic NS • decrease in heart rate, lower blood pressure, increased GI tract secretion and mobility

25. Autonomic Dysreflexia • Exaggerated response of sympathetic NS in cases of spinal cord injury above T6 • Certain sensory impulses trigger mass stimulation of sympathetic nerves below the injury • Result • vasoconstriction which elevates blood pressure • parasympathetic NS tries to compensate by slowing heart rate & dilating blood vessels above the injury • pounding headaches, sweating warm skin above the injury and cool dry skin below • can cause seizures, strokes & heart attacks