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Chapter 13

Chapter 13. Peripheral Nervous System. PNS – the link to the outside world. White matter transmitted impulses to and from the brain PNS includes all neural structures outside the brain and spinal cord: Sensory receptors Peripheral nerves Ganglia Motor endings

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Chapter 13

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  1. Chapter 13 Peripheral Nervous System

  2. PNS – the link to the outside world • White matter transmitted impulses to and from the brain • PNS includes all neural structures outside the brain and spinal cord: • Sensory receptors • Peripheral nerves • Ganglia • Motor endings • Also includes: Somatic NS, ANS (sympathetic & parasympathetic)

  3. Sensory Receptors & Sensation • Stimuli from environment results in graded (local) potentials that in turn trigger nerve impulses along afferent PNS fibers • In the brain, sensation (awareness) and perception (interpretation of the “meaning” of the stimulus) • Classify sensory receptors by: 1) type of stimulus detected, 2) body location, & 3) their structural complexity

  4. Classification by Stimulus Type 1)Mechanoreceptors – deform w/ mechanical forces: touch, pressure, vibration, stretch, itch 2)Thermoreceptors – Tpo changes 3)Photoreceptors – retina in the eye 4)Chemoreceptors – chemicals of taste 7 smell 5)Nociceptors – respond to stimuli that result in pain, which can be any sensory receptor if overstimulated

  5. Classification by Location • Exteroceptors – stimuli arise outside the body, so receptors are close to the surface: touch, pressure, pain and Tpo plus special senses vision, hearing, equilibrium, taste, smell • Interoceptors – aka visceroceptors from organs and BV’s, monitor chemical change, stretch, Tpo and we feel discomfort, hunger or thirst • Proprioceptors – in skeletal m., tendons, joints, & ligaments, which advise the brain of our positions in space and our movements

  6. Classification by Structural Complexity • Simple or complex – latter are generally special sense organs for vision, taste, hearing, smell and equilibrium. Simple are receptors of general senses: A) Free Nerve Endings – “naked” • Naked – everywhere (epithelium, CT), C-fibers, pain, Tpo • Merkel (tactile) disc – epidermis for light touch • Hair follicle receptors – detect bending hair or are light touch detecting mosquitoes – “itch receptor” B) Encapsulated Dendritic Endings - most are mechano-receptors w/ >1 terminals of sensory nerves in CT

  7. Encapsulated - Cont. • Meissner’s corpuscle in dermal papillae of sensitive skin (nipples, fingertips, soles/palms) – light touch analogous to hair follicle in hairless skin • Pacinian corpuscle in deep dermis & sub-Q, respond to deep pressure but only at first stimulus, so their value is in vibration detection. Largest receptor • Ruffini corpuscle in dermis, sub-Q, & joint capsules, with a “spray” of receptor endings responding to deep & continuous pressure • Muscle spindles – proprioceptors of skeletal m.peri-mysium – modified muscle fibers called intrafusal fibers in CT – stretch receptor that initiates a reflex stop

  8. Encapsulated - Cont. 5) Golgi tendon organs – proprioceptors in tendons where they join the muscle body – stimulated by muscle contraction and tendon stretch – activation of these causes muscle to relax (inhibited) 6) Joint kinesthetic receptors – proprioceptors that monitor stretch in synovial joint capsules – 4 of the categories contribute to this group: Ruffini, Golgi Tendon, Pacinian, & Free (naked) all of which provide information on joint position

  9. Simple Receptors: Unencapsulated Table 13.1.1

  10. Simple Receptors: Encapsulated Table 13.1.2

  11. From Sensation to Perception • Survival depends upon sensation and perception • Sensation is the awareness of changes in the internal and external environment • Perception is the conscious interpretation of those stimuli

  12. Organization of the Somatosensory System • Input comes from exteroceptors, proprioceptors, and interoceptors • The three main levels of neural integration in the somatosensory system are: • Receptor level – the sensor receptors • Circuit level – ascending pathways • Perceptual level – neuronal circuits in the cerebral cortex

  13. Somatosensory System Processing – 3 Levels • Receptor level – stimulus must: • Have specificity i.e. mechano won’t respond to light • Be applied in receptor field – smaller field the better able the brain can locate the stimulus • Be converted to graded (receptor) potential that may be excitatory or inhibitory. Those that summate to cause AP are called generator potentials that release NT • Generator potential must reach threshold to open Na+ gated channels on the axon to send impulse to CNS * Info about the stimulus: strength, duration, pattern is all in the frequency of arriving impulses (faster = stronger)

  14. Cont. 2) Tonic receptors just keep signaling at a steady rate (unless inhibited or activated), like the equilibrium receptors of the inner ear 3) Phasic receptors are “off” unless activated by some change in the environment and they just “report” changes in the internal or external Hearing , smell and some other sensory receptors exhibit adaptation by being less sensitive as stimuli continue – phasic are fast, tonic are slow and pain & proprioception don’t at all

  15. Processing at the Circuit Level • Task is to deliver impulse to the appropriate cerebral cortex region to localize stimulus and for perception • Uses 1st, 2nd, or 3rd order neurons to get info to thalamus, cerebellum and cerebral cortex • Non-specific pathways – pain, Tpo, coarse touch so it is “general” info heavily tied to emotional aspect of perception (pleasure or pain) • Specific pathways – info from discriminating aspect of touch, vibration, pressure, & conscious proprioception

  16. Processing at the Perceptual Level • Cortical areas receive AP’s regardless of whatever stimulates a receptor –projection to the usual area for that stimulus so it can be interpreted as a specific modality i.e. hearing, taste “Caller ID for the brain” • Perceptual detection – simplest, requires receptor summation • Magnitude estimation – how much stimulus (↑ frequency) • Spatial discrimination – ID site or pattern of stimulus two point discrimination shows closeness – tactile receptor density • Feature abstraction – sensation from several stimulus properties i.e. velvet feels warm, smooth, compressible – texture/shape ID • Quality discrimination – submodality, i.e. sour, sweet, bitter etc. • Pattern recognition – take in the scene & detect familiar (song)

  17. Nerve structure – axon surrounded by endoneurium, w/ bundle (fascicle) surrounded by perinerurium, w/ all fascicles surrounded by the epineurium

  18. PNS Nerves • Afferent – Efferent – Mixed (both fibers in most nerves) • Mixed nerves carry both somatic & autonomic (visceral) nervous system fibers, therefore they are: • Classified as: somatic afferent or efferent, and visceral afferent or efferent and are further classified as either cranial or spinal depending upon where they arise • Ganglia – collections of neuron cell bodies of PNS nerves • Those associated with afferent nerve fibers are of sensory neurons (dorsal root ganglia), while those of efferent fibers are cell bodies of autonomic motor neurons

  19. Nerve Regeneration • Cut axons may regenerate, but NOT cell bodies • Cut axonal end seals itself and the end undergoes Wallerian degeneration as axon and myelin sheath disintegrate and fragment the axon w/in a week, but the neurilemma (Schwann cell) remains intact w/in the endoneurium • Schwann cells then migrate into the injury site and begin releasing growth factors and N-CAMS and that guide new axon through a regeneration tube at 1.5mm/day, but distance limits success and it never occurs naturally in the CNS

  20. Regeneration of Nerve Fibers Figure 13.4

  21. Regeneration of Nerve Fibers Figure 13.4

  22. Cranial Nerves • Twelve pairs of cranial nerves arise from the brain • They have sensory, motor, or both sensory and motor functions • Each nerve is identified by a number (I through XII) and a name • Four cranial nerves carry parasympathetic fibers that serve muscles and glands

  23. Summary of Function of Cranial Nerves Figure 13.5b

  24. Cranial Nerve I: Olfactory • Arises from the olfactory epithelium • Passes through the cribriform plate of the ethmoid bone • Fibers run through the olfactory bulb and terminate in the primary olfactory cortex • Functions solely by carrying afferent impulses for the sense of smell

  25. Cranial Nerve I: Olfactory Figure I from Table 13.2

  26. Cranial Nerve II: Optic • Arises from the retina of the eye • Optic nerves pass through the optic canals and converge at the optic chiasm • They continue to the thalamus where they synapse • From there, the optic radiation fibers run to the visual cortex • Functions solely by carrying afferent impulses for vision

  27. Cranial Nerve II: Optic Figure II from Table 13.2

  28. Cranial Nerve III: Oculomotor • Fibers extend from the ventral midbrain, pass through the superior orbital fissure, and go to the extrinsic eye muscles • Functions in raising the eyelid, directing the eyeball, constricting (parasympathetic) the iris, and controlling lens shape • Parasympathetic cell bodies are in the ciliary ganglia

  29. Oculomotor Deficit • Lack of pupillary response • External strabismus – opposite of being “cross-eyed” • Drooping eyelid - ptosis

  30. Cranial Nerve III: Oculomotor Figure III from Table 13.2

  31. Cranial Nerve IV: Trochlear • Fibers emerge from the dorsal midbrain and enter the orbits via the superior orbital fissures; innervate the superior oblique muscle • Primarily a motor nerve that directs the eyeball

  32. Cranial Nerve IV: Trochlear Figure IV from Table 13.2

  33. Cranial Nerve V: Trigeminal • Three divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3) • Fibers run from the face to the pons via the superior orbital fissure (V1), the foramen rotundum (V2), and the foramen ovale (V3) • Conveys sensory impulses from various areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication

  34. Cranial Nerve V: Trigeminal Figure V from Table 13.2

  35. Cranial Nerve VI: Abdcuens • Fibers leave the inferior pons and enter the orbit via the superior orbital fissure • Primarily a motor nerve innervating the lateral rectus muscle Figure VI from Table 13.2

  36. Cranial Nerve VII: Facial • Fibers leave the pons, travel through the internal acoustic meatus, and emerge through the stylomastoid foramen to the lateral aspect of the face • Mixed nerve with five major branches • Motor functions include facial expression, and the transmittal of autonomic impulses to lacrimal and salivary glands • Sensory function is taste from the anterior two-thirds of the tongue

  37. Bell’s Palsy

  38. Xerophthalmia (“dry eye”)

  39. Cranial Nerve VII: Facial Figure VII from Table 13.2

  40. Cranial Nerve VIII: Vestibulocochlear • Fibers arise from the hearing and equilibrium apparatus of the inner ear, pass through the internal acoustic meatus, and enter the brainstem at the pons-medulla border • Two divisions – cochlear (hearing) and vestibular (balance) • Functions are solely sensory – equilibrium and hearing

  41. Cranial Nerve VIII: Vestibulocochlear Figure VIII from Table 13.2

  42. Cranial Nerve IX: Glossopharyngeal • Fibers emerge from the medulla, leave the skull via the jugular foramen, and run to the throat • Nerve IX is a mixed nerve with motor and sensory functions • Motor – innervates part of the tongue and pharynx, and provides motor fibers to the parotid salivary gland • Sensory – fibers conduct taste and general sensory impulses from the tongue and pharynx

  43. Cranial Nerve IX: Glossopharyngeal Figure IX from Table 13.2

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