1. PNS Chapter 13

2. Peripheral Nervous System (PNS) Provides links from and to world outside body All neural structures outside brain – Sensory receptors – Peripheral nerves and associated ganglia – Efferent motor endings

3. Sensory Receptors Specialized to respond to changes in environment (stimuli) Activation results in graded potentials that trigger nerve impulses Sensation (awareness of stimulus) and perception (interpretation of meaning of stimulus) occur in brain

4. Classification of Receptors Based on – Type of stimulus they detect – Location in body – Structural complexity

5. Classification by Stimulus Type Mechanoreceptors—respond to touch, pressure, vibration, and stretch Thermoreceptors—sensitive to changes in temperature Photoreceptors—respond to light energy (e.g., retina) Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood chemistry) Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)

6. Classification by Location Exteroceptors – Respond to stimuli arising outside body – Most special sense organs Interoceptors (visceroceptors) – Respond to stimuli arising in internal viscera and blood vessels Proprioceptors – Inform brain of body movements

7. Classification by Receptor Structure General senses (simple receptors) – Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense – Modified dendritic endings of sensory neurons Non-encapsulated (free nerve endings) Encapsulated Special senses – Vision, hearing, equilibrium, smell, and taste (Chapter 15)

8. Sensory Integration Somatosensory system – part of sensory system serving body wall and limbs – Receives inputs from Exteroceptors, proprioceptors, and interoceptors – Input relayed toward head, but processed along way – Levels of neural integration in sensory systems: 1.Receptor level—sensory receptors 2.Circuit level—processing in ascending pathways 3.Perceptual level—processing in cortical sensory areas

9. Adaptation of Sensory Receptors Adaptation is change in sensitivity in presence of constant stimulus – Phasic (fast-adapting) receptors signal beginning or end of stimulus Examples - receptors for pressure, touch, and smell – Tonic receptors adapt slowly or not at all Examples - nociceptors and most proprioceptors

10. Processing at the Perceptual Level Interpretation of sensory input depends on specific location of target neurons in sensory cortex Aspects of sensory perception: – Perceptual detection—ability to detect a stimulus (requires summation of impulses) – Magnitude estimation—intensity coded in frequency of impulses – Spatial discrimination—identifying site or pattern of stimulus (studied by two-point discrimination test)

11. Perception of Pain Warns of actual or impending tissue damage  protective action Impulses travel on fibers that release neurotransmitters glutamate and substance P Some pain impulses are blocked by inhibitory endogenous opioids (e.g., endorphins) All perceive pain at same stimulus intensity, but pain tolerance varies – "Sensitive to pain" means low pain tolerance, not low pain threshold – Pain tolerance, and response to pain medication, is rooted in genetics.

12. Homeostatic Imbalance Hyperalgesia (pain amplification), – NMDA receptors-allow spinal cord to "learn" hyperalgesia Phantom limb pain – felt in limb no longer present – epidural use during anesthesia to prevent

13. Visceral and Referred Pain Stimulation of visceral organ receptors – Felt as vague aching, gnawing, burning – Activated by tissue stretching, ischemia, chemicals, muscle spasms Referred pain – Pain from one body region perceived from different region – Visceral and somatic pain fibers travel in same nerves; brain assumes stimulus from common (somatic) region

14. Structure of a Nerve Connective tissue coverings include: – Endoneurium—loose connective tissue that encloses axons and their myelin sheaths – Perineurium—coarse connective tissue that bundles fibers into fascicles – Epineurium—tough fibrous sheath around a nerve

15. Classification of Nerves Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers – Pure sensory (afferent) or motor (efferent) nerves are rare – Types of fibers? Peripheral nerves classified as cranial or spinal nerves Ganglia – Contain neuron cell bodies associated with nerves

16. Regeneration of Nerve Fibers Mature neurons are amitotic If the soma of a damaged nerve is intact, axon will regenerate Involves coordinated activity among: – Macrophages—remove debris – Schwann cells—form regeneration tube and secrete growth factors – Axons—regenerate damaged part CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration

17. Figure 13.4 (1 of 4) Endoneurium Droplets of myelin Fragmented axon Schwann cells Site of nerve damage The axon becomes fragmented at the injury site. 1

18. Figure 13.4 (2 of 4) Schwann cellMacrophage Macrophages clean out the dead axon distal to the injury. 2

19. Figure 13.4 (3 of 4) Fine axon sprouts or filaments Aligning Schwann cells form regeneration tube 3 Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells.

20. Figure 13.4 (4 of 4) Schwann cell Site of new myelin sheath formation 4 The axon regenerates and a new myelin sheath forms. Single enlarging axon filament

21. Cranial Nerves Twelve pairs of nerves associated with the brain Most are mixed in function; two pairs are purely sensory Each nerve is identified by a number (I through XII) and a name “ On occasion, our trusty truck acts funny—very good vehicle anyhow ”

22. Figure 13.5 (a) Frontal lobe Temporal lobe Infundibulum Facial nerve (VII) Vestibulo- cochlear nerve (VIII) Glossopharyngeal nerve (IX) Vagus nerve (X) Accessory nerve (XI) Hypoglossal nerve (XII) (a) Filaments of olfactory nerve (I) Olfactory bulb Olfactory tract Optic chiasma Optic nerve (II) Optic tract Oculomotor nerve (III) Trochlear nerve (IV) Trigeminal nerve (V) Abducens nerve (VI) Cerebellum Medulla oblongata

23. Figure 13.5 (b) *PS = parasympathetic (b) Cranial nerves I – VI I II III IV V VI Olfactory Optic Oculomotor Trochlear Trigeminal Abducens Yes (smell) Yes (vision) No Yes (general sensation) No Yes No Yes No Cranial nerves VII – XII Sensory function Motor function PS* fibers Sensory function Motor function PS* fibers VII VIII IX X XI XII Facial Vestibulocochlear Glossopharyngeal Vagus Accessory Hypoglossal Yes (taste) Yes (hearing and balance) Yes (taste) No Yes Some Yes No Yes No

24. Table 13.2 I: The Olfactory Nerves Purely sensory (olfactory) function Afferent impulses for sense of smell Injury diagnosis: partial or total loss of smell (anosmia)

25. Table 13.2 II: The Optic Nerves Pass through the optic canals, converge and partially cross over at the optic chiasma Purely sensory (visual) function Injury diagnosis: blindness or partial loss of vision in affected eye

26. Table 13.2 III: The Oculomotor Nerves Functions in raising the eyelid, directing the eyeball, constricting the iris (parasympathetic), and controlling lens shape Injury: eye cannot be moved (up, down, inward) or rotates laterally when at rest, upper eyelid droops, double vision, and difficulty focusing on close objects

27. Table 13.2 IV: The Trochlear Nerves Primarily a motor nerve that directs the eyeball Injury: double vision, reduced ability to rotate eye inferolaterally Tested w/ III

28. V: The Trigeminal Nerves Three divisions – Ophthalmic (V 1 ) – Maxillary (V 2 ) – Mandibular (V 3 ) – Convey sensory impulses from various areas of the face (V 1 ) and (V 2 ), and supplies motor fibers (V 3 ) for mastication Injury: produces excruciating pain for a few seconds to a minute, recurring many times a day and by various causes (brushing teeth, breeze hitting the face)

29. VI: The Abducens Nerves Primarily a motor nerve, innervating the lateral rectus muscle Injury: eye cannot be moved laterally, at rest eyeball rotates medially

30. VII: The Facial Nerves Chief motor nerves of the face with 5 major branches – Temporal, zygomatic, buccal, mandibular, cervical Motor functions include facial expression, parasympathetic impulses to lacrimal and salivary glands Sensory function (taste) from the anterior two-thirds of the tongue Injury: Bell’s palsy

31. VIII: The Vestibulocochlear Nerves Mostly sensory function; small motor component for adjustment of sensitivity of receptors Injury: – Cochlear: deafness – Vestibular: dizziness, rapid involuntary eye movements, loss of balance, nausea, vomiting

32. IX: The Glossopharyngeal Nerves Motor functions: innervate part of the tongue and pharynx for swallowing, and provide parasympathetic fibers to the parotid salivary glands Sensory functions: fibers conduct taste and general sensory impulses from the pharynx and posterior tongue, and impulses from carotid chemoreceptors and baroreceptors Injury: Impaired swallowing or taste

33. X: The Vagus Nerves Fibers from the medulla exit the skull via the jugular foramen Most motor fibers are parasympathetic fibers that help regulate the activities of the heart, lungs, and abdominal viscera Sensory fibers carry impulses from thoracic and abdominal viscera, baroreceptors, chemoreceptors, and taste buds of posterior tongue and pharynx

34. Injury: Hoarseness or loss of voice, difficulty swallowing, impaired digestive system motility. Total destruction is incompatible with life X: The Vagus Nerves

35. XI: The Accessory Nerves Rootlets pass into the cranium via each foramen magnum Accessory nerves exit the skull via the jugular foramina to innervate the trapezius and sternocleidomastoid muscles Injury: head turns toward side of injury, shrugging of that shoulder difficult

36. XII: The Hypoglossal Nerves Innervate extrinsic and intrinsic muscles of the tongue that contribute to swallowing and speech Injury: difficulties in speech/swallowing, problems with the tongue.

37. Spinal Nerves 31 pairs of mixed nerves named according to their point of issue from the spinal cord – 8 cervical (C 1 –C 8 ) – 12 thoracic (T 1 –T 12 ) – 5 Lumbar (L 1 –L 5 ) – 5 Sacral (S 1 –S 5 ) – 1 Coccygeal (C 0 )

38. Spinal Nerves: Roots Each spinal nerve connects to the spinal cord via two roots – Ventral roots Contain motor (efferent) fibers from the ventral horn motor neurons – Dorsal roots Contain sensory (afferent) fibers from sensory neurons in the dorsal root ganglia Dorsal + Ventral = spinal nerves

39. Spinal Nerves: Rami Each spinal nerve branches into mixed rami – Dorsal ramus – Ventral ramus - larger – Meningeal branch – tiny, reenters vertebral canal, innervates meninges and blood vessels – Rami communicantes (autonomic pathways) join ventral rami in thoracic region

41. Spinal Nerves: Rami All ventral rami (except T 2 –T 12 ) form nerve plexuses (cervical, brachial, lumbar, and sacral) Back innervated by dorsal rami via several branches Ventral rami of T 2 –T 12 as intercostal nerves supply muscles of ribs, anterolateral thorax, and abdominal wall Spinal roots get longer as move inferiorly in cord – Lumbar and sacral roots extend as cauda equina

42. Spinal Nerves: Plexuses Within plexus fibers criss-cross – Each branch contains fibers from several spinal nerves – Fibers from as single ventral ramus go to body periphery via several routes Each limb muscle innervated by more than one spinal nerve

43. Cervical Plexus and the Neck Formed by ventral rami of C 1 –C 4 Most branches form cutaneous nerves – Innervate skin of neck, ear, back of head, and shoulders – Other branches innervate neck muscles Phrenic nerve Major motor and sensory nerve of diaphragm (receives fibers from C 3 –C 5 ) Irritation  hiccups

44. Brachial Plexus and Upper Limb Formed by ventral rami of C 5 –C 8 and T 1 (and often C 4 and/or T 2 ) Gives rise to nerves that innervate upper limb

45. © 2013 Pearson Education, Inc. Anterior divisions Roots (ventral rami): Posterior divisions TrunksRoots Dorsal scapular Nerve to subclavius Suprascapular Posterior divisions Lateral Posterior Medial Axillary Musculo- cutaneous Radial Median Ulnar Upper Middle Lower Long thoracic Medial pectoral Lateral pectoral Upper subscapular Lower subscapular Thoracodorsal Medial cutaneous nerves of the arm and forearm C4C4 C5C5 C6C6 C7C7 C8C8 T1T1 Trunks Roots (rami C 5 –T 1 ), trunks, divisions, and cords Cords Figure 13.10a The brachial plexus.

46. Brachial Plexus: Five Important Nerves Axillary Musculocutaneous Median Ulnar Radial

47. Lumbar Plexus Arises from L 1 –L 4 Innervates thigh, abdominal wall, and psoas muscle Femoral nerve—innervates quadriceps and skin of anterior thigh and medial surface of leg Obturator nerve—passes through obturator foramen to innervate adductor muscles

48. Figure 13.11 The lumbar plexus. Ventral rami Iliohypogastric Ilioinguinal Genitofemoral Lateral femoral cutaneous Obturator Femoral Lumbosacral trunk Iliohypogastric Ilioinguinal Femoral Lateral femoral cutaneous Obturator Anterior femoral cutaneous Saphenous Ventral rami: Ventral rami and major branches of the lumbar plexus Distribution of the major nerves from the lumbar plexus to the lower limb L1L1 L2L2 L3L3 L4L4 L5L5

49. Sacral Plexus Arises from L 4 –S 4 Serves the buttock, lower limb, pelvic structures, and perineum Sciatic nerve – Longest and thickest nerve of body – Innervates hamstring muscles, adductor magnus, and most muscles in leg and foot – Composed of two nerves: tibial and common fibular

51. Innervation of Skin Dermatome: the area of skin innervated by the cutaneous branches of a single spinal nerve All spinal nerves except C 1 participate in dermatomes Most dermatomes overlap, so destruction of a single spinal nerve will not cause complete numbness

52. Figure 13.14 Receptor Sensory neuron Integration center Motor neuron Effector Spinal cord (in cross section) Interneuron Stimulus Skin 1 2 3 4 5 Reflex Arc

53. Spinal Reflexes Spinal somatic reflexes – Integration center is in the spinal cord – Effectors are skeletal muscle Testing of somatic reflexes is important clinically to assess the condition of the nervous system

54. Stretch and Golgi Tendon Reflexes For skeletal muscle activity to be smoothly coordinated, proprioceptor input is necessary – Muscle spindles inform the nervous system of the length of the muscle – Golgi tendon organs inform the brain as to the amount of tension in the muscle and tendons

55. Stretch Reflexes Maintain muscle tone in large postural muscles Cause muscle contraction in response to increased muscle length (stretch) How a stretch reflex works: – Stretch activates the muscle spindle – sensory neurons synapse directly with motor neurons in the spinal cord – motor neurons cause the stretched muscle to contract

56. Stretch Reflexes All stretch reflexes are monosynaptic and ipsilateral Reciprocal inhibition also occurs – fibers synapse with interneurons that inhibit the motor neurons of antagonistic muscles Example: In the patellar reflex, the stretched muscle (quadriceps) contracts and the antagonists (hamstrings) relax

57. Figure 13.17 (1 of 2) Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. The events by which muscle stretch is damped Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition). Initial stimulus (muscle stretch) Cell body of sensory neuron Sensory neuron Muscle spindle Antagonist muscle Spinal cord 1 2 3a3b

58. Figure 13.17 (2 of 2) The patellar (knee-jerk) reflex—a specific example of a stretch reflex Muscle spindle Quadriceps (extensors) Hamstrings (flexors) Patella Patellar ligament Spinal cord (L 2 –L 4 ) Tapping the patellar ligament excites muscle spindles in the quadriceps. The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. Excitatory synapse Inhibitory synapse +–+– 1 2 3a 3b 1 2 3a 3b

59. Golgi Tendon Reflexes Polysynaptic reflexes Help to prevent damage due to excessive stretch Important for smooth onset and termination of muscle contraction

60. Golgi Tendon Reflexes Produce muscle relaxation (lengthening) in response to tension – Contraction or passive stretch activates Golgi tendon organs – Afferent impulses are transmitted to spinal cord – Contracting muscle relaxes and the antagonist contracts (reciprocal activation) – Information transmitted simultaneously to the cerebellum is used to adjust muscle tension

61. Figure 13.18 + Excitatory synapse – Inhibitory synapse Quadriceps strongly contracts. Golgi tendon organs are activated. Afferent fibers synapse with interneurons in the spinal cord. Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. Efferent impulses to antagonist muscle cause it to contract. Interneurons Spinal cord Quadriceps (extensors) Golgi tendon organ Hamstrings (flexors) 1 2 3a 3b

62. Flexor and Crossed-Extensor Reflexes Flexor (withdrawal) reflex – Initiated by a painful stimulus – Causes automatic withdrawal of the threatened body part – Ipsilateral and polysynaptic

63. Flexor and Crossed-Extensor Reflexes Crossed extensor reflex – Occurs with flexor reflexes in weight-bearing limbs to maintain balance – Consists of an ipsilateral flexor reflex and a contralateral extensor reflex The stimulated side is withdrawn (flexed) The contralateral side is extended

64. Figure 13.19 Afferent fiber Efferent fibers Extensor inhibited Flexor stimulated Site of stimulus: a noxious stimulus causes a flexor reflex on the same side, withdrawing that limb. Site of reciprocal activation: At the same time, the extensor muscles on the opposite side are activated. Arm movements Interneurons Efferent fibers Flexor inhibited Extensor stimulated + Excitatory synapse – Inhibitory synapse