Chapter 15, part 1

1. Chapter 15, part 1 Neural Integration I: Sensory Pathways and the Somatic Nervous System

2. Learning Objectives • Specify the components of the afferent and efferent divisions of the nervous system, and explain what is meant by the somatic nervous system. • Explain why receptors respond to specific stimuli and how the organization of a receptor affects its sensitivity. • Identify the major sensory pathways.

3. Learning Objectives • Explain how we can distinguish among sensations that originate in different areas of the body. • Describe the components, processes and functions of the somatic motor pathways. • Describe the levels of information processing involved in motor control.

4. SECTION 15-1An Overview of Sensory Pathways and the Somatic Nervous System

5. Neural pathways • Afferent pathways • Sensory information coming from the sensory receptors through peripheral nerves to the spinal cord and on to the brain • Efferent pathways • Motor commands coming from the brain and spinal cord, through peripheral nerves to effecter organs

6. Figure 15.1 An Overview of Neural Integration Figure 15.1

7. SECTION 15-2Sensory Receptors and their Classification

8. Sensory receptor • Specialized cell or cell process that monitors specific conditions • Arriving information is a sensation • Awareness of a sensation is a perception

9. Senses • General senses • Pain • Temperature • Physical distortion • Chemical detection • Receptors for general senses scattered throughout the body • Special senses • Located in specific sense organs • Structurally complex

10. Sensory receptors • Each receptor cell monitors a specific receptive field—the larger the field, the poorer the spatial resolution

11. Receptive Fields

12. Receptors transduce stimulus signals to neural signals • Transduction • A large enough stimulus changes the receptor potential, reaching generator potential

13. Receptors • Tonic receptors • Always active • Slow acting receptors • Phasic receptors • Provide information about the intensity and rate of change of a stimulus • Fast acting receptors • Adaptation • Reduction in sensitivity in the presence of a constant stimulus

14. Tonic and Phasic Responses and Adaptation

15. Two Examples of Adaptation

16. The general senses • Three types of nociceptor • Provide information on pain as related to extremes of temperature • Provide information on pain as related to extremes of mechanical damage • Provide information on pain as related to extremes of dissolved chemicals • Myelinated type A fibers carry fast pain • Slower type C fibers carry slow pain

17. Myelinated sheathing and Axon diameter

18. Figure 15.2 Receptors and Receptive Fields Figure 15.2

19. Thermoceptors and mechaniceptors • Found in the dermis • Mechaniceptors • Sensitive to distortion of their membrane • Tactile receptors (six types) • Baroreceptors • Proprioceptors (three groups)

20. Figure 15.3 Tactile Receptors in the Skin Figure 15.3a-f

21. Touch • Types of classification: • Capsulated/non-capsulated • Fast-adapting/slow-adapting; amount of myelin • Superficial/Deep layer

22. Touch Receptor Cell Types • Merkel cell: Slow adapting, superficial, non-capsulated • Meissner cell: Fast adapting, superficial, capsulated • Pacinian: Fast adapting, deep, capsulated • Ruffini: Slow adapting, deep, non-capsulated

23. Chemoreceptors • Chemoreceptors—function is to allow for regulation of pH, PCO2, and PO2 • Carotid bodies • Aortic bodies

24. Figure 15.5 Chemoreceptors Figure 15.5

25. Figure 15.4 Baroreceptors and the Regulation of Visceral Function Figure 15.4

26. Nociceptors (Pain Receptors) • Useful to indicate something is amiss (adaptive advantage to know when something is hurting you) • Normally produced by intense stimuli that can damage tissue • BUT, hypersensitivity to pain can occur

27. Hypersensitivity to Pain • Hyperalgesia: Sensitization such that normal pain becomes excruciating pain • Allodynia: Normal, non-pain (e.g. light touch) becomes painful • How and why would hypersensitivity occur?

28. Hypersensitivity to Pain • Can occur due to nociceptor sensitization—may result from changes in membrane permeability, membrane receptors, post- and pre-synaptic modulation, etc. • Peripheral sensitization: occurs in PNS; increased responsiveness/lower threshold to stimuli • E.g. sunburn • Central sensitization: occurs in CNS; increased excitability of neurons—often triggered by increase in nociceptor activity • E.g. Following an injury • Actually a manifestation of abnormal sensory processing within the CNS (not on the periphery) • E.g. migraines, post-surgical hypersensitization, “tender” areas surrounding injury • And MAYBE fibromyalgia, irritable bowel syndrome

29. Silent Nociceptors • Often can involve so-called “silent nociceptors”—nociceptors that normally don’t respond except under conditions of sensitization • Up to 30% of nociceptors are silent • May add to pain further, as the functioning density of nociceptors has essentially increased

30. SECTION 15-3The Organization of Sensory Pathways

31. First, second, and third order neurons • First order neurons • Sensory neurons that deliver sensory information to the CNS • Second order neurons • First order neurons synapse on these in the brain or spinal cord • Third order neurons • Found in the thalamus • Second order neurons synapse on these

32. Somatic sensory pathways • Three major pathways carry sensory information • Posterior column pathway • Anterolateral pathway • Spinocerebellar pathway

33. Figure 15.6 Sensory Pathways and Ascending Tracts in the Spinal Cord Figure 15.6

34. Posterior column pathway • Carries fine touch, pressure and proprioceptive sensations • Axons ascend within the fasciculus gracilis and fasciculus cuneatus • Relay information to the thalamus via the medial lemniscus • Decussation—”crossing over”

35. Figure 15.8 The Posterior Column Pathway and the Spinothalamic Tracts Figure 15.8a, b

36. Anterolateral pathway • Carries poorly localized sensations of touch, pressure, pain, and temperature • Axons decussate in the spinal cord and ascend within the anterior and lateral spinothalamic tracts • Headed toward the ventral nuclei of the thalamus

37. Figure 15.8 The Posterior Column Pathway and the Spinothalamic Tracts Figure 15.8c

38. Sensory Homunculi

39. Posterior and Anterolateral Pathways

40. Spinocerebellar pathway • Includes the posterior and anterior spinocerebellar tracts • Carries sensation to the cerebellum concerning position of muscles, tendons and joints

41. Figure 15.9 The Spinocerebellar Pathway Figure 15.9

42. Visceral sensory pathways • Carry information collected by interoceptors • Information from cranial nerves V, VII, IX and X delivered to solitary nucleus in medulla oblongata • Dorsal roots of spinal nerves T1 – L2 carry visceral sensory information from organs between the diaphragm and pelvis • Dorsal roots of spinal nerves S2 – S4 carry sensory information below this area

43. SECTION 15-4The Somatic Nervous System

44. Somatic motor pathways • Upper motor neuron • Cell body lies in a CNS processing center • Lower motor neuron • Cell body located in a motor nucleus of the brain or spinal cord

45. Figure 15.10 Descending (Motor) Tracts in the Spinal Cord Figure 15.10

46. The corticospinal pathway • Provides voluntary skeletal muscle control • Corticobulbar tracts terminate at cranial nerve nuclei • Corticospinal tracts synapse on motor neurons in the anterior gray horns of the spinal cord • Visible along medulla as pyramids

47. Pyramids • Most of the axons decussate to enter the descending lateral corticospinal tracts • Those that do not cross over enter the anterior corticospinal tracts • Provide rapid direct method for controlling skeletal muscle

48. Figure 15.11 The Corticospinal Pathway Figure 15.11

49. medial and lateral pathways • The medial and lateral pathways • Issue motor commands as a result of subconscious processing • Medial pathway • Primarily controls gross movements of the trunk and proximal limbs • Includes the vestibulospinal tracts, tectospinal tracts and reticulospinal tracts

50. lateral pathways • Lateral pathway • Controls muscle tone and movements of the distal muscles of the upper limbs • Rubrospinal tracts