Somatic Sensory System

1. Somatic Sensory System Sensation arising from skin muscle joints Allow you to survive in your environment and make appropriate motor responses

2. Somatic Sensation • The ability to feel your physical environment, to ache, feel temperature, pain, to know where your body position is • Pressure, position of joints/muscles, temperature, distension of bladder, stomach • Over-stimulation of body can be damaging

3. Sensory Receptors • Spread throughout body • Senses 4 types of information aka sensory modality • Touch, pressure, vibration • Body position, propioceptive receptors • Temperature • Pain-Nociception

4. Encoded Information • Intensity • Duration • Position • Direction

5. Propioception • Sensory Information regarding joint and limb position • Allow you to know your body position without looking in the mirror • Muscle spindles- intrafusal fibers and • Golgi Tendon organs are specialized structures that are innervated by DRGNs and send information to the spinal cord

6. Touch • Skin: largest sensory organ • Epidermis (outer layer) & dermis (inner layer) • Sensitive to raised dot: 0.006mmH x 0.04mmW; braille dot is 167 times bigger • Skin receptors • Hairy & glabrous (hairless)

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8. Mechanoreceptors • Found in skin for sensing contact with physical world • Found in bladder, blood vessels, heart digestive organs and teeth to sense pressure • Mechanoreceptors are innervated by myelinated axons • Axons have mechanosensitive ion channels gated by stretch & tension changes

9. Types of Mechanoreceptors Found in Skin • Pacinian corpuscle • Found in dermis • Meissner corpuscle • Found in ridges of glabrous skin • Merkel’s disc in epidermis • Rufini endings • Found in hairy & glabrous skin

11. Primary Afferent Axon • Axons of varying diameters with soma in DRG and enter Spinal cord through dorsal roots into dorsal horn • Different diameter axons carry different types of somatosensory information • Project locally in spinal cord and have long ascending branches to contact secondary somatosensory axons

12. Receptive Field • Area of skin that is monitored by a single mechanoreceptor • Meissner & merkel have small RF (2-3mm) • Pacinian & ruffini have large RF (entire finger/ ½ palm)

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14. Dermatomes • The area of skin innervated by the right and left dorsal roots (1st order neuron) of a single spinal segment • When mapped, dermatomes form sets of bands representing surface of body innervated by axons in one level of sc.

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18. 2-point discrimination • Ability to discern 2 closely position points as 2 rather than 1. • Varies 20 fold throughout body • Fingertips have highest resolution • Due to high density of mechanoreceptors • Receptor subtypes with small receptive fields • More cortical neurons dedicated to deciphering sensory information

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21. First and Second Order Neurons • Primary and secondary somatosensory or afferent neurons • The primary neuron has the sensory receptor • The secondary neurons gets information from the primary and can project information or modify locally activity of primary neurons.

23. Second order Neurons aka Interneurons • Neurons that receive synaptic input form DRG neurons • Reside in dorsal horn and trigger reflex responses • Also ascend to brainstem and thalamus • Also reside in brain stem and are involved in perception

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27. Dorsal Column-Medial Leminiscus Pathway • Touch/vibration & position (proprioception) info travels to brain separate from pain/temperature • Afferent/central axon of large sensory (AB) fibers ascend ipsilaterally in dorsal columns with tactile info and limb position info • DC also have 2nd order ascending axons from dorsal horn neurons

29. Dorsal column nuclei • Axons terminate in DCN in medulla • Then decussate (cross) and ascend as medial lemniscus tract through pons & midbrain to synapse in ventral posterior nucleus (VPN) of thalamus • VPN axons then project to primary somatosensory neurons in parietal cortex (S1)

31. Somotopic Organization

35. Somotopy • Mapping of body areas sensation onto the cortex • Somotopic map called homunculus that shows that the largest number of neurons in S1 receive sensory information from hand and mouth

38. Somatosensory Cortex • In Parietal lobe, posterior to central sulcus • Carries on higher order processing of sensory information. Called S1 • Receives synaptic input from VP nucleus of thalamus • Respond to somatosensory info • Lesions in S1 impair somatic sensation • Electrically stimulate S1 and you “feel” a sensation on the appropriate body part

40. S1 Cortex • Reciprocal (bidirectional) connections between cortical areas • Association pathways • Restriction of information: some cortex areas specialize in decoding texture, size & shape • Thalamic input is to cortical layer IV which send axons to other layers in same area

41. S2 and Parietal Posterior Cortex • S2 is lateral to S1 and is association area • PPC is posterior to S1 and is involved in perception/recognition of sensation • Neurons in S2 and PPC have complex receptive fields which can include sensory information as well as attention and visual and movement planning.

42. Posterior Parietal Cortex • Injury causes neglect syndrome: Do not recognize body part as your own so you do not dress it, wash it. • Agnosia: Inability to recognize objects including your own body parts. • Astereoagnosia: inability to recognize something by touch, but recognized by sight

43. Trigeminal Touch Pathway • Two trigeminal nerves CN5 • Each divided into 3 PNs that innervate face, mouth and anterior 2/3 tongue & dura mater

44. Sensory CNs • CN 7 facial, 9 glossopharyngeal, 10 vagus • Large diameter axons carry tactile info from skin mechanoreceptors • Synapse on ipsilateral trigeminal nucleus in pons • Decussate and project to medial VP nucleus of thalamus • Project to somatosensory cortex

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