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The Peripheral Nervous System (PNS)

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  1. The Peripheral Nervous System (PNS) P A R T A

  2. Peripheral Nervous System (PNS) • PNS – all neural structures outside the brain and spinal cord • Includes sensory receptors, peripheral nerves, associated ganglia, and motor endings • Provides links to and from the external environment

  3. PNS in the Nervous System Figure 13.1

  4. Sensory Receptors • Structures specialized to respond to stimuli • Activation of sensory receptors results in depolarizations that trigger impulses to the CNS • The realization of these stimuli, sensation and perception, occur in the brain

  5. Receptor Classification by Stimulus Type • Mechanoreceptors – respond to touch, pressure, vibration, stretch, and itch • 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

  6. Receptor Class by Location: Exteroceptors • Respond to stimuli arising outside the body • Found near the body surface • Sensitive to touch, pressure, pain, and temperature • Include the special sense organs

  7. Receptor Class by Location: Interoceptors • Respond to stimuli arising within the body • Found in internal viscera and blood vessels • Sensitive to chemical changes, stretch, and temperature changes

  8. Receptor Class by Location: Proprioceptors • Respond to degree of stretch of the organs they occupy • Found in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles • Constantly “advise” the brain of one’s movements

  9. Receptor Classification by Structural Complexity • Receptors are structurally classified as either simple or complex • Most receptors are simple and include encapsulated and unencapsulated varieties • Complex receptors are special sense organs

  10. Simple Receptors: Unencapsulated • Free dendritic nerve endings • Respond chiefly to temperature and pain • Merkel (tactile) discs • Hair follicle receptors

  11. Simple Receptors: Encapsulated • Meissner’s corpuscles (tactile corpuscles) • Pacinian corpuscles (lamellated corpuscles) • Muscle spindles, Golgi tendon organs, and Ruffini’s corpuscles • Joint kinesthetic receptors

  12. Unencapsulated Receptors Table 13.1.1

  13. Simple Receptors:Encapsulated Table 13.1.2

  14. From Sensation to Perception • Sensation is the awareness of changes in the internal and external environment • Perception is the conscious interpretation of those stimuli

  15. 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

  16. Figure 13.2

  17. Processing at the Receptor Lever • The receptor must have specificity for the stimulus energy • The receptor’s receptive field must be stimulated • Transduction • Conversion of the energy of a stimulus into the energy of a nerve signal

  18. Processing at the Receptor Lever • Receptor potential • It is a graded potential happening on a receptor • Depolarization or hyperpolarization • Generator potential • It is a receptor potential strong enough to cause an action potential in an afferent fiber

  19. Adaptation of Sensory Receptors • Adaptation is a reduction in sensitivity in the presence of a stimulus • Receptor membranes become less responsive • Receptor potentials decline in frequency or stop

  20. Adaptation of Sensory Receptors • Tonic receptors • Have little peripheral adaptation • Chemical interoceptors • Pain receptors • Macula in the vestibular apparatus • Proprioceptors

  21. Adaptation of Sensory Receptors • Phasic receptors • Are fast adapting receptors • Pressure • Touch • Smell

  22. Processing at the Circuit Level • Chains of three neurons that conduct sensory impulses to the cerebral cortex • First-order neurons – soma reside in dorsal root or cranial ganglia, and conduct impulses from the skin to the spinal cord or brain stem

  23. Processing at the Circuit Level • Second-order neurons – soma reside in the dorsal horn of the spinal cord or medullary nuclei and transmit impulses to the thalamus or cerebellum • Third-order neurons – located in the thalamus and conduct impulses to the somatosensory cortex of the cerebrum

  24. Processing at the Perceptual Level • The thalamus projects fibers to: • The somatosensory cortex • Sensory association areas • The exact point in the cortex that is activated will refer to where in the body the stimulus is happening • The result is an internal, conscious image of the stimulus

  25. Main Aspects of Sensory Perception • Perceptual detection – detecting that a stimulus has occurred and requires summation • Magnitude estimation =intensity of the stimulus • Frequency of action potentials

  26. Main Aspects of Sensory Perception • Spatial discrimination – identifies the location of the stimulus. It depends on the size of the receptor field. • Two-point discrimination test – smaller fields equals finer two-point discrimination test

  27. Main Aspects of Sensory Perception • Feature abstraction – used to identify a specific feature of the stimulus (texture or shape) • Quality discrimination – the ability to identify submodalities of a sensation (e.g., sweet or sour tastes) • Pattern recognition – ability to recognize patterns in stimuli (e.g., melody, familiar face)

  28. Structure of a Nerve • Nerve – peripheral axons enclosed by connective tissue • Connective tissue coverings include: • Endoneurium – loose connective tissue that surrounds axons • Perineurium – coarse connective tissue that bundles fibers into fascicles • Epineurium – tough fibrous sheath around a nerve

  29. Structure of a Nerve Figure 13.3b

  30. Classification of Nerves • Sensory (afferent) – carry impulse to the CNS • Motor (efferent) – carry impulses from CNS • Mixed nerves – carry somatic and autonomic (visceral) impulses • Most common type

  31. Peripheral Nerves • The four types of mixed nerves are: • Somatic • Sensory • Motor • Visceral • Sensory • Motor • Peripheral nerves can be cranial or spinal

  32. Regeneration of Nerve Fibers • Mature neurons are amitotic • If the soma remains intact, damage can be repaired • Steps • Separated ends seal themselves • Wallerian degeneration of the distal axon by macrophages • Formation of a regeneration tube by the Schwann cell • Guide the axon growth distally

  33. Regeneration of Nerve Fibers Figure 13.4

  34. Regeneration of Nerve Fibers Figure 13.4

  35. Cranial Nerves • Twelve pairs of cranial nerves arise from the brain • They have sensory, motor, or both sensory and motor (mixed nerves) functions • Each nerve is identified by a number (I through XII) and a name

  36. Cranial Nerves Figure 13.5a

  37. Summary of Function of Cranial Nerves Figure 13.5b

  38. 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 • Function is the sense of smell

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

  40. 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 carry impulses for vision

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

  42. Cranial Nerve III: Oculomotor • Motor for movements of the eyes • Parasympathetic fibers innervate the intrinsic muscles of the eye • Constricting the iris, and controlling lens shape

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

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

  45. Cranial Nerve V: Trigeminal • Three divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3) • Conveys sensory impulses from various areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication

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

  47. Cranial Nerve VI: Abducens • Primarily a somatic motor nerve Figure VI from Table 13.2

  48. Cranial Nerve VII: Facial • Somatic Motor to the muscles of facial expression, and the transmittal of • Visceral motor to lacrimal and salivary glands • Sensory function is taste from the anterior two-thirds of the tongue

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