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

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

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

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

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

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

Parietal Lobe Post 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

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

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

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 Referred Pain Fig. 7-7 Ganong Fig. 16.4

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

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

Subconscious Proprioception via spinocerebellar pathways Fig. 13.4

Video Assignment The 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.