Autonomic Nervous System and Visceral Reflexes

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

2. 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

3. 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

4. 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

5. 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

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

7. Efferent Pathways

8. Preganglionic Pathways

9. 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

10. Efferent Pathways

11. 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

12. 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)

13. Neurotransmitters and Receptors

14. 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

15. 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

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

17. 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

18. 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

19. Dual Innervation of the Iris

20. 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

21. Sense Organs • Sensory receptors • properties and types • General senses • Chemical senses • Hearing and equilibrium • Vision

22. Properties of Receptors • Sensory transduction • convert stimulus energy into nerve energy • Receptor potential • local electrical change in receptor cell • Adaptation • conscious sensation declines with continued stimulation

23. Receptors Transmit Information • Modality - type of stimulus • Location • each sensory receptor receives input from its receptive field • sensory projection - brain identifies site of stimulation • Intensity • frequency, number of fibers and which fibers • Duration - change in firing frequency over time • phasic receptor - burst of activity and quickly adapt (smell and hair receptors) • tonic receptor - adapt slowly, generate impulses continually (proprioceptor)

24. Receptive Fields

25. Classification of Receptors • By modality: • chemo-, thermo-, mechano-, photo- receptors and nociceptors • By origin of stimuli • interoceptors - detect internal stimuli • proprioceptors - sense body position and movements • exteroceptors - detect external stimuli • By distribution • general senses - widely distributed • special senses - limited to head

26. Unencapsulated Nerve Endings • Dendrites not wrapped in connective tissue • General sense receptors • for pain and temperature • Tactile discs • associated with cells at base of epidermis • Hair receptors • monitor movement of hair

27. Encapsulated Nerve Endings • Dendrites wrapped by glial cells or connective tissue • tactile corpuscles - phasic • light touch and texture • krause end bulb - phasic • tactile; in mucous membranes • lamellated corpuscles - phasic • deep pressure, stretch, tickle and vibration • ruffini corpuscles - tonic • heavy touch, pressure, joint movements and skin stretching

28. Somesthetic Projection Pathways • 1st order neuron (afferent neuron) • from body, enter the dorsal horn of spinal cord via spinal nerves • from head, enter pons and medulla via cranial nerve • touch, pressure and proprioception on large, fast, myelinated axons • heat and cold on small, unmyelinated, slow fibers • 2nd order neuron • decussation to opposite side in spinal cord or medulla/pons • end in thalamus, except for proprioception (cerebellum) • 3rd order neuron • thalamus to primary somesthetic cortex of cerebrum

29. Pain • Nociceptors – allow awareness of tissue injuries • found in all tissues except the brain • Fast pain travels in myelinated fibers at 30 m/sec • sharp, localized, stabbing pain perceived with injury • Slow pain travels unmyelinated fibers at 2 m/sec • longer-lasting, dull, diffuse feeling • Somatic pain from skin, muscles and joints • Visceral pain from stretch, chemical irritants or ischemia of viscera (poorly localized) • Injured tissues release chemicals that stimulate pain fibers (bradykinin, histamine, prostaglandin)

30. Projection Pathway for Pain • General pathway – conscious pain • 1st order neuron cell bodies in dorsal root ganglion of spinal nerves or cranial nerves V, VII, IX, and X • 2nd order neurons decussate and send fibers up spinothalamic tract or through medulla to thalamus • gracile fasciculus carries visceral pain signals • 3rd order neurons from thalamus reach primary somesthetic cortex as sensory homunculus • Spinoreticular tract • pain signals reach reticular formation, hypothalamus and limbic • trigger visceral, emotional, and behavioral reactions

31. Pain Signal Destinations

32. Referred Pain • Misinterpreted pain • brain “assumes” visceral pain is coming from skin • heart pain felt in shoulder or arm because both send pain input to spinal cord segments T1 to T5

33. Referred Pain

34. CNS Modulation of Pain • Intensity of pain - affected by state of mind • Endogenous opiods (enkephalins, endorphins and dynorphins) • produced by CNS and other organs under stress • in dorsal horn of spinal cord (spinal gating) • act as neuromodulators block transmission of pain

35. Spinal Gating • Stops pain signals at dorsal horn • descending analgesic fibers from reticular formation travel down reticulospinal tract to dorsal horn • secrete inhibitory substances that block pain fibers from secreting substance P • pain signals never ascend • dorsal horn fibers inhibited by input from mechanoreceptors • rubbing a sore arm reduces pain

36. Spinal Gating of Pain Signals

37. Chemical Sense - Taste • Gustation - sensation of taste • results from action of chemicals on taste buds • Lingual papillae • filiform (no taste buds) • important for texture • foliate (no taste buds) • fungiform • at tips and sides of tongue • vallate (circumvallate) • at rear of tongue • contains 1/2 of taste buds

38. Taste Bud Structure • Taste cells • apical microvilli serve as receptor surface • synapse with sensory nerve fibers at their base • Supporting cells • Basal cells

39. Physiology of Taste • Molecules must dissolve in saliva • 5 primary sensations - throughout tongue • Sweet - concentrated on tip • Salty - lateral margins • Sour - lateral margins • Bitter - posterior • Umami - taste of amino acids (MSG) • Influenced by food texture, aroma, temperature, and appearance • mouthfeel - detected by lingual nerve in papillae • Hot pepper stimulates free nerve endings (pain)

40. Physiology of Taste • Mechanisms of action • activate 2nd messenger systems • sugars, alkaloids and glutamates bind to receptors • depolarize cells directly • sodium and acids penetrate cells

41. Projection Pathways for Taste • Innervation of taste buds • facial nerve (VII) - anterior 2/3’s of tongue • glossopharyngeal nerve (IX) - posterior 1/3 • vagus nerve (X) - palate, pharynx, epiglottis

42. Chemical Sense - Smell • Olfactory mucosa • contains receptor cells for olfaction • highly sensitive • up to 10,000 odors • on 5cm2 of superior concha and nasal septum

43. Olfactory Epithelial Cells • Olfactory cells • olfactory hairs neurons with 20 cilia • bind odor molecules in thin layer of mucus • axons pass through cribriform plate • survive 60 days • Supporting cells • Basal cells • divide

44. Physiology of Smell • Molecules bind to receptor on olfactory hair • hydrophilic - diffuse through mucus • hydrophobic - transport by odorant-binding protein • Activate G protein and cAMP system • Opens ion channels for Na+ or Ca2+ • creates a receptor potential • Action potential travels to brain • Receptors adapt quickly • due to synaptic inhibition in olfactory bulbs

45. Olfactory Pathway • Olfactory cells synapse in olfactory bulb • on mitral and tufted cell dendrites • in spherical clusters called glomeruli • each glomeruli dedicated to single odor

46. The Nature of Sound • Sound - audible vibration of molecules • vibrating object pushes air molecules

47. Pitch and Loudness • Pitch - frequency vibrates specific parts of ear • hearing range is 20 (low pitch) - 20,000 Hz (cycles/sec) • speech is 1500-4000 where hearing is most sensitive • Loudness – amplitude; intensity of sound energy

48. Outer Ear

49. Outer Ear • Fleshy auricle (pinna) directs air vibrations down external auditory meatus • cartilagenous and bony, S-shaped tunnel ending at eardrum • glandular secretions and dead cells form cerumen (earwax)

50. Anatomy of Middle Ear