The document summarizes ascending and descending tracts in the spinal cord that transmit sensory and motor information between the spinal cord and brain. It describes the three neuron chain of ascending tracts that transmit exteroceptive, interoceptive, and proprioceptive sensory information from the peripheral receptors to the brain. It outlines the major ascending tracts - lateral and anterior spinothalamic tracts for pain/temperature and touch/pressure, and the posterior white column and cuneate and gracilis fasciculi for proprioception. It also summarizes the descending motor tracts including the corticospinal, rubrospinal, vestibulospinal, reticulospinal and tectospinal tracts that

1. Ascending& Descending tracts Dr. Israa M. Sulaiman Department of Anatomy IMS/MSU

2. The ascending tracts

3. By the end of the lecture, students should be able to define the ascending tract enumerate the tracts according to their functional components explain general outline of neuronal chain of ascending tracts illustrate and trace the neuronal chain of each tract apply anatomical knowledge to correlate with the clinical condition in case of injury to these tracts

4. contents function of nervous system in general sensory system overview spinal cord and nerve tracts ascending tracts organization in general ascending tracts functional components

5. nervous system communication receive information transform it into impulses ( transduction ) transmit impulses to the CNS correlate / coordinate transmit impulses to the effector organs response / action

6. CENTRAL NERVOUS SYSTEM integration / processing / modulating stimulus receptor neurone motor / descending tracts effector organ / response PNS transmission lower motor neurone sensory / ascending tracts

7. Sensory system sensory information three basic information Exteroceptive information Interoceptive information Proprioceptive information

8. sensory information are received and carried by ascending tracts exteroceptive sensation origin:- outside the body e.g. temp, touch, light, sound, chemicals, mechanical receptors:- surface layer of skin, mucosa proprioceptive sensation origin:- within the body e.g. muscles, joints, tendons receptors – deeper layer of skin, tendons, joints, GTO, muscle spindles, ligaments

10. sensory information from the peripheral sensory endings is conducted through the nervous system by a series of neurones

11. information conscious sensation reach the cerebral cortex unconscious sensation reach to the areas other than cortex

12. spinal cord Grey matter mostly made up of cell bodies of neurone White matter composed of nerve fibres ( ascending and descending tracts ) embedded in neuroglial cells

13. nerve fibres enter the spinal cord through posterior nerve root after entering the spinal cord sorted out and segregated into nerve bundles, tracts ( origin, function, termination )

14. ascending tracts bundles of nerve fibres linking spinal cord with higher centres of the brain convey information from soma / viscera to higher level of neuraxis

15. ascending sensory pathway are organized in three neuronal chain - First order neurone - Second order neurone - Third order neurone

16. First order neurone cell body in posterior root ganglion peripheral process connects with sensory receptor ending central process enter the spinal cord through the posterior root synapse with second order neuron in spinal gray matter

17. dorsal root dorsal root ganglion spinal nerve dorsal horn FIRST ORDER NEURON

18. Second order neurone cell body in posterior gray column of spinal cord axon crosses the midline ( decussate ) ascend & synapse with third order neuron in VPL nucleus of thalamus

19. SECOND ORDER NEURON cross the mid line in front of central canal VPL 1 st 2 nd

20. Third order neurone cell body in the thalamus give rise to projection fibres to the cerebral cortex, postcentral gyrus ( sensory area )

21. ascending sensory pathway ( in general form ) from sensory endings to cerebral cortex ( note the three neurons chain )

22. ascending tracts in spinal cord

23. Tracts & their functional components lateral spinothalamic tract pain, temperature anterior spinothalamic tract touch, pressure

24. posterior white column conscious proprioceptive sense, discriminative touch, vibratory sense spinocerebellar tract / cuneocerebellar tract unconscious information from muscle, joints, skin, subcutaneous tissues

25. Main somatosensory pathways sensation receptors pathways destination Pain and temperature Free nerve endings Lateral STT Spinal lemniscus Postcentral gyrus Light touch and pressure Free nerve endings Anterior STT Spinal lemniscus Postcentral gyrus Discriminative touch, vibratory sense, conscious muscle joint sense Meissner’s corpuscle, pacinian corpuscles, muscle spindles, tendon organs Fasciculus gracilis and cuneatus Medial lemniscus Postcentral gyrus

26. Lateral spinothalamic tract pain and thermal impulses ( input from free nerve endings, thermal receptors ) transmitted to spinal cord in delta A and C fibres central process enters the spinal cord through posterior nerve root , proceed to the tip of the dorsal gray column

27. the central process of 1 st order neuron synapse with cell body of 2 nd order neuron in substantia gelatinosa of posterior gray column of the spinal cord

28. the axon of 2 nd order neuron cross to the opposite side in the anterior gray and white commissure and ascend in contralateral white column as lateral spinothalamic tract end by synapsing with 3 rd order neuron in the ventral posterolateral nucleus of thalamus

29. axon of the 3 rd order neuron passes through the posterior limb of internal capsule and corona radiata to reach the postcentral gyrus of cerebral cortex ( area 3, 1 and 2 )

30. pain and temperature pathways

31. Clinical application destruction of LSTT loss of pain and thermal sensation on the contralateral side below the level of the lesion patient will not respond to pinprick recognize hot and cold

32. Anterior spinothalamic tract light touch and pressure impulses ( input from free nerve endings, Merkel’s tactile disks ) First order neuron dorsal root ganglion( all level ) Second order neuron in the dorsal horn, cross to the opposite side (decussate) ascend in the contralateral ventral column as ASTT end in VPL nucleus of thalamus Third order neuron in the VPL nucleus of thalamus project to cerebral cortex ( area 3, 1 and 2 )

33. touch and pressure pathways

34. Clinical application destruction of ASTT loss of touch and pressure sense below the level of lesion on the contralateral side of the body

35. Fasciculus gracilis and fasciculus cuneatus discriminative touch, vibratory sense and conscious muscle joint sense ( inputs from pacinian corpuscles, Messiner’s corpuscles, joint receptors, muscle spindles and Golgi tendon organs ) axon of 1 st order neuron enter the spinal cord passes directly to the posterior white column of the same side ( without synapsing )

36. long ascending fibres travel upward in the posterior column of the same side as fasciculus gracilis and fasciculus cuneatus ( FG – carrying fibres from lower thoracic, lumbar and sacral regions / including lower limbs ) ( FC - only in thoracic and cervical segments / including upper limb fibres ) synapse on the 2 nd order neuron in the nucleus gracilis and cuneatus of medulla oblongata of the same side.

37. lower 6 thoracic segments lumbar segments sacral segments cervical segments upper 6 thoracic segments fasciculus gracilis fasciculus cuneatus [ nucleus G & C ] in medulla G C

38. axons of 2 nd order neuron “ internal arcuate fibres ” cross the median plane ( sensory decussation ) ascend as medial lemniscus through medulla oblongata, pons, and midbrain synapse on the 3 rd order neuron in ventral posteriolateral nucleus of thalamus axon of 3 rd order neuron leaves and passes through the internal capsule, corona radiata to reach the postcentral gyrus of cerebral cortex area 3, 1 and 2 )

39. pathways for conscious proprioception discriminative touch vibratory sense

40. Clinical application destruction of fasciculus gracilia and cuneatus loss of muscle joint sense, position sense, vibration sense and tactile discrimination on the same side below the level of the lesion (extremely rare to have a lesion of the spinal cord to be localized as to affect one sensory tract only )

41. Posterior & anterior spinocerebellar tract transmit unconscious proprioceptive information to the cerebellum receive input from muscle spindles, GTOs and pressure receptors involved in coordination of posture and movement of individual muscles of the lower limb

42. First order neuron in dorsal root ganglion axons end in nucleus dorsalis of Clarke Second order neuron cell body in nucleus dorsalis of Clarke give rise to axons ascending to the cerebellum of the same side ( anterior – crossed & uncrossed fibres / posterior – uncrossed fibres)

43. muscle joint sense pathways to cerebellum

44. Spinotectal tract passes pain, thermal, tactile information to superior colliculus for spinovisual reflexes cross the median plane synapse in the superior colliculus integrate visual and somatic sensory information ( it brings about the movement of eye and head towards the source of information ) Spinoreticular tract uncrossed fibres, synapse with neurones of reticular formation (important role in influencing level of consciousness ) Spino-olivary tract

45. spinotectal tract spinoreticular tract spino-oloivary tract

46. clinical application relief of pain posterior rhizotomy (posterior root) cordotomy (lateral STT) Injury hemisection of spinal cord diseases tabes dorsalis / syringomyelia / vascular

47. Hemisection of the spinal cord ( Brown Sequard’s syndrome ) Dorsal column damage Lateral column damage Anterolateral column damage Damage to local cord segment and nerve roots

48. spinal cord hemisection

49. below the level of lesion on the side of lesion lateral column damage UMNL dorsal column damage loss of position sense loss of vibratory sense loss of tactile discrimination anterolateral system damage loss of sensation of pain and temperature on the side opposite the lesion local segment side of lesion Dorsal Root irritate destruction Ventral root flaccid paralysis

50. Lesions of central gray matter seen in syringomyelia ( progressive cavitation around or near the central canal of spinal cord especially in cervical segments ) interrupt fibres of lateral spinothalamic tract that passes in front of the central canal loss of pain and temperature sensibility on both sides ( proprioception and light touch is spared ) sensory dissociation

52. Posterior root lesions seen in tabes dorsalis ( neurosyphilis ) bilateral degeneration of posterior root and posterior funiculus ( particularly in lower segments of spinal cord ) Clinically Initial stage Irritation - paraesthesia Intermittant of attack of sharp pain Later decreased sensitivity to pain loss of muscle stretch reflexes loss of position sense, posture senses positive Romberg sign ( visual compensation ) walk with legs apart, high stepping gait

53. blood supply of spinal cord Anterior spinal artery Posterior spinal arteries Segmental spinal arteries - radicular arteries Feeder arteries - Adamkiewicz

54. posterior 3 rd of spinal cord dorsal column penetrating branches anterior and part of gray matter circumferential branches anterior white matter

55. dorsal 1/3 rd resulting from occlusion of the posterior spinal artery ventral 2/3 rd resulting from occlusion of the anterior spinal artery

56. Descending tracts By the end of this lecture, students should be able to: define the tract enumerate the tracts according to their functional components illustrate and trace the neuronal chain of each tract apply their knowledge of anatomy to correlate with the clinical condition in relation to the injury to these tracts

57. Motor system areas of the nervous system that are responsible for controlling movements

58. cerebellum premotor cortex motor cortex motor unit muscle spindle pyramidal tract sp cd internreurons & central pattern generator extrapyramidal tracts premotor cortex SMA PMC basal ganglia cortical sensory area MOTOR SYSTEM I II III IV V

59. Level I initiation, planning, programming of movements in response to desire to move ( probably originate in the limbic system and posterior parietal cortex ) desire is translated into movements ( basal ganglia and their cortical projections in the frontal lobe-SMA, PMC ) Level II coordination of movements cerebellum ( compare the intended movement / actual movement )

60. Level III descending pathways pyramidal tract - CoSt originates in the motor, premotor and somatosensory corticies synapse direclty on MN , IN extrapyramidal tract – VeSt, ReSt, TeSt, RuSt originate from subcortical structures receive inputs from motor cortex complex distribution, synapse on MN, IN

61. Level IV motor organization in spinal cord alpha & gamma neurons Renshaw’s cells interneurons / CPGs descending tracts CoSt, RuSt distal musculature – fine skilled movement VeSt- ReSt- TeSt axial, proximal musculature – balance, posture Level V final common pathway

62. primary structure responsible for translating desire into a movement is the basal ganglia

63. Introduction brain exerts powerful and subtle influences upon the activity of the voluntary musculature ( modulate, regulate, bias the activities of LMN ) through the descending pathways

64. descending tracts segregated bundles of nerve fibres in the white matter of the spinal cord descending from the supraspinal centres referred to as upper motor neurons ( UMN ) are concerned with somatic and visceral motor activity cells of origin lie in cerebral cortex and brain stem regulate the LMN activity

65. motor homunculus cerebral cortex

66. lower motor neurons ( LMN ) motor neurons that innervate the voluntary muscles in anterior gray column of spinal cord / motor nuclei of brainstem innervate skeletal muscles form final common pathway LMN

67. LMN constantly bombarded by nerve impulses( excitatory or inhibitory ) that descend from cerebral cortex, pons, midbrain and medulla sensory inputs from the posterior root

68. upper motor neurons ( UMN ) the descending supraspinal pathways that influence the activity of the LMN e.g. CoSt, CoBt , RuSt, TeSt, ReSt, VeSt

69. UMN control voluntary motor activity maintenance of posture & equilibrium control of muscle tone and reflex activity generally exerts their effect on groups of muscles ( not on one specific muscle ) reciprocally on agonist and antagonist muscle group

70. cerebral cortex – midbrain - pons - medulla oblongata descending tracts LMN sensory inputs

72. Corticospinal tract arises from the pyramidal cells of cerebral cortex fibres travel through corona radiata posterior limb of the internal capsule cerebral peduncle ( middle 3/ 5 th ) pons medulla oblongata ( passed through the pyramids )

73. at the caudal part of medulla oblongata most of the fibres 90 % cross the mid line ( motor decussation) descend in the lateral column as LCST terminate on LMN of anterior gray column at all spinal level remaining uncrossed fibres descend as ACST eventually fibres cross the mid line and terminate on LMN of anterior gray column of respective spinal cord segments

74. motor decussation medulla oblongata

75. corticospinal tract for fine skilled movements

76. Rubrospinal tract nerve cells in red nucleus ( tegmentum of midbrain at the level of superior colliculus ) nerve fibres / axons cross the mid line descend as rubrospinal tract through pons and medulla oblongata terminate anterior gray column of spinal cord ( facilitate the activity of flexor muscles )

78. Tectospinal tract nerve cells in superior colliculus of the midbrain nerve fibres/ axons cross the mid line descend close to medial longitudinal fasciculus terminate in the anterior gray column of upper cervical segments of spinal cord ( responsible for reflex movement of head & neck in response to visual stimuli )

80. Vestibulospinal tract nerve cells in vestibular nucleus (in the pons and medulla oblongata received afferents from inner ear and cerebellum axons descend uncrossed through medulla and through the length of spinal cord synapse with neuron in the anterior gray column of the spinal cord ( balance by facilitate the activity of the extensor muscles )

82. Reticulospinal tract nerve cells in reticular formation fibres pass through midbrain, pons, and medulla oblongata end at the anterior gray column of spinal cord control activity of motor neurons (influence voluntary movement and reflex activity )

83. reticulospinal tract

84. clinical application pyramidal tract refer to corticospinal tracts extrapyramidal tract other than corticospinal tract ( VeSt, ReSt, TeSt, RuSt )

85. upper motor neuron lesion Babinski sign ( extensor plantar response ) Superficial abdominal reflexes ( absent ) Cremasteric reflex ( absent ) Loss of performance of fine skilled voluntary movement

86. lower motor neuron lesion flaccid paralysis atrophy of muscles loss of reflexes muscular fasciculation muscular contracture

87. extrapyramidal tract lesions severe paralysis with little or no atrophy spasticity or hypertonicity exaggeration of deep muscular reflexes and clonus clasp-knife reaction

88. These motor pathways are complex and multisynaptic, and regulate: Axial muscles that maintain balance and posture Muscles controlling coarse movements of the proximal portions of limbs Head, neck, and eye movement

89. dorsal column lateral STT anterior STT pyramidal tracts extrapyramidal tracts nerve roots

90. spinal cord hemisection

91. thank you