The Somatosensory System CH7 Blumenfield

1. The Somatosensory SystemCH7 Blumenfield By: Laurence Poliquin-Lasnier R2 neurology

2. Outline • Sensory neuron • Main somatosensory pathways • Posterior column-medial lemniscus • Spinothalamic tract • Somatosensory cortex • Central modulation of pain • Thalamus • Spinal cord syndromes • Bladder, bowel and sexual function

3. Main somatosensory pathways • Posterior column-medial lemniscus • Proprioception, vibration, fine discriminative touch • Spinothalamic tract • Pain, temperature, crude touch • Via unipolar sensory neuron

4. Unipolar sensory neuron

5. Sensory neuron fiber types

7. Sensory neuron • Sensory neuron cell body located in dorsal root ganglia • A peripheral region innervated by sensory fibers from a single nerve root = dermatome

8. Outline • Sensory neuron • Main somatosensory pathways • Posterior column-medial lemniscus • Spinothalamic tract • Somatosensory cortex • Central modulation of pain • Thalamus • Spinal cord syndromes • Bladder, bowel and sexual function

9. Posterior column-Medial lemniscus

10. Posterior column-Medial lemniscus • Large myelinated axons • Proprioception, vibration, fine touch

11. Posterior column-Medial lemniscus Ascend through: • Gracile Fasciculus: legs + lower trunk • Cuneate Fasciculus: arms, neck, trunk above T6 • 1st order sensory neuron synapses synapse onto 2nd order neurons in the nucleus gracilis and nucleus cuneatus at th level of the medulla • Axons of these 2nd order neurons decussate as internal arcuate fibers and form the medial lemniscus on the other side of the medulla

12. Posterior column-Medial lemniscus • 2nd order neurons synapse into the ventral posterior lateral (VPL) nucleus of the thalamus • 3rd order neurons then project to the posterior limb of the internal capsule to reach the primary somatosensory cortex in the post-central gyrus

13. Sensory homunculus

14. Outline • Sensory neuron • Main somatosensory pathways • Posterior column-medial lemniscus • Spinothalamic tract • Somatosensory cortex • Central modulation of pain • Thalamus • Spinal cord syndromes • Bladder, bowel and sexual function

15. Spinothalamic tract

16. SpinothalamicTract • Small diameter • Unmyelinated • Pain and temperature

17. Spinothalamic tract • Enter spinal cord via dorsal root ganglia • 1st order neuron synapse in the grey matter of the dorsal horn marginal zone (lamina1) and deeper in the dorsal horn (lamina 5) • Some axon collaterals ascend or descend for a few segments in lissauer tract before entering the central gray • 2nd order neuron cross over in the spinal cord anterior commissure to ascend in the anterolateral white matter • It takes 2-3 spinal segments for the decussating fibers to reach the opposite side ( so sensory level of spinal cord lesion starts a few levels below the lesion)

18. Spinothalamic tract • Anterolateral pathway reaches medulla • Run between the olives and the inferior cerebellar peduncles • Enters pontinetegmentum • 2nd order neuron synapses in the thalamus to 3rd order neuron • 3rd order neuron to somatosensory cortex in the postcentralgyrus • Secondary somatosensory association cortex in parietal operculum (somatotopic organization) and association area in posterior parietal lobule

19. Anterolateral pathway: 3 tracts • Spinothalamic (I, V) • Discriminative aspects of pain, location, intensity • Synapse on VPL (different area than DCML), relay to specific SSC target (Brodmann 3,1,2) • Spinoreticular (VI, VII, VIII) • Emotional and arousal aspects of pain • Reticular formation projects to intralaminar thalamic nuclei (centromedian), which then project diffusely to the entire cerebral cortex (behavioural arousal) • Spinomesencephalic (I, V) • To periaqueductal grey and superior colliculi • Pain modulation

20. Outline • Sensory neuron • Main somatosensory pathways • Posterior column-medial lemniscus • Spinothalamic tract • Somatosensory cortex • Central modulation of pain • Thalamus • Spinal cord syndromes • Bladder, bowel and sexual function

21. Central modulation of pain • Gate control theory • Sensory input from large diameter non pain A-β fibers reduce pain transmission through the dorsal horn • Periaqueductal gray receives input from: hypothalamus, amygdala, cortex • Inhibits pain transmission in the dorsal horn via relay in rostral ventral medulla (RVM) • RVM includes serotonergic neurons of the raphe nuclei that project to the spinal cord and modulate pain • RVM sends input (via substance P) to the locus ceruleus to spinal cord dorsal horn (via NE)

22. Central modulation of pain • Opiate receptors and endogenous opiate peptides located at key points in the pain modulatory pathways • Enkephalin and dynorphin -> PAG, RVM, dorsal column • β-endorphin -> hypothalamus

23. Outline • Sensory neuron • Main somatosensory pathways • Posterior column-medial lemniscus • Spinothalamic tract • Somatosensory cortex • Central modulation of pain • Thalamus • Spinal cord syndromes • Bladder, bowel and sexual function

24. Thalamus • Major sensory relay station • Deep gray matter structure part of the diencephalon • Convey different types of input to the cortex • Sensory • Motor from cerebellum and basal ganglia • Limbic • Modulatory inputs involved in aroual and sleep-wake cycle

25. Thalamus • Divided by internal medullary lamina (a Y shaped structure) into: • Medial nuclear group • Lateral nuclear group • Anterior nuclear group • Nuclei within internal medulary lamina called intralaminar nuclei

26. Thalamus

27. Thalamus • 3 categories of nuclei: • Relay nuclei • Intralaminar nuclei • Reticular nucleus

28. Thalamus: Relay nuclei • Lie mainly in lateral thalamus • All primary sensory modalities have relays in the lateral thalamus en route to their specific cortical target, with one exception -> olfaction • Reciprocal innervationw/ cortex

29. Thalamus: Relay nuclei -> Lateral nuclear group

30. Thalamus: Relay nuclei -> other groups

31. Thalamus

34. Clinical concept: dysfunction in pain pathways • Negative symptom = sensory loss • Positive symptoms = paresthesias = added sensation • Dysesthesia = unpleasant abnormal sensation • Allodynia = painful sensation provoked by minor stimulus eg.: light touch • Posterior column: tingling, numb, tight band, walking on clouds • Anterolateral: sharp, burning pain

35. Outline • Sensory neuron • Main somatosensory pathways • Posterior column-medial lemniscus • Spinothalamic tract • Somatosensory cortex • Central modulation of pain • Thalamus • Spinal cord syndromes • Bladder, bowel and sexual function

36. Spinal cord lesions Spinal shock: • Flaccid paralysis below the lesion • Loss of DTR • Autonomic dysfunction • Decreased sympathetic outflow to vascular smooth muscles -> Hypotension • Absent sphincter tone • Over weeks to months, spasticity and UMN signs develop Cord compression: • If non-ambulatory at tx, 80% remain so • If ambulatory at tx, 80% will remain mobile

37. Sensory loss: patterns and localization • Primary somatosensory cortex • Contralateral face, arm, leg, trunk • Two point discrimination, extinction, stereognosis, graphestesia • Thalamus (VPL or VPL) • Contralateral face, arm, leg, trunk • Relative preservation of cortical features • Lateral pons and medulla • Pain and temperature • Ipsi face and contra hemibody • Medial medulla • Medial lemniscus = vibration, position sense

38. Spinal cord syndromes • Transverse cord syndrome • Sensory level with loss of all sensory modalities • DDx: trauma, tumor, MS, transverse myelitis

39. Spinal cord syndromes • Hemicord syndrome • “Brown-Sequard” • Damage to lateral corticospinal tract = ipsi UMN weakness • Damage to post. column = ipsi loss of vibration and position sense • Damage to anterolateral system = contra pain and temperature • May have a strip of 1-2 segments of ipsi loss of pain and temp caused by damage to post horn cell before their axons have crossed over

40. Spinal cord syndromes • Central cord syndrome • Suspended sensory loss to pain and temp • Cape-like pattern if cervical cord • Suspended dermatomes if at other level • LMN deficit if damage to anterior horn cells

41. Spinal cord syndromes • May get sacral sparing as spinothalamic tract = more medial cervical region and more lateral sacral region • Causes of central cord syndrome: • Spinal cord contusion, post-traumatic syringomyelia, intrinsic spinal cord tumor

42. Spinal cord syndromes • Posterior cord syndrome • Loss of vibration and position sense below the lesion • May get UMN weakness if it encroaches lateral corticospinal tract • Causes: trauma, extrinsic compression, MS, Vitamin B12 deficiency, tabesdorsalis (tertiary syphilis), HTLV-1

43. Spinal cord syndromes • Anterior cord syndrome • Damage to anterolateral pathway = loss of pain and temp below lesion • Damage to anterior horn cell may produce LMN weakness at the level of the lesion • If larger lesion, corticospinal tract involved -> UMN weakness

44. Spinal cord syndrome Anterior spinal artery syndrome: • Back of neck pain of sudden onset • Rapidly progressive flaccid and areflexic paraplegia • Loss of pain and temperature to a sensory level • Preservation of JPS and vibration sensation • Urinary incontinence

45. Outline • Sensory neuron • Main somatosensory pathways • Posterior column-medial lemniscus • Spinothalamic tract • Somatosensory cortex • Central modulation of pain • Thalamus • Spinal cord syndromes • Bladder, bowel and sexual function

46. Anatomy of bowel, bladder and sexual function • Complex interplay between sensory, motor (voluntary and involuntary) and autonomic pathways at multiple levels of the nervous system • Frontal “micturition inhibiting area”, sensorimotor sphincter control area, BG, vermis, pontinemicturition center • S2-S4 • Sensory (bladder, rectum, urethra, genitalia) • Ascends via posterior & anterolateral columns • Motor • ant. horn cell  pelvic floor • Onuf’s nucleus =sphincteromotor nucleus  urethral and anal sphincters contraction • Parasympatheticsdetrusor contraction • Sympathetics T11-L1 (intermediolateral cell column)detrusor relaxation, bladder neck contraction • Need bilateral pathways involved to get clinical syndrome

47. Bladder function: detrusor reflex (voiding) and urethral reflex (storage) • Voluntary relaxation of external urethral sphincter • Inhibition of sympathetics to bladder neck (relaxes) • Parasympathetic activation for detrusor (dome) contraction • Self-perpetuate as long as urine flows • When urine stops, , urethral sphincters contract triggering detrusor relaxation Detrusor reflex mediated by intrinsic spinal cord circuits, pontinemicturition center, cerebellar and BG pathways

48. Incontinence • Lesions affecting bilateral medial frontal micturition centers result in reflex activation of pontine and spinal micturition centers when the bladder is full • Normal emptying but not under voluntary control • Causes of frontal type incontinence: hydrocephalus, parasagittalmeningioma, traumatic brain injury, neurodegenerative disorders

49. Incontinence • Lesion below pontinemicturition center but above conus (S2-S4) • Flaccid, acontractile (atonic) bladder ->retention • Evolves over months into hyperreflexic spastic bladder -> retention 2ary dyssynergia and feeling of urgency 2ary reflex bladder contractions • Peripheral nerve lesion or lesion at S2-S4 • Flaccid atonic bladder ->overflow incontinence • Loss of parasympathetic outflow to detrusor or loss of afferent sensory information

50. Bowel function • Also mediated by medial frontal lobes • 3 components: • Internal smooth muscle sphincter + GI motility (parasympathetics) • External striated sphincter (Onuf) • Pelvic floor muscles (S2-S4 anterior horn cells) • Etiologies: damage at any level • Acute lesions flaccid sphincter and loss of sacral PS  constipation