The Peripheral Nervous System

1. The PeripheralNervous System

2. The Peripheral Nervous System • Nervous structures outside the brain and spinal cord • Nerves allow the CNS to receive information and take action • Functional components of the PNS • Sensory inputs and motor outputs • Categorized as somatic or visceral • Sensory inputs also classified as general or special

3. Functional Organization of the PNS

4. Basic Structural Components of the PNS • Sensory receptors – pick up stimuli from inside or outside the body • Motor endings – axon terminals of motor neurons • Innervate effectors (muscle fibers and glands) • Nerves and ganglia • Nerves – bundles of peripheral axons • Ganglia – clusters of peripheral neuronal cell bodies

5. Structural Organization of PNS in Region of a Spinal Nerve

6. Peripheral Endings • Afferent: Sensory Receptors • Efferent: Somatic Motor • Efferent: Autonomic Nervous System

7. Peripheral Sensory Receptors Afferent: Sensory • Structures that pick up sensory stimuli • Initiate signals in sensory axons

8. Peripheral Sensory Receptors • Two main categories of sensory receptors • Special nerve endings of sensory neurons • Monitor general sensory information • Independent receptor cells – specialized epithelial cells or small neurons • Monitor most types of special sensory information

9. Sensory Receptors

10. Peripheral Sensory Receptors • Sensory receptors also classified according to: • Location • Type of stimulus detected • Structure

11. Classification by Location • Exteroceptors – sensitive to stimuli arising from outside the body • Located at or near body surfaces • Include receptors for touch, pressure, pain, and temperature • Interoceptors – (visceroceptors) receive stimuli from internal viscera • Monitor a variety of stimuli • Proprioceptors – monitor degree of stretch • Located in musculoskeletal organs

12. Classification by Modality • Mechanoreceptors – respond to mechanical forces • Thermoreceptors – respond to temperature changes • Chemoreceptors – respond to chemicals in solution • Photoreceptors – respond to light – located in the eye • Nociceptors – respond to harmful stimuli that result in pain

13. Classification by Structure • General sensory receptors • Widely distributed • Nerve endings of sensory neurons monitor: • Touch, pressure, vibration, stretch • Pain, temperature, proprioception • Divided into two groups • Free nerve endings • Encapsulated nerve endings

14. Free Nerve Endings • Abundant in epithelia and underlying connective tissue • Respond to pain and temperature • Monitor affective senses • Two specialized types of free nerve endings • Merkel discs – lie in the epidermis • Slowly adapting receptors for light touch • Hair follicle receptors – wrap around hair follicles • Rapidly adapting receptors

15. Unencapsulated Nerve Endings

16. Encapsulated Nerve Endings • Consist of one or more end fibers of sensory neurons • Enclosed in connective tissue • Mechanoreceptors • Include four main types

17. Encapsulated Nerve Endings • Encapsulated nerve endings: dendrites with special supporting structures (mechanoreceptors and proprioceptors)

18. Encapsulated Nerve Endings • Meissner’s corpuscles • Pacinian corpuscles • Ruffini’s corpuscles • Proprioceptors

19. Encapsulated Receptors

20. Proprioceptors • Monitor stretch in locomotory organs • Three types of proprioceptors

21. Three Types of Proprioceptors • Muscle spindles – measure the changing length of a muscle • Imbedded in the perimysium between muscle fascicles • Golgi tendon organs – located near the muscle-tendon junction • Monitor tension within tendons • Joint kinesthetic receptors • Sensory nerve endings within the joint capsules

22. Proprioceptors

23. Structure of Receptors in Skin

24. Somatic Efferent: Innervation of Skeletal Muscles

25. Innervation of Skeletal Muscle • Motor axons innervate skeletal muscles • Neuromuscular junctions (motor end plates) • Similar to synapses between neurons • Acetylcholine diffuses across the synaptic cleft • Binds with molecules on the sarcolemma • Motor axons branch to innervate muscle fibers

26. The Neuromuscular Junction

27. Innervation of Skeletal Muscle • Motor unit – a motor neuron and all the muscle fibers it innervates

28. Efferent: Autonomic Nervous System

29. Innervation of Visceral Muscle and Glands • Simpler than neuromuscular junctions of skeletal muscle • Near the smooth muscle or gland it innervates • Visceral motor axon swells into a row of varicosities • Visceral motor responses • Slower than somatic motor reflexes

30. Innervation of Smooth Muscle

31. Cranial Nerves

32. Cranial Nerves • Attach to the brain and pass through foramina of the skull • Numbered from I–XII • Cranial nerves I and II attach to the forebrain • All others attach to the brain stem • Primarily serve head and neck structures • The vagus nerve (X) extends into the abdomen

33. The 12 Pairs of Cranial Nerves

34. CN I: Olfactory Nerves • Sensory nerves of smell • Sense of smell • Damage causes impaired sense of smell

35. CN II: Optic Nerve • Sensory nerve of vision • Provides vision • Damage causes blindness in visual field

36. CN III: Oculomotor Nerve • Innervates four of the extrinsic eye muscles • Somatic and Autonomic motor function • Eye movement (Superior, inferior, medial rectus muscles and inferior oblique muscle), opening of eyelid (levator palpebrae superioris), constriction of pupil (circular muscle), focusing (ciliary muscle and accomodation) • Damage causes drooping eyelid, dilated pupil, double vision, difficulty focusing and inability to move eye in certain directions

37. CN IV: Trochlear Nerve • Innervates an extrinsic eye muscle • Eye movement (superior oblique muscle) • Damage causes double vision and inability to rotate eye inferolaterally

38. CN V: Trigeminal Nerve • Provides sensory innervation to the face • Motor innervation to chewing muscles • Ophthalmic branch – sensations from nasal cavity, skin of forehead, upper eyelid, eyebrow, nose • Maxillary branch – sensations from lower eyelid, upper lips and gums, teeth of the maxilla, cheek, nose, palate, pharynx • Mandibular branch – sensations from teeth of the mandible, lower gums and lips, palate, tongue. Motor function of temporalis and masseter muscles. • Damage produces loss of sensation and impaired chewing

39. Trigeminal Nerve

40. CN VI: Abducens Nerve • Abducts the eyeball • Provides eye movement (lateral rectus m.) • Damage results in inability to rotate eye laterally and at rest eye rotates medially

41. CN VII: Facial Nerve • Innervates muscles of facial expression • Sensory innervation of face • Taste • Somatic Motor - facial expressions • Autonomic Motor - salivary and lacrimal glands, mucous membranes of nasal and palatine mucosa • Special Sensory - taste on anterior 2/3’s of tongue • Damage produces sagging facial muscles and disturbed sense of taste (no sweet and salty)

42. Branches of Facial Nerve Clinical test: Test anterior 2/3’s of tongue with substances such as sugar, salt, vinegar, and quinine; test response of tear glands to ammonia fumes; test motor functions by asking subject to close eyes, smile, whistle, frown, raise eyebrows, etc.

43. CN VIII: Vestibulocochlear Nerve • Sensory nerve of hearing and balance • Special Sensory • Provides hearing (cochlear branch) and sense of balance (vestibular branch) • Damage produces deafness, dizziness, nausea, loss of balance and nystagmus

44. CN IX: Glossopharyngeal Nerve • Sensory and motor innervation of structures of the tongue and pharynx • Taste • Somatic motor – Swallowing and voice production via pharyngeal muscles • Autonomic motor - salivation, gagging, control of BP and respiration • Sensations from posterior 1/3 of tongue including taste • Sensations from baroreceptors and chemoreceptors • Damage results in loss of bitter and sour taste and impaired swallowing, blood pressure anomalies (with CN X).

45. CN X: Vagus Nerve • A mixed sensory and motor nerve • Main parasympathetic nerve • “Wanders” into thorax and abdomen

46. Vagus Nerve X • Sensations from skin at back of ear, external acoustic meatus, part of tympanic membrane, larynx, trachea, espophagus, thoracic and abdominal viscera • Sensations from bararoceptors and chemoreceptors • Special sensory – taste from epiglottis and pharynx • Somatic motor – Swallowing and voice production via pharyngeal muscles • Autonomic motor – smooth muscle of abdominal viscera, visceral glands secretions, relaxation of airways, and normal or decreased heart rate. • Damage causes hoarseness or loss of voice, impaired swallowing, GI dysfunction, blood pressure anomalies (with CN IX), fatal if both are cut

47. CN XI: Accessory Nerve • An accessory part of the vagus nerve • Somatic motor function of pharynx, larynx, neck muscles • Swallowing, head, neck and shoulder movement via trapezius and sternocleidomastoid and pharyngeal muscles • Damage causes impaired head, neck, shoulder movement

48. CN XII: Hypoglossal Nerve • Runs inferior to the tongue • Innervates the tongue muscles • Tongue movements for speech, food manipulation and swallowing • If both are damaged – can’t protrude tongue • If one side is damaged – tongue deviates towards injured side

49. Cranial Nerve Disorders • Trigeminal neuralgia (tic douloureux) • recurring episodes of intense stabbing pain in trigeminal nerve area (near mouth or nose) • pain triggered by touch, drinking, washing face • treatment may require cutting nerve • Bell’s palsy • disorder of facial nerve causes paralysis of facial muscles on one side • may appear abruptly with full recovery within 3-5 weeks

50. Spinal Nerves