The ANS and Visceral Sensory Neurons

1. The ANS and Visceral Sensory Neurons Chapter 15

2. The ANS and Visceral Sensory Neurons • The ANS (autonomic = self-governing) – a system of motor neurons - general visceral motor division of the PNS - distinct from the somatic motor division that innervates skeletal muscles • Innervates - smooth muscle, cardiac muscle, and glands - essential for maintaining the stability of the body’s internal environment • Regulates visceral functions: heart rate (HR), blood pressure (BP), digestion, urination

3. The ANS and Visceral Sensory Neurons Figure 15.1

4. Terms • Synapse – junction between 2 neurons that communicates the message from the presynaptic neuron to the postsynaptic neuron • Ganglion (pl. ganglia) – a cluster of neuronal cell bodies in the PNS • Preganglionic neuron – cell body lies within the CNS - its axon, the preganglionic fiber synapses with the 2nd motor neuron, the ganglionic neuron, in a peripheral autonomic ganglion • Postganglionic fiber (axon) of the ganglionic neuron extends to the visceral organs

5. Comparison of Autonomic and Somatic Motor Systems • Somatic motor system - one motor neuron extends from the CNS to the skeletal muscle being innervated - each motor unit consists of a single neuron plus the skeletal muscle cells it innervates - axons are well myelinated, conduct impulses rapidly

6. ANS • Chain of 2 motor neurons - preganglionic neuron and ganglionic neuron - preganglionic neuron signals the ganglionic neuron, in a peripherial autonomic ganglion - postganglionic fiber extends to the visceral organ - stimulates the effector organ • Conduction is slower than somatic nervous system due to - thinly myelinated or unmyelinated axons - motor neuron synapses in a ganglion

7. Autonomic and Somatic Motor Systems Figure 15.2

8. Divisions of the ANS • Sympathetic and parasympathetic divisions - chains of 2 motor neurons - innervate mostly the same visceral organs - cause opposite effects • Balance between the 2 divisions keep body systems running smoothly

9. Sympathetic and Parasympathetic Divisions • Sympathetic – ‘fight, flight, or fright’ - activated during extreme situations (3 Es) - exercise, excitement, emergencies • Parasympathetic – ‘rest and digest’ - concerned with conserving energy

10. Anatomical Differences • Issue from different regions of the CNS • Sympathetic - aka the thoracolumbar division • Parasympathetic – aka the craniosacral division Figure 15.3

11. Anatomical Differences • Length of postganglionic fibers: sympathetic are long and parasympathetic are short • Branching of axons: sympathetic axons are highly branched to influence many organs while parasympathetic axons have few branches so have a localized effect • Sympathetic release norepinephrine (adrenergic) • Parasympathetic release acetylcholine (cholinergic)

12. Anatomical Differences Figure 15.4a

13. Anatomical Differences Figure 15.4b

14. Table 15.1

15. The Parasympathetic Division • Cranial outflow – comes from the brain - innervates organs of the head, neck, thorax, and abdomen • Sacral outflow – innervation supplies remaining abdominal and pelvic organs

16. The Parasympathetic Division Figure 15.5

17. Cranial Outflow (Parasympathetic) • Is contained in several cranial nerves • Preganglionic fibers run via the oculomotor nerve (III), facial nerve (VII), glossopharyngeal nerve (IX) and vagus nerve (X) • Cell bodies of CNs located in cranial nerve nuclei in the brain stem

18. Outflow via CN III • Parasympathetic fibers innervate smooth muscles in the eye – cause pupil constriction • Preganglionic cell bodies – located in the oculomotor nucleus in the midbrain • Ganglionic cell bodies – lie in the ciliary ganglion

19. Outflow via the CN VII • Parasympathetic fibers stimulate secretion of glands in the head • Lacrimal nucleus – located in the pons - synapse in the pterygopalatine ganglion • Superior salivatory nucleus – located in the pons - synapse in the submandibular ganglion

20. Outflow via CN IX • Parasympathetic fibers – stimulate secretion of glands in the head - lacrimal nucleus located in the pons • Synapse in the pterygopalatine ganglion - superior salivatory nucleus – located in the pons • Synapse in the submandibular ganglion

21. Outflow via the Vagus Nerve (X) • Fibers innervate visceral organs of the thorax and most of the abdomen • Stimulates – digestion, reduction in HR and reduction in BP • Preganglionic cell bodies – located in dorsal motor nucleus in the medulla • Ganglionic neurons – confined within the walls of organs being innervated

22. Path of the Vagus Nerve • Sends branches through autonomic nerve plexuses: - from the cardiac plexus to the heart - from the pulmonary plexus to the lungs - from the esophageal plexus to the esophagus and into the stomach wall - from the celiac plexus and the superior mesenteric plexus to the other abdominal organs (intestines, liver, pancreas, etc.)

23. Figure 15.6

24. Sacral Outflow • Emerges from S2 – S4 • Innervates organs of the pelvis and lower abdomen • Preganglionic cell bodies – located in visceral motor region of spinal gray matter • Axons run in ventral roots to ventral rami – form splanchnic nerves - run through the inferior hypogastric plexus

25. The Sympathetic Division • Basic organization – issues from T1 – L2 • Preganglionic cell bodies lie in the visceral motor region of the SC - form the lateral gray horn • More complex - supplies visceral organs and structures of superficial body regions • Contains more ganglia than the parasympathetic division that fall into 2 classes: 1) sympathetic trunk ganglia and 2) collateral ganglia

26. Sympathetic Trunk Ganglia • Located on both sides of the vertebral column • Linked by short nerves into sympathetic trunks • Joined to ventral rami by white and gray rami communicantes • About one sympathetic trunk ganglion for each spinal nerve - fusion of ganglia  fewer ganglia (22-24) than spinal nerves

27. Sympathetic Trunk Ganglia Figure 15.8

28. Collateral Ganglia • Or prevertebral ganglia differ in 3 ways: 1) unpaired, and not segmentally arranged 2) occur only in abdomen and pelvis 3) lie anterior to the vertebral column, mostly on the abdominal aorta • Main ganglia – celiac, superior mesenteric, inferior mesenteric, and inferior hypogastric ganglia

29. Sympathetic Division Figure 15.7

30. Sympathetic Pathways • Preganglionic neurons in the thoracolumbar SC send their motor axons through the adjacent ventral root into the spinal nerve, white ramus communicans, and associated sympathetic trunk ganglion • Synapse with the ganglionic neurons, either within a sympathetic trunk ganglion or in a collateral ganglion, and the postganglionic axon extends to the visceral organ • In many cases preganglionic fiber ascend or descend in the sympathetic trunk before synapsing – allows sympathetic outputs, which come only from the thoracolumbar region to supply the superior and inferior body regions

31. Sympathetic Pathways to the Body Periphery • Innervate – sweat glands, arrector pili muscles, and peripheral blood vessel • Preganglionic fibers enter the sympathetic trunk ganglia nd synapse there with ganglionic cell bodies

32. To the Body Periphery Figure 15.9

33. Sympathetic Pathways to the Head • Preganglionic fibers originate in spinal cord at T1 – T4 • Fibers ascend in the sympathetic trunk • Synapse in superior cervical ganglion • Postganglionic fibers associate with large arteries - carried by these structures to glands, smooth muscle, and vessels throughout the head

34. To the Head Figure 15.10

35. Sympathetic Pathways to Thoracic Organs • Preganglionic fibers originate at spinal levels T1 – T6 • Some fibers synapse in nearest sympathetic trunk ganglion • Postganglionic fibers run directly to the organ supplied • Sympathetic fibers to heart have a less direct route - increase HR; dilate bronchioles; dilate BVs to the heart wall; inhibit muscles and glands in the esophagus and digestive system

36. To Thoracic Organs Figure 15.11

37. Sympathetic Pathways to Abdominal Organs • Preganglionic fibers originate in spinal cord T5 – L2 • Pass through adjacent sympathetic trunk ganglia - then travel in thoracic splanchnic nerves - synapse in prevertebral ganglia on the abdominal aorta and celiac and superior mesenteric ganglia • Inhibit activity of muscles and glands in visceral organs

38. To the Abdominal Organs Figure 15.12

39. Sympathetic Pathways to the Pelvic Organs • Preganglionic fibers originate in spinal cord T10 – L2 • Some fibers synapse in sympathetic trunk • Other preganglionic fibers synapse in prevertebral ganglia • Postganglionic fibers proceed from plexuses to pelvic organs

40. To the Pelvic Organs Figure 15.13

41. Role of Adrenal Medulla in the Sympathetic Division • Major organ of the sympathetic nervous system • Constitutes largest sympathetic ganglia • Secretes great quantities of catecholamines – norepinephrine and epinephrine • Stimulated to secrete by preganglionic sympathetic fibers

42. The Adrenal Medulla Figure 15.14

43. Table 15.2 (1 of 3)

44. Table 15.2 (2 of 3)

45. Table 15.2 (3 of 3)

46. Visceral Sensory Neurons • General visceral sensory neurons monitor stretch, temperature, chemical changes, and irritation • Cell bodies are located in the dorsal root ganglion • Viscera pain – perceived to be somatic in origin  referred pain

47. A Map of Referred Pain Figure 15.15

48. Visceral Reflexes • Visceral sensory and autonomic neurons – participate in visceral reflex arcs - defecation and micturition reflexes • Some are simple spinal reflexes • Others do not involve the CNS – strictly peripheral reflexes

49. Figure 15.16

50. Central Control of the ANS • Control by the brain stem and spinal cord • Reticular formation exerts most direct influence - medulla oblongata and periaqueductal gray matter • Control by the hypothalamus and amygdala - hypothalamus main integration center of the ANS - amygdala main limbic region for emotions • Control by the cerebral cortex