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Somatic Sensory Pathways

Somatic Sensory Pathways. Two Pathways: 3 neurons in pathway 1 st , 2 nd and 3 rd order neurons (from PNS through CNS) 2 nd fiber crosses over (ipsilateral to contralateral) dorsal column / medial lemniscus system anterolateral system. Fig. 7- 2 Ganong. Somatic Sensory Pathways.

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Somatic Sensory Pathways

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  1. Somatic Sensory Pathways Two Pathways: • 3 neurons in pathway • 1st, 2nd and 3rd order neurons (from PNS through CNS) • 2nd fiber crosses over (ipsilateral to contralateral) • dorsal column / medial lemniscus system • anterolateral system

  2. Fig. 7- 2 Ganong Somatic Sensory Pathways Fig. 13.4

  3. Dorsal Column / Medial Lemniscus System • Signals ascend spinal cord via the dorsal (white) columns. • large myelinated fibers (types I and II) • fine touch and proprioception • rapid transmission • high degree of spatial orientation Fig. 13.5

  4. Posterior Column / Medial Lemniscus System • 1st order sensory fibers ascend ipsilaterally in dorsal (posterior) columns to the medulla, and synapse in the medulla. • 2nd order fibers cross over in the medulla and travel through the brain stem via the medial lemniscus to the thalamus. • 3rd order fibers project to the somatosensory areas of the cerebral cortex. • post central gyrus of the parietal lobe

  5. Anterolateral System • Signals ascend the spinal cord via the anterior and lateral spinothalamic tracts. • small myelinated and unmyelinated fibers (types III and IV) • crude touch, pain, temperature • slow transmission • high volume of information • low spatial orientation Fig. 13.5

  6. Anterolateral System • 1st order sensory neurons enter spinal cord and synapse in the posterior horn. • 2nd order fibers cross over in the spinal cord, and ascend spinal cord to the thalamus via the anterior (ventral) and lateral spinothalamic tracts. • 3rd order fibers project to the somatosensory areas of the cerebral cortex. • post central gyrus of the parietal lobe

  7. Parietal LobePost Central Gyrus • homunculus • visual representation of the human body on the cerebral cortex • Because of crossover, the left side of the body projects to the right side of the cerebrum and vice versa. Fig. 14.21

  8. Somatic Sensation • Perception depends less on the source of the signal and more on what part of the brain is stimulated. • 2 examples from pain perception • phantom pain • referred pain

  9. Phantom Pain • Amputees feel pain in their (missing) limbs. • filling of cerebral cortical “vacuum” by other afferent signals? • activation of cut nerve endings?

  10. Referred Pain • Pain from viscera is perceived as coming from the skin. • e.g., heart attack: pain in left arm and shoulder • related to dermatomes Fig. 7-7 Ganong Fig. 16.4

  11. Regulation of Pain Sensation • Gate Theory • In the dorsal horn, the synapse between the first and second order neuron is a gateway for pain. • Pain may be blocked by “closing” this gate. • counter irritants • brain’s analgesia system: one of its sites of action gate Fig. 16.5 • collateral from touch receptor Fig. 7-2 Ganong

  12. Sensory Collaterals to the Brain Stem Stimulate the Reticular Activating System Fig. 16.3

  13. Subconscious Proprioception • via spinocerebellar pathways Fig. 13.4

  14. Video AssignmentThe Brain – Parts 1 and 2 • Part 1: The Enlightened Machine • Part 2: Vision and Movement • DVD and VHS versions are available. • Each part lasts about an hour. • See p.13 of Lecture Outlines for checkout information. • As you watch the videos, pay attention to who has what diseases.

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